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Ethene Dimerization on Zeolite-Hosted Ni Ions: Reversible Mobilization of the Active Site

Rasmus Yding Brogaard*
Rasmus Yding Brogaard
Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium
*E-mail: [email protected]
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http://orcid.org/0000-0001-7882-9183
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Mustafa Kømurcu
Mustafa Kømurcu
Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
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Michael Martin Dyballa
Michael Martin Dyballa
Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
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http://orcid.org/0000-0002-8883-1145
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Alexandru Botan
Alexandru Botan
Department of Physics, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
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Veronique Van Speybroeck
Veronique Van Speybroeck
Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium
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http://orcid.org/0000-0003-2206-178X
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Unni Olsbye*
Unni Olsbye
Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
*E-mail: [email protected]
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http://orcid.org/0000-0003-3693-2857
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Kristof De Wispelaere*
Kristof De Wispelaere
Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium
*E-mail: [email protected]
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Cite this: ACS Catal. 2019, 9, 6, 5645–5650

Publication Date (Web):May 22, 2019

https://doi.org/10.1021/acscatal.9b00721

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Abstract

The active site in ethene oligomerization catalyzed by Ni-zeolites is proposed to be a mobile Ni(II) complex, based on density functional theory-based molecular dynamics (DFT-MD) simulations corroborated by continuous-flow experiments on Ni-SSZ-24 zeolite. The results of the simulations at operating conditions show that ethene molecules reversibly mobilize the active site as they exchange with the zeolite as ligands on Ni during reaction. Microkinetic modeling was conducted on the basis of free-energy profiles derived with DFT-MD for oligomerization on these mobile [(ethene)₂-Ni-alkyl]⁺ species. The model reproduces the experimentally observed high selectivity to dimerization and indicates that the mechanism is consistent with the observed second-order rate dependence on ethene pressure.

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Homogeneous and heterogeneous catalysts are traditionally distinguished by the active site being either in solution or anchored on a support. However, recent works have revealed heterogeneous catalysts where the active sites are mobilized: selective catalytic reduction of nitrogen oxides with ammonia in Cu-SSZ-13 zeolite (1) and with hydrocarbons in Ag-ZSM-5 zeolite, (2) reactant-mobilized protons in methanol to hydrocarbons conversion in H-ZSM-5, (3) and ethylene dimerization/hydrogenation on Rh sites in Y zeolite. (4) In this work, we present findings on a reaction, ethene oligomerization in Ni-SSZ-24 zeolite, that indicate that the active site is reversibly mobilized and anchored to the support during the catalytic cycle.

There is increasing interest in selective oligomerization of alkenes, and in particular heterogeneous catalysts for ethene dimerization, as it is both a fundamentally challenging topic of catalysis and a versatile reaction for the chemical industry, such as forming butene from biobased ethylene. (5) Ni-aluminosilicates have long been known as promising heterogeneous catalysts. (6) Intriguingly, they work without the alkylaluminoxane cocatalysts required for homogeneous and metal–organic framework (MOF)-based heterogeneous catalysts. (7) Nevertheless, there is accumulating evidence provided by us and others that the reaction follows the Cossee–Arlman mechanism (Scheme 1) (8−11) known from homogeneous catalysis. The three-dimensional structure of the Ni-aluminosilicates plays a central role in alkene oligomerization: experimental results show that dimer formation is first order in alkene pressure on Ni-silicaalumina (10,12) but second order in Ni-zeolites. (12,13) 1-Butene formation is also second order in mesoporous Ni-silicaalumina at low temperatures, where it was demonstrated that condensation of ethene in the pores stabilized catalytic activity. (14) These observations suggest that surrounding molecules can have a significant influence on alkene oligomerization in Ni-aluminosilicates, an influence we for the first time elucidate with molecular simulations mimicking operating conditions.

Scheme 1. Cossee–Arlman Catalytic Cycle of Ethene Oligomerization in Ni(II) Cationic Complexes

We simulated ethene oligomerization in Ni-SSZ-24 using density-functional-theory-based molecular dynamics (DFT-MD) simulations and advanced free-energy sampling techniques. Such an approach has proven to account for the complexity of catalytic reactions at operating conditions by allowing for dynamic active sites and the influence of surrounding molecules. (3,15) The latter was found to be critical to investigate the nature of the active site in the presence of multiple ethene molecules. The computational results show a remarkable resemblance between the active sites in Ni-zeolites and homogeneous catalysts for alkene oligomerization, as ethene molecules reversibly mobilize the Ni site inside the zeolite pores. Continuous-flow experiments were conducted to verify the corresponding kinetic parameters predicted from theory. This leads us to predict that the active site within the Ni-zeolite dynamically crosses the border between homogeneous and heterogeneous catalysis throughout the reaction.

The computational part of this work takes its starting point in the Cossee–Arlman cycle, shown in Scheme 1, inferring that the Ni-ethyl species initially forms in situ by reaction between Ni(II) ions and ethene (Section S1). (11) We seek to unravel the dynamics of the active site at operating conditions by simulating the catalytic cycle in Ni-SSZ-24 zeolite with DFT-MD (see details in Computational and Experimental Methods), starting from the simplest Cossee–Arlman species [ethene-Ni-ethyl]⁺ surrounded by 2 ethene molecules in the micropore of the simulation cell (Section S2.2). This ethene loading corresponds to equilibrium at typical experimental conditions of 120 °C and 25 bar ethene pressure, (10,12) as evidenced by Grand Canonical Monte Carlo simulations (Section S3). We have investigated several reaction steps and obtain good agreement in the free-energy barriers of the few of these steps that were investigated in our previous work (Section S2.4). (8) We collected the free-energy profiles of the elementary steps that make up the most favorable pathways for ethene oligomerization in the diagram shown in Figure 1. Note that alkene adsorption/desorption in this diagram occurs from/to the zeolite pore encompassing the adsorbates. By using the equilibrium loading of two ethene molecules surrounding the [ethene-Ni-ethyl]⁺ species, we ensure that in this state the free energy of ethene in the pore is equivalent to that of gas phase ethene at 120 °C and 25 bar.

Figure 1. Free-energy diagram for ethene oligomerization pathways in Ni-SSZ-24 zeolite at 25 bar and 120 °C, based on free-energy profiles of elementary steps obtained with DFT-MD umbrella sampling simulations. The reference state is the equilibrium loading of ethene surrounding [ethene-Ni-ethyl]⁺, as discussed in the text. The numbers indicate intrinsic free energies of activation in kJ/mol. As indicated in the diagram, the [ethyl-Ni-butene]⁺ species can also be reached by ethene desorption; see Section S2.3 for a detailed discussion of how the diagram was constructed.

Remarkably, the catalytic cycle of oligomerization proceeds on mobile complexes. Such a species is created from [ethene-Ni-ethyl]⁺ by coordination of an additional ethene to Ni, thereby detaching the Ni complex from the ion-exchange position of the zeolite to create a mobile active site. The mobility is observed in the distribution of Ni–Al distances in Figure 2: the distribution is shifted toward larger values and is wider for the detached [(ethene)₂-Ni-ethyl]⁺ than the anchored species. In the mobile species Ni can achieve its preferred square-planar coordination, realizing the maximal potential of the metal site by opening up a favorable pathway for C–C coupling (vide infra). Figure 2 shows the simulated free-energy profiles for coordination of ethenes on Ni-alkyl species originally anchored to the zeolite. The results show that [ethene-Ni-ethyl]⁺ is stabilized by coordination of up to two additional ethene molecules. In [(ethene)₂-Ni-ethyl]⁺, one oxygen coordination is replaced by ethene and the second by an agostic bond with a β hydrogen of the ethyl group, creating a square planar coordination of Ni (Figure 2, right inset). The same is the case for the butyl analogue (Figure S8). This square-planar geometry is preferred for Ni as a d⁸ metal when coordinated to strong-field ligands, (16) such as alkyl and hydride. (17)

Figure 2. Free-energy profiles for ethene coordination to [ethene-Ni-ethyl]⁺ (left) and [ethene-Ni-butyl]⁺ (right) at 25 bar and 120 °C, derived from DFT-MD simulations. The left inset shows histograms of Ni–Al distances, illustrating that the doubly and triply ethene-coordinated Ni-ethyl species are detached from the framework and more mobile than their anchored counterpart. The associated charge-separation energy is estimated to 78 kJ/mol (Section S2.5). The right insets show snapshots from the simulations of the Ni-ethyl species, including the trajectories of the Ni (light blue) and the α alkyl C (dark blue) atoms (Figure S8 shows the butyl analogues).

The mobile Ni complexes resemble a homogeneous oligomerization catalyst in solution; the noncovalent interactions with the zeolite framework parallel the nondirectional stabilization by a solvent shell. However, the positively charged complex is tethered to the zeolite framework by the negative charge of the ion-exchange site. Note that there is a potential energy gain on formation of the [(ethene)₂-Ni-ethyl]⁺ detached species (28 kJ/mol, Section S2.6.1). Ethene coordination thus counteracts the energy required to separate the positive Ni ion from the framework (78 kJ/mol, Figure 2). As complex formation is also favored in free energy (19 kJ/mol, Figure 2), the entropy loss of coordinating ethene is partly compensated by the configurational entropy of the mobile [(ethene)₂-Ni-ethyl]⁺ complex; translation (Figure S9) and rotation (inset of Figure 2). This rotation may also explain why ethene coordination to [ethene-Ni-butyl]⁺ is favored in potential energy (38 kJ/mol, Section S2.6.1) but slightly disfavored in free energy (3 kJ/mol, Figure 2); the Ni-butyl species is bulkier than the ethyl analogue, limiting rotation in the zeolite micropore (Figure S8).

C–C coupling is the central reaction step in ethene oligomerization, which in the mobile species proceeds through migratory insertion of ethene in the Ni-alkyl bond. We find the intrinsic free-energy barrier in [(ethene)₂-Ni-ethyl]⁺ to be significantly lower than in both the anchored [ethene-Ni-ethyl]⁺ species and the mobile [(ethene)₃-Ni-ethyl]⁺ species (Table 1). We explain this trend using Hammond’s postulate (18) considering the structures shown in Figure 3; C–C coupling in [ethene-Ni-ethyl]⁺ is an endergonic reaction (Figure S11) leading to a large activation barrier with a late transition state that resembles a [Ni-butyl]⁺ product with a γ-agostic hydrogen bond. In [(ethene)₂-Ni-ethyl]⁺, the reaction is exergonic (Figure S13), leading to a low activation barrier with an early transition state that resembles the reactant with the favorable square-planar coordination of Ni. In [(ethene)₃-Ni-ethyl]⁺ the transition state (trigonal bipyramid) resembles neither the reactant (half trigonal bipyramid) nor the product ([(ethene)₂-Ni-butyl]⁺, square planar) and is hence significantly higher in free energy than either state (Figure S14). The Ni-butyl analogues exhibit the same trend (Figures S11 and S13).

Table 1. Intrinsic Free-Energy Barriers (kJ/mol) of Ethene Desorption and C–C Coupling in [(ethene)_x-Ni-ethyl]⁺ Species in SSZ-24 Zeolite, Obtained from DFT-MD Simulations at 120 °C (Sections S2.7 and S2.8)

	[ethene-Ni-ethyl]⁺	[(ethene)₂-Ni-ethyl]⁺	[(ethene)₃-Ni-ethyl]⁺
C–C coupling	77	37	76
ethene desorption	72	54	20

Figure 3. Snapshots from DFT-MD umbrella simulations of the initial (IS) and transition states (TS) of C–C coupling in [(ethene)_x-Ni-ethyl]⁺ species in SSZ-24 zeolite. Planes of square and trigonal coordination are shown in blue and yellow, respectively.

Following C–C coupling, the reaction proceeds from [(ethene)₂-Ni-butyl]⁺ via transfer of a β hydrogen of the butyl chain to form 1-butene, which subsequently desorbs. Like for C–C coupling, the hydrogen transfer has the lowest barrier in the mobile [(ethene)₂-Ni-butyl]⁺ species (20 kJ/mol, Figures 1 and S21), as hydrogen is already involved in an agostic bond to Ni prior to reaction (see Ni-ethyl analogue in Figure 3). In contrast, the anchored [ethene-Ni-butyl]⁺ species undergoes a significant rearrangement to reach the transition state, reflected in a higher barrier (38 kJ/mol, Figures 1 and S18).

The last step toward dimerization is desorption of 1-butene. Analogously to ethene (Table 1), the 1-butene desorption barrier decreases with ethene coordination and is hence lowest in [(ethene)₂-ethyl-Ni-1-butene]⁺ (Table S5). This can be explained by the late transition state of desorption: in this species, the 1-butene weakly interacts with the favored square-planar [(ethene)₂-Ni-ethyl]⁺ (Figure S22). In the latter species, the catalytic cycle is closed.

In summary, we have used DFT-MD to predict that both ethene dimerization and trimerization are catalyzed by positively charged Ni complexes mobilized by ethene coordination. The transition states involved in butene formation are slightly lower in free energy than that for hexene formation (Figure 1), indicating a preference toward butene over higher oligomers. Indeed, when employing the free-energy profiles in a microkinetic model (Section S4), we obtain a rate of formation of 1-butene that is 2 orders of magnitude higher than that of 1-hexene (Section S4.3).

We now consider experimental results to verify our computational findings. Ethene oligomerization was investigated in continuous-flow experiments on Ni-SSZ-24 zeolite (see details in Computational and Experimental Methods). The catalyst achieves a selectivity of more than 98% to butenes at ethene pressures ranging from 4 to 26 bar (Section S5.3), in agreement with the computational result that the production rate of butene is 2 orders of magnitude higher than that of hexene as a representative of higher oligomers. Figure 4 shows that the rate of butene (and hexene) formation is second order in ethene pressure and that the apparent activation energy is virtually pressure-independent. These findings strongly suggest that the oligomerization mechanism is the same over a wide pressure range. The observed reaction order agrees with microkinetic modeling based on the free-energy profiles shown in Figure 1, where oligomerization occurs on mobile active sites. Note that a second order is only obtained from the model when [ethyl-Ni-alkene]⁺ species are the most abundant adsorbates, the catalyst resting states (Table S7, Section S4.3). We note that the predicted coverage of [ethyl-Ni-alkene]⁺ and hence the reaction order is sensitive to the free energy of these species (Table S7), which is associated with some uncertainty (Section S2.3). However, a zeolite-anchored [ethyl-Ni-alkene]⁺ resting state is consistent with recent experimental X-ray absorption data, which indicate that in the catalyst resting state in Ni-Beta zeolite, Ni is in tetrahedral coordination with oxygens in the first coordination shell. (11) In the context of resting states it is instructive to consider the concept of the energetic span introduced by Kozuch and Shaik: the largest free-energy difference between an adsorbate and a transition state along the catalytic cycle, corresponding to the apparent free energy of activation. (19) While being cautious about using the free energies from Figure 1 directly (Section S2.3), we derive an energetic span for butene formation of 71 kJ/mol between the [ethyl-Ni-butene]⁺ and the transition state of hydrogen transfer, which is in fairly good agreement with the 58 kJ/mol derived from the experiments (Section S5.3.1).

Figure 4. Results from continuous-flow experiments on ethene oligomerization in Ni-SSZ-24: (a) Arrhenius plot for butene formation at 4 and 26 bar ethene pressure, (b) reaction order of butene and hexene formation at 150 °C, and (c) butene isomer selectivity at 150 °C. All results were obtained at differential ethene conversions of 1–7%.

