Carboxylic Acid Decarbonylation on Nickel: The Critical Role of the Acid Binding Geometry

Although renewable energy sources like solar and wind provide alternatives to fossil fuels for electricity generation, liquid transportation fuels are still required for marine and aviation travel, the electrification of which remains years or decades away. For these applications, biofuels produced through the deoxygenation of biomass provide a viable alternative. To elucidate the mechanism behind the deoxygenation of oleaginous biomass to hydrocarbons via decarbonylation (DCN), this work presents a joint experimental and computational investigation wherein propanoic acid (PAc) and a Ni[111] surface were used to model fatty acids and Ni-based catalysts. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements show the binding mode of PAc on Ni/Al₂O₃ to be predominantly bidentate, which informed density functional theory (DFT) studies. The bidentate adsorption mode of PAc plays a key role in determining the DCN mechanism since α-carbon dehydrogenation of PAc was deemed unfeasible, in contrast with previous reports. A mean-field microkinetic model reveals the following dominant pathway at 573 K: PAc dehydroxylation affords a CH₃CH₂CO* intermediate that after two α-carbon dehydrogenation steps forms CH₃CCO*, which then undergoes CO abstraction and hydrogenation to ultimately yield ethane. The overall reaction has a turnover frequency (TOF) of 2.75 × 10^-11 s^-1, with the dehydroxylation of CH₃CH₂COOH* being the rate-determining step with a degree of rate control of one. The apparent activation barrier determined here is much lower than that previously reported, which could be due to the binding mode of PAc and considered experimental conditions. The effect of partial pressures of gaseous intermediates on the overall rate of reaction showed a negative order with respect to H₂ partial pressure, while a positive order was observed with respect to PAc partial pressure. Overall, this work shows the importance of experimentally determining the binding mode of carboxylic acids on the catalyst surface to inform DFT work designed to elucidate the DCN reaction mechanisms.