Dehydrogenation and Transfer Hydrogenation of Alkenones to Phenols and Ketones on Carbon-Supported Noble Metals

The catalytic dehydrogenation of substituted alkenones on noble metal catalysts supported on carbon (Pt/C, Pd/C, Rh/C, and Ru/C) was investigated in an organic phase under inert conditions. The dehydrogenation and semihydrogenation of the enone starting materials resulted in aromatic compounds (primary products), saturated cyclic ketones (secondary products), and cyclic alcohols (minor products). Pd/C exhibits the highest catalytic activity, followed by Pt/C and Rh/C. Aromatic compounds remain the primary products, even in the presence of hydrogen donors. Joint experimental and theoretical analyses showed that the four catalytic materials stabilize a common dienol intermediate on the metal surfaces, formed by keto–enol tautomerization. This intermediate subsequently forms aromatic products upon dehydrogenation. The binding orientation of the enone reactants on the catalytic surface is strongly metal-dependent, as the M–O bond distance changes substantially according to the metal. The longer M–O bonds (Pt: 2.84 Å > Pd: 2.23 Å > Rh: 2.17 Å > Ru: 2.07 Å) correlate with faster reaction rates and more favorable keto–enol tautomerization, as shorter distances correspond to a more stabilized starting material. Tautomerization is shown to occur via a stepwise surface-assisted pathway. Overall, each of the studied metals exhibits a distinct balance of enthalpy and entropy of activation (ΔH°‡, ΔS°‡), offering unique possibilities in the realm of enone dehydrogenation reactions that can be achieved by suitable selection of catalytic materials.


GC-MS
The GC-MS sample was prepared by adding 1mL of p-xylene as the solvent, 10 μL mesitylene as an internal standard and 50 μL of the reaction mixture.The equipment used was an Agilent 7890B gas chromatograph equipped with a flame ionization detector (FID) and an Agilent 5977A mass spectrometer.1 μL of the liquid sample was injected into a HP-5 MS column (30 m  0.32 mm  0.25 μm) at an inlet temperature of 280 °C using a split ratio of 50 (He).The following heating programs were used; step/heating rate (°C min -1 )/temperature (°C)/ hold time (min): 1/-/50/0 -2/4/70/2 -3/1/80/1 -4/15/150/0.Identification of the components was performed by using the retention times of commercially available pure substances.Quantification of reactants was analysed via the FID-signal.For MS analysis the database NIST Spectrum Library 2.0 was used.For the product distribution (isomer) analysis of substrate 1, the following procedure was used: column/ CycloSil-B (30 m  250 μm  0.25 μm).The FID was operated at 250°C.Carrier gas/ N2, 0.8 mL min -1 ; injection: 1μl at 250°C, split 30:1, mesitylene (10 μL) as internal standard.Temperature program: starting temperature was 60°C which was held for 3 min, at a rate of 50°C min -1 the temperature was increased to 100°C and was held for 16 min, at a rate of 30 °C min -1 the temperature increased further to 240°C and was held for 3 min.
Atomic Absorption Spectroscopy (AAS) Elemental analysis of the samples were performed by atomic absorption spectroscopy on an ICE 3500 AAS (Thermo Fisher Scientific) equipped with a GF 95 graphite furnace to determine the Pd, Pt, Rh content of the catalysts.The samples were dissolved in a solution of perchloric acid (72%) and nitro-hydrochloric acid at its boiling point before the measurement.The Ru content analysis was performed by atomic absorption spectroscopy on an Agilent AAS FS 280 200 Series (Flame AAS) (with CEM SP-Discover microwave oven).The samples were dissolved in a solution of ccHCl (3 mL for 100 mL total solution), ccHNO3 (1 mL) (3% ccHCl, 1% ccHNO3 for the digestion in the microwave) and 5mL LaCl3 (5% LaCl3) and were heated up for 180°C (5% of LaCl3 solution (10% La) as buffer solution for the AAS).

BET surface analysis
The specific surface area of the support was determined from nitrogen adsorptiondesorption isotherms recorded on an automated PMI Sorptomatic 1990 instrument at liquid nitrogen temperature (77 K).The samples were outgassed in vacuum (p = 110 -3 mbar) for 2 h at 475 K prior to adsorption.The specific surface areas were calculated by applying the B.E.T. theory; the t-plot method was used to determine the micropore volumes and mesopore surface areas, while mesopore volumes were determined using the B.J.H. theory.

