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Conversion of Polyolefin Waste to Liquid Alkanes with Ru-Based Catalysts under Mild Conditions

Cite this: JACS Au 2021, 1, 1, 8–12
Publication Date (Web):December 21, 2020
https://doi.org/10.1021/jacsau.0c00041

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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Chemical upcycling of waste polyolefins via hydrogenolysis offers unique opportunities for selective depolymerization compared to high temperature thermal deconstruction. Here, we demonstrate the hydrogenolysis of polyethylene into liquid alkanes under mild conditions using ruthenium nanoparticles supported on carbon (Ru/C). Reactivity studies on a model n-octadecane substrate showed that Ru/C catalysts are highly active and selective for the hydrogenolysis of C(sp3)–C(sp3) bonds at temperatures ranging from 200 to 250 °C. Under optimal conditions of 200 °C in 20 bar H2, polyethylene (average Mw ∼ 4000 Da) was converted into liquid n-alkanes with yields of up to 45% by mass after 16 h using a 5 wt % Ru/C catalyst with the remaining products comprising light alkane gases (C1–C6). At 250 °C, nearly stoichiometric yields of CH4 were obtained from polyethylene over the catalyst. The hydrogenolysis of long chain, low-density polyethylene (LDPE) and a postconsumer LDPE plastic bottle to produce C7–C45 alkanes was also achieved over Ru/C, demonstrating the feasibility of this reaction for the valorization of realistic postconsumer plastic waste. By identifying Ru-based catalysts as a class of active materials for the hydrogenolysis of polyethylene, this study elucidates promising avenues for the valorization of plastic waste under mild conditions.

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The development of polyolefins has enabled the production of safe and sturdy single-use plastic packaging for transportation and storage, sterile medical devices, and countless other transformative consumer products. Because the raw materials for producing polyolefins such as polyethylene (PE) and polypropylene (PP) are abundant and inexpensive, the manufacture of these plastics is immense and continues to grow. Worldwide, approximately 380 million tons of plastics are generated annually, 57% percent of which are polyolefins. (1) Projections estimate that by 2050, plastic production will reach over 1.1 billion tons per year. (2) From a chemistry perspective, the strong sp3 carbon–carbon bonds in polyolefins that provide desirable material properties also make them highly recalcitrant to degradation. Mechanical recycling is one method of utilizing plastic waste; however, only around 16% of plastic is actually recycled (3) and usually ends up being “downcycled” into lower-value materials with diminished properties. (4) The majority of single-use plastics end up in landfills or the environment, harming the ecosystem and affecting the natural environment. (1,3,5)

Thermochemical pathways such as pyrolysis and thermal cracking enable polyolefin depolymerization by breaking the strong C–C bonds in PE to produce small molecules that could be used as fuel or integrated into chemical refineries. However, these processes are energy intensive and suffer from low control over product selectivity. (6−8) Alternatively, C–C bond cleavage via hydrogenolysis allows for selective depolymerization of polyolefins into liquid alkanes with targeted molecular weight ranges. This reaction has been studied in the context of short-chain alkane (9−14) and lignin (15,16) conversion but has not been extensively explored for the depolymerization of polyolefins with high molecular weights. Dufaud and Basset studied the degradation of model PE (C20–C50) and PP over a zirconium hydride supported on silica–alumina and found moderate activity under mild conditions (190 °C). (17) Celik et al. investigated hydrogenolysis using well-dispersed Pt nanoparticles supported on SrTiO3 nanocuboids for the depolymerization of PE (Mn = 8000–158 000 Da) at 170 psi H2 and 300 °C for 96 h under solvent-free conditions, obtaining high yields of liquid hydrocarbons. (18) The authors argued that this catalyst is superior to Pt/Al2O3 because it produced fewer light hydrocarbons and promoted favorable adsorption of PE on Pt sites. Open questions remain whether the higher yield of light hydrocarbons over Pt/Al2O3 was due to the acidity of the support, a promotional effect of the isomerization and hydrocracking pathways, or increased activity for terminal C–C bond cleavage, and whether similar activity could be obtained at lower temperatures. Indeed, further investigation of other supported noble metals for hydrogenolysis at even milder conditions is needed.

Here, we demonstrate the selective depolymerization of polyethylene to processable liquid hydrocarbons under mild conditions in the absence of solvent using Ru nanoparticles supported on carbon. First, we screened a series of noble metal catalysts using n-octadecane—a model compound for linear polyethylene—and identified Ru nanoparticles supported on carbon as the most active. Next, we investigated the C–C bond cleavage selectivity as a function of conversion to understand how product distributions change as a function of extent of reaction. We then implemented optimized reaction conditions for the hydrogenolysis of a model PE substrate (average Mw 4000 Da) and commercial low-density polyethylene (LDPE). Finally, we demonstrated that the Ru/C catalyst is capable of converting LDPE from a real postconsumer plastic bottle. The identification of Ru-based catalysts as a class of materials for highly active hydrogenolysis is important for developing effective depolymerization processes of waste plastics to produce processable and transportable liquid that could be used as fuels, chemicals, or synthons for the next generation of infinitely recyclable polymers. (19)

Ruthenium-based catalysts such as Ru/CeO2, Ru/SiO2, Ru/Al2O3, and Ru/TiO2 have been shown to be active both for the hydrogenolysis of light alkanes (9−14) and lignin. (20,21) Our group has also shown that cobalt-based catalysts have tunable activity for C–O vs C–C bond hydrogenolysis of oxygenated arenes. (22) These previous observations prompted us to test a series of noble metal catalysts including Ru- and Co-based catalysts, as well as other transition metals, for the hydrogenolysis of n-octadecane in 25 mL Parr stainless steel pressurized reaction vessels under temperatures and H2 pressures ranging from 200 to 250 °C and 30–50 bar, respectively (see Table 1). Products were identified by gas chromatography mass spectrometry (GC-MS) and quantified using a GC equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). Additional methods, catalyst characterization, and product characterization are provided in the Supporting Information (Figures S1–S4, Tables S1–S5). At 250 °C, 1% Pt/γ-Al2O3, NiO, 5 wt % Ni on carbon, and γ-Al2O3 showed little to no activity. Co-based catalysts (entries 1 and 2) and Rh-based catalysts (entry 9) showed moderate hydrogenolysis activity, converting n-octadecane into a range of C1–C17 alkanes. Notably, Ru-based catalysts stood out for their high hydrogenolysis activity (entries 5–8, 10, and 11). Specifically, the 5 wt % Ru/C catalyst (entry 10) reached an n-octadecane conversion of 92% at 200 °C, generating a mixture of liquid and gaseous alkanes, while at 250 °C, 100% of the n-octadecane was converted into CH4. Due to the high activity at low temperatures and relatively low catalyst loadings, the 5 wt % Ru/C catalyst was selected for further investigation. Using a catalyst that is active at low temperatures decreases both the energy requirements for polyethylene processing and the thermodynamic driving force toward terminal C–C bond cleavage to produce CH4.

Table 1. Hydrogenolysis of n-Octadecane Using Heterogeneous Transition Metal Catalystsa
entrycatalysttemp (°C)mass catalyst (mg)conversion, C18 (mol %)products (liquid)products (gaseous)
1Co3O42001040nonenone
2Co3O425010946n-alkanes (C14–C17)n/a
3γ-Al2O32501030nonenone
41% Pt/γ-Al2O325047.00nonenone
5RuO225043.0100noneCH4
6RuO220020.49n-alkanes (C9–C17)CH4
75% Ru/Al2O320026.765n-alkanes (C8–C17)light alkanes (C1–C4)
85% Ru/Al2O325026.3100nonelight alkanes (C1–C4)
95% Rh/C25022.621n-alkanes (C8–C17)light alkanes (C1–C6)
105% Ru/C20025.092n-alkanes (C6–C17)light alkanes (C1–C5)
115% Ru/C25025.0100noneCH4
12NiO25025.00nonenone
135% Ni/C25025.0<4trace n-alkanes (C8–C17)trace light alkanes (C1–C3)
a

Reaction conditions: 700 mg of n-octadecane, 14 h, 30 bar H2 (entries 1, 10), 50 bar H2 (entries 2–9, 11–13).

