Selective decarbonylation of palmitic acid over Ni-Cu alloy catalysts and its kinetic studies

Due to escalating carbon emissions, increasing attention is being focused on the synthesis of biomass-derived liquid fuels with a low CO 2 footprint. Currently, the dominant biobased liquid fuel in the diesel range is green diesel, produced via deoxygenation of non-edible