Finally, the experimental results indicate that both 1- and 2-butenes are primary products (Section S5.3), as was also observed on Ni-Beta. (11) This supports the Cossee–Arlman mechanism, as the [ethyl-Ni-butene]⁺ species can isomerize between 1- and 2-butene through β-hydrogen elimination on the Ni site prior to desorption (Figure S17), analogously to homogeneous catalysts. (20) Note also that 1-butene selectivity increases with pressure (Figure 4), contrary to what would be expected for isomerization at Brønsted acid sites, but paralleling the results of previous work by Agirrezabal-Telleria and Iglesia on mesoporous Ni-silicaalumina. (14) Interestingly, the same work demonstrated stabilization of catalytic activity at subambient temperatures when ethene condenses in the mesopores. (14) Such conditions likely favor coordination of multiple ethene molecules to form the mobile sites described here (Section S6). These sites may be involved in the stabilization, as they facilitate butene desorption over further oligomerization.

In conclusion, we have conducted an experimental and computational investigation of ethene oligomerization in Ni-SSZ-24 zeolite. The computational results allow us to make detailed predictions about the reaction at operating conditions; two ethene molecules adsorb on Ni-alkyl species, creating mobile active sites in the zeolite pores. The mobilization is reversible, as ethene dynamically exchanges with oxygens of the zeolite support as ligand on Ni during the reaction. Microkinetic modeling based on calculated free-energy profiles reproduces the experimentally observed relative production rate of butene over higher oligomers of 10². The model indicates that the proposed mechanism leads to a second-order rate dependence on ethene pressure as experimentally observed here and elsewhere. (12,14) An analogous mechanism may explain the second-order rate dependence observed in 1-butene dimerization. (13) Such reactant-mobilized active sites have also been observed in selective catalytic reduction of nitrogen oxides with ammonia in Cu-SSZ-13 zeolite (1) and with hydrocarbons in Ag-ZSM-5, (2) in methanol to hydrocarbons conversion in H-ZSM-5, (3) and ethylene dimerization/hydrogenation on Rh sites in Y zeolite. (4) Together these works present materials that appear as hybrids between heterogeneous and homogeneous catalysts, highlighting an intriguing regime of catalysis (21) that likely extends beyond the examples known to date.

Computational and Experimental Methods

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Density functional theory (DFT)-based Molecular Dynamics (DFT-MD) simulations were carried out with CP2K, (22) versions 2.6 and 3.0. (23) Energies and forces were calculated from DFT based on a mixed Gaussian and plane wave approach (24) using the revPBE functional (25) with Grimme D3 dispersion correction. (26) DFT-MD calculations were carried out nonspinpolarized, as our previous work determined the singlet state to be the most stable for the Ni(II) species investigated here. (8) The majority of the simulations employed a super cell constructed by repeating the AFI unit cell twice along the channel direction (1 × 1 × 2) and making one Si/Al substitution (Si/Al = 47), while a larger super cell (Si/Al = 72) was used in a few simulations (Figure S23). Enhanced free-energy sampling in the DFT-MD simulations was done using umbrella sampling (27,28) with quadratic potentials by interfacing CP2K with PLUMED 2. (29) Free-energy profiles of elementary reaction steps were computed from the output of umbrella simulations along collective variables, using the Weighted Histogram Analysis Method (WHAM). (27) Monte Carlo simulations were carried out using LAMMPS, (30) while microkinetic modeling was conducted with CatMAP. (31) Graphics of molecular structures from the DFT-MD simulations were created using POV-Ray 3.6 and VMD 1.9.1. (32)

The synthesis of SSZ-24 was conducted as described in literature. (33) The zeolite was ion-exchanged five consecutive times with 0.1 M Ni(NO₃)₂(aq), washed and calcined to form Ni-SSZ-24, Si/Al = 80, Ni/Al = 0.34. Kinetic experiments were carried out in a continuous-flow, fixed-bed, stainless steel reactor. Standard measurement conditions were 150 °C and 30 bar total pressure, P(ethene)=26 bar, P(inert)=4 bar, with a total flow rate of 30 mL/min (contact time at STP: 6.7 mg_cat·min·mL_ethene^–1). A small fraction of the effluent was analyzed on an online gas chromatograph. Carbon mass balances closed witin 2%. Further computational and experimental details can be found in the Supporting Information.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00721.

Complete body of DFT-MD results, results of Monte Carlo simulations, setup and results of microkinetic modeling, additional experimental details and results (PDF)
CatMAP files (ZIP)

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Author Information

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Corresponding Authors
- Rasmus Yding Brogaard - Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway; Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium; Present Address: R.Y.B.: Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark; http://orcid.org/0000-0001-7882-9183; Email: [email protected]
- Unni Olsbye - Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway; http://orcid.org/0000-0003-3693-2857; Email: [email protected]
- Kristof De Wispelaere - Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium; Email: [email protected]
Authors
- Mustafa Kømurcu - Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
- Michael Martin Dyballa - Department of Chemistry, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway; http://orcid.org/0000-0002-8883-1145
- Alexandru Botan - Department of Physics, Centre for Materials and Nanoscience (SMN), University of Oslo, P.O. Box 1033, Blindern, NO-0315 Oslo, Norway
- Veronique Van Speybroeck - Center for Molecular Modeling (CMM), Ghent University, Tech Lane Ghent Science Park Campus A, Technologiepark 46, 9052 Zwijnaarde, Belgium; http://orcid.org/0000-0003-2206-178X
Notes
The authors declare no competing financial interest.

Acknowledgments

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This research was funded in part by the Research Council of Norway through the OLIGOM (224980) and CONFINE (250795) projects. R.Y.B. thanks the University of Oslo for the Young Scientist Mobility Grant. V.V.S. and R.Y.B acknowledge funding from the European Union’s Horizon 2020 research and innovation program (consolidator ERC grant agreement No. 647755 - DYNPOR (2015-2020)). The work is also supported by the Research Board of Ghent University (BOF). The Norwegian Metacenter for High Performance Computing (NOTUR) and Prof. Stian Svelle are thanked for computing resources (nn4683k). Ghent University (Stevin Supercomputer Infrastructure) and the VSC (Flemish Supercomputer Center), funded by the Research Foundation Flanders (FWO), are also thanked for computational resources. K.D.W. is a fellow funded by the FWO (FWO16-PDO-047).