Based on these preliminary results, the hydrogenolysis of n-octadecane over 5 wt % Ru/C at 200 °C was investigated as a function of time to track changes in the product distribution of n-alkanes with increasing conversion. As shown in Figure 1, the product distribution after 2 h includes C8–C17n-alkanes, and increasing the extent of reaction shifts the product distribution to lower molecular weights, implicating sequential cleavage events of both terminal and nonterminal C–C bonds. After 16 h, 100% of the n-octadecane was consumed, and the products shifted from liquid-range alkanes to gaseous products in the C1–C6 range. Based on these data, we surmised the hydrogenolysis of longer-chain polyethylene over Ru/C would also proceed via both terminal C–C bond cleavage to produce CH4 and internal C–C bond cleavage to produce shorter chain alkanes.

Figure 1

Figure 1. Hydrogenolysis of n-octadecane as a function of time over 5 wt % Ru/C. Reaction conditions: 700 mg n-octadecane (∼50 mmol carbon), 25 mg 5 wt % Ru/C, 30 bar H2, 200 °C.

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The effect of temperature and hydrogen pressure on the hydrogenolysis of model polyethylene (Sigma-Aldrich, average Mw 4000 Da, average Mn 1700 Da, Table S1) was investigated over Ru/C to identify suitable reaction conditions for the hydrogenolysis of realistic postconsumer polyethylene waste (Figure 2, Figures S5–S7). At all temperatures investigated between 200 and 250 °C and using a 4:1 PE:catalyst mass ratio, polyethylene conversion reached 100% and generated exclusively gaseous products. At 200 °C, the head space contained mainly CH4, ethane, propane, and butane. Upon increasing the temperature to 225 °C, only CH4 and ethane were produced, and at 250 °C, the product yield was pure CH4 in near stoichiometric yields. The effect of hydrogen pressure on polyethylene hydrogenolysis products over Ru/C at 200 °C (using a 28:1 PE:catalyst mass ratio) is shown in Figure 2b, with a more detailed compositional analysis shown in Figure 2c. At increasing hydrogen pressures, the yield of gas relative to liquid and solid increases, suggesting a sequence shifting from solid alkanes (>C45) to solvated liquid alkanes (C8–C45), and finally to light alkane gases (C1–C7).

Figure 2

Figure 2. Optimizing reaction conditions for the hydrogenolysis of polyethylene (average Mw 4000): (a) effect of temperature on overall product yields, 100 mg PE (average Mw 4000), 16 h, 30 bar H2, 25 mg 5 wt % Ru/C, 200–250 °C, 100% conversion. (b) Effect of H2 pressure on overall product composition, 200 °C, 16 h, 700 mg PE (average Mw 4000), 25 mg 5 wt % Ru/C. (c) Liquid composition of C8–C45 products shown in panel b.

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Based on these data, we selected reaction conditions of 20 bar H2, 200 °C as optimal for producing a distribution of n-alkanes in the processable liquid range when using 700 mg polyethylene and 25 mg of catalyst. Investigations of hydrogenolysis of Ir-based catalysts with both experiment and density functional theory (DFT) calculations have suggested that hydrogen inhibits hydrogenolysis rates, the extent of which is affected by the structure of the alkane due to changes in enthalpy and entropy of the transition state. (23) A study of the mechanism of hydrogenolysis of light alkanes over Ru/CeO2 also suggested that H2 pressure had a marked effect on selectivity for C–C bond scission, where high H2 pressure is necessary to avoid CH4 formation. (11) These opposing effects explain why an intermediate H2 pressure of 20 bar was ideal for achieving high selectivity to liquid-range n-alkanes in our experiments. As observed from both the model compound studies with n-octadecane and with polyethylene, the product distribution and selectivity for hydrogenolysis can be tuned by manipulating reaction temperature, H2 pressure, and residence time.

Implementation of hydrogenolysis technology for the catalytic upcycling of genuine postconsumer polyolefin waste will require flexibility in the molecular weight and composition of the feedstock, including extent of branching, moisture, and contaminants. To this end, we demonstrated the hydrogenolysis of two additional sources of polyethylene: low-density polyethylene (Aldrich, Table S2) with a melt index 25 g/10 min (190 °C/2.16 kg), denoted here as LDPE MI25, and a postconsumer LDPE plastic bottle. The former is a material commonly used in toys, lids, and closures, (24) and the latter was previously used as a solvent bottle containing water (VWR). The results from these reactions are summarized in Figure 3 and Figures S8–S14. The mass yields of liquid (C8–C45) and gaseous (C1–C7) n-alkanes for these substrates are shown in Figure 3a. For each of these substrates, both liquid and gaseous products are obtained, while the gray represents the unaccounted products in the mass balance. Only gaseous and liquid products were observed for the reactions over polyethylene (average Mw 4000 Da, denoted PE 4K), LDPE MI25, and the LDPE plastic bottle. The gaseous product distributions for the three substrates are shown in Figure 3b, and the C8–C45+ products are shown in Figure 3c–e for PE 4K, LDPE MI25, and the postconsumer LDPE plastic bottle, respectively. A small number of branched alkanes was also observed (Figure S11) over LDPE MI25, likely due to carbon chain branches in the substrate. While less substrate was used for the hydrogenolysis of the LDPE plastic bottle (200 mg) compared to the model polyethene (1400 mg), the formation of liquid products in spite of the higher complexity of this material and lack of any pretreatment is promising and indicates the feasibility of this method for the production of liquid products from postconsumer polyolefins. Furthermore, we observed that at longer reaction times (16 h) at 225 °C, the LDPE plastic bottle could be converted into CH4 with nearly 100% selectivity (Figure S14), which represents a promising avenue to produce natural gas from postconsumer plastic waste.

Figure 3

Figure 3. (a) Product distributions for polyethylene hydrogenolysis over 5 wt % Ru/C with PE 4K (200 °C, 1.4 g of PE 4K, 56 mg of 5 wt % Ru/C, 22 bar H2, 16 h), LDPE MI25 (225 °C, 1.4 g of LDPE, 50 mg of 5 wt % Ru/C, 22 bar H2, 16 h), and a postconsumer LDPE plastic bottle (225 °C, 200 mg of LDPE, 25 mg of 5 wt % Ru/C, 22 bar H2, 2 h); (b) gaseous product distribution for PE 4K, LDPE MI25, and an LDPE plastic bottle; (c) C8–C45+ product distribution for PE 4K; (d) C8–C45+ product distribution for LDPE MI25; (e) C8–C45+ product distribution for an LDPE plastic bottle.

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The depolymerization of a well-characterized NIST linear polyethylene Standard Reference Material (SRM 1475, avg Mw ∼ 52 000 Da, avg Mn ∼ 18 310 Da) (25) was also investigated over 5 wt % Ru/C and was found to undergo complete conversion to liquid and gaseous alkanes under similar reaction conditions (Figures S15–S17). In addition, polyethylene hydrogenolysis experiments over recycled catalyst also demonstrated that the catalyst can be reused with minimal change in activity (Figure S18, Table S6). Characterization of the fresh and spent catalyst (XRD, TEM) is shown in Figures S2–S4. The slight increase in average particle diameter from 1.62 ± 0.36 to 2.10 ± 0.43 nm for the spent catalyst indicates some aggregation of nanoparticles over the course of the reaction, although changes in dispersion are minimal, and catalytic activity is maintained. For a more rigorous investigation of catalyst stability over time, the hydrogenolysis of n-dodecane, a model compound for PE, was studied in a gas phase flow reactor over 5 wt % Ru/C at 225 °C and showed high activity over 60 h of time on stream with minimal catalyst deactivation and changes to product distribution (Figure S19). The calculated lower bound for the number of catalytic turnovers (TON > 15 000) suggests that the activity of 5 wt % Ru/C is well within the range of industrially relevant catalysts (Supporting Information). (26)

In this work, we demonstrated that Ru-based catalysts exhibit high activity and stability for the hydrogenolysis of solid polyolefin plastics under mild conditions to produce processable liquid alkanes. Temperature, hydrogen pressure, and reaction time can all be manipulated to control product distribution and selectivity, enabling the production of liquid products or complete hydrogenolysis to pure CH4. The identification of Ru-based materials as a class of heterogeneous catalysts for the efficient depolymerization of polyethylene into liquid alkanes, or pure CH4 compatible with existing natural-gas infrastructure, opens doors for future investigations into improving reactivity, understanding selectivity, and exploration of a variety of feedstocks and compositions. Further studies involving the effects of substrate branching, reactivity, and characterization of other hydrocarbon polymers such as polypropylene and polystyrene, mechanistic interrogation of active site requirements for selective internal C–C bond cleavage, models for predicting distributions of C–C bond cleavage, utilization of experimentally determined kinetic parameters to inform cleavage probability at various locations along the carbon chain, characterization of the Ru catalyst surface and mass transfer in the polymer melt, and technoeconomic analysis of the process are currently underway in our laboratory.