H2 Chemisorption
The active carbon supported metals (Pd, Pt, Rh, Ru) were pre-treated at 573 K under 0.1 MPa H2 for 1 h, followed by evacuation in vacuum for 1 h.After the temperature was cooled to 298 K, the H2 chemisorption and physisorption were subsequently determined in a pressure of H2 from 5 to 350 Torr.Then, the physisorbed H2 was removed by outgassing the sample at 298 K for 1 h.The concentration of chemisorbed hydrogen on the metal was obtained by extrapolating the isotherm to zero Torr of H2 pressure.The metal (Pd, Pt, Rh, Ru) dispersion and TOF were deduced by assuming an average H/metal ratio of 1.
Transmission electron microscopy (TEM) TEM measurements were performed on a JEOL JEM-2011 instrument at 120 kV.The average particle size and its standard deviation were calculated based on the Pd, Pt, Rh, Ru particle size distribution of 300 metal particles measured in at least five different particle domains of the catalyst.The average particle sizes were 2.8 ± 0.7 nm for Rh/C and 3.8 ± 0.9 nm for Ru/C.For Pd/C (7.1 ± 1.3 nm) 2 and Pt/C (3.8 ± 0.6 nm), 2 the characterization data was described within our previous study.

Mode of calculations
Conversion = (mole of converted reactant / mole of the starting reactant) ( 100 (%)).Yield = the ratio of the amount of reaction product and the amount of a starting material ( 100 (%)).Selectivity = the ratio of the amount of reaction product and the amount of a converted feedstock material ( 100 (%)).Rates/ formation rates were deduced from the slope of the linear fit to the conversion/corresponding yield versus reaction time plot in the linear region.TOF = mole of converted reactant / (mole of accessible metal sites  reaction time) (mol mol(surf.metal) -1 s -1 which is shortened as s -1 ).Accessible metal sites (mol g(cat) -1 ) were calculated by the normalization of the catalyst amount to metal dispersion and metal loading (as an example: for Pd/C (10 wt%): 106.42 mg (1 mmol Pd/C), the accessible metal sites for 0.1 g Pd/C is 0.015 mmol Pd).The carbon balance = (mole of carbon in the product / mole of carbon of starting reactant) ( 100 (%)).
Experiments with different agitating speeds were carried out to determine the impact of stirring speed on the reaction to exclude mass transfer limitation.As shown in Figure S2 below, mass transfer limitation is not taking place in the stirring speed were the reactions carried out.In all reactions we used the stirring speed of 250 rpm.

S8 Kinetic measurements, activation energy (Ea) determination
The overall rates for conversion of substrate 1 are given in Table S1-Table S4.The formation rates for mcresol from substrate 1 on Pt, Pd, Rh and Ru were calculated from the yield (%) of m-cresol at given reaction times (Table S5-Table S8).Table S9 shows the formation of m-cresol from substrate 2 on Pt/C.The corresponding rate (formation rate) determination for 3-methylcyclohexan-1-one from substrate 1 on Pt, Pd, Rh and Ru were calculated from the yield (%) of 3-methylcyclohexan-1-one at given reaction times (Table S10-Table S13).All of the points used for the rate calculations were measured from separate experiments.No in situ sampling was applied.Table S17.Reaction order of 3-methyl-2-cyclohexene-1-one (1) (using overall conversion) (substrate 1 (0.5-2.0 mmol), Rh/C (5 wt%, 0.1 mmol Rh), p-xylene (1.5 mL), 140 °C, under Ar and atmospheric pressure).Substrate 1 (mmol) c (mol L -1 ) ln(c) r (mol g(cat)

Computational details
Periodic DFT calculations DFT calculations using periodic boundary conditions were performed in the Vienna Ab Initio Simulation Program (VASP), version 5.4. 4 The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional 5 was used along with the corresponding projector augmented wave (PAW) potentials. 6Dispersion was included in the calculations using the D3 correction with Becke-Johnson damping (D3-BJ). 7First order Methfessel-Paxton smearing 8 was used to describe the partial orbital occupancies with a width (σ) of 0.1 eV.A plane-wave cutoff energy of 500 eV was used in all calculations.The SCF energy convergence criterion was 10 -7 eV, while the optimizations were considered to be converged when the total energy change was smaller than 10 -6 eV.The results of the periodic DFT calculations were visualized using VESTA. 9ulk optimizations of the four metals were performed using a 14 × 14 × 14 gamma-centered k-point grid.All atomic positions and lattice vectors were optimized and used to generate the metal slabs for the surface calculations.The metal slabs were built using four unit cells in each direction, and the bottom two layers were frozen in all geometry optimizations.The (111) facet was used to model the surface of the three face-centered cubic (fcc) metals (Pt, Pd, Ph), while the analogous (0001) facet was used for hexagonal close packed (hcp) Ru.
All supercell calculations used a 3 × 3 × 1 gamma-centered k-point grid, and the projection operators were evaluated in real space.A 30 Å vacuum spacer was used for all surface calculations, while free molecules were optimized inside a 20 Å × 20 Å × 20 Å cube.All lattice vectors were kept constant in surface and free molecule calculations.The lattice vectors of the supercells used for all surface calculations are given in Table S18.
Table S18.Components of the lattice vectors (a, b, c) of the supercells used for periodic calculations on each metal surface.S19, and the relative energies of each intermediate are given in Table S20.Calculations on metal clusters (transition states).Transition state calculations were performed in Gaussian 16, revision C.01, 10 using small 32-atom metal clusters to model the Pt and Pd surfaces.The top layer of these clusters consisted of 20 metal atoms, while the second layer had 12 atoms.This cluster size was chosen such that adsorbates never interacted directly with the edge of the surface.During the geometry optimizations, the bottom layer and the outer metal atoms from the top layer were kept frozen (Figure S6).Each cluster structure was built directly from optimized structures of 3-methyl-2-cyclohexen-1-one ( 1) and its enol counterpart on Pt and Pd from the periodic boundary calculations.Several conformations were considered for each molecule in case the lowestenergy transition state did not proceed from the minimum energy binding mode.The cluster structures were prepared and visualized using Gaussview, Version 6. 11 SCF convergence issues were encountered when using the PBE functional, so we used the B3LYP functional 12 with D3-BJ dispersion 13 for the cluster calculations.The DEF2SVP basis set and pseudopotential 14 were used on Pt and Pd, while the 6-31G(d,p) basis set 15 was used on all nonmetal atoms.The default convergence criteria were used for geometry optimizations along with the default "tight" SCF convergence criterion and "ultrafine" integration grid.Frequency calculations were used to verify that all structures corresponded to a stationary point and to compute thermochemical values.In particular, the harmonic vibrational frequencies were used along with the rigid rotor approximation to compute the Gibbs free energy of each species.Intrinsic reaction coordinate (IRC) calculations were used to confirm the identity of all transition states.).In contrast, the barrier of 64.9 kJ mol -1 on Pd is comparable to that of b1 (63.2 kJ mol -1 ), so both pathways may be accessible.