References

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Moors, Samuel L. C.; De Wispelaere, Kristof; Van der Mynsbrugge, Jeroen; Waroquier, Michel; Van Speybroeck, Veronique
ACS Catalysis (2013), 3 (11), 2556-2567CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The methylation of arenes is a key step in the prodn. of hydrocarbons from methanol over acidic zeolites. The authors performed ab initio static and mol. dynamics free energy simulations of benzene methylation in H-ZSM-5 to det. the factors that influence the reaction kinetics. Special emphasis is given to the effect of the surrounding methanol mols. on the methylation kinetics. For higher methanol loadings, methylation may also occur from a protonated methanol cluster, indicating that the exact location of the Broensted acid site is not essential for the zeolite-catalyzed methylation reaction. However, methylations from a protonated methanol cluster exhibit higher free energy barriers than a methylation from a single methanol mol. Finally, comparison with a pure methanol solvent reaction environment indicates that the main role of the zeolite during the methylation of benzene is to provide the acidic proton and to create a polar environment for the reaction. The metadynamics approach, which is specifically designed to sample rare events, allows exploring new reaction pathways, which take into account the flexibility of the framework and addnl. guest mols. in the pores and channels of the zeolite framework. This approach goes beyond the often applied static calcns. to det. reaction kinetics.
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Serna, P.; Gates, B. C. Zeolite-Supported Rhodium Complexes and Clusters: Switching Catalytic Selectivity by Controlling Structures of Essentially Molecular Species. J. Am. Chem. Soc. 2011, 133, 4714– 4717, DOI: 10.1021/ja111749s
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Zeolite-Supported Rhodium Complexes and Clusters: Switching Catalytic Selectivity by Controlling Structures of Essentially Molecular Species
Serna, Pedro; Gates, B. C.
Journal of the American Chemical Society (2011), 133 (13), 4714-4717CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Precise synthesis and characterization of site-isolated rhodium complexes and extremely small rhodium clusters supported on zeolite HY allow control of the catalyst selectivity in the conversion of ethene to n-butene or ethane, resp., as a result of tuning the structure of the active sites at a mol. level.
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(a) Hulea, V. Toward Platform Chemicals from Bio-Based Ethylene: Heterogeneous Catalysts and Processes. ACS Catal. 2018, 8, 3263– 3279, DOI: 10.1021/acscatal.7b04294
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5a
Toward Platform Chemicals from Bio-Based Ethylene: Heterogeneous Catalysts and Processes
Hulea, Vasile
ACS Catalysis (2018), 8 (4), 3263-3279CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
With stricter ecol. regulation and redn. of fossil feedstock, the research works have been oriented to renewable resources. Various bio-based mols. were proposed for replacing the platform chems. based on crude oil, but there are not yet well established processes for producing these chems. at large scale. As a result, in the short to mid-term, a main impact will be expected from the prodn. of bio-based bulk chems. having identical structure with that of the today's bulk chems. Among them, ethylene, which is a key intermediate for the prodn. of platform mols. The com. method for producing ethylene is based on steam thermal cracking, but emerging method using alternative sources (natural gas, coal, biomass) are very promising processes. For example, the high-vol. prodn. of ethylene by ethanol dehydration became economically feasible application. This review summarizes the advances of catalysts, processes and fundamental understanding of reaction mechanisms in ethylene conversion to other high value hydrocarbons, including propylene, butenes and aroms. BTX.
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(b) Nicholas, C. P. Applications of Light Olefin Oligomerization to the Production of Fuels and Chemicals. Appl. Catal., A 2017, 543, 82– 97, DOI: 10.1016/j.apcata.2017.06.011
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5b
Applications of light olefin oligomerization to the production of fuels and chemicals
Nicholas, Christopher P.
Applied Catalysis, A: General (2017), 543 (), 82-97CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
The oligomerization of the light olefins ethene, propene, and butenes into fuels and chems. was investigated and com. practiced for many years. While the area appears on the surface to be mature, many advances were made in recent years. In this feature article, I discuss the mechanisms of reaction and showcase catalysts and processes useful for oligomerization from both the open and patent literature. Com. practiced processes are spotlighted. Among the catalysts utilized in the art are acidic catalysts such as solid phosphoric acid and zeolites, as well as metal based catalysts including aluminum alkyls, and nickel and zirconium based complexes and solids. A short section on catalysts and processes which utilize a metallacycle mechanism to achieve high selectivity to 1-hexene or 1-octene is followed by a discussion of multi-functional materials possessing both acid and metal active sites. Finally, processes where oligomerization is a key step in a multi-step or multi-reaction process are discussed.
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Finiels, A.; Fajula, F.; Hulea, V. Nickel-based Solid Catalysts for Ethylene Oligomerization - a Review. Catal. Sci. Technol. 2014, 4, 2412– 2426, DOI: 10.1039/C4CY00305E
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6
Nickel-based solid catalysts for ethylene oligomerization - a review
Finiels, Annie; Fajula, Francois; Hulea, Vasile
Catalysis Science & Technology (2014), 4 (8), 2412-2426CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
A review. Ni-based inorg. porous materials are the most important heterogeneous catalysts for ethylene oligomerization. In spite of extensive research carried out in previous decades on these catalysts, only a few aspects have been reviewed in the literature. We illustrate here the main properties of these materials and their catalytic performances in oligomerization reactions performed under mild conditions. The major groups of oligomerization catalysts, including NiO on various carriers and Ni-exchanged zeolites, sulfated-alumina or silica-alumina, are described in the first part of this review. The nature of the active sites and their role in the ethylene oligomerization process are pointed out. In the second part, the literature dealing with the catalyst performances under various reaction parameters is reviewed. The influence of catalyst properties (in particular the catalyst pore size) and reaction conditions (i.e. temp. and pressure) on the productivity and the product distribution in oligomerization is methodically discussed. The excellent performances of Ni-exchanged ordered mesoporous silica-alumina, which are the best oligomerization catalysts known today, are carefully emphasized.
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(a) Canivet, J.; Aguado, S.; Schuurman, Y.; Farrusseng, D. MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts Through One-Pot Postsynthetic Modification. J. Am. Chem. Soc. 2013, 135, 4195– 4198, DOI: 10.1021/ja312120x
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7a
MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification
Canivet, Jerome; Aguado, Sonia; Schuurman, Yves; Farrusseng, David
Journal of the American Chemical Society (2013), 135 (11), 4195-4198CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The one-pot postfunctionalization allows anchoring a mol. nickel complex into a mesoporous metal-org. framework (Ni@(Fe)MIL-101). It is generating a very active and reusable catalyst for the liq.-phase ethylene dimerization to selectively form 1-butene. Higher selectivity for 1-butene is found using the Ni@(Fe)MIL-101 catalyst than reported for mol. nickel diimino complexes.
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(b) Madrahimov, S. T.; Gallagher, J. R.; Zhang, G.; Meinhart, Z.; Garibay, S. J.; Delferro, M.; Miller, J. T.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)Ni^II Complexes. ACS Catal. 2015, 5, 6713– 6718, DOI: 10.1021/acscatal.5b01604
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7b
Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes
Madrahimov, Sherzod T.; Gallagher, James R.; Zhang, Guanghui; Meinhart, Zachary; Garibay, Sergio J.; Delferro, Massimiliano; Miller, Jeffrey T.; Farha, Omar K.; Hupp, Joseph T.; Nguyen, SonBinh T.
ACS Catalysis (2015), 5 (11), 6713-6718CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
NU-1000-(bpy)NiII, a highly porous MOF material possessing well-defined (bpy)NiII moieties, was prepd. through solvent-assisted ligand incorporation (SALI). Treatment with Et2AlCl affords a single-site catalyst with excellent catalytic activity for ethylene dimerization (intrinsic activity for butenes that is up to an order of magnitude higher than the corresponding (bpy)NiCl2 homogeneous analog) and stability (can be reused at least three times). The high porosity of this catalyst results in outstanding levels of activity at ambient temp. in gas-phase ethylene dimerization reactions, both under batch and continuous flow conditions.
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(c) Bernales, V.; League, A. B.; Li, Z.; Schweitzer, N. M.; Peters, A. W.; Carlson, R. K.; Hupp, J. T.; Cramer, C. J.; Farha, O. K.; Gagliardi, L. Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal-Organic Framework for Ethylene Dimerization. J. Phys. Chem. C 2016, 120, 23576– 23583, DOI: 10.1021/acs.jpcc.6b07362
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7c
Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal-Organic Framework for Ethylene Dimerization
Bernales, Varinia; League, Aaron B.; Li, Zhanyong; Schweitzer, Neil M.; Peters, Aaron W.; Carlson, Rebecca K.; Hupp, Joseph T.; Cramer, Christopher J.; Farha, Omar K.; Gagliardi, Laura
Journal of Physical Chemistry C (2016), 120 (41), 23576-23583CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The catalytic performance of a cobalt(II) single-site catalyst supported on the zirconia-like nodes of the metal org.-framework (MOF) NU-1000 is herein characterized by quantum chem. methods and compared to an iso-structural analog incorporating nickel(II) as the active transition metal. The mechanisms of at. layer deposition in MOFs and of catalysis are examd. using d. functional theory. We compare the catalytic activity of Co and Ni installed on the zirconia-like nodes for ethylene dimerization, considering three plausible pathways. Multiconfigurational wave function theory methods are employed to further characterize the electronic structures of key transition states and intermediates. Finally, we report confirmation of Co catalytic activity for ethylene dimerization from expts. that were prompted by the computational prediction.
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(d) Metzger, E. D.; Comito, R. J.; Hendon, C. H.; Dincă, M. Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l. J. Am. Chem. Soc. 2017, 139, 757– 762, DOI: 10.1021/jacs.6b10300
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7d
Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l
Metzger, Eric D.; Comito, Robert J.; Hendon, Christopher H.; Dinca, Mircea
Journal of the American Chemical Society (2017), 139 (2), 757-762CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
A recently developed metal-org. framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of com. homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calcns. to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.
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(e) Liu, J.; Ye, J.; Li, Z.; Otake, K.-i.; Liao, Y.; Peters, A. W.; Noh, H.; Truhlar, D. G.; Gagliardi, L.; Cramer, C. J.; Farha, O. K.; Hupp, J. T. Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization. J. Am. Chem. Soc. 2018, 140, 11174– 11178, DOI: 10.1021/jacs.8b06006
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7e
Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization
Liu, Jian; Ye, Jingyun; Li, Zhanyong; Otake, Ken-ichi; Liao, Yijun; Peters, Aaron W.; Noh, Hyunho; Truhlar, Donald G.; Gagliardi, Laura; Cramer, Christopher J.; Farha, Omar K.; Hupp, Joseph T.
Journal of the American Chemical Society (2018), 140 (36), 11174-11178CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
To modify its steric and electronic properties as a support for heterogeneous catalysts, electron-withdrawing and electron-donating ligands, hexafluoroacetylacetonate (Facac-) and acetylacetonate (Acac-), were introduced to the metal-org. framework (MOF), NU-1000, via a process akin to at. layer deposition (ALD). In the absence of Facac- or Acac-, NU-1000-supported, AIM-installed Ni(II) sites yield a mixt. of C4, C6, C8, and polymeric products in ethylene oligomerization. (AIM = ALD-like deposition in MOFs). In contrast, both Ni-Facac-AIM-NU-1000 and Ni-Acac-AIM-NU-1000 exhibit quant. catalytic selectivity for C4 species. Exptl. findings are supported by d. functional theory calcns., which show increases in the activation barrier for the C-C coupling step, due mainly to rearrangement of the siting of Facac- or Acac- to partially ligate added nickel. The results illustrate the important role of structure-tuning support modifiers in controlling the activity of MOF-sited heterogeneous catalysts and in engendering catalytic selectivity. The results also illustrate the ease with which crystallog. well-defined modifications of the catalyst support can be introduced when the node-coordinating mol. modifier is delivered via the vapor phase.
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Brogaard, R. Y.; Olsbye, U. Ethene Oligomerization in Ni-containing Zeolites: Theoretical Discrimination of Reaction Mechanisms. ACS Catal. 2016, 6, 1205– 1214, DOI: 10.1021/acscatal.5b01957
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Ethene Oligomerization in Ni-Containing Zeolites: Theoretical Discrimination of Reaction Mechanisms
Brogaard, Rasmus Y.; Olsbye, Unni
ACS Catalysis (2016), 6 (2), 1205-1214CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Ni-contg. porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addn., it remains unclear why the aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses d. functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calcns. employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calcns. targeted Ni-contg. SSZ-24 zeolite as a representative catalyst and considered Ni+, Ni2+ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee-Arlman mechanisms that have been proposed in the literature, in addn. to a new proton-transfer mechanism. Free energy profiles were derived at a typical exptl. reaction temp. of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee-Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calcd. intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available exptl. data. We further propose a mechanism for formation of the active nickel-alkyl species by reaction between ethene and isolated Ni2+ ions. The results hence provide a solid starting point for exptl. investigations of the catalytic cycle, to validate our predictions and ultimately det. the atom-scale properties that control catalytic activity.
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Forget, S.; Olivier-Bourbigou, H.; Delcroix, D. Homogeneous and Heterogeneous Nickel-Catalyzed Olefin Oligomerization: Experimental Investigation for a Common Mechanistic Proposition and Catalyst Optimization. ChemCatChem 2017, 9, 2408– 2417, DOI: 10.1002/cctc.201700348
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9
Homogeneous and Heterogeneous Nickel-Catalyzed Olefin Oligomerization: Experimental Investigation for a Common Mechanistic Proposition and Catalyst Optimization
Forget, Severine; Olivier-Bourbigou, Helene; Delcroix, Damien
ChemCatChem (2017), 9 (12), 2408-2417CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
Only a few catalytic transformations can be efficiently catalyzed by both homogeneous or heterogeneous technologies, with short olefin oligomerization promoted by a nickel-based catalysts among them. Homogeneous and heterogeneous catalysis are often opposed in terms of activity, active-site description or recyclability, traditionally mentioned as "homogeneous vs. heterogeneous". Unlike previous studies, we propose to emphasize the similarities between both catalysis by comparing industrially representative results obtained in continuous flow-mode under similar mild conditions. A detailed anal. of both primary products of olefins oligomerization, and a set of secondary products obtained in exptl. assays completed with recently published results of DFT calcns. prompted us to postulate a common mechanistic pathway for nickel-catalyzed ethylene oligomerization. Heterogeneous metallic ethylene oligomerization, for which nickel active sites identification has indeed long been under debate, seems to follow the Cossee-Arlman mechanism, well accepted as major mechanism in the homogeneous counterpart. This anal. allowed us to design an unprecedented heterogeneous catalyst active for ethylene and butene oligomerization.
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Toch, K.; Thybaut, J.; Marin, G. Ethene Oligomerization on Ni-SiO₂-Al₂O₃: Experimental Investigation and Single-Event MicroKinetic Modeling. Appl. Catal., A 2015, 489, 292– 304, DOI: 10.1016/j.apcata.2014.10.036
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10
Ethene oligomerization on Ni-SiO2-Al2O3: Experimental investigation and Single-Event MicroKinetic modeling
Toch, K.; Thybaut, J. W.; Marin, G. B.
Applied Catalysis, A: General (2015), 489 (), 292-304CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
An exptl. investigation of the intrinsic ethene oligomerization kinetics on an amorphous 1.8 wt.% Ni-SiO2-Al2O3 was performed. The temp. ranged from 443 to 503 K, the total pressure from 1.5 to 3.5 MPa, the inlet ethene partial pressure from 0.15 to 0.35 MPa and the space time was between 4.8 and 14.4 kgcat s mol-1C2. Due to the absence of strong acid sites, the catalytic activity toward oligomerization originated solely from the nickel ion sites at the investigated conditions and resulted in an Anderson Schulz Flory product distribution. The oligomerization rate amounted from 0.007 to 0.027 mol s-1 kg-1cat with butene selectivities of 80-90%. A Single-Event MicroKinetic (SEMK) model for ethene oligomerization, based upon an insertion-termination mechanism, was constructed. The kinetic parameter ests. were all statistically significant with a precise phys. meaning. The model was found to be globally significant and adequate, i.e., it was able to describe all exptl. data without any systematic deviation. Using the model, it was detd. that mainly ethene, because of its abundance, and octene, because of its higher carbon no., were physisorbed. The corresponding catalyst occupancy by physisorbed species typically ranged between 10% and 50%. Coordination of the alkenes at the active nickel ions resulted in about 90% of these ions being present under the form of a nickel-ethene species.
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Joshi, R.; Zhang, G.; Miller, J. T.; Gounder, R. Evidence for the Coordination-Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves. ACS Catal. 2018, 8, 11407– 11422, DOI: 10.1021/acscatal.8b03202
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Evidence for the Coordination-Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves
Joshi, Ravi; Zhang, Guanghui; Miller, Jeffrey T.; Gounder, Rajamani
