Kinetics and mechanism of m-cresol hydrodeoxygenation on a Pt/SiO2 catalyst

The Langmuir-Hinshelwood kinetic model was employed to investigate the hydrodeoxygenation (HDO) of m-cresol in the vapor phase over Pt/SiO2, at 300 degrees C. The model takes into account the competitive adsorption and secondary reactions of all possible reactants and products. Therefore, independent measurements of the conversion of the reaction intermediates 3-methyl-cyclohexanone and 3-methyl-cyclohexanol were done at various levels of conversion and incorporated in the kinetic fitting of the model. The kinetic analysis of these reactions over the Pt/SiO2 catalyst confirms that hydrogenation and dehydrogenation reactions occur at faster rates than the hydrodeoxygenation steps. The hydrogenation/dehydration followed by dehydrogenation (HYD route) was shown to be a minor deoxygenation pathway. The actual hydrodeoxygenation path is proposed to proceed via a tautomerization route that starts with a fast and reversible tautomerization of m-cresol to an unstable ketone intermediate (3-methyl-3,5-cyclohexadienone) over the catalyst surface. In the case of the Pt/SiO2 catalyst, the C=C bonds of the unstable ketone intermediate are fully hydrogenated to the saturated ketone (3-methyl-cyclohexanone). Sequential hydrogenation of this ketone produces a saturated alcohol (3-methyl-cyclohexanol). In the absence of surface acidity, this path only leads to the two ring-saturated products. When the support is acidic enough, dehydration of the alcohol to 3-methyl-1-cyclohexene followed by dehydrogenation leads to toluene. The kinetic analysis conducted here shows that this path is very limited over Pt/ SiO2. By contrast, the toluene selectivity can be dramatically enhanced when using TiO2 and ZrO2 as the catalyst supports. This enhancement is not through the hydrogenation/dehydration/dehydrogenation path, but rather through the direct dehydroxylation mentioned above, which is favored by the incorporation of oxophilic sites in the catalysts. The oxophilic sites increase the rate of chemoselective hydrogenation of the carbonyl group of the m-cresol tautomer (3-methyl-3,5-cyclohexadienone) to a very reactive unsaturated alcohol (3-methyl-3,5-cyclohexadienol), which can be easily dehydrated to toluene, even in the absence of any significant catalyst acidity. (C) 2014 Elsevier Inc. All rights reserved.