With heterogeneous catalysts, additional parameters for exploration include utilizing tailored acid or acid–base supports to promote tandem isomerization and hydrogenolysis, zeolites and microporous materials to impose confinement effects, Raney-type catalysts to improve catalytic activity, and the synthesis of bimetallic catalysts to improve selectivity, activity, and stability, and to control C–O, C–C, and C═C bond scission in mixed plastics feeds. Exhaustive studies into the effects of moisture and contaminants (27) as well as engineering product removal strategies to decrease the formation of light alkanes will be critical for the industrialization of this reaction, enabling the integration of polyolefin upcycling technology into the global economy and ultimately providing an economic incentive for the removal of waste plastics from the landfill and environment.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.0c00041.

  • Materials and methods (Figure S1, Table S1–S3), catalyst characterization (Figure S2–S3, Tables S4–S5), product characterization (GC-MS, GC-FID, GC-MS, supporting images, Figures S5–S17), recyclability studies (Figure S18, Table S6), catalyst stability studies in flow reactor, and TON calculation (Figure S19) (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Julie E. Rorrer - Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States;  Orcidhttp://orcid.org/0000-0003-4401-8520
    • Gregg T. Beckham - Renewable Resources and Enabling Sciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States;  Orcidhttp://orcid.org/0000-0002-3480-212X
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Funding was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office (AMO) and Bioenergy Technologies Office (BETO). This work was performed as part of the Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) Consortium and was supported by AMO and BETO under Contract DE-AC36-08GO28308 with the National Renewable Energy Laboratory (NREL), operated by Alliance for Sustainable Energy, LLC. The BOTTLE Consortium includes members from MIT, funded under Contract DE-AC36-08GO28308 with NREL. The authors thank Sujay Bagi for assistance with transmission electron microscopy and Kathryn Beers and Sara Orski at NIST for providing polyethylene standard reference material (SRM 1475). The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