ACS Catalysis (2018), 8 (12), 11407-11422CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The mechanistic origin of alkene dimerization on Ni sites supported on aluminosilicates has been ascribed to both coordination-insertion (i.e., Cossee-Arlman) and metallacycle-based cycles; the latter is often invoked in the absence of externally supplied cocatalysts or activators that generate Ni-hydride or Ni-alkyl species to initiate coordination-insertion cycles. Detg. the prevalent reaction mechanism at Ni sites is often complicated by the formation and consumption of alkene dimer products via oligomerization and other parallel reactions (e.g., cracking, isomerization) that occur at Bronsted acid sites on supports. Here, ethene dimerization (453 K) was studied on Beta zeolites synthesized to contain predominantly exchanged Ni2+ sites according to site balances detd. by cation exchange, and to Ni structure detd. by CO IR, UV-visible, and Ni K-edge X-ray absorption spectroscopies. The catalytic behavior of Ni2+ sites was isolated by suppressing contributions from residual H+ sites on support structures, either by selectively poisoning them with Li+ cations or NH4+ species, or by weakening them using a zincosilicate support. Bronsted acid sites form linear dimers (1-butene, cis-2-butene, trans-2-butene) in thermodynamically equilibrated ratios, in addn. to their skeletal isomers (isobutene) and products of subsequent oligomerization-cracking cycles; hence, isobutene formation rates serve as a kinetic marker for the presence of H+ sites. After residual H+ sites deactivate during initial reaction times or when they are suppressed prior to reaction, linear butene isomers form in nonequilibrated ratios that are invariant with ethene site-time, reflecting primary butene double-bond isomerization events catalyzed at Ni2+-derived active sites. Ni-zeolites pretreated in oxidative environments (5 kPa O2, 773 K) show transient activation periods during initial reaction times at dil. ethene pressures (<0.4 kPa) but not at higher ethene pressures (>0.4 kPa) or in the presence of co-fed hydrogen (5 kPa). This behavior is consistent with in situ ethene-assisted formation of [Ni(II)-H]+ intermediates, which isotopically scramble H2-D2 mixts. (453 K) and are quantified from surface H/D exchange reactions (453 K). Taken together, these findings provide unambiguous evidence for the coordination-insertion mechanism as the dominant route for alkene dimerization at Ni2+ cations exchanged onto mol. sieves.
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Henry, R.; Komurcu, M.; Ganjkhanlou, Y.; Brogaard, R. Y.; Lu, L.; Jens, K.-J.; Berlier, G.; Olsbye, U. Ethene Oligomerization on Nickel Microporous and Mesoporous-Supported Catalysts: Investigation of the Active Sites. Catal. Today 2018, 299, 154– 163, DOI: 10.1016/j.cattod.2017.04.029
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12
Ethene oligomerization on nickel microporous and mesoporous-supported catalysts: Investigation of the active sites
Henry, Reynald; Komurcu, Mustafa; Ganjkhanlou, Yadolah; Brogaard, Rasmus Y.; Lu, Li; Jens, Klaus-Joachim; Berlier, Gloria; Olsbye, Unni
Catalysis Today (2018), 299 (), 154-163CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
Nickel-contg. mesoporous (using Al2O3 or SiO2-Al2O3 support) and microporous (using nano- or micro-cryst. Beta zeolite support) catalysts were prepd. by ion exchange and characterized by XRD, SEM, N2-adsorption, MP-AES and FTIR. The samples were subjected to testing as ethene oligomerization catalysts at T = 120 °C, Ptotal = 29 bar, Pethene = 11.6-25.1 bar. All catalysts were active for ethene oligomerization, and linear butenes were the main gaseous products. However, catalyst deactivation due to retained long-chain alkenes was obsd. Ethene partial pressure variation expts. showed that the reaction order in ethene for butene formation was 1.5-1.7 for the mesoporous, and 2.0 for the microporous catalysts. Contact time variation expts. carried out with the microporous catalysts at Pethene = 18 bar showed that product selectivity was independent of ethene conversion, and suggested that 1-butene and 2-butenes are primary products. This result is consistent with the Cossee-Arlman mechanism. The nature of the active sites was investigated with FTIR spectroscopy with CO as probe mol. The results point to Ni2+-counterions as the preeminent active sites, while we suggest that Ni-sites grafted on silanol groups and NiO particles are spectators.
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Ehrmaier, A.; Liu, Y.; Peitz, S.; Jentys, A.; Chin, Y.-H. C.; Sanchez-Sanchez, M.; Bermejo-Deval, R.; Lercher, J. Dimerization of Linear Butenes on Zeolite-Supported Ni²⁺. ACS Catal. 2019, 9, 315– 324, DOI: 10.1021/acscatal.8b03095
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Dimerization of Linear Butenes on Zeolite-Supported Ni2+
Ehrmaier, Andreas; Liu, Yue; Peitz, Stephan; Jentys, Andreas; Chin, Ya-Huei Cathy; Sanchez-Sanchez, Maricruz; Bermejo-Deval, Ricardo; Lercher, Johannes
ACS Catalysis (2019), 9 (1), 315-324CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Nickel- and alkali-earth-modified LTA based zeolites catalyze the dimerization of 1-butene in the absence of Bronsted acid sites. The catalyst reaches over 95% selectivity to n-octenes and methylheptenes. The ratio of these two dimers is markedly influenced by the parallel isomerization of 1-butene to 2-butene, shifting the methylheptene/octene ratio from 0.7 to 1.4 as the conversion increases to 35%. At this conversion, the thermodn. equil. of 90% cis- and trans-2-butenes is reached. Conversion of 2-butene results in methylheptene and dimethylhexene with rates that are 1 order of magnitude lower than those with 1-butene. The catalyst is deactivated rapidly by strongly adsorbed products in the presence of 2-butene. The presence of π-allyl-bound butene and Ni-alkyl intermediates was obsd. by IR spectroscopy, suggesting both to be reaction intermediates in isomerization and dimerization. Product distribution and apparent activation barriers suggest 1-butene dimerization to occur via a 1'-adsorption of the first butene mol. and a subsequent 1'- or 2'-insertion of the second butene to form octene and methylheptene, resp. The reaction order of 2 for 1-butene and its high surface coverage suggest that the rate-detg. step involves two weakly adsorbed butene mols. in addn. to the more strongly held butene.
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Agirrezabal-Telleria, I.; Iglesia, E. Stabilization of Active, Selective, and Regenerable Ni-based Dimerization Catalysts by Condensation of Ethene within Ordered Mesopores. J. Catal. 2017, 352, 505– 514, DOI: 10.1016/j.jcat.2017.06.025
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Stabilization of active, selective, and regenerable Ni-based dimerization catalysts by condensation of ethene withinordered mesopores
Agirrezabal-Telleria, Iker; Iglesia, Enrique
Journal of Catalysis (2017), 352 (), 505-514CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
This study reports the high and stable ethene dimerization turnover rates conferred upon Ni-based active sites at subambient temps. by the condensation of liq. ethene reactants within ordered mesopores in Al-MCM-41. Such active and stable catalysts do not require the activators or co-catalysts essential for organometallic Ni-based catalysts and their robust porous framework allows their full regeneration by thermal treatments in inert or oxidizing environments when deactivation occurs. Dimerization rates (per Ni) are independent of Ni content below Ni2+/H+ exchange ratios of unity, consistent with isolated (Ni-OH)+ cations as active moieties, and with the formation of inactive NiO oligomers at higher Ni contents. Both CO and 2,6 di-tertbutylpyridine, used as titrants of Ni centers and protons, resp., fully suppressed reactivity, indicating that the Ni2+ and OH- centers in (Ni-OH)+ are involved in stabilizing the kinetically-relevant C-C formation transition state. Deactivation consts. decreased abruptly to undetectable values at those temps. and ethene pressures that formed an extended liq. phase within the MCM-41 mesopores that confine the Ni active sites. This remarkable shift from rapid to undetectable deactivation reflects how intrapore liqs. preferentially stabilize the late transition states that mediate the desorption of bound 1-butenes before subsequent isomerization and C-C bond formation events during one surface sojourn. Their preferential desorption inhibits the formation of regioisomers and larger oligomers, thus inhibiting deactivation while also leading to high C4 selectivities among products and to the predominant presence of 1-butene among linear C4 alkenes. Such unprecedented stability and high selectivities allow accurate kinetic measurements and their rigorous mechanistic interpretation in terms of transition state formalisms of chem. dynamics in thermodn. non-ideal media. Dimerization turnovers involve kinetically-relevant C-C bond formation between unbound ethene mols. and an ethene bound at low coverages on (Ni-OH)+ moieties. Such catalytic sequences lead to dimerization rates with a nearly second-order dependence in ethene fugacity, but with an effective rate const. that benefits from the stabilization of the C-C bond formation transition state as intrapore liqs. form. The essential elimination of deactivation events and secondary reactions by such liqs. leads to dimerization rates and 1-butene selectivities that are much higher at 240-260 K with liq.-filled mesopores than at 448 K, conditions that maintain all species in their gaseous form. In addn. to their significant practical impact, these results provide compelling evidence for the ability of an extended liq. to create a thermodynamically non-ideal environment that markedly alters reaction pathways through the selective stabilization of specific transition states based on their location along the reaction coordinate for a given elementary step.
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(a) Van Speybroeck, V.; De Wispelaere, K.; Van der Mynsbrugge, J.; Vandichel, M.; Hemelsoet, K.; Waroquier, M. First Principle Chemical Kinetics in Zeolites: the Methanol-to-Olefin Process as a Case Study. Chem. Soc. Rev. 2014, 43, 7326– 7357, DOI: 10.1039/C4CS00146J
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15a
First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study
Van Speybroeck, Veronique; De Wispelaere, Kristof; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Waroquier, Michel
Chemical Society Reviews (2014), 43 (21), 7326-7357CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
To optimally design next generation catalysts a thorough understanding of the chem. phenomena at the mol. scale is a prerequisite. Apart from qual. knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate consts. Mol. modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to det. chem. kinetics from first principles, thus by using no exptl. input and by modeling the catalyst and reacting species at the mol. level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theor. concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramol. nature, for which not only the Bronsted acidic site is important but also org. species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theor. modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate consts. can be predicted for reactions occurring at a single active site. The comparison with exptl. data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect mol. dynamics methods complemented with addnl. techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chem. reactions in complex mol. environments.
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(b) De Wispelaere, K.; Ensing, B.; Ghysels, A.; Meijer, E. J.; Van Speybroeck, V. Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics. Chem. - Eur. J. 2015, 21, 9385– 9396, DOI: 10.1002/chem.201500473
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15b
Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics
De Wispelaere, Kristof; Ensing, Bernd; Ghysels, An; Meijer, Evert Jan; Van Speybroeck, Veronique
Chemistry - A European Journal (2015), 21 (26), 9385-9396CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
The methanol-to-olefin process is a showcase example of complex zeolite-catalyzed chem. At real operating conditions, many factors affect the reactivity, such as framework flexibility, adsorption of various guest mols., and competitive reaction pathways. In this study, the strength of first principle mol. dynamics techniques to capture this complexity is shown by means of two case studies. Firstly, the adsorption behavior of methanol and water in H-SAPO-34 at 350 °C is investigated. Hereby an important degree of framework flexibility and proton mobility was obsd. Secondly, the methylation of benzene by methanol through a competitive direct and stepwise pathway in the AFI topol. was studied. Both case studies clearly show that a first-principle mol. dynamics approach enables unprecedented insights into zeolite-catalyzed reactions at the nanometer scale to be obtained.
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(c) De Wispelaere, K.; Wondergem, C. S.; Ensing, B.; Hemelsoet, K.; Meijer, E. J.; Weckhuysen, B. M.; Van Speybroeck, V.; Ruiz-Martínez, J. Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy. ACS Catal. 2016, 6, 1991– 2002, DOI: 10.1021/acscatal.5b02139
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15c
Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy
De Wispelaere, Kristof; Wondergem, Caterina S.; Ensing, Bernd; Hemelsoet, Karen; Meijer, Evert Jan; Weckhuysen, Bert M.; Van Speybroeck, Veronique; Ruiz-Martinez, Javier
ACS Catalysis (2016), 6 (3), 1991-2002CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The role of water in the methanol-to-olefins (MTO) process over H-SAPO-34 has been elucidated by a combined theor. and exptl. approach, encompassing advanced mol. dynamics simulations and in situ microspectroscopy. First-principles calcns. at the mol. level point out that water competes with methanol and propene for direct access to the Bronsted acid sites. This results in less efficient activation of these mols., which are crucial for the formation of the hydrocarbon pool. Furthermore, lower intrinsic methanol reactivity toward methoxide formation has been obsd. These observations are in line with a longer induction period obsd. from in situ UV-vis microspectroscopy expts. These expts. revealed a slower and more homogeneous discoloration of H-SAPO-34, while in situ confocal fluorescence microscopy confirmed the more homogeneous distribution and larger amt. of MTO intermediates when cofeeding water. As such, it is shown that water induces a more efficient use of the H-SAPO-34 catalyst crystals at the microscopic level. The combined exptl.-theor. approach gives a profound insight into the role of water in the catalytic process at the mol. and single-particle level.
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(d) De Wispelaere, K.; Vanduyfhuys, L.; Van Speybroeck, V. Modelling and Simulation in the Science of Micro- and Meso-Porous Materials; Elsevier, 2017; Chapter entitled Entropy Contributions to Transition State Modelling.
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(e) Li, G.; Pidko, E. A. The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective. ChemCatChem 2019, 11, 134– 156, DOI: 10.1002/cctc.201801493
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15e
The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective
Li, Guanna; Pidko, Evgeny A.
ChemCatChem (2019), 11 (1), 134-156CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Zeolites have a broad spectrum of applications as robust microporous catalysts for various chem. transformations. The reactivity of zeolite catalysts can be tailored by introducing heteroatoms either into the framework or at the extra-framework positions that gives rise to the formation of versatile Bronsted acid, Lewis acid and redox-active catalytic sites. Understanding the nature and catalytic role of such sites is crucial for guiding the design of new and improved zeolite-based catalysts. This work presents an overview of recent computational studies devoted to unraveling the mol. level details of catalytic transformations inside the zeolite pores. The role of modern computational chem. in addressing the structural problem in zeolite catalysis, understanding reaction mechanisms and establishing structure-activity relations is discussed. Special attention is devoted to such mechanistic phenomena as active site cooperativity, multifunctional catalysis as well as confinement-induced and multi-site reactivity commonly encountered in zeolite catalysis.
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16
Gray, H. B.; Ballhausen, C. J. A Molecular Orbital Theory for Square Planar Metal Complexes. J. Am. Chem. Soc. 1963, 85, 260– 265, DOI: 10.1021/ja00886a002
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A molecular-orbital theory for square planar metal complexes
Gray, Harry B.; Ballhausen, C. J.
Journal of the American Chemical Society (1963), 85 (), 260-5CODEN: JACSAT; ISSN:0002-7863.
The bonding in square planar metal complexes is described in terms of mol. orbitals. The relative single-electron mol. orbital energies are estd. for the different types of π-electron systems. Both the d-d and charge transfer spectra of the halide and cyanide complexes of Ni++, Pd++, and Au+++ are discussed in terms of the derived mol. orbital scheme. The magnetic susceptibilities of diamagnetic d8-metal complexes are considered, and it is concluded that a substantial "ring current" exists in K2Ni(CN)4.H2O.
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Chatt, J.; Hayter, R. G. 167. Ligand Field Strengths of the Halide, Methyl, Phenyl, and Hydride Anions. J. Chem. Soc. 1961, 772– 774, DOI: 10.1039/jr9610000772
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Ligand field strengths of the halide, methyl, phenyl, and hydride anions
Chatt, J.; Hayter, R. G.
Journal of the Chemical Society (1961), (), 772-4CODEN: JCSOA9; ISSN:0368-1769.
The visible and UV spectra of compds. of the formula trans-[RuXY{C2H4(PMe2)2}2] (X = Y = Cl, Br, I, or CN; X = H, Me, Ph, or p-tolyl, and Y = Cl) were detd. and interpreted to show that the ligand field strengths of H and the org. groups are large, but less than that of CN-.
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Hammond, G. S. A. Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334– 338, DOI: 10.1021/ja01607a027
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18
A correlation of reaction rates
Hammond, George S.
Journal of the American Chemical Society (1955), 77 (), 334-8CODEN: JACSAT; ISSN:0002-7863.
Although there is not completely general correlation between the rates and free energies of chem. reactions there are many instances in which the most stable products are formed at the most rapid rates from a series of similarly constituted reactants. In certain other reactions, the less stable products seem to be formed at the more rapid rates with considerable regularity. In the latter cases it appears that the transition states are formed with the min. amt. of reorganization of the reactants. By invoking the simple postulate that if 2 states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the mol. structures, decision can be made whether the reactants or products or neither are good structural models for estg. the effect of structural variations on the free energies of transition states. The postulates was applied to the production of free radicals, carbanions, carbonium ions, to the reactions of carbonium ions, and to the benzilic acid rearrangement.
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Kozuch, S.; Shaik, S. How to Conceptualize Catalytic Cycles? The Energetic Span Model. Acc. Chem. Res. 2011, 44, 101– 110, DOI: 10.1021/ar1000956
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19
How to Conceptualize Catalytic Cycles? The Energetic Span Model
Kozuch, Sebastian; Shaik, Sason
