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 3-methylcyclohexanone 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 m-cresol 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 3-methylcyclohexanone is the dominant product over Pt/SiO 2 at low conversions. By contrast, the direct dehydroxylation of m-cresol 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.