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    The global plastic prodn. increased over years due to the vast applications of plastics in many sectors. The continuous demand of plastics caused the plastic wastes accumulation in the landfill consumed a lot of spaces that contributed to the environmental problem. The rising in plastics demand led to the depletion of petroleum as part of non-renewable fossil fuel since plastics were the petroleum-based material. Some alternatives that have been developed to manage plastic wastes were recycling and energy recovery method. However, there were some drawbacks of the recycling method as it required high labor cost for the sepn. process and caused water contamination that reduced the process sustainability. Due to these drawbacks, the researchers have diverted their attentions to the energy recovery method to compensate the high energy demand. Through extensive research and technol. development, the plastic waste conversion to energy was developed. As petroleum was the main source of plastic manufg., the recovery of plastic to liq. oil through pyrolysis process had a great potential since the oil produced had high calorific value comparable with the com. fuel. This paper reviewed the pyrolysis process for each type of plastics and the main process parameters that influenced the final end product such as oil, gaseous and char. The key parameters that were reviewed in this paper included temps., type of reactors, residence time, pressure, catalysts, type of fluidizing gas and its flow rate. In addn., several viewpoints to optimize the liq. oil prodn. for each plastic were also discussed in this paper.
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  8. 8
    Kunwar, B.; Cheng, H. N.; Chandrashekaran, S. R.; Sharma, B. K. Plastics to fuel: a review. Renewable Sustainable Energy Rev. 2016, 54, 421– 428,  DOI: 10.1016/j.rser.2015.10.015
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    Plastics to fuel: a review
    Kunwar, Bidhya; Cheng, H. N.; Chandrashekaran, Sriram R.; Sharma, Brajendra K.
    Renewable & Sustainable Energy Reviews (2016), 54 (), 421-428CODEN: RSERFH; ISSN:1364-0321. (Elsevier Ltd.)
    The thermal and catalytic processes of converting waste plastics into fuels are promising techniques to eliminate the refuse which otherwise is harmful to the environment, and decrease the dependence on fossil fuels. Thermal degrdn. decomps. plastic into three fractions: gas, crude oil, and solid residue. Crude oil from non-catalytic pyrolysis is usually composed of higher b.p. hydrocarbons. The optimization of conversion parameters such as the choice of catalyst, reactor design, pyrolysis temp., and plastic-to-catalyst ratio plays a very important role in the efficient generation of gasoline and diesel grade fuel. The use of a catalyst for thermal conversion lowers the energy required for conversion, and the catalyst choice is important for efficient fuel prodn. The suitable selection of catalysts can increase the yield of crude oil with lower hydrocarbon content. Co-pyrolysis of plastics with coal or shale oil improves crude oil quality by decreasing its viscosity. A large no. of publications have appeared on various processes, and continued improvements and/or innovations are expected in the future. Further investigations on the catalytic systems are required in order to advance the field, particularly to enhance the added value of fuels and to minimize the use of energy. This review aims to provide both the highlights of the remarkable achievements of this field and the milestones that need to be achieved in the future.
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  9. 9
    Bond, G. C.; Rajaram, R. R.; Burch, R. Hydrogenolysis of Propane, n-Butane, and Isobutane over Variously Pretreated Ru/TI02 Catalysts. J. Phys. Chem. 1986, 90, 4877– 4881,  DOI: 10.1021/j100411a032
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    9
    Hydrogenolysis of propane, n-butane, and isobutane over variously pretreated ruthenium/titanium dioxide catalysts
    Bond, Geoffrey C.; Rajaram, Raj R.; Burch, Robert
    Journal of Physical Chemistry (1986), 90 (20), 4877-81CODEN: JPCHAX; ISSN:0022-3654.
    The hydrogenolysis of propane, n-butane, and isobutane has been investigated using Ru/TiO2 catalysts contg. 0.5 and 5 wt % Ru pretreated in various ways. Redn. of RuCl3/TiO2 by H2 at 758 K gives catalysts of low to moderate activity; they are thought to be substantially contaminated by Cl-, and not in the low-activity strong metal-support interaction (SMSI) state, which is however attained on redn. at 893 K. Oxidn. of 623 K after the first redn. at either temp., followed by mild redn., affords catalysts of high activity, free of Cl-, and in a non-SMSI state. This treatment leads to highly dispersed Ru particles, which exhibit high methane selectivities; anal. of products distributions using the Kempling-Anderson formalism shows in the case of butane that oxidized catalysts have a marked preference for terminal C-C bond breaking. Subsequent redns. at high temp. lead to catalysts of low activity in the SMSI state, but residual activity after 758 K redn. is due to small particles which have not succumbed to SMSI and after 893 K redn. to large particles persisting from the first redn. Isobutane is much less reactive than the n-alkanes, perhaps because it requires a larger active center for its chemisorption.
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  10. 10
    Bond, G. C.; Yide, X. Effect of Reduction and Oxidation on the Activity of Ruthenium/Titania Catalysts for n- Butane Hydrogenolysis. J. Chem. Soc., Chem. Commun. 1983, 1248– 1249,  DOI: 10.1039/c39830001248
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    Nakagawa, Y.; Oya, S. I.; Kanno, D.; Nakaji, Y.; Tamura, M.; Tomishige, K. Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes. ChemSusChem 2017, 10 (1), 189– 198,  DOI: 10.1002/cssc.201601204
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    11
    Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes
    Nakagawa, Yoshinao; Oya, Shin-ichi; Kanno, Daisuke; Nakaji, Yosuke; Tamura, Masazumi; Tomishige, Keiichi
    ChemSusChem (2017), 10 (1), 189-198CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The dependence of the C-C hydrogenolysis activity on reaction parameters and the structure of the substrate alkanes was investigated for Ru/CeO2 catalyst with very small (dispersion: H/Ru = 0.89) Ru particles. The substrate concn. and reaction temp. did not have a significant effect on the selectivity pattern, except that methane prodn. was promoted at high temps. However, the hydrogen pressure had a marked effect on the selectivity pattern. Ctertiary-C bond dissocn., terminal Csecondary-Cprimary bond dissocn., and fragmentation to form excess methane had neg. reaction order with respect to hydrogen partial pressure, whereas Csecondary-Csecondary bond dissocn. had an approx. zero reaction order. Therefore, a high hydrogen pressure is essential for the regioselective hydrogenolysis of Csecondary-Csecondary bonds in squalane. Ru/SiO2 catalyst with larger Ru particles showed similar changes in the product distribution during the change in hydrogen pressure. The reaction mechanism for each type of C-C bond dissocn. is proposed based on reactivity trends and DFT calcns. The proposed intermediate species for the internal Csecondary-Csecondary dissocn., terminal Csecondary-Cprimary dissocn., and Ctertiary-C dissocn. is alkyls, alkylidynes, and alkenes, resp.
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  12. 12
    Flaherty, D. W.; Hibbitts, D. D.; Iglesia, E. Metal-Catalyzed C-C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability. J. Am. Chem. Soc. 2014, 136 (27), 9664– 76,  DOI: 10.1021/ja5037429
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    12
    Metal-Catalyzed C-C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability
    Flaherty, David W.; Hibbitts, David D.; Iglesia, Enrique
    Journal of the American Chemical Society (2014), 136 (27), 9664-9676CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Me substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from satd. surfaces, and form λ H2(g) mols. Activation enthalpies (ΔH⧺) and entropies (ΔS⧺) and λ values for 3C-xC cleavage are larger than for 2C-2C or 2C-1C bonds, irresp. of the compn. of metal clusters or the cyclic/acyclic structure of the reactants. 3C-xC bonds cleave through α,β,γ- or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estd. for such structures using statistical mechanics. But less substituted C-C bonds involve α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with 3C atoms, which have one C-H bond and thus can form only one C-M bond. 3C-xC cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger ΔH⧺ values but also larger ΔS⧺ values (by forming more H2(g)) than for 2C-2C and 2C-1C bonds, irresp. of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temps. and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.
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    Egawa, C.; Iwasawa, Y. Ethane hydrogenolysis on a Ru(1,1,10) surface. Surf. Sci. 1988, 198 (1), L329– L334,  DOI: 10.1016/0039-6028(88)90465-7
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    Kempling, J. C.; Anderson, R. B. Hydrogenolysis of n-Butane on Supported Ruthenium. Ind. Eng. Chem. Process Des. Dev. 1970, 9 (1), 116– 120,  DOI: 10.1021/i260033a021
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    Dong, L.; Lin, L.; Han, X.; Si, X.; Liu, X.; Guo, Y.; Lu, F.; Rudić, S.; Parker, S. F.; Yang, S.; Wang, Y. Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon-Carbon Linkages. Chem. 2019, 5 (6), 1521– 1536,  DOI: 10.1016/j.chempr.2019.03.007
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    15
    Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon-Carbon Linkages
    Dong, Lin; Lin, Longfei; Han, Xue; Si, Xiaoqin; Liu, Xiaohui; Guo, Yong; Lu, Fang; Rudic, Svemir; Parker, Stewart F.; Yang, Sihai; Wang, Yanqin
    Chem (2019), 5 (6), 1521-1536CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Conversion of lignin into monocyclic hydrocarbons as commodity chems. and drop-in fuels is a highly desirable target for biorefineries. However, this is severely hindered by the presence of stable interunit carbon-carbon linkages in native lignin and those formed during lignin extn. Herein, we report a new multifunctional catalyst, Ru/NbOPO4, that achieves the first example of catalytic cleavage of both interunit C-C and C-O bonds in one-pot lignin conversions to yield 124%-153% of monocyclic hydrocarbons, which is 1.2-1.5 times the yields obtained from the established nitrobenzene oxidn. method. This catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic arenes over repeated cycles. The mechanism of the activation and cleavage of 5-5 C-C bonds in biphenyl, as a lignin model adopting the most robust C-C linkages, has been revealed via in situ inelastic neutron scattering coupled with modeling. This study breaks the conventional theor. limit on lignin monomer prodn.
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  16. 16