Accounts of Chemical Research (2011), 44 (2), 101-110CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review; a computational study of a catalytic cycle generates state energies (the E-representation), whereas expts. lead to rate consts. (the k-representation). Based on transition state theory (TST), these are equiv. representations. Nevertheless, until recently, there has been no simple way to calc. the efficiency of a catalytic cycle, i.e., its turnover frequency (TOF), from a theor. obtained energy profile. In this Account, we introduce the energetic span model that enables one to evaluate TOFs in a straightforward manner and in affinity with the Curtin-Hammett principle. As shown herein, the model implies a change in our kinetic concepts. Analogous to Ohm's law, the catalytic chem. current (the TOF) can be defined by a chem. potential (independent of the mechanism) divided by a chem. resistance (dependent on the mechanism and the nature of the catalyst). This formulation is based on Eyring's TST and corresponds to a steady-state regime. In many catalytic cycles, only one transition state and one intermediate det. the TOF. We call them the TOF-detg. transition state (TDTS) and the TOF-detg. intermediate (TDI). These key states can be located, from among the many states available to a catalytic cycle, by assessing the degree of TOF control (XTOF); this last term resembles the structure-reactivity coeff. in classical phys. org. chem. The TDTS-TDI energy difference and the reaction driving force define the energetic span (δE) of the cycle. Whenever the TDTS appears after the TDI, δE is the energy difference between these two states; when the opposite is true, we must also add the driving force to this difference. Having δE, the TOF is expressed simply in the Arrhenius-Eyring fashion, wherein δE serves as the apparent activation energy of the cycle. An important lesson from this model is that neither one transition state nor one reaction step possess all the kinetic information that dets. the efficiency of a catalyst. Addnl., the TDI and TDTS are not necessarily the highest and lowest states, nor do they have to be adjoined as a single step. As such, we can conclude that a change in the conceptualization of catalytic cycles is in order: in catalysis, there are no rate-detg. steps, but rather rate-detg. states. We also include a study on the effect of reactant and product concns. In the energetic span approxn., only the reactants or products that are located between the TDI and TDTS accelerate or inhibit the reaction. In this manner, the energetic span model creates a direct link between exptl. quantities and theor. results. The versatility of the energetic span model is demonstrated with several catalytic cycles of organometallic reactions.
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Ittel, S. D.; Johnson, L. K.; Brookhart, M. Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000, 100, 1169– 1204, DOI: 10.1021/cr9804644
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Late-Metal Catalysts for Ethylene Homo- and Copolymerization
Ittel, Steven D.; Johnson, Lynda K.; Brookhart, Maurice
Chemical Reviews (Washington, D. C.) (2000), 100 (4), 1169-1203CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review with 427 refs. on the use of late transition metal complexes as catalysts for ethylene polymn.
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Shamzhy, M.; Opanasenko, M.; Concepción, P.; Martínez, A. New Trends in Tailoring Active Sites in Zeolite-Based Catalysts. Chem. Soc. Rev. 2019, 48, 1095– 1149, DOI: 10.1039/C8CS00887F
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21
New trends in tailoring active sites in zeolite-based catalysts
Shamzhy, Mariya; Opanasenko, Maksym; Concepcion, Patricia; Martinez, Agustin
Chemical Society Reviews (2019), 48 (4), 1095-1149CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
This review addresses the recent developments and trends in tailoring the nature and local properties of active sites in zeolite-based catalysts, with a special focus on novel extra-large pore, layered (2D), nanocryst., and hierarchical (mesoporous) zeolites with enhanced pore accessibility. In the first part of the review, we discuss the latest achievements in the bottom-up (direct synthesis) and top-down (post-synthesis) approaches for isomorphous substitution in zeolites enabling control over the type (Bronsted, Lewis, or both), amt., strength, and location of acid sites. The benefits in catalysis provided by such zeolites with tuned acidity and improved accessibility are shown for different acid-catalyzed reactions involving bulky mols., as in the synthesis of fine chems. and biomass transformations. The incorporation of metal species of different sizes (increasing from single atoms to clusters and to nanoparticles) in zeolites allows expanding the set of reactions catalyzed by these materials. The main prepn. strategies for designing metal-zeolite catalysts, esp. those offering control over the size of the metal species, and their catalytic behavior in industrially relevant and emerging sustainable catalytic processes are dealt with in the second part of the review. Particular attention is paid to the stabilization of size-controlled small metal clusters and nanoparticles through their encapsulation in the voids of zeolite frameworks as well as to the dynamic behavior of the metal species under reactive environments with important implications in catalysis. The need for using advanced operando spectroscopic and imaging tools to unveil the precise nature and functioning of the active sites in working zeolites is emphasized. The information gathered in this review is expected to provide guidance for developing more efficient zeolite-based catalysts for existing and new applications.
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Hutter, J.; Iannuzzi, M.; Schiffmann, F.; VandeVondele, J. CP2K: Atomistic Simulations of Condensed Matter Systems. WIREs Comput. Mol. Sci. 2014, 4, 15– 25, DOI: 10.1002/wcms.1159
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cp2k: atomistic simulations of condensed matter systems
Hutter, Juerg; Iannuzzi, Marcella; Schiffmann, Florian; VandeVondele, Joost
Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (1), 15-25CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
A review. Cp2k has become a versatile open-source tool for the simulation of complex systems on the nanometer scale. It allows for sampling and exploring potential energy surfaces that can be computed using a variety of empirical and first principles models. Excellent performance for electronic structure calcns. is achieved using novel algorithms implemented for modern and massively parallel hardware. This review briefly summarizes the main capabilities and illustrates with recent applications the science cp2k has enabled in the field of atomistic simulation. WIREs Comput Mol Sci 2014, 4:15-25. doi: 10.1002/wcms.1159 The authors have declared no conflicts of interest in relation to this article. For further resources related to this article, please visit the WIREs website.
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CP2K, version 3.0 or higher, the CP2K developers group. CP2K is freely available from http://www.cp2k.org.
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VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Quickstep: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103– 128, DOI: 10.1016/j.cpc.2004.12.014
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QUICKSTEP: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach
VandeVondele, Joost; Krack, Matthias; Mohamed, Fawzi; Parrinello, Michele; Chassaing, Thomas; Hutter, Juerg
Computer Physics Communications (2005), 167 (2), 103-128CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
We present the Gaussian and plane waves (GPW) method and its implementation in which is part of the freely available program package CP2K. The GPW method allows for accurate d. functional calcns. in gas and condensed phases and can be effectively used for mol. dynamics simulations. We show how derivs. of the GPW energy functional, namely ionic forces and the Kohn-Sham matrix, can be computed in a consistent way. The computational cost of computing the total energy and the Kohn-Sham matrix is scaling linearly with the system size, even for condensed phase systems of just a few tens of atoms. The efficiency of the method allows for the use of large Gaussian basis sets for systems up to 3000 atoms, and we illustrate the accuracy of the method for various basis sets in gas and condensed phases. Agreement with basis set free calcns. for single mols. and plane wave based calcns. in the condensed phase is excellent. Wave function optimization with the orbital transformation technique leads to good parallel performance, and outperforms traditional diagonalisation methods. Energy conserving Born-Oppenheimer dynamics can be performed, and a highly efficient scheme is obtained using an extrapolation of the d. matrix. We illustrate these findings with calcns. using commodity PCs as well as supercomputers.
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(a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868, DOI: 10.1103/PhysRevLett.77.3865
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25a
Generalized gradient approximation made simple
Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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(b) Zhang, Y.; Yang, W. Comment on “Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1998, 80, 890– 890, DOI: 10.1103/PhysRevLett.80.890
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25b
Comment on "Generalized Gradient Approximation Made Simple"
Zhang, Yingkai; Yang, Weitao
Physical Review Letters (1998), 80 (4), 890CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
A Comment on the Letter by John P. Perdew, Kieron Burke, and Matthias Ernzerhof, Phys. 77, 3865 (1996). The authors of the Letter offer a Reply.
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(a) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104, DOI: 10.1063/1.3382344
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26a
A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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(b) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456– 1465, DOI: 10.1002/jcc.21759
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Effect of the damping function in dispersion corrected density functional theory
Grimme, Stefan; Ehrlich, Stephan; Goerigk, Lars
Journal of Computational Chemistry (2011), 32 (7), 1456-1465CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
It is shown by an extensive benchmark on mol. energy data that the math. form of the damping function in DFT-D methods has only a minor impact on the quality of the results. For 12 different functionals, a std. "zero-damping" formula and rational damping to finite values for small interat. distances according to Becke and Johnson (BJ-damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coeffs. is used. The BJ-damping requires one fit parameter more for each functional (three instead of two) but has the advantage of avoiding repulsive interat. forces at shorter distances. With BJ-damping better results for nonbonded distances and more clear effects of intramol. dispersion in four representative mol. structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermol. distances. For noncovalent interaction energies BJ-damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of cor. GGAs for non-covalent interactions. According to the thermodn. benchmarks BJ-damping is more accurate esp. for medium-range electron correlation problems and only small and practically insignificant double-counting effects are obsd. It seems to provide a phys. correct short-range behavior of correlation/dispersion even with unmodified std. functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying d. functional. © 2011 Wiley Periodicals, Inc.; J. Comput. Chem., 2011.
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Kumar, S.; Rosenberg, J. M.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A. The Weighted Histogram Analysis Method for Free-energy Calculations on Biomolecules. I. The method. J. Comput. Chem. 1992, 13, 1011– 1021, DOI: 10.1002/jcc.540130812
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The weighted histogram analysis method for free-energy calculations on biomolecules. I. The method
Kumar, Shankar; Bouzida, Djamal; Swendsen, Robert H.; Kollman, Peter A.; Rosenberg, John M.
Journal of Computational Chemistry (1992), 13 (8), 1011-21CODEN: JCCHDD; ISSN:0192-8651.
The Weighted Histogram Anal. Method (WHAM), an extension of Ferrenberg and Swendsen's Multiple Histogram Technique, has been applied for the first time on complex biomol. Hamiltonians. The method is presented here as an extension of the Umbrella Sampling method for free-energy and Potential of Mean Force calcns. This algorithm possesses the following advantages over methods that are currently employed: (1) it provides a built-in est. of sampling errors thereby yielding objective ests. of the optimal location and length of addnl. simulations needed to achieve a desired level of precision; (2) it yields the "best" value of free energies by taking into account all the simulations so as to minimize the statistical errors; (3) in addn. to optimizing the links between simulations, it also allows multiple overlaps of probability distributions for obtaining better ests. of the free-energy differences. By recasting the Ferrenberg-Swendsen Multiple Histogram equations in a form suitable for mol. mechanics type Hamiltonians, we have demonstrated the feasibility and robustness of this method by applying it to a test problem of the generation of the Potential of Mean Force profile of the pseudorotation phase angle of the sugar ring in deoxyadenosine.
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Ensing, B.; Laio, A.; Parrinello, M.; Klein, M. L. A Recipe for the Computation of the Free Energy Barrier and the Lowest Free Energy Path of Concerted Reactions. J. Phys. Chem. B 2005, 109, 6676– 6687, DOI: 10.1021/jp045571i
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A Recipe for the Computation of the Free Energy Barrier and the Lowest Free Energy Path of Concerted Reactions
Ensing, Bernd; Laio, Alessandro; Parrinello, Michele; Klein, Michael L.
Journal of Physical Chemistry B (2005), 109 (14), 6676-6687CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
The recently introduced hills method (Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562) is a powerful tool to compute the multidimensional free energy surface of intrinsically concerted reactions. We have extended this method by focusing our attention on localizing the lowest free energy path that connects the stable reactant and product states. This path represents the most probable reaction mechanism, similar to the zero temp. intrinsic reaction coordinate, but also includes finite temp. effects. The transformation of the multidimensional problem to a one-dimensional reaction coordinate allows for accurate convergence of the free energy profile along the lowest free energy path using std. free energy methods. Here we apply the hills method, our lowest free energy path search algorithm, and umbrella sampling to the prototype SN2 reaction. The hills method replaces the in many cases difficult problem of finding a good reaction coordinate with choosing relatively simple collective variables, such as the bond lengths of the broken and formed chem. bonds. The second part of the paper presents a guide to using the hills method, in which we test and fine-tune the method for optimal accuracy and efficiency using the umbrella sampling results as a ref.
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Tribello, G. A.; Bonomi, M.; Branduardi, D.; Camilloni, C.; Bussi, G. PLUMED 2: New Feathers for an Old Bird. Comput. Phys. Commun. 2014, 185, 604– 613, DOI: 10.1016/j.cpc.2013.09.018
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PLUMED 2: New feathers for an old bird
Tribello, Gareth A.; Bonomi, Massimiliano; Branduardi, Davide; Camilloni, Carlo; Bussi, Giovanni
Computer Physics Communications (2014), 185 (2), 604-613CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
Enhancing sampling and analyzing simulations are central issues in mol. simulation. Recently, we introduced PLUMED, an open-source plug-in that provides some of the most popular mol. dynamics (MD) codes with implementations of a variety of different enhanced sampling algorithms and collective variables (CVs). The rapid changes in this field, in particular new directions in enhanced sampling and dimensionality redn. together with new hardware, require a code that is more flexible and more efficient. We therefore present PLUMED 2 here-a complete rewrite of the code in an object-oriented programming language (C++). This new version introduces greater flexibility and greater modularity, which both extends its core capabilities and makes it far easier to add new methods and CVs. It also has a simpler interface with the MD engines and provides a single software library contg. both tools and core facilities. Ultimately, the new code better serves the ever-growing community of users and contributors in coping with the new challenges arising in the field.
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Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1– 19, DOI: 10.1006/jcph.1995.1039
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Fast parallel algorithms for short-range molecular dynamics
Plimpton, Steve
Journal of Computational Physics (1995), 117 (1), 1-19CODEN: JCTPAH; ISSN:0021-9991.
Three parallel algorithms for classical mol. dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-at. forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for mol. dynamics models which can be difficult to parallelize efficiently - those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a std. Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers - the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C90 processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex mol. dynamics simulations are also discussed.
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Medford, A. J.; Shi, C.; Hoffmann, M. J.; Lausche, A. C.; Fitzgibbon, S. R.; Bligaard, T.; Nørskov, J. K. CatMAP: A Software Package for Descriptor-Based Microkinetic Mapping of Catalytic Trends. Catal. Lett. 2015, 145, 794– 807, DOI: 10.1007/s10562-015-1495-6
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CatMAP: A Software Package for Descriptor-Based Microkinetic Mapping of Catalytic Trends
Medford, Andrew J.; Shi, Chuan; Hoffmann, Max J.; Lausche, Adam C.; Fitzgibbon, Sean R.; Bligaard, Thomas; Noerskov, Jens K.
Catalysis Letters (2015), 145 (3), 794-807CODEN: CALEER; ISSN:1011-372X. (Springer)
Abstr.: Descriptor-based anal. is a powerful tool for understanding the trends across various catalysts. In general, the rate of a reaction over a given catalyst is a function of many parameters-reaction energies, activation barriers, thermodn. conditions, etc. The high dimensionality of this problem makes it very difficult and expensive to solve completely, and even a full soln. would not give much insight into the rational design of new catalysts. The descriptor-based approach seeks to det. a few "descriptors" upon which the other parameters are dependent. By doing this it is possible to reduce the dimensionality of the problem-preferably to 1 or 2 descriptors-thus greatly reducing computational efforts and simultaneously increasing the understanding of trends in catalysis. The "CatMAP" Python module seeks to standardize and automate many of the math. routines necessary to move from "descriptor space" to reaction rates for heterogeneous (electro) catalysts. The module is designed to be both flexible and powerful, and is available for free online. A "reaction model" can be fully defined by a configuration file, thus no new programming is necessary to change the complexity or assumptions of a model. Furthermore, various steps in the process of moving from descriptors to reaction rates have been abstracted into sep. Python classes, making it easy to change the methods used or add new functionality. This work discusses the structure of the code and presents the underlying algorithms and math. expressions both generally and via an example for the CO oxidn. reaction.
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Humphrey, W.; Dalke, A.; Schulten, K. VMD - Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33– 38, DOI: 10.1016/0263-7855(96)00018-5
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VDM: visual molecular dynamics
