Different Product Distributions and Mechanistic Aspects of the Hydrodeoxygenation of m-Cresol over Platinum and Ruthenium Catalysts

Abstract

Experimental measurements of the conversion of m-cresol over Pt and Ru/SiO 2 catalysts show very different product distributions, even when the reaction is conducted at similarly low conversions and the same operating conditions (300 °C, 1 atm). That is, although ring hydrogenation to 3methylcyclohexanone is dominant over Pt, deoxygenation to toluene and C−C cleavage to C 1 −C 5 hydrocarbons prevail over Ru. For understanding the differences in reaction mechanisms responsible for this contrasting behavior, the conversion of mcresol over the Pt(111) and Ru(0001) surfaces has been analyzed using density functional theory (DFT) methods. The DFT results show that the direct dehydroxylation of m-cresol is unfavorable over the Pt(111) surface with an energy barrier of 242 kJ/mol. In turn, the calculations suggest that the reaction could proceed through a keto tautomer intermediate, which undergoes hydrogenation of the carbonyl group followed by dehydration to form toluene and water. At the same time, a low energy barrier for the ring hydrogenation path toward 3-methylcyclohexanone compared to the energy barrier for the deoxygenation path toward toluene over the Pt(111) surface is in agreement with the experimental observations, which show that 3methylcyclohexanone is the dominant product over Pt/SiO 2 at low conversions. By contrast, the direct dehydroxylation of mcresol becomes more favorable than the tautomerization route over the more oxophilic Ru(0001) surface. In this case, the deoxygenation path exhibits an energy barrier lower than that for the ring hydrogenation, which is also in agreement with experimental results that show higher selectivity to the deoxygenation product toluene. Finally, it is proposed that a partially unsaturated hydrocarbon surface species C 7 H 7 * is formed during the direct dehydroxylation of m-cresol over Ru(0001), becoming the crucial intermediate for the C−C bond breaking products C 1 −C 5 hydrocarbons, which are observed experimentally over the Ru/SiO 2 catalyst.

Hydrodeoxygenation of Phenol over Zirconia‐Supported Catalysts: The Effect of Metal Type on Reaction Mechanism and Catalyst Deactivation

“…The results obtained in the present work revealed that the reaction pathway for HDO of phenol strongly depends on the type of metal. Similar results have been reported in the literature for HDO of different model molecules ,,,…”

“…The results of DRIFTS are cast in terms of two mechanisms, as supported by prior DFT and experimental investigations: (1) direct dehydroxylation of phenol to a partially unsaturated hydrocarbon surface species (e.g., C 6 H 5 *), with subsequent hydrogenation to benzene (by analogy with Ref. ), which is favored by metal surfaces such as Ru(0 0 0 1) or (2) a mechanism involving a keto tautomer intermediate, which may undergo selective hydrogenation of the carbonyl to 2,4‐cyclohexadienone (depending on the support), followed by dehydration to benzene. This was suggested to occur on Pd/ZrO 2 catalysts by experimental observations, as well as on Pt(1 1 1) …”

“…A detailed investigation about the HDO of m ‐cresol in the gas phase over silica‐supported catalysts (Pd, Pt, Ru, Ni, Fe, NiFe) is available in the literature . Products of hydrogenation (e.g., 3‐methylcyclohexanone and 3‐methylcyclohexanol) were mainly formed over the Pt/SiO 2 , Pd/SiO 2 , and Ni/SiO 2 catalysts whereas toluene was the main product on Fe and Ni–Fe bimetallic catalysts.…”

A Perspective on Catalytic Strategies for Deoxygenation in Biomass Pyrolysis

“…The selectivity towards 3‐methylcyclohexanone was 64.9 %, with the selectivities towards the other products being 27.3 % for toluene and 7.7 % for 3‐methylcyclohexanol. Toluene was also a major product when using a Ru/SiO 2 catalyst with a selectivity of 38.5 % . However, Ru/SiO 2 also catalyzed the cleavage of C−C bonds, yielding methane as another major product (43.4 % selectivity).…”

Advances in Upgrading Lignin Pyrolysis Vapors by Ex Situ Catalytic Fast Pyrolysis

“…Tan and co‐workers have compared the reaction pathways of Ru/SiO 2 and Pt/SiO 2 at 300 °C and atmospheric pressure of H 2 . Deoxygenation to toluene and C−C cleavage to C 1 –C 5 hydrocarbons prevail over Ru, whereas ring hydrogenation to form 3‐methylcyclohexanone is dominant over Pt . The catalytic activity of Ni, Pd, and Pt for the HDO of m ‐cresol over silica‐supported catalysts at 250 °C and atmospheric H 2 pressure has been investigated by Chen and co‐workers .…”