    Xiao, L.-P.; Wang, S.; Li, H.; Li, Z.; Shi, Z.-J.; Xiao, L.; Sun, R.-C.; Fang, Y.; Song, G. Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide. ACS Catal. 2017, 7 (11), 7535– 7542,  DOI: 10.1021/acscatal.7b02563
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    16
    Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide
    Xiao, Ling-Ping; Wang, Shuizhong; Li, Helong; Li, Zhaowei; Shi, Zheng-Jun; Xiao, Liang; Sun, Run-Cang; Fang, Yunming; Song, Guoyong
    ACS Catalysis (2017), 7 (11), 7535-7542CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Lignin represents the most abundant source of renewable arom. resources, and the depolymn. of lignin has been identified as a prominent challenge to produce low-mol.-mass arom. chems. Herein, we report a nanostructured MoOx/CNT, which can serve as an efficient catalyst in hydrogenolysis of enzymic mild acidolysis lignins (EMALs) derived from various lignocellulosic biomass, thus giving monomeric phenols in high yields (up to 47 wt. %). This catalyst showed high selectivity toward phenolic compds. having an unsatd. substituent, because the cleavage of C-O bonds in β-O-4 units is prior to redn. of double bonds by MoOx/CNT under a H2 atmosphere, which was confirmed by examn. of lignin model compd. reactions. The effects of some key parameters such as the influence of solvent, temp., reaction time, and catalyst recyclability were also examd. in view of monomer yields and av. mol. wt. This method constitutes an economically responsible pathway for lignin valorization, which is comparable to the performance of precious-metal catalytic systems in terms of activity, reusability, and biomass feedstock compatibility.
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    Dufaud, V.; Basset, J.-M. Catalytic Hydrogenolysis at Low Temperature and Pressure of Polyethylene and Polypropylene to Diesels or Lower Alkanes by a Zirconium Hydride Supported on Silica-Alumina: A Step Toward Polyolefin Degradation by the Microscopic Reverse of Ziegler± Natta Polymerization. Angew. Chem., Int. Ed. 1998, 37 (6), 806– 810,  DOI: 10.1002/(SICI)1521-3773(19980403)37:6<806::AID-ANIE806>3.0.CO;2-6
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    17
    Catalytic hydrogenolysis at low temperature and pressure of polyethylene and polypropylene to diesels or lower alkanes by a zirconium hydride supported on silica-alumina: a step toward polyolefin degradation by the microscopic reverse of Ziegler-Natta polymerization
    Dufaud, Veronique; Basset, Jean-Marie
    Angewandte Chemie, International Edition (1998), 37 (6), 806-810CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
    Zirconium hydride supported on silica-alumina cleaves polyethylene at a low hydrogen pressure (1 atm) and moderate temp. (150°). The catalyst gives 100% conversion of high-mol.-wt. polyethylene into satd. oligomers over a period of 5 h and or into lower alkanes over a period of <10 h.
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    Celik, G.; Kennedy, R. M.; Hackler, R. A.; Ferrandon, M.; Tennakoon, A.; Patnaik, S.; LaPointe, A. M.; Ammal, S. C.; Heyden, A.; Perras, F. A.; Pruski, M.; Scott, S. L.; Poeppelmeier, K. R.; Sadow, A. D.; Delferro, M. Upcycling Single-Use Polyethylene into High-Quality Liquid Products. ACS Cent. Sci. 2019, 5 (11), 1795– 1803,  DOI: 10.1021/acscentsci.9b00722
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    18
    Upcycling Single-Use Polyethylene into High-Quality Liquid Products
    Celik, Gokhan; Kennedy, Robert M.; Hackler, Ryan A.; Ferrandon, Magali; Tennakoon, Akalanka; Patnaik, Smita; LaPointe, Anne M.; Ammal, Salai C.; Heyden, Andreas; Perras, Frederic A.; Pruski, Marek; Scott, Susannah L.; Poeppelmeier, Kenneth R.; Sadow, Aaron D.; Delferro, Massimiliano
    ACS Central Science (2019), 5 (11), 1795-1803CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degrdn. of plastics are causing increasing concern about their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chem. and energy value. In this study, energy-rich polyethylene (PE) macromols. are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by at. layer deposition. Pt/SrTiO3 completely converts PE (Mn = 8000-158,000 Da) or a single-use plastic bag (Mn = 31,000 Da) into high-quality liq. products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the no. averaged mol. wt. (Mn) and the narrow polydispersity (D) of the final liq. product. Solid-state NMR of 13C-enriched PE adsorption studies and d. functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concns. of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons. Single-use polyethylene is converted into value-added high-quality liq. products by a catalytic upcycling process using platinum nanoparticles supported on perovskites.
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    Hong, M.; Chen, E. Y. Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained gamma-Butyrolactone. Angew. Chem., Int. Ed. 2016, 55 (13), 4188– 93,  DOI: 10.1002/anie.201601092
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    19
    Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained γ-Butyrolactone
    Hong, Miao; Chen, Eugene Y.-X.
    Angewandte Chemie, International Edition (2016), 55 (13), 4188-4193CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The first effective organopolymn. of the biorenewable "non-polymerizable" γ-butyrolactone (γ-BL) to a high-mol.-wt. metal-free recyclable polyester is reported. The superbase tert-Bu-P4 is found to directly initiate this polymn. through deprotonation of γ-BL to generate reactive enolate species. When combined with a suitable alc., the tert-Bu-P4-based system rapidly converts γ-BL into polyesters with high monomer conversions (up to 90 %), high mol. wts. (Mn up to 26.7 kg mol-1), and complete recyclability (quant. γ-BL recovery).
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    Hu, Y.; Jiang, G.; Xu, G.; Mu, X. Hydrogenolysis of lignin model compounds into aromatics with bimetallic Ru-Ni supported onto nitrogen-doped activated carbon catalyst. Mol. Catal. 2018, 445, 316– 326,  DOI: 10.1016/j.mcat.2017.12.009
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    20
    Hydrogenolysis of lignin model compounds into aromatics with bimetallic Ru-Ni supported onto nitrogen-doped activated carbon catalyst
    Hu, Yinghui; Jiang, Guangce; Xu, Guoqiang; Mu, Xindong
    Molecular Catalysis (2018), 445 (), 316-326CODEN: MCOADH ISSN:. (Elsevier B.V.)
    Lignin is the most abundant and renewable resources for prodn. of natural aroms. In this paper, new bimetallic catalytic system of Ru and Ni supported onto nitrogen-doped activated carbon (Ru-Ni-AC/N) was developed and its performances on hydrogenolysis of lignin model compds. under mild reaction conditions (1.0 MPa, 230 °C, in aq.) were investigated. The results indicate that Ru-Ni-AC/N was a highly active, selective and stable catalyst for the conversion of lignin model compds. into aroms., e.g. phenol, benzene and their derivs. As verified by BET, XRD, HRTEM, XPS, H2-TPR and ICP-MS, the strong synergistic effects between (1) Ru and Ni and (2) metals and N-groups were contributed to its excellent aroms. selectivity. What's more, the introduction of electron rich N atoms on AC was beneficial to the stabilization of metal particles, which greatly enhanced the durability of the catalyst.
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  21. 21
    Li, T.; Lin, H.; Ouyang, X.; Qiu, X.; Wan, Z. In Situ Preparation of Ru@N-Doped Carbon Catalyst for the Hydrogenolysis of Lignin To Produce Aromatic Monomers. ACS Catal. 2019, 9 (7), 5828– 5836,  DOI: 10.1021/acscatal.9b01452
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    21
    In Situ Preparation of Ru@N-Doped Carbon Catalyst for the Hydrogenolysis of Lignin To Produce Aromatic Monomers
    Li, Tianjin; Lin, Hongfei; Ouyang, Xinping; Qiu, Xueqing; Wan, Zechen
    ACS Catalysis (2019), 9 (7), 5828-5836CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A high-performance catalyst is the key to selectively convert lignin into arom. monomers. In this study, Ru@N-doped carbon catalysts were prepd. in situ by a two-stage pyrolysis of a mixt. of D-glucosamine hydrochloride (GAH) and ruthenium trichloride with melamine acting as a soft template. The layered graphitic carbon nitride (g-C3N4) and the well-wrinkled, mesoporous graphitic carbon with doped nitrogen atoms were formed after the pyrolysis at 600 and 800 °C, resp. For the latter catalyst, the incorporation of either pyridinic or pyrrolic N atoms on the graphitic carbon support could not only homogeneously disperse and stabilize the Ru nanoparticles (NPs) but also create the defect-rich carbon structure. Compared with com. Pd/C and Ru/C catalysts, the Ru@N-doped carbon catalyst contributed to higher catalytic activity for lignin depolymn. The yield of the arom. monomers reached ∼30.5% over the optimal catalyst at 300 °C.
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  22. 22
    Shetty, M.; Zanchet, D.; Green, W. H.; Roman-Leshkov, Y. Cooperative Co0/CoII Communications Sites Stabilized by a Perovskite Matrix Enable Selective C-O and C-C bond Hydrogenolysis of Oxygenated Arenes. ChemSusChem 2019, 12, 2171– 2175,  DOI: 10.1002/cssc.201900664
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    22
    Cooperative Co0/CoII Sites Stabilized by a Perovskite Matrix Enable Selective C-O and C-C bond Hydrogenolysis of Oxygenated Arenes
    Shetty, Manish; Zanchet, Daniela; Green, William H.; Roman-Leshkov, Yuriy
    ChemSusChem (2019), 12 (10), 2171-2175CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Strontium-substituted lanthanum cobaltite (La0.8Sr0.2CoO3) matrix-stabilized Co0/CoII catalytic sites were prepd., which present tunable C-O and C-C hydrogenolysis activity for the vapor-phase upgrading of oxygenated arenes. CoII sites assocd. with oxygen vacancies were favored at low temps. and performed selective C-O hydrogenolysis, in which Sr-substitution facilitated oxygen vacancy formation, leading to approx. 10 times higher reactivity compared to undoped LaCoO3. Co0 sites were favored at high temps. and performed extensive C-C bond hydrogenolysis, generating a wide range of alkanes. The lower reaction order with PH2 (1.1±0.1) for C-C hydrogenolysis than for C-O hydrogenolysis (2.0±0.1) led to a high selectivity towards C-C hydrogenolysis at low PH2. The Co3O4 surfaces featured a narrower temp. window for obtaining the resp. optimal CoII and Co0 pairs compared to analogous perovskite surfaces; whereas, the perovskite matrix stabilizes these pairs for selective C-O and C-C hydrogenolysis. This stabilization effect offers an addnl. handle to control reactivity in oxide catalysts.
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  23. 23
    Hibbitts, D. D.; Flaherty, D. W.; Iglesia, E. Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes. J. Phys. Chem. C 2016, 120 (15), 8125– 8138,  DOI: 10.1021/acs.jpcc.6b00323
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    23
    Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes
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    Figure 1