Humphrey, William; Dalke, Andrew; Schulten, Klaus
Journal of Molecular Graphics (1996), 14 (1), 33-8, plates, 27-28CODEN: JMGRDV; ISSN:0263-7855. (Elsevier)
VMD is a mol. graphics program designed for the display and anal. of mol. assemblies, in particular, biopolymers such as proteins and nucleic acids. VMD can simultaneously display any no. of structures using a wide variety of rendering styles and coloring methods. Mols. are displayed as one or more "representations," in which each representation embodies a particular rendering method and coloring scheme for a selected subset of atoms. The atoms displayed in each representation are chosen using an extensive atom selection syntax, which includes Boolean operators and regular expressions. VMD provides a complete graphical user interface for program control, as well as a text interface using the Tcl embeddable parser to allow for complex scripts with variable substitution, control loops, and function calls. Full session logging is supported, which produces a VMD command script for later playback. High-resoln. raster images of displayed mols. may be produced by generating input scripts for use by a no. of photorealistic image-rendering applications. VMD has also been expressly designed with the ability to animate mol. dynamics (MD) simulation trajectories, imported either from files or from a direct connection to a running MD simulation. VMD is the visualization component of MDScope, a set of tools for interactive problem solving in structural biol., which also includes the parallel MD program NAMD, and the MDCOMM software used to connect the visualization and simulation programs, VMD is written in C++, using an object-oriented design; the program, including source code and extensive documentation, is freely available via anonymous ftp and through the World Wide Web.
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Kubota, Y.; Maekawa, H.; Miyata, S.; Tatsumi, T.; Sugi, Y. Hydrothermal Synthesis of Metallosilicate SSZ-24 from Metallosilicate Beta as Precursors. Microporous Mesoporous Mater. 2007, 101, 115– 126, DOI: 10.1016/j.micromeso.2006.11.037
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Hydrothermal synthesis of metallosilicate SSZ-24 from metallosilicate beta as precursors
Kubota, Yoshihiro; Maekawa, Hiroyoshi; Miyata, Shunsuke; Tatsumi, Takashi; Sugi, Yoshihiro
Microporous and Mesoporous Materials (2007), 101 (1-2), 115-126CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
Aluminosilicate version of SSZ-24 ([Al]-SSZ-24) zeolite (AFI topol.) was synthesized by a new route including phase-transformation, wherein aluminosilicate analog of beta zeolite ([Al]-BEA) was used as a precursor and N(16)-methylsparteinium hydroxide (MeSPA+OH-) was used as structure-directing agent (SDA). [Al]-BEA precursors synthesized by various routes, mainly hydrothermal synthesis (HTS) and steam-assisted crystn. (SAC) methods, were employed for the synthesis. In general, [Al]-SSZ-24 samples with the SiO2/Al2O3 ratios comparable to the input SiO2/Al2O3 ratio were readily synthesized by using the precursor route when the SiO2/Al2O3 was as high as 200. The limit of aluminum introduction depended on the origin of BEA precursor. After optimization, pure [Al]-SSZ-24 with SiO2/Al2O3 = 109 was successfully obtained. During crystn., a phase transformation from [Al]-BEA to [Al]-SSZ-24 was obsd., suggesting dissoln. of beta phase to a min. structural unit followed by its redirection towards SSZ-24 phase in the presence of the SDA. Field emission SEM (FE-SEM) showed hexagonal rod-shaped crystals of about 1 μm in size. NH3-TPD measurement showed the presence of enough amts. of acidic sites as compared to the expected amt. based on the elemental anal. The catalytic performance of [Al]-SSZ-24 synthesized from [Al]-BEA was compared with that of the [Al]-SSZ-24 synthesized via [B]-SSZ-24 by isomorphous substitution of Al for B. The [Al]-SSZ-24 synthesized by the former method from [Al]-BEA showed comparable or better shape-selectivity than that synthesized by the latter method in the isopropylation of biphenyl. The applicability of this "BEA precursor method" to other metallosilicates (e.g. titanosilicate) was also demonstrated.
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Abstract
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Scheme 1
Scheme 1. Cossee–Arlman Catalytic Cycle of Ethene Oligomerization in Ni(II) Cationic Complexes
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Figure 1
Figure 1. Free-energy diagram for ethene oligomerization pathways in Ni-SSZ-24 zeolite at 25 bar and 120 °C, based on free-energy profiles of elementary steps obtained with DFT-MD umbrella sampling simulations. The reference state is the equilibrium loading of ethene surrounding [ethene-Ni-ethyl]⁺, as discussed in the text. The numbers indicate intrinsic free energies of activation in kJ/mol. As indicated in the diagram, the [ethyl-Ni-butene]⁺ species can also be reached by ethene desorption; see Section S2.3 for a detailed discussion of how the diagram was constructed.
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Figure 2
Figure 2. Free-energy profiles for ethene coordination to [ethene-Ni-ethyl]⁺ (left) and [ethene-Ni-butyl]⁺ (right) at 25 bar and 120 °C, derived from DFT-MD simulations. The left inset shows histograms of Ni–Al distances, illustrating that the doubly and triply ethene-coordinated Ni-ethyl species are detached from the framework and more mobile than their anchored counterpart. The associated charge-separation energy is estimated to 78 kJ/mol (Section S2.5). The right insets show snapshots from the simulations of the Ni-ethyl species, including the trajectories of the Ni (light blue) and the α alkyl C (dark blue) atoms (Figure S8 shows the butyl analogues).
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Figure 3
Figure 3. Snapshots from DFT-MD umbrella simulations of the initial (IS) and transition states (TS) of C–C coupling in [(ethene)_x-Ni-ethyl]⁺ species in SSZ-24 zeolite. Planes of square and trigonal coordination are shown in blue and yellow, respectively.
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Figure 4
Figure 4. Results from continuous-flow experiments on ethene oligomerization in Ni-SSZ-24: (a) Arrhenius plot for butene formation at 4 and 26 bar ethene pressure, (b) reaction order of butene and hexene formation at 150 °C, and (c) butene isomer selectivity at 150 °C. All results were obtained at differential ethene conversions of 1–7%.
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This article references 33 other publications.
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  Journal of the American Chemical Society (2017), 139 (2), 757-762CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  A recently developed metal-org. framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of com. homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calcns. to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.
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  .
  (e) Liu, J.; Ye, J.; Li, Z.; Otake, K.-i.; Liao, Y.; Peters, A. W.; Noh, H.; Truhlar, D. G.; Gagliardi, L.; Cramer, C. J.; Farha, O. K.; Hupp, J. T. Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization. J. Am. Chem. Soc. 2018, 140, 11174– 11178, DOI: 10.1021/jacs.8b06006
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  7e
  Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization
  Liu, Jian; Ye, Jingyun; Li, Zhanyong; Otake, Ken-ichi; Liao, Yijun; Peters, Aaron W.; Noh, Hyunho; Truhlar, Donald G.; Gagliardi, Laura; Cramer, Christopher J.; Farha, Omar K.; Hupp, Joseph T.
  Journal of the American Chemical Society (2018), 140 (36), 11174-11178CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  To modify its steric and electronic properties as a support for heterogeneous catalysts, electron-withdrawing and electron-donating ligands, hexafluoroacetylacetonate (Facac-) and acetylacetonate (Acac-), were introduced to the metal-org. framework (MOF), NU-1000, via a process akin to at. layer deposition (ALD). In the absence of Facac- or Acac-, NU-1000-supported, AIM-installed Ni(II) sites yield a mixt. of C4, C6, C8, and polymeric products in ethylene oligomerization. (AIM = ALD-like deposition in MOFs). In contrast, both Ni-Facac-AIM-NU-1000 and Ni-Acac-AIM-NU-1000 exhibit quant. catalytic selectivity for C4 species. Exptl. findings are supported by d. functional theory calcns., which show increases in the activation barrier for the C-C coupling step, due mainly to rearrangement of the siting of Facac- or Acac- to partially ligate added nickel. The results illustrate the important role of structure-tuning support modifiers in controlling the activity of MOF-sited heterogeneous catalysts and in engendering catalytic selectivity. The results also illustrate the ease with which crystallog. well-defined modifications of the catalyst support can be introduced when the node-coordinating mol. modifier is delivered via the vapor phase.
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8. 8
  Brogaard, R. Y.; Olsbye, U. Ethene Oligomerization in Ni-containing Zeolites: Theoretical Discrimination of Reaction Mechanisms. ACS Catal. 2016, 6, 1205– 1214, DOI: 10.1021/acscatal.5b01957
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  8
  Ethene Oligomerization in Ni-Containing Zeolites: Theoretical Discrimination of Reaction Mechanisms
  Brogaard, Rasmus Y.; Olsbye, Unni
  ACS Catalysis (2016), 6 (2), 1205-1214CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Ni-contg. porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addn., it remains unclear why the aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses d. functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calcns. employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calcns. targeted Ni-contg. SSZ-24 zeolite as a representative catalyst and considered Ni+, Ni2+ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee-Arlman mechanisms that have been proposed in the literature, in addn. to a new proton-transfer mechanism. Free energy profiles were derived at a typical exptl. reaction temp. of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee-Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calcd. intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available exptl. data. We further propose a mechanism for formation of the active nickel-alkyl species by reaction between ethene and isolated Ni2+ ions. The results hence provide a solid starting point for exptl. investigations of the catalytic cycle, to validate our predictions and ultimately det. the atom-scale properties that control catalytic activity.
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9. 9
  Forget, S.; Olivier-Bourbigou, H.; Delcroix, D. Homogeneous and Heterogeneous Nickel-Catalyzed Olefin Oligomerization: Experimental Investigation for a Common Mechanistic Proposition and Catalyst Optimization. ChemCatChem 2017, 9, 2408– 2417, DOI: 10.1002/cctc.201700348
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  9
  Homogeneous and Heterogeneous Nickel-Catalyzed Olefin Oligomerization: Experimental Investigation for a Common Mechanistic Proposition and Catalyst Optimization
  Forget, Severine; Olivier-Bourbigou, Helene; Delcroix, Damien
  ChemCatChem (2017), 9 (12), 2408-2417CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Only a few catalytic transformations can be efficiently catalyzed by both homogeneous or heterogeneous technologies, with short olefin oligomerization promoted by a nickel-based catalysts among them. Homogeneous and heterogeneous catalysis are often opposed in terms of activity, active-site description or recyclability, traditionally mentioned as "homogeneous vs. heterogeneous". Unlike previous studies, we propose to emphasize the similarities between both catalysis by comparing industrially representative results obtained in continuous flow-mode under similar mild conditions. A detailed anal. of both primary products of olefins oligomerization, and a set of secondary products obtained in exptl. assays completed with recently published results of DFT calcns. prompted us to postulate a common mechanistic pathway for nickel-catalyzed ethylene oligomerization. Heterogeneous metallic ethylene oligomerization, for which nickel active sites identification has indeed long been under debate, seems to follow the Cossee-Arlman mechanism, well accepted as major mechanism in the homogeneous counterpart. This anal. allowed us to design an unprecedented heterogeneous catalyst active for ethylene and butene oligomerization.
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10. 10
  Toch, K.; Thybaut, J.; Marin, G. Ethene Oligomerization on Ni-SiO₂-Al₂O₃: Experimental Investigation and Single-Event MicroKinetic Modeling. Appl. Catal., A 2015, 489, 292– 304, DOI: 10.1016/j.apcata.2014.10.036
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  10
  Ethene oligomerization on Ni-SiO2-Al2O3: Experimental investigation and Single-Event MicroKinetic modeling
  Toch, K.; Thybaut, J. W.; Marin, G. B.
  Applied Catalysis, A: General (2015), 489 (), 292-304CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
  An exptl. investigation of the intrinsic ethene oligomerization kinetics on an amorphous 1.8 wt.% Ni-SiO2-Al2O3 was performed. The temp. ranged from 443 to 503 K, the total pressure from 1.5 to 3.5 MPa, the inlet ethene partial pressure from 0.15 to 0.35 MPa and the space time was between 4.8 and 14.4 kgcat s mol-1C2. Due to the absence of strong acid sites, the catalytic activity toward oligomerization originated solely from the nickel ion sites at the investigated conditions and resulted in an Anderson Schulz Flory product distribution. The oligomerization rate amounted from 0.007 to 0.027 mol s-1 kg-1cat with butene selectivities of 80-90%. A Single-Event MicroKinetic (SEMK) model for ethene oligomerization, based upon an insertion-termination mechanism, was constructed. The kinetic parameter ests. were all statistically significant with a precise phys. meaning. The model was found to be globally significant and adequate, i.e., it was able to describe all exptl. data without any systematic deviation. Using the model, it was detd. that mainly ethene, because of its abundance, and octene, because of its higher carbon no., were physisorbed. The corresponding catalyst occupancy by physisorbed species typically ranged between 10% and 50%. Coordination of the alkenes at the active nickel ions resulted in about 90% of these ions being present under the form of a nickel-ethene species.
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11. 11
  Joshi, R.; Zhang, G.; Miller, J. T.; Gounder, R. Evidence for the Coordination-Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves. ACS Catal. 2018, 8, 11407– 11422, DOI: 10.1021/acscatal.8b03202
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  11
  Evidence for the Coordination-Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves
  Joshi, Ravi; Zhang, Guanghui; Miller, Jeffrey T.; Gounder, Rajamani
  ACS Catalysis (2018), 8 (12), 11407-11422CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The mechanistic origin of alkene dimerization on Ni sites supported on aluminosilicates has been ascribed to both coordination-insertion (i.e., Cossee-Arlman) and metallacycle-based cycles; the latter is often invoked in the absence of externally supplied cocatalysts or activators that generate Ni-hydride or Ni-alkyl species to initiate coordination-insertion cycles. Detg. the prevalent reaction mechanism at Ni sites is often complicated by the formation and consumption of alkene dimer products via oligomerization and other parallel reactions (e.g., cracking, isomerization) that occur at Bronsted acid sites on supports. Here, ethene dimerization (453 K) was studied on Beta zeolites synthesized to contain predominantly exchanged Ni2+ sites according to site balances detd. by cation exchange, and to Ni structure detd. by CO IR, UV-visible, and Ni K-edge X-ray absorption spectroscopies. The catalytic behavior of Ni2+ sites was isolated by suppressing contributions from residual H+ sites on support structures, either by selectively poisoning them with Li+ cations or NH4+ species, or by weakening them using a zincosilicate support. Bronsted acid sites form linear dimers (1-butene, cis-2-butene, trans-2-butene) in thermodynamically equilibrated ratios, in addn. to their skeletal isomers (isobutene) and products of subsequent oligomerization-cracking cycles; hence, isobutene formation rates serve as a kinetic marker for the presence of H+ sites. After residual H+ sites deactivate during initial reaction times or when they are suppressed prior to reaction, linear butene isomers form in nonequilibrated ratios that are invariant with ethene site-time, reflecting primary butene double-bond isomerization events catalyzed at Ni2+-derived active sites. Ni-zeolites pretreated in oxidative environments (5 kPa O2, 773 K) show transient activation periods during initial reaction times at dil. ethene pressures (<0.4 kPa) but not at higher ethene pressures (>0.4 kPa) or in the presence of co-fed hydrogen (5 kPa). This behavior is consistent with in situ ethene-assisted formation of [Ni(II)-H]+ intermediates, which isotopically scramble H2-D2 mixts. (453 K) and are quantified from surface H/D exchange reactions (453 K). Taken together, these findings provide unambiguous evidence for the coordination-insertion mechanism as the dominant route for alkene dimerization at Ni2+ cations exchanged onto mol. sieves.
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12. 12
  Henry, R.; Komurcu, M.; Ganjkhanlou, Y.; Brogaard, R. Y.; Lu, L.; Jens, K.-J.; Berlier, G.; Olsbye, U. Ethene Oligomerization on Nickel Microporous and Mesoporous-Supported Catalysts: Investigation of the Active Sites. Catal. Today 2018, 299, 154– 163, DOI: 10.1016/j.cattod.2017.04.029
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  12
  Ethene oligomerization on nickel microporous and mesoporous-supported catalysts: Investigation of the active sites
  Henry, Reynald; Komurcu, Mustafa; Ganjkhanlou, Yadolah; Brogaard, Rasmus Y.; Lu, Li; Jens, Klaus-Joachim; Berlier, Gloria; Olsbye, Unni
  Catalysis Today (2018), 299 (), 154-163CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
  Nickel-contg. mesoporous (using Al2O3 or SiO2-Al2O3 support) and microporous (using nano- or micro-cryst. Beta zeolite support) catalysts were prepd. by ion exchange and characterized by XRD, SEM, N2-adsorption, MP-AES and FTIR. The samples were subjected to testing as ethene oligomerization catalysts at T = 120 °C, Ptotal = 29 bar, Pethene = 11.6-25.1 bar. All catalysts were active for ethene oligomerization, and linear butenes were the main gaseous products. However, catalyst deactivation due to retained long-chain alkenes was obsd. Ethene partial pressure variation expts. showed that the reaction order in ethene for butene formation was 1.5-1.7 for the mesoporous, and 2.0 for the microporous catalysts. Contact time variation expts. carried out with the microporous catalysts at Pethene = 18 bar showed that product selectivity was independent of ethene conversion, and suggested that 1-butene and 2-butenes are primary products. This result is consistent with the Cossee-Arlman mechanism. The nature of the active sites was investigated with FTIR spectroscopy with CO as probe mol. The results point to Ni2+-counterions as the preeminent active sites, while we suggest that Ni-sites grafted on silanol groups and NiO particles are spectators.
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13. 13
  Ehrmaier, A.; Liu, Y.; Peitz, S.; Jentys, A.; Chin, Y.-H. C.; Sanchez-Sanchez, M.; Bermejo-Deval, R.; Lercher, J. Dimerization of Linear Butenes on Zeolite-Supported Ni²⁺. ACS Catal. 2019, 9, 315– 324, DOI: 10.1021/acscatal.8b03095
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  13
  Dimerization of Linear Butenes on Zeolite-Supported Ni2+
  Ehrmaier, Andreas; Liu, Yue; Peitz, Stephan; Jentys, Andreas; Chin, Ya-Huei Cathy; Sanchez-Sanchez, Maricruz; Bermejo-Deval, Ricardo; Lercher, Johannes