2
Figure S1.Particle size distribution of Rh/C and Ru/C via TEM analysis.

Figure S4 .
Figure S4.Reaction order determination of substrate 1, using overall conversion on Rh/C in p-xylene at 140°C under inert conditions.All data points of experiments were taken from separate measurements, no in situ sampling was applied.

Figure S5 .
Figure S5.Full pathway for consecutive ring dehydrogenation steps followed by H transfer from the surface to O.Each elementary step is numbered (i-x), and intermediates are labeled continuing the scheme from Figures 6 and 7. Pathways involving intermediate d are excluded because of its low favorability relative to b and c.All systems contain the same number of hydrogens, but metal-bound hydrogens are omitted from the scheme for clarity.The energetics of each elementary step (i-x) on all four metals is provided in TableS19, and the relative energies of each intermediate are given in TableS20.

Figure S6 .
Figure S6.Top (A) and side (B) views of a cluster calculation for the step b1 transition state on Pt.The top layer of the cluster consists of 20 atoms, while the bottom layer has 12 atoms.The bottom layer and atoms at the edge of the cluster were frozen during the optimizations (green outline), while all other atoms were allowed to relax (red outline).The same cluster size and constraint scheme were used for all calculations.

Figure S7 .
Figure S7.(A) Representative relaxed potential energy scan for the direct tautomerization pathway on Pt.Scans were performed from several initial binding conformations on both Pt and Pd, and this was the lowest energy direct pathway that was identified.The maximum energy of 250 kJ mol -1 is completely outside of the experimental range of activation energies.The dashed line is used to guide the eye.(B) Structure of the maximum energy point in the relaxed scan.

Figure S8 .
Figure S8.Transition state for dehydrogenation of the meta carbon of 1 on (A) Pt and (B) Pd.The C-H bond distance is indicated.This step has a free energy barrier of 133.8 kJ mol -1 on Pt, which is much larger than that of step b1 (67.3 kJ mol -1 ).In contrast, the barrier of 64.9 kJ mol -1 on Pd is comparable to that of b1 (63.2 kJ mol -1 ), so both pathways may be accessible.

Table S1 .
Overall rates at different reaction temperatures for substrate 1 on Pt/C.

Table S2 .
Overall rates at different reaction temperatures for substrate 1 on Pd/C.

Table S3 .
Overall rates at different reaction temperatures for substrate 1 on Rh/C.

Table S4 .
Overall rates at different reaction temperatures for substrate 1 on Ru/C.

Table S5 .
Rates at different reaction temperatures for substrate 1 to m-cresol on Pt/C.

Table S6 .
Rates at different reaction temperatures for substrate 1 to m-cresol on Pd/C.

Table S7 .
Rates at different reaction temperatures for substrate 1 to m-cresol Rh/C.

Table S8 .
Rates at different reaction temperatures for substrate 1 to m-cresol on Ru/C.

Table S9 .
Formation rate of m-cresol from substrate 2 on Pt/C at 140°C.

Table S16 .
Calculation of [ln (TOF h kB -1 T -1 ) R] values on Pt/C, Pd/C, Rh/C and Ru/C for the determination of activation entropy and enthalpy in p-xylene solvent (substrate 1 to 3-methylcyclohexane-1-one).

Table S19 .
Reaction energy for each of the elementary steps of the dehydrogenation pathway shown in FigureS5.Note that dehydrogenation steps are broadly energetically favorable on all four metal surfaces.

Table S20 .
Electronic energy of each intermediate in the dehydrogenation pathway shown in Figure S5 relative to the energy of reactant 1.