    Figure 1. Hydrogenolysis of n-octadecane as a function of time over 5 wt % Ru/C. Reaction conditions: 700 mg n-octadecane (∼50 mmol carbon), 25 mg 5 wt % Ru/C, 30 bar H2, 200 °C.

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    Figure 2

    Figure 2. Optimizing reaction conditions for the hydrogenolysis of polyethylene (average Mw 4000): (a) effect of temperature on overall product yields, 100 mg PE (average Mw 4000), 16 h, 30 bar H2, 25 mg 5 wt % Ru/C, 200–250 °C, 100% conversion. (b) Effect of H2 pressure on overall product composition, 200 °C, 16 h, 700 mg PE (average Mw 4000), 25 mg 5 wt % Ru/C. (c) Liquid composition of C8–C45 products shown in panel b.

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    Figure 3

    Figure 3. (a) Product distributions for polyethylene hydrogenolysis over 5 wt % Ru/C with PE 4K (200 °C, 1.4 g of PE 4K, 56 mg of 5 wt % Ru/C, 22 bar H2, 16 h), LDPE MI25 (225 °C, 1.4 g of LDPE, 50 mg of 5 wt % Ru/C, 22 bar H2, 16 h), and a postconsumer LDPE plastic bottle (225 °C, 200 mg of LDPE, 25 mg of 5 wt % Ru/C, 22 bar H2, 2 h); (b) gaseous product distribution for PE 4K, LDPE MI25, and an LDPE plastic bottle; (c) C8–C45+ product distribution for PE 4K; (d) C8–C45+ product distribution for LDPE MI25; (e) C8–C45+ product distribution for an LDPE plastic bottle.

