Influence of Re addition to Ni/SiO2 catalyst on the reaction network and deactivation during hydrodeoxygenation of m-cresol

Abstract

The reaction network of hydrodeoxygenation of m-cresol was investigated over Ni/SiO2 and Ni-Re/SiO2 catalysts at 300 °C and 1 atm H2. m-Cresol conversion proceeds through three major primary pathways: phenyl ring hydrogenation to 3-methylcyclohexanone and 3-methylcyclohexanol, CC hydrogenolysis to CH4 and phenol, and deoxygenation to toluene. Re addition promotes dehydrogenation of the initially formed methylcyclohexanone and methylcyclohexanol to surface intermediates followed by deoxygenation toward formation of toluene. CC hydrogenolysis happens on unsaturated compounds with phenyl ring, instead of saturated compounds, since dehydrogenation is requested prior to CC cleavage. The turnover frequency (TOF) for CH4 formation follows the order of m-cresol < toluene << benzene, indicating that CC hydrogenolysis is accelerated after deoxygenation and/or demethylation. Increasing reaction temperature promotes both deoxygenation and hydrogenolysis. The toluene selectivity keeps low even at higher temperature of 340 °C on monometallic Ni catalyst due to successive CC hydrogenolysis to CH4. Re addition not only promotes deoxygenation to toluene by providing active Ni-Re neighboring sites, but also inhibits the aromatics hydrogenolysis to CH4, resulting in high toluene selectivity on bimetallic catalyst. Both catalysts show a similar deactivation trend and coke is speculated to be the major reason for deactivation.