  ACS Catalysis (2019), 9 (1), 315-324CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Nickel- and alkali-earth-modified LTA based zeolites catalyze the dimerization of 1-butene in the absence of Bronsted acid sites. The catalyst reaches over 95% selectivity to n-octenes and methylheptenes. The ratio of these two dimers is markedly influenced by the parallel isomerization of 1-butene to 2-butene, shifting the methylheptene/octene ratio from 0.7 to 1.4 as the conversion increases to 35%. At this conversion, the thermodn. equil. of 90% cis- and trans-2-butenes is reached. Conversion of 2-butene results in methylheptene and dimethylhexene with rates that are 1 order of magnitude lower than those with 1-butene. The catalyst is deactivated rapidly by strongly adsorbed products in the presence of 2-butene. The presence of π-allyl-bound butene and Ni-alkyl intermediates was obsd. by IR spectroscopy, suggesting both to be reaction intermediates in isomerization and dimerization. Product distribution and apparent activation barriers suggest 1-butene dimerization to occur via a 1'-adsorption of the first butene mol. and a subsequent 1'- or 2'-insertion of the second butene to form octene and methylheptene, resp. The reaction order of 2 for 1-butene and its high surface coverage suggest that the rate-detg. step involves two weakly adsorbed butene mols. in addn. to the more strongly held butene.
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14. 14
  Agirrezabal-Telleria, I.; Iglesia, E. Stabilization of Active, Selective, and Regenerable Ni-based Dimerization Catalysts by Condensation of Ethene within Ordered Mesopores. J. Catal. 2017, 352, 505– 514, DOI: 10.1016/j.jcat.2017.06.025
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  14
  Stabilization of active, selective, and regenerable Ni-based dimerization catalysts by condensation of ethene withinordered mesopores
  Agirrezabal-Telleria, Iker; Iglesia, Enrique
  Journal of Catalysis (2017), 352 (), 505-514CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
  This study reports the high and stable ethene dimerization turnover rates conferred upon Ni-based active sites at subambient temps. by the condensation of liq. ethene reactants within ordered mesopores in Al-MCM-41. Such active and stable catalysts do not require the activators or co-catalysts essential for organometallic Ni-based catalysts and their robust porous framework allows their full regeneration by thermal treatments in inert or oxidizing environments when deactivation occurs. Dimerization rates (per Ni) are independent of Ni content below Ni2+/H+ exchange ratios of unity, consistent with isolated (Ni-OH)+ cations as active moieties, and with the formation of inactive NiO oligomers at higher Ni contents. Both CO and 2,6 di-tertbutylpyridine, used as titrants of Ni centers and protons, resp., fully suppressed reactivity, indicating that the Ni2+ and OH- centers in (Ni-OH)+ are involved in stabilizing the kinetically-relevant C-C formation transition state. Deactivation consts. decreased abruptly to undetectable values at those temps. and ethene pressures that formed an extended liq. phase within the MCM-41 mesopores that confine the Ni active sites. This remarkable shift from rapid to undetectable deactivation reflects how intrapore liqs. preferentially stabilize the late transition states that mediate the desorption of bound 1-butenes before subsequent isomerization and C-C bond formation events during one surface sojourn. Their preferential desorption inhibits the formation of regioisomers and larger oligomers, thus inhibiting deactivation while also leading to high C4 selectivities among products and to the predominant presence of 1-butene among linear C4 alkenes. Such unprecedented stability and high selectivities allow accurate kinetic measurements and their rigorous mechanistic interpretation in terms of transition state formalisms of chem. dynamics in thermodn. non-ideal media. Dimerization turnovers involve kinetically-relevant C-C bond formation between unbound ethene mols. and an ethene bound at low coverages on (Ni-OH)+ moieties. Such catalytic sequences lead to dimerization rates with a nearly second-order dependence in ethene fugacity, but with an effective rate const. that benefits from the stabilization of the C-C bond formation transition state as intrapore liqs. form. The essential elimination of deactivation events and secondary reactions by such liqs. leads to dimerization rates and 1-butene selectivities that are much higher at 240-260 K with liq.-filled mesopores than at 448 K, conditions that maintain all species in their gaseous form. In addn. to their significant practical impact, these results provide compelling evidence for the ability of an extended liq. to create a thermodynamically non-ideal environment that markedly alters reaction pathways through the selective stabilization of specific transition states based on their location along the reaction coordinate for a given elementary step.
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15. 15
  (a) Van Speybroeck, V.; De Wispelaere, K.; Van der Mynsbrugge, J.; Vandichel, M.; Hemelsoet, K.; Waroquier, M. First Principle Chemical Kinetics in Zeolites: the Methanol-to-Olefin Process as a Case Study. Chem. Soc. Rev. 2014, 43, 7326– 7357, DOI: 10.1039/C4CS00146J
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  15a
  First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study
  Van Speybroeck, Veronique; De Wispelaere, Kristof; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Waroquier, Michel
  Chemical Society Reviews (2014), 43 (21), 7326-7357CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  To optimally design next generation catalysts a thorough understanding of the chem. phenomena at the mol. scale is a prerequisite. Apart from qual. knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate consts. Mol. modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to det. chem. kinetics from first principles, thus by using no exptl. input and by modeling the catalyst and reacting species at the mol. level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theor. concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramol. nature, for which not only the Bronsted acidic site is important but also org. species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theor. modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate consts. can be predicted for reactions occurring at a single active site. The comparison with exptl. data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect mol. dynamics methods complemented with addnl. techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chem. reactions in complex mol. environments.
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  .
  (b) De Wispelaere, K.; Ensing, B.; Ghysels, A.; Meijer, E. J.; Van Speybroeck, V. Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics. Chem. - Eur. J. 2015, 21, 9385– 9396, DOI: 10.1002/chem.201500473
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  15b
  Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics
  De Wispelaere, Kristof; Ensing, Bernd; Ghysels, An; Meijer, Evert Jan; Van Speybroeck, Veronique
  Chemistry - A European Journal (2015), 21 (26), 9385-9396CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The methanol-to-olefin process is a showcase example of complex zeolite-catalyzed chem. At real operating conditions, many factors affect the reactivity, such as framework flexibility, adsorption of various guest mols., and competitive reaction pathways. In this study, the strength of first principle mol. dynamics techniques to capture this complexity is shown by means of two case studies. Firstly, the adsorption behavior of methanol and water in H-SAPO-34 at 350 °C is investigated. Hereby an important degree of framework flexibility and proton mobility was obsd. Secondly, the methylation of benzene by methanol through a competitive direct and stepwise pathway in the AFI topol. was studied. Both case studies clearly show that a first-principle mol. dynamics approach enables unprecedented insights into zeolite-catalyzed reactions at the nanometer scale to be obtained.
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  .
  (c) De Wispelaere, K.; Wondergem, C. S.; Ensing, B.; Hemelsoet, K.; Meijer, E. J.; Weckhuysen, B. M.; Van Speybroeck, V.; Ruiz-Martínez, J. Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy. ACS Catal. 2016, 6, 1991– 2002, DOI: 10.1021/acscatal.5b02139
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  15c
  Insight into the Effect of Water on the Methanol-to-Olefins Conversion in H-SAPO-34 from Molecular Simulations and in Situ Microspectroscopy
  De Wispelaere, Kristof; Wondergem, Caterina S.; Ensing, Bernd; Hemelsoet, Karen; Meijer, Evert Jan; Weckhuysen, Bert M.; Van Speybroeck, Veronique; Ruiz-Martinez, Javier
  ACS Catalysis (2016), 6 (3), 1991-2002CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The role of water in the methanol-to-olefins (MTO) process over H-SAPO-34 has been elucidated by a combined theor. and exptl. approach, encompassing advanced mol. dynamics simulations and in situ microspectroscopy. First-principles calcns. at the mol. level point out that water competes with methanol and propene for direct access to the Bronsted acid sites. This results in less efficient activation of these mols., which are crucial for the formation of the hydrocarbon pool. Furthermore, lower intrinsic methanol reactivity toward methoxide formation has been obsd. These observations are in line with a longer induction period obsd. from in situ UV-vis microspectroscopy expts. These expts. revealed a slower and more homogeneous discoloration of H-SAPO-34, while in situ confocal fluorescence microscopy confirmed the more homogeneous distribution and larger amt. of MTO intermediates when cofeeding water. As such, it is shown that water induces a more efficient use of the H-SAPO-34 catalyst crystals at the microscopic level. The combined exptl.-theor. approach gives a profound insight into the role of water in the catalytic process at the mol. and single-particle level.
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  .
  (d) De Wispelaere, K.; Vanduyfhuys, L.; Van Speybroeck, V. Modelling and Simulation in the Science of Micro- and Meso-Porous Materials; Elsevier, 2017; Chapter entitled Entropy Contributions to Transition State Modelling.
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  (e) Li, G.; Pidko, E. A. The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective. ChemCatChem 2019, 11, 134– 156, DOI: 10.1002/cctc.201801493
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  15e
  The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective
  Li, Guanna; Pidko, Evgeny A.
  ChemCatChem (2019), 11 (1), 134-156CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Zeolites have a broad spectrum of applications as robust microporous catalysts for various chem. transformations. The reactivity of zeolite catalysts can be tailored by introducing heteroatoms either into the framework or at the extra-framework positions that gives rise to the formation of versatile Bronsted acid, Lewis acid and redox-active catalytic sites. Understanding the nature and catalytic role of such sites is crucial for guiding the design of new and improved zeolite-based catalysts. This work presents an overview of recent computational studies devoted to unraveling the mol. level details of catalytic transformations inside the zeolite pores. The role of modern computational chem. in addressing the structural problem in zeolite catalysis, understanding reaction mechanisms and establishing structure-activity relations is discussed. Special attention is devoted to such mechanistic phenomena as active site cooperativity, multifunctional catalysis as well as confinement-induced and multi-site reactivity commonly encountered in zeolite catalysis.
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16. 16
  Gray, H. B.; Ballhausen, C. J. A Molecular Orbital Theory for Square Planar Metal Complexes. J. Am. Chem. Soc. 1963, 85, 260– 265, DOI: 10.1021/ja00886a002
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  A molecular-orbital theory for square planar metal complexes
  Gray, Harry B.; Ballhausen, C. J.
  Journal of the American Chemical Society (1963), 85 (), 260-5CODEN: JACSAT; ISSN:0002-7863.
  The bonding in square planar metal complexes is described in terms of mol. orbitals. The relative single-electron mol. orbital energies are estd. for the different types of π-electron systems. Both the d-d and charge transfer spectra of the halide and cyanide complexes of Ni++, Pd++, and Au+++ are discussed in terms of the derived mol. orbital scheme. The magnetic susceptibilities of diamagnetic d8-metal complexes are considered, and it is concluded that a substantial "ring current" exists in K2Ni(CN)4.H2O.
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17. 17
  Chatt, J.; Hayter, R. G. 167. Ligand Field Strengths of the Halide, Methyl, Phenyl, and Hydride Anions. J. Chem. Soc. 1961, 772– 774, DOI: 10.1039/jr9610000772
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  Ligand field strengths of the halide, methyl, phenyl, and hydride anions
  Chatt, J.; Hayter, R. G.
  Journal of the Chemical Society (1961), (), 772-4CODEN: JCSOA9; ISSN:0368-1769.
  The visible and UV spectra of compds. of the formula trans-[RuXY{C2H4(PMe2)2}2] (X = Y = Cl, Br, I, or CN; X = H, Me, Ph, or p-tolyl, and Y = Cl) were detd. and interpreted to show that the ligand field strengths of H and the org. groups are large, but less than that of CN-.
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  Hammond, G. S. A. Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334– 338, DOI: 10.1021/ja01607a027
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  A correlation of reaction rates
  Hammond, George S.
  Journal of the American Chemical Society (1955), 77 (), 334-8CODEN: JACSAT; ISSN:0002-7863.
  Although there is not completely general correlation between the rates and free energies of chem. reactions there are many instances in which the most stable products are formed at the most rapid rates from a series of similarly constituted reactants. In certain other reactions, the less stable products seem to be formed at the more rapid rates with considerable regularity. In the latter cases it appears that the transition states are formed with the min. amt. of reorganization of the reactants. By invoking the simple postulate that if 2 states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the mol. structures, decision can be made whether the reactants or products or neither are good structural models for estg. the effect of structural variations on the free energies of transition states. The postulates was applied to the production of free radicals, carbanions, carbonium ions, to the reactions of carbonium ions, and to the benzilic acid rearrangement.
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19. 19
  Kozuch, S.; Shaik, S. How to Conceptualize Catalytic Cycles? The Energetic Span Model. Acc. Chem. Res. 2011, 44, 101– 110, DOI: 10.1021/ar1000956
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  How to Conceptualize Catalytic Cycles? The Energetic Span Model
  Kozuch, Sebastian; Shaik, Sason
  Accounts of Chemical Research (2011), 44 (2), 101-110CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review; a computational study of a catalytic cycle generates state energies (the E-representation), whereas expts. lead to rate consts. (the k-representation). Based on transition state theory (TST), these are equiv. representations. Nevertheless, until recently, there has been no simple way to calc. the efficiency of a catalytic cycle, i.e., its turnover frequency (TOF), from a theor. obtained energy profile. In this Account, we introduce the energetic span model that enables one to evaluate TOFs in a straightforward manner and in affinity with the Curtin-Hammett principle. As shown herein, the model implies a change in our kinetic concepts. Analogous to Ohm's law, the catalytic chem. current (the TOF) can be defined by a chem. potential (independent of the mechanism) divided by a chem. resistance (dependent on the mechanism and the nature of the catalyst). This formulation is based on Eyring's TST and corresponds to a steady-state regime. In many catalytic cycles, only one transition state and one intermediate det. the TOF. We call them the TOF-detg. transition state (TDTS) and the TOF-detg. intermediate (TDI). These key states can be located, from among the many states available to a catalytic cycle, by assessing the degree of TOF control (XTOF); this last term resembles the structure-reactivity coeff. in classical phys. org. chem. The TDTS-TDI energy difference and the reaction driving force define the energetic span (δE) of the cycle. Whenever the TDTS appears after the TDI, δE is the energy difference between these two states; when the opposite is true, we must also add the driving force to this difference. Having δE, the TOF is expressed simply in the Arrhenius-Eyring fashion, wherein δE serves as the apparent activation energy of the cycle. An important lesson from this model is that neither one transition state nor one reaction step possess all the kinetic information that dets. the efficiency of a catalyst. Addnl., the TDI and TDTS are not necessarily the highest and lowest states, nor do they have to be adjoined as a single step. As such, we can conclude that a change in the conceptualization of catalytic cycles is in order: in catalysis, there are no rate-detg. steps, but rather rate-detg. states. We also include a study on the effect of reactant and product concns. In the energetic span approxn., only the reactants or products that are located between the TDI and TDTS accelerate or inhibit the reaction. In this manner, the energetic span model creates a direct link between exptl. quantities and theor. results. The versatility of the energetic span model is demonstrated with several catalytic cycles of organometallic reactions.
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20. 20
  Ittel, S. D.; Johnson, L. K.; Brookhart, M. Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000, 100, 1169– 1204, DOI: 10.1021/cr9804644
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  Late-Metal Catalysts for Ethylene Homo- and Copolymerization
  Ittel, Steven D.; Johnson, Lynda K.; Brookhart, Maurice
  Chemical Reviews (Washington, D. C.) (2000), 100 (4), 1169-1203CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review with 427 refs. on the use of late transition metal complexes as catalysts for ethylene polymn.
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21. 21
  Shamzhy, M.; Opanasenko, M.; Concepción, P.; Martínez, A. New Trends in Tailoring Active Sites in Zeolite-Based Catalysts. Chem. Soc. Rev. 2019, 48, 1095– 1149, DOI: 10.1039/C8CS00887F
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  New trends in tailoring active sites in zeolite-based catalysts
  Shamzhy, Mariya; Opanasenko, Maksym; Concepcion, Patricia; Martinez, Agustin
  Chemical Society Reviews (2019), 48 (4), 1095-1149CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  This review addresses the recent developments and trends in tailoring the nature and local properties of active sites in zeolite-based catalysts, with a special focus on novel extra-large pore, layered (2D), nanocryst., and hierarchical (mesoporous) zeolites with enhanced pore accessibility. In the first part of the review, we discuss the latest achievements in the bottom-up (direct synthesis) and top-down (post-synthesis) approaches for isomorphous substitution in zeolites enabling control over the type (Bronsted, Lewis, or both), amt., strength, and location of acid sites. The benefits in catalysis provided by such zeolites with tuned acidity and improved accessibility are shown for different acid-catalyzed reactions involving bulky mols., as in the synthesis of fine chems. and biomass transformations. The incorporation of metal species of different sizes (increasing from single atoms to clusters and to nanoparticles) in zeolites allows expanding the set of reactions catalyzed by these materials. The main prepn. strategies for designing metal-zeolite catalysts, esp. those offering control over the size of the metal species, and their catalytic behavior in industrially relevant and emerging sustainable catalytic processes are dealt with in the second part of the review. Particular attention is paid to the stabilization of size-controlled small metal clusters and nanoparticles through their encapsulation in the voids of zeolite frameworks as well as to the dynamic behavior of the metal species under reactive environments with important implications in catalysis. The need for using advanced operando spectroscopic and imaging tools to unveil the precise nature and functioning of the active sites in working zeolites is emphasized. The information gathered in this review is expected to provide guidance for developing more efficient zeolite-based catalysts for existing and new applications.
