Deoxygenation of methyl laurate as a model compound to hydrocarbons on transition metal phosphide catalysts

Abstract

A series of silica-supported metal (Ni, Co, Fe, Mo and W) phosphides were investigated for the deoxygenation of methyl laurate as a model compound to hydrocarbons. To insight into the mechanism, the deoxygenations of the intermediates (lauric acid, dodecanal and dodecanol) were also evaluated. For comparison, Ni/SiO2 and POx/SiO2 were also studied. At 573K, 2.0MPa and methyl laurate weight-hourly space velocity of 5.2h−1, the catalyst activity followed the order: Ni2P>MoP>CoP-Co2P>WP>Fe2P-FeP; Ni≈Ni2P>Ni12P5>Ni3P. The high activity is ascribed to the increases in surface metal site density, the electron density of metal site and Brönsted acidity. In contrast to metallic Ni, the nickel phosphides had much lower activities for methanation and cracking reaction due to the electronic and geometrical effects of P. The main products on the metallic Ni and the Ni, Co and Fe phosphides were C11 hydrocarbons formed via decarbonylation pathway, whereas the predominating products on MoP and WP were C12 hydrocarbons produced via hydrodeoxygenation pathway. The higher the electron density of metal site is, the more predominant the decarbonylation pathway. The increase in the interaction between metal site and oxygen promoted the hydrodeoxygenation pathway. Compared to metallic Ni and the Ni and Co phosphides that primarily gave n-alkanes, Fe, Mo and W phosphides additionally yielded alkenes and iso-hydrocarbons, which is attributed to their lower hydrogenation ability and higher Brönsted acidity. In all, we propose that the deoxygenation mechanism is mainly determined by the electron property of metal site and Brönsted acidity. Also, there might be a synergism between the metal and Brönsted acid site for the deoxygenation of methyl laurate.