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      The hydrogenolysis of propane, n-butane, and isobutane has been investigated using Ru/TiO2 catalysts contg. 0.5 and 5 wt % Ru pretreated in various ways. Redn. of RuCl3/TiO2 by H2 at 758 K gives catalysts of low to moderate activity; they are thought to be substantially contaminated by Cl-, and not in the low-activity strong metal-support interaction (SMSI) state, which is however attained on redn. at 893 K. Oxidn. of 623 K after the first redn. at either temp., followed by mild redn., affords catalysts of high activity, free of Cl-, and in a non-SMSI state. This treatment leads to highly dispersed Ru particles, which exhibit high methane selectivities; anal. of products distributions using the Kempling-Anderson formalism shows in the case of butane that oxidized catalysts have a marked preference for terminal C-C bond breaking. Subsequent redns. at high temp. lead to catalysts of low activity in the SMSI state, but residual activity after 758 K redn. is due to small particles which have not succumbed to SMSI and after 893 K redn. to large particles persisting from the first redn. Isobutane is much less reactive than the n-alkanes, perhaps because it requires a larger active center for its chemisorption.
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    10. 10
      Bond, G. C.; Yide, X. Effect of Reduction and Oxidation on the Activity of Ruthenium/Titania Catalysts for n- Butane Hydrogenolysis. J. Chem. Soc., Chem. Commun. 1983, 1248– 1249,  DOI: 10.1039/c39830001248
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    11. 11
      Nakagawa, Y.; Oya, S. I.; Kanno, D.; Nakaji, Y.; Tamura, M.; Tomishige, K. Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes. ChemSusChem 2017, 10 (1), 189– 198,  DOI: 10.1002/cssc.201601204
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      11
      Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes
      Nakagawa, Yoshinao; Oya, Shin-ichi; Kanno, Daisuke; Nakaji, Yosuke; Tamura, Masazumi; Tomishige, Keiichi
      ChemSusChem (2017), 10 (1), 189-198CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The dependence of the C-C hydrogenolysis activity on reaction parameters and the structure of the substrate alkanes was investigated for Ru/CeO2 catalyst with very small (dispersion: H/Ru = 0.89) Ru particles. The substrate concn. and reaction temp. did not have a significant effect on the selectivity pattern, except that methane prodn. was promoted at high temps. However, the hydrogen pressure had a marked effect on the selectivity pattern. Ctertiary-C bond dissocn., terminal Csecondary-Cprimary bond dissocn., and fragmentation to form excess methane had neg. reaction order with respect to hydrogen partial pressure, whereas Csecondary-Csecondary bond dissocn. had an approx. zero reaction order. Therefore, a high hydrogen pressure is essential for the regioselective hydrogenolysis of Csecondary-Csecondary bonds in squalane. Ru/SiO2 catalyst with larger Ru particles showed similar changes in the product distribution during the change in hydrogen pressure. The reaction mechanism for each type of C-C bond dissocn. is proposed based on reactivity trends and DFT calcns. The proposed intermediate species for the internal Csecondary-Csecondary dissocn., terminal Csecondary-Cprimary dissocn., and Ctertiary-C dissocn. is alkyls, alkylidynes, and alkenes, resp.
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    12. 12
      Flaherty, D. W.; Hibbitts, D. D.; Iglesia, E. Metal-Catalyzed C-C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability. J. Am. Chem. Soc. 2014, 136 (27), 9664– 76,  DOI: 10.1021/ja5037429
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      12
      Metal-Catalyzed C-C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability
      Flaherty, David W.; Hibbitts, David D.; Iglesia, Enrique
      Journal of the American Chemical Society (2014), 136 (27), 9664-9676CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Me substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from satd. surfaces, and form λ H2(g) mols. Activation enthalpies (ΔH⧺) and entropies (ΔS⧺) and λ values for 3C-xC cleavage are larger than for 2C-2C or 2C-1C bonds, irresp. of the compn. of metal clusters or the cyclic/acyclic structure of the reactants. 3C-xC bonds cleave through α,β,γ- or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estd. for such structures using statistical mechanics. But less substituted C-C bonds involve α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with 3C atoms, which have one C-H bond and thus can form only one C-M bond. 3C-xC cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger ΔH⧺ values but also larger ΔS⧺ values (by forming more H2(g)) than for 2C-2C and 2C-1C bonds, irresp. of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temps. and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.
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    13. 13
      Egawa, C.; Iwasawa, Y. Ethane hydrogenolysis on a Ru(1,1,10) surface. Surf. Sci. 1988, 198 (1), L329– L334,  DOI: 10.1016/0039-6028(88)90465-7
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      Kempling, J. C.; Anderson, R. B. Hydrogenolysis of n-Butane on Supported Ruthenium. Ind. Eng. Chem. Process Des. Dev. 1970, 9 (1), 116– 120,  DOI: 10.1021/i260033a021
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      Dong, L.; Lin, L.; Han, X.; Si, X.; Liu, X.; Guo, Y.; Lu, F.; Rudić, S.; Parker, S. F.; Yang, S.; Wang, Y. Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon-Carbon Linkages. Chem. 2019, 5 (6), 1521– 1536,  DOI: 10.1016/j.chempr.2019.03.007
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      15
      Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon-Carbon Linkages
      Dong, Lin; Lin, Longfei; Han, Xue; Si, Xiaoqin; Liu, Xiaohui; Guo, Yong; Lu, Fang; Rudic, Svemir; Parker, Stewart F.; Yang, Sihai; Wang, Yanqin
      Chem (2019), 5 (6), 1521-1536CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Conversion of lignin into monocyclic hydrocarbons as commodity chems. and drop-in fuels is a highly desirable target for biorefineries. However, this is severely hindered by the presence of stable interunit carbon-carbon linkages in native lignin and those formed during lignin extn. Herein, we report a new multifunctional catalyst, Ru/NbOPO4, that achieves the first example of catalytic cleavage of both interunit C-C and C-O bonds in one-pot lignin conversions to yield 124%-153% of monocyclic hydrocarbons, which is 1.2-1.5 times the yields obtained from the established nitrobenzene oxidn. method. This catalyst also exhibits high stability and selectivity (up to 68%) to monocyclic arenes over repeated cycles. The mechanism of the activation and cleavage of 5-5 C-C bonds in biphenyl, as a lignin model adopting the most robust C-C linkages, has been revealed via in situ inelastic neutron scattering coupled with modeling. This study breaks the conventional theor. limit on lignin monomer prodn.
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    16. 16
      Xiao, L.-P.; Wang, S.; Li, H.; Li, Z.; Shi, Z.-J.; Xiao, L.; Sun, R.-C.; Fang, Y.; Song, G. Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide. ACS Catal. 2017, 7 (11), 7535– 7542,  DOI: 10.1021/acscatal.7b02563
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      16
      Catalytic Hydrogenolysis of Lignins into Phenolic Compounds over Carbon Nanotube Supported Molybdenum Oxide
      Xiao, Ling-Ping; Wang, Shuizhong; Li, Helong; Li, Zhaowei; Shi, Zheng-Jun; Xiao, Liang; Sun, Run-Cang; Fang, Yunming; Song, Guoyong
      ACS Catalysis (2017), 7 (11), 7535-7542CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Lignin represents the most abundant source of renewable arom. resources, and the depolymn. of lignin has been identified as a prominent challenge to produce low-mol.-mass arom. chems. Herein, we report a nanostructured MoOx/CNT, which can serve as an efficient catalyst in hydrogenolysis of enzymic mild acidolysis lignins (EMALs) derived from various lignocellulosic biomass, thus giving monomeric phenols in high yields (up to 47 wt. %). This catalyst showed high selectivity toward phenolic compds. having an unsatd. substituent, because the cleavage of C-O bonds in β-O-4 units is prior to redn. of double bonds by MoOx/CNT under a H2 atmosphere, which was confirmed by examn. of lignin model compd. reactions. The effects of some key parameters such as the influence of solvent, temp., reaction time, and catalyst recyclability were also examd. in view of monomer yields and av. mol. wt. This method constitutes an economically responsible pathway for lignin valorization, which is comparable to the performance of precious-metal catalytic systems in terms of activity, reusability, and biomass feedstock compatibility.
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    17. 17
      Dufaud, V.; Basset, J.-M. Catalytic Hydrogenolysis at Low Temperature and Pressure of Polyethylene and Polypropylene to Diesels or Lower Alkanes by a Zirconium Hydride Supported on Silica-Alumina: A Step Toward Polyolefin Degradation by the Microscopic Reverse of Ziegler± Natta Polymerization. Angew. Chem., Int. Ed. 1998, 37 (6), 806– 810,  DOI: 10.1002/(SICI)1521-3773(19980403)37:6<806::AID-ANIE806>3.0.CO;2-6
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      17
      Catalytic hydrogenolysis at low temperature and pressure of polyethylene and polypropylene to diesels or lower alkanes by a zirconium hydride supported on silica-alumina: a step toward polyolefin degradation by the microscopic reverse of Ziegler-Natta polymerization
      Dufaud, Veronique; Basset, Jean-Marie
      Angewandte Chemie, International Edition (1998), 37 (6), 806-810CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
      Zirconium hydride supported on silica-alumina cleaves polyethylene at a low hydrogen pressure (1 atm) and moderate temp. (150°). The catalyst gives 100% conversion of high-mol.-wt. polyethylene into satd. oligomers over a period of 5 h and or into lower alkanes over a period of <10 h.
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    18. 18
      Celik, G.; Kennedy, R. M.; Hackler, R. A.; Ferrandon, M.; Tennakoon, A.; Patnaik, S.; LaPointe, A. M.; Ammal, S. C.; Heyden, A.; Perras, F. A.; Pruski, M.; Scott, S. L.; Poeppelmeier, K. R.; Sadow, A. D.; Delferro, M. Upcycling Single-Use Polyethylene into High-Quality Liquid Products. ACS Cent. Sci. 2019, 5 (11), 1795– 1803,  DOI: 10.1021/acscentsci.9b00722
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      18
      Upcycling Single-Use Polyethylene into High-Quality Liquid Products
      Celik, Gokhan; Kennedy, Robert M.; Hackler, Ryan A.; Ferrandon, Magali; Tennakoon, Akalanka; Patnaik, Smita; LaPointe, Anne M.; Ammal, Salai C.; Heyden, Andreas; Perras, Frederic A.; Pruski, Marek; Scott, Susannah L.; Poeppelmeier, Kenneth R.; Sadow, Aaron D.; Delferro, Massimiliano
      ACS Central Science (2019), 5 (11), 1795-1803CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degrdn. of plastics are causing increasing concern about their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chem. and energy value. In this study, energy-rich polyethylene (PE) macromols. are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by at. layer deposition. Pt/SrTiO3 completely converts PE (Mn = 8000-158,000 Da) or a single-use plastic bag (Mn = 31,000 Da) into high-quality liq. products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the no. averaged mol. wt. (Mn) and the narrow polydispersity (D) of the final liq. product. Solid-state NMR of 13C-enriched PE adsorption studies and d. functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concns. of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons. Single-use polyethylene is converted into value-added high-quality liq. products by a catalytic upcycling process using platinum nanoparticles supported on perovskites.
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    19. 19