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  Hutter, J.; Iannuzzi, M.; Schiffmann, F.; VandeVondele, J. CP2K: Atomistic Simulations of Condensed Matter Systems. WIREs Comput. Mol. Sci. 2014, 4, 15– 25, DOI: 10.1002/wcms.1159
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  cp2k: atomistic simulations of condensed matter systems
  Hutter, Juerg; Iannuzzi, Marcella; Schiffmann, Florian; VandeVondele, Joost
  Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (1), 15-25CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
  A review. Cp2k has become a versatile open-source tool for the simulation of complex systems on the nanometer scale. It allows for sampling and exploring potential energy surfaces that can be computed using a variety of empirical and first principles models. Excellent performance for electronic structure calcns. is achieved using novel algorithms implemented for modern and massively parallel hardware. This review briefly summarizes the main capabilities and illustrates with recent applications the science cp2k has enabled in the field of atomistic simulation. WIREs Comput Mol Sci 2014, 4:15-25. doi: 10.1002/wcms.1159 The authors have declared no conflicts of interest in relation to this article. For further resources related to this article, please visit the WIREs website.
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23. 23
  CP2K, version 3.0 or higher, the CP2K developers group. CP2K is freely available from http://www.cp2k.org.
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  VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J. Quickstep: Fast and Accurate Density Functional Calculations using a Mixed Gaussian and Plane Waves Approach. Comput. Phys. Commun. 2005, 167, 103– 128, DOI: 10.1016/j.cpc.2004.12.014
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  QUICKSTEP: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach
  VandeVondele, Joost; Krack, Matthias; Mohamed, Fawzi; Parrinello, Michele; Chassaing, Thomas; Hutter, Juerg
  Computer Physics Communications (2005), 167 (2), 103-128CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
  We present the Gaussian and plane waves (GPW) method and its implementation in which is part of the freely available program package CP2K. The GPW method allows for accurate d. functional calcns. in gas and condensed phases and can be effectively used for mol. dynamics simulations. We show how derivs. of the GPW energy functional, namely ionic forces and the Kohn-Sham matrix, can be computed in a consistent way. The computational cost of computing the total energy and the Kohn-Sham matrix is scaling linearly with the system size, even for condensed phase systems of just a few tens of atoms. The efficiency of the method allows for the use of large Gaussian basis sets for systems up to 3000 atoms, and we illustrate the accuracy of the method for various basis sets in gas and condensed phases. Agreement with basis set free calcns. for single mols. and plane wave based calcns. in the condensed phase is excellent. Wave function optimization with the orbital transformation technique leads to good parallel performance, and outperforms traditional diagonalisation methods. Energy conserving Born-Oppenheimer dynamics can be performed, and a highly efficient scheme is obtained using an extrapolation of the d. matrix. We illustrate these findings with calcns. using commodity PCs as well as supercomputers.
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25. 25
  (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868, DOI: 10.1103/PhysRevLett.77.3865
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  Generalized gradient approximation made simple
  Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
  Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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  (b) Zhang, Y.; Yang, W. Comment on “Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1998, 80, 890– 890, DOI: 10.1103/PhysRevLett.80.890
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  Comment on "Generalized Gradient Approximation Made Simple"
  Zhang, Yingkai; Yang, Weitao
  Physical Review Letters (1998), 80 (4), 890CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
  A Comment on the Letter by John P. Perdew, Kieron Burke, and Matthias Ernzerhof, Phys. 77, 3865 (1996). The authors of the Letter offer a Reply.
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26. 26
  (a) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104, DOI: 10.1063/1.3382344
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  A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
  Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
  Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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  .
  (b) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456– 1465, DOI: 10.1002/jcc.21759
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  Effect of the damping function in dispersion corrected density functional theory
  Grimme, Stefan; Ehrlich, Stephan; Goerigk, Lars
  Journal of Computational Chemistry (2011), 32 (7), 1456-1465CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
  It is shown by an extensive benchmark on mol. energy data that the math. form of the damping function in DFT-D methods has only a minor impact on the quality of the results. For 12 different functionals, a std. "zero-damping" formula and rational damping to finite values for small interat. distances according to Becke and Johnson (BJ-damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coeffs. is used. The BJ-damping requires one fit parameter more for each functional (three instead of two) but has the advantage of avoiding repulsive interat. forces at shorter distances. With BJ-damping better results for nonbonded distances and more clear effects of intramol. dispersion in four representative mol. structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermol. distances. For noncovalent interaction energies BJ-damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of cor. GGAs for non-covalent interactions. According to the thermodn. benchmarks BJ-damping is more accurate esp. for medium-range electron correlation problems and only small and practically insignificant double-counting effects are obsd. It seems to provide a phys. correct short-range behavior of correlation/dispersion even with unmodified std. functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying d. functional. © 2011 Wiley Periodicals, Inc.; J. Comput. Chem., 2011.
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27. 27
  Kumar, S.; Rosenberg, J. M.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A. The Weighted Histogram Analysis Method for Free-energy Calculations on Biomolecules. I. The method. J. Comput. Chem. 1992, 13, 1011– 1021, DOI: 10.1002/jcc.540130812
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  The weighted histogram analysis method for free-energy calculations on biomolecules. I. The method
  Kumar, Shankar; Bouzida, Djamal; Swendsen, Robert H.; Kollman, Peter A.; Rosenberg, John M.
  Journal of Computational Chemistry (1992), 13 (8), 1011-21CODEN: JCCHDD; ISSN:0192-8651.
  The Weighted Histogram Anal. Method (WHAM), an extension of Ferrenberg and Swendsen's Multiple Histogram Technique, has been applied for the first time on complex biomol. Hamiltonians. The method is presented here as an extension of the Umbrella Sampling method for free-energy and Potential of Mean Force calcns. This algorithm possesses the following advantages over methods that are currently employed: (1) it provides a built-in est. of sampling errors thereby yielding objective ests. of the optimal location and length of addnl. simulations needed to achieve a desired level of precision; (2) it yields the "best" value of free energies by taking into account all the simulations so as to minimize the statistical errors; (3) in addn. to optimizing the links between simulations, it also allows multiple overlaps of probability distributions for obtaining better ests. of the free-energy differences. By recasting the Ferrenberg-Swendsen Multiple Histogram equations in a form suitable for mol. mechanics type Hamiltonians, we have demonstrated the feasibility and robustness of this method by applying it to a test problem of the generation of the Potential of Mean Force profile of the pseudorotation phase angle of the sugar ring in deoxyadenosine.
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28. 28
  Ensing, B.; Laio, A.; Parrinello, M.; Klein, M. L. A Recipe for the Computation of the Free Energy Barrier and the Lowest Free Energy Path of Concerted Reactions. J. Phys. Chem. B 2005, 109, 6676– 6687, DOI: 10.1021/jp045571i
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  A Recipe for the Computation of the Free Energy Barrier and the Lowest Free Energy Path of Concerted Reactions
  Ensing, Bernd; Laio, Alessandro; Parrinello, Michele; Klein, Michael L.
  Journal of Physical Chemistry B (2005), 109 (14), 6676-6687CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  The recently introduced hills method (Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562) is a powerful tool to compute the multidimensional free energy surface of intrinsically concerted reactions. We have extended this method by focusing our attention on localizing the lowest free energy path that connects the stable reactant and product states. This path represents the most probable reaction mechanism, similar to the zero temp. intrinsic reaction coordinate, but also includes finite temp. effects. The transformation of the multidimensional problem to a one-dimensional reaction coordinate allows for accurate convergence of the free energy profile along the lowest free energy path using std. free energy methods. Here we apply the hills method, our lowest free energy path search algorithm, and umbrella sampling to the prototype SN2 reaction. The hills method replaces the in many cases difficult problem of finding a good reaction coordinate with choosing relatively simple collective variables, such as the bond lengths of the broken and formed chem. bonds. The second part of the paper presents a guide to using the hills method, in which we test and fine-tune the method for optimal accuracy and efficiency using the umbrella sampling results as a ref.
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29. 29
  Tribello, G. A.; Bonomi, M.; Branduardi, D.; Camilloni, C.; Bussi, G. PLUMED 2: New Feathers for an Old Bird. Comput. Phys. Commun. 2014, 185, 604– 613, DOI: 10.1016/j.cpc.2013.09.018
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  PLUMED 2: New feathers for an old bird
  Tribello, Gareth A.; Bonomi, Massimiliano; Branduardi, Davide; Camilloni, Carlo; Bussi, Giovanni
  Computer Physics Communications (2014), 185 (2), 604-613CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
  Enhancing sampling and analyzing simulations are central issues in mol. simulation. Recently, we introduced PLUMED, an open-source plug-in that provides some of the most popular mol. dynamics (MD) codes with implementations of a variety of different enhanced sampling algorithms and collective variables (CVs). The rapid changes in this field, in particular new directions in enhanced sampling and dimensionality redn. together with new hardware, require a code that is more flexible and more efficient. We therefore present PLUMED 2 here-a complete rewrite of the code in an object-oriented programming language (C++). This new version introduces greater flexibility and greater modularity, which both extends its core capabilities and makes it far easier to add new methods and CVs. It also has a simpler interface with the MD engines and provides a single software library contg. both tools and core facilities. Ultimately, the new code better serves the ever-growing community of users and contributors in coping with the new challenges arising in the field.
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  Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1– 19, DOI: 10.1006/jcph.1995.1039
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  Fast parallel algorithms for short-range molecular dynamics
  Plimpton, Steve
  Journal of Computational Physics (1995), 117 (1), 1-19CODEN: JCTPAH; ISSN:0021-9991.
  Three parallel algorithms for classical mol. dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-at. forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for mol. dynamics models which can be difficult to parallelize efficiently - those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a std. Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers - the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C90 processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex mol. dynamics simulations are also discussed.
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  Medford, A. J.; Shi, C.; Hoffmann, M. J.; Lausche, A. C.; Fitzgibbon, S. R.; Bligaard, T.; Nørskov, J. K. CatMAP: A Software Package for Descriptor-Based Microkinetic Mapping of Catalytic Trends. Catal. Lett. 2015, 145, 794– 807, DOI: 10.1007/s10562-015-1495-6
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  CatMAP: A Software Package for Descriptor-Based Microkinetic Mapping of Catalytic Trends
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  Catalysis Letters (2015), 145 (3), 794-807CODEN: CALEER; ISSN:1011-372X. (Springer)
  Abstr.: Descriptor-based anal. is a powerful tool for understanding the trends across various catalysts. In general, the rate of a reaction over a given catalyst is a function of many parameters-reaction energies, activation barriers, thermodn. conditions, etc. The high dimensionality of this problem makes it very difficult and expensive to solve completely, and even a full soln. would not give much insight into the rational design of new catalysts. The descriptor-based approach seeks to det. a few "descriptors" upon which the other parameters are dependent. By doing this it is possible to reduce the dimensionality of the problem-preferably to 1 or 2 descriptors-thus greatly reducing computational efforts and simultaneously increasing the understanding of trends in catalysis. The "CatMAP" Python module seeks to standardize and automate many of the math. routines necessary to move from "descriptor space" to reaction rates for heterogeneous (electro) catalysts. The module is designed to be both flexible and powerful, and is available for free online. A "reaction model" can be fully defined by a configuration file, thus no new programming is necessary to change the complexity or assumptions of a model. Furthermore, various steps in the process of moving from descriptors to reaction rates have been abstracted into sep. Python classes, making it easy to change the methods used or add new functionality. This work discusses the structure of the code and presents the underlying algorithms and math. expressions both generally and via an example for the CO oxidn. reaction.
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  Humphrey, W.; Dalke, A.; Schulten, K. VMD - Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33– 38, DOI: 10.1016/0263-7855(96)00018-5
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  VMD is a mol. graphics program designed for the display and anal. of mol. assemblies, in particular, biopolymers such as proteins and nucleic acids. VMD can simultaneously display any no. of structures using a wide variety of rendering styles and coloring methods. Mols. are displayed as one or more "representations," in which each representation embodies a particular rendering method and coloring scheme for a selected subset of atoms. The atoms displayed in each representation are chosen using an extensive atom selection syntax, which includes Boolean operators and regular expressions. VMD provides a complete graphical user interface for program control, as well as a text interface using the Tcl embeddable parser to allow for complex scripts with variable substitution, control loops, and function calls. Full session logging is supported, which produces a VMD command script for later playback. High-resoln. raster images of displayed mols. may be produced by generating input scripts for use by a no. of photorealistic image-rendering applications. VMD has also been expressly designed with the ability to animate mol. dynamics (MD) simulation trajectories, imported either from files or from a direct connection to a running MD simulation. VMD is the visualization component of MDScope, a set of tools for interactive problem solving in structural biol., which also includes the parallel MD program NAMD, and the MDCOMM software used to connect the visualization and simulation programs, VMD is written in C++, using an object-oriented design; the program, including source code and extensive documentation, is freely available via anonymous ftp and through the World Wide Web.
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  Kubota, Y.; Maekawa, H.; Miyata, S.; Tatsumi, T.; Sugi, Y. Hydrothermal Synthesis of Metallosilicate SSZ-24 from Metallosilicate Beta as Precursors. Microporous Mesoporous Mater. 2007, 101, 115– 126, DOI: 10.1016/j.micromeso.2006.11.037
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  Hydrothermal synthesis of metallosilicate SSZ-24 from metallosilicate beta as precursors
  Kubota, Yoshihiro; Maekawa, Hiroyoshi; Miyata, Shunsuke; Tatsumi, Takashi; Sugi, Yoshihiro
  Microporous and Mesoporous Materials (2007), 101 (1-2), 115-126CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
  Aluminosilicate version of SSZ-24 ([Al]-SSZ-24) zeolite (AFI topol.) was synthesized by a new route including phase-transformation, wherein aluminosilicate analog of beta zeolite ([Al]-BEA) was used as a precursor and N(16)-methylsparteinium hydroxide (MeSPA+OH-) was used as structure-directing agent (SDA). [Al]-BEA precursors synthesized by various routes, mainly hydrothermal synthesis (HTS) and steam-assisted crystn. (SAC) methods, were employed for the synthesis. In general, [Al]-SSZ-24 samples with the SiO2/Al2O3 ratios comparable to the input SiO2/Al2O3 ratio were readily synthesized by using the precursor route when the SiO2/Al2O3 was as high as 200. The limit of aluminum introduction depended on the origin of BEA precursor. After optimization, pure [Al]-SSZ-24 with SiO2/Al2O3 = 109 was successfully obtained. During crystn., a phase transformation from [Al]-BEA to [Al]-SSZ-24 was obsd., suggesting dissoln. of beta phase to a min. structural unit followed by its redirection towards SSZ-24 phase in the presence of the SDA. Field emission SEM (FE-SEM) showed hexagonal rod-shaped crystals of about 1 μm in size. NH3-TPD measurement showed the presence of enough amts. of acidic sites as compared to the expected amt. based on the elemental anal. The catalytic performance of [Al]-SSZ-24 synthesized from [Al]-BEA was compared with that of the [Al]-SSZ-24 synthesized via [B]-SSZ-24 by isomorphous substitution of Al for B. The [Al]-SSZ-24 synthesized by the former method from [Al]-BEA showed comparable or better shape-selectivity than that synthesized by the latter method in the isopropylation of biphenyl. The applicability of this "BEA precursor method" to other metallosilicates (e.g. titanosilicate) was also demonstrated.
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Ethene Dimerization on Zeolite-Hosted Ni Ions: Reversible Mobilization of the Active Site

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