Enhancement of fatty acids hydrodeoxygenation selectivity to diesel-range alkanes over the supported Ni-MoOx catalyst and elucidation of the active phase

Metal oxide modified nickel catalysts are highly desirable for the conversion of biomass-derived compounds to biofuels and valuable chemicals because of their low-cost and unique synergistic effects in catalytic reaction. Herein, a series of metal oxide modified nickel catalysts were scrutinized in the hydrodeoxygenation of steatic acid to diesel-range alkanes without any carbon loss as a model reaction. The results showed that MoOx modified Ni/SiO2 catalyst was the most active and selective for the hydrodeoxygenation of stearic acid under mild reaction conditions (260 °C and 3.0 MPa H2 partial pressure). The catalyst with an Ni/Mo molar ratio of 1, and that was reduced at an optimized temperature (500 °C), exhibited the best performance; this catalyst achieved a high selectivity (>95%) to diesel-range alkanes at 100% stearic acid conversion. A high selectivity (>60%) to C18 alkane and a less than 30% selectivity to C17 alkane were observed at 260 °C and 3.0 MPa H2 partial pressure. By selecting an appropriate support, the selectivity to C18 alkane can reach 95% at 100% stearic acid conversion over Ni–Mo/H-ZSM-5 catalysts. In contrast to Ni/SiO2, Ni–Mo/SiO2 was more efficient for CO hydrogenation and less active for C–C bond cleavage, which afforded a higher selectivity to long-chain hydrocarbons without any carbon loss. Detailed characterization, control experiments, and kinetic studies indicate that the high activity and selectivity to the C18 alkane arises from a synergy between Ni and MoOx. The Ni sites at the interface between the Ni metal and MoOx species play a role in the generation of hydride (Hδ−) species from H2 dissociation, and MoOx plays a role in promotion of fatty acids adsorption through adsorbing carboxylic groups at the oxygen vacancy of MoOx. The deep understanding of such synergic catalysis will provide significant clues for the rational design of bimetallic catalysts towards the production of diesel-range alkanes without any carbon loss from the hydrodeoxygenation of fatty acids/esters.