      Hong, M.; Chen, E. Y. Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained gamma-Butyrolactone. Angew. Chem., Int. Ed. 2016, 55 (13), 4188– 93,  DOI: 10.1002/anie.201601092
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      19
      Towards Truly Sustainable Polymers: A Metal-Free Recyclable Polyester from Biorenewable Non-Strained γ-Butyrolactone
      Hong, Miao; Chen, Eugene Y.-X.
      Angewandte Chemie, International Edition (2016), 55 (13), 4188-4193CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The first effective organopolymn. of the biorenewable "non-polymerizable" γ-butyrolactone (γ-BL) to a high-mol.-wt. metal-free recyclable polyester is reported. The superbase tert-Bu-P4 is found to directly initiate this polymn. through deprotonation of γ-BL to generate reactive enolate species. When combined with a suitable alc., the tert-Bu-P4-based system rapidly converts γ-BL into polyesters with high monomer conversions (up to 90 %), high mol. wts. (Mn up to 26.7 kg mol-1), and complete recyclability (quant. γ-BL recovery).
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    20. 20
      Hu, Y.; Jiang, G.; Xu, G.; Mu, X. Hydrogenolysis of lignin model compounds into aromatics with bimetallic Ru-Ni supported onto nitrogen-doped activated carbon catalyst. Mol. Catal. 2018, 445, 316– 326,  DOI: 10.1016/j.mcat.2017.12.009
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      20
      Hydrogenolysis of lignin model compounds into aromatics with bimetallic Ru-Ni supported onto nitrogen-doped activated carbon catalyst
      Hu, Yinghui; Jiang, Guangce; Xu, Guoqiang; Mu, Xindong
      Molecular Catalysis (2018), 445 (), 316-326CODEN: MCOADH ISSN:. (Elsevier B.V.)
      Lignin is the most abundant and renewable resources for prodn. of natural aroms. In this paper, new bimetallic catalytic system of Ru and Ni supported onto nitrogen-doped activated carbon (Ru-Ni-AC/N) was developed and its performances on hydrogenolysis of lignin model compds. under mild reaction conditions (1.0 MPa, 230 °C, in aq.) were investigated. The results indicate that Ru-Ni-AC/N was a highly active, selective and stable catalyst for the conversion of lignin model compds. into aroms., e.g. phenol, benzene and their derivs. As verified by BET, XRD, HRTEM, XPS, H2-TPR and ICP-MS, the strong synergistic effects between (1) Ru and Ni and (2) metals and N-groups were contributed to its excellent aroms. selectivity. What's more, the introduction of electron rich N atoms on AC was beneficial to the stabilization of metal particles, which greatly enhanced the durability of the catalyst.
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    21. 21
      Li, T.; Lin, H.; Ouyang, X.; Qiu, X.; Wan, Z. In Situ Preparation of Ru@N-Doped Carbon Catalyst for the Hydrogenolysis of Lignin To Produce Aromatic Monomers. ACS Catal. 2019, 9 (7), 5828– 5836,  DOI: 10.1021/acscatal.9b01452
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      21
      In Situ Preparation of Ru@N-Doped Carbon Catalyst for the Hydrogenolysis of Lignin To Produce Aromatic Monomers
      Li, Tianjin; Lin, Hongfei; Ouyang, Xinping; Qiu, Xueqing; Wan, Zechen
      ACS Catalysis (2019), 9 (7), 5828-5836CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A high-performance catalyst is the key to selectively convert lignin into arom. monomers. In this study, Ru@N-doped carbon catalysts were prepd. in situ by a two-stage pyrolysis of a mixt. of D-glucosamine hydrochloride (GAH) and ruthenium trichloride with melamine acting as a soft template. The layered graphitic carbon nitride (g-C3N4) and the well-wrinkled, mesoporous graphitic carbon with doped nitrogen atoms were formed after the pyrolysis at 600 and 800 °C, resp. For the latter catalyst, the incorporation of either pyridinic or pyrrolic N atoms on the graphitic carbon support could not only homogeneously disperse and stabilize the Ru nanoparticles (NPs) but also create the defect-rich carbon structure. Compared with com. Pd/C and Ru/C catalysts, the Ru@N-doped carbon catalyst contributed to higher catalytic activity for lignin depolymn. The yield of the arom. monomers reached ∼30.5% over the optimal catalyst at 300 °C.
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    22. 22
      Shetty, M.; Zanchet, D.; Green, W. H.; Roman-Leshkov, Y. Cooperative Co0/CoII Communications Sites Stabilized by a Perovskite Matrix Enable Selective C-O and C-C bond Hydrogenolysis of Oxygenated Arenes. ChemSusChem 2019, 12, 2171– 2175,  DOI: 10.1002/cssc.201900664
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      22
      Cooperative Co0/CoII Sites Stabilized by a Perovskite Matrix Enable Selective C-O and C-C bond Hydrogenolysis of Oxygenated Arenes
      Shetty, Manish; Zanchet, Daniela; Green, William H.; Roman-Leshkov, Yuriy
      ChemSusChem (2019), 12 (10), 2171-2175CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Strontium-substituted lanthanum cobaltite (La0.8Sr0.2CoO3) matrix-stabilized Co0/CoII catalytic sites were prepd., which present tunable C-O and C-C hydrogenolysis activity for the vapor-phase upgrading of oxygenated arenes. CoII sites assocd. with oxygen vacancies were favored at low temps. and performed selective C-O hydrogenolysis, in which Sr-substitution facilitated oxygen vacancy formation, leading to approx. 10 times higher reactivity compared to undoped LaCoO3. Co0 sites were favored at high temps. and performed extensive C-C bond hydrogenolysis, generating a wide range of alkanes. The lower reaction order with PH2 (1.1±0.1) for C-C hydrogenolysis than for C-O hydrogenolysis (2.0±0.1) led to a high selectivity towards C-C hydrogenolysis at low PH2. The Co3O4 surfaces featured a narrower temp. window for obtaining the resp. optimal CoII and Co0 pairs compared to analogous perovskite surfaces; whereas, the perovskite matrix stabilizes these pairs for selective C-O and C-C hydrogenolysis. This stabilization effect offers an addnl. handle to control reactivity in oxide catalysts.
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      Hibbitts, D. D.; Flaherty, D. W.; Iglesia, E. Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes. J. Phys. Chem. C 2016, 120 (15), 8125– 8138,  DOI: 10.1021/acs.jpcc.6b00323
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      23
      Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes
      Hibbitts, David D.; Flaherty, David W.; Iglesia, Enrique
      Journal of Physical Chemistry C (2016), 120 (15), 8125-8138CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      C-C cleavage in C2-C10n-alkanes involves quasi-equilibrated C-H activation steps to form dehydrogenated intermediates on surfaces satd. with H atoms. These reactions are inhibited by H2 to similar extents for C-C bonds of similar substitution in all acyclic and cyclic alkanes and, thus, show similar kinetic dependences on H2 pressure. Yet, turnover rates depend sensitively on chain length because of differences in activation enthalpies (ΔH⧺) and entropies (ΔS⧺) whose mechanistic origins remain unclear. D. functional theory (DFT) ests. of ΔH⧺ and ΔG⧺ for C-C cleavage via >150 plausible elementary steps for propane and n-butane reactants on Ir show that hydrogenolysis occurs via α,β-bound RC*-C*R'⧺ transition states (R = H, CxH2x+1) in which two H atoms are removed from each C*. Calcd. ΔH⧺ values decrease with increasing alkane chain length (C2-C8), consistent with expt., because attractive van der Waals interactions with surfaces preferentially stabilize larger transition states. A concomitant increase in ΔS⧺, evident from expts., is not captured by periodic DFT methods, which treat low-frequency vibrational modes inaccurately, but statistical mechanics treatments describe such effects well for RC*-C*R⧺ species, as previously reported. These findings, together with parallel studies of the cleavage of more substituted C-C bonds in branched and cyclic alkanes, account for the reasons that chain length and substitution influence ΔH⧺ and ΔS⧺ values and the dependence of rates on H2 pressure and consequently explain differences in hydrogenolysis reactivities and selectivities across all alkanes.
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      DOW LDPE 993I Low Density Polyethylene Resin; The Dow Chemical Company, 2011.
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      There is no corresponding record for this reference.
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      Hoeve, C. A. J.; Wagner, H. L.; Brown, J. E.; Christensen, R. G.; Frolen, L. J.; Maurey, J. R.; Ross, G. S.; Verdier, P. H. The Characterization of Linear Polyethylene SRM 1475; National Bureau of Standards: Washington D.C., 1971.
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      Kozuch, S.; Martin, J. M. L. Turning Over” Definitions in Catalytic Cycles. ACS Catal. 2012, 2 (12), 2787– 2794,  DOI: 10.1021/cs3005264
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      "Turning Over" Definitions in Catalytic Cycles
      Kozuch, Sebastian; Martin, Jan M. L.
      ACS Catalysis (2012), 2 (12), 2787-2794CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The turnover frequency (TOF) and no. (TON) are the most important quantities to assess the efficiency of a catalyst, the first being a measure of its speed, the second of its robustness. However, common discrepancies in their implementation and typical errors in their estn. hinder the possibility to use them as universal values for comparative purposes. Is it possible to define a TOF and TON that can provide a std. measure of catalytic efficiency.
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      Ragaert, K.; Delva, L.; Van Geem, K. Mechanical and chemical recycling of solid plastic waste. Waste Manage. (Oxford, U. K.) 2017, 69, 24– 58,  DOI: 10.1016/j.wasman.2017.07.044
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      Mechanical and chemical recycling of solid plastic waste
      Ragaert, Kim; Delva, Laurens; Van Geem, Kevin
      Waste Management (Oxford, United Kingdom) (2017), 69 (), 24-58CODEN: WAMAE2; ISSN:0956-053X. (Elsevier Ltd.)
      A review. This review presents a comprehensive description of the current pathways for recycling of polymers, via both mech. and chem. recycling. The principles of these recycling pathways are framed against current-day industrial reality, by discussing predominant industrial technologies, design strategies and recycling examples of specific waste streams. Starting with an overview on types of solid plastic waste (SPW) and their origins, the manuscript continues with a discussion on the different valorization options for SPW. The section on mech. recycling contains an overview of current sorting technologies, specific challenges for mech. recycling such as thermo-mech. or lifetime degrdn. and the immiscibility of polymer blends. It also includes some industrial examples such as polyethylene terephthalate (PET) recycling, and SPW from post-consumer packaging, end-of-life vehicles or electr(on)ic devices. A sep. section is dedicated to the relationship between design and recycling, emphasizing the role of concepts such as Design from Recycling. The section on chem. recycling collects a state-of-the-art on techniques such as chemolysis, pyrolysis, fluid catalytic cracking, hydrogen techniques and gasification. Addnl., this review discusses the main challenges (and some potential remedies) to these recycling strategies and ground them in the relevant polymer science, thus providing an academic angle as well as an applied one.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtlOhtLrO&md5=565a0d7f947308f749b9167393b28062

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.0c00041.

    • Materials and methods (Figure S1, Table S1–S3), catalyst characterization (Figure S2–S3, Tables S4–S5), product characterization (GC-MS, GC-FID, GC-MS, supporting images, Figures S5–S17), recyclability studies (Figure S18, Table S6), catalyst stability studies in flow reactor, and TON calculation (Figure S19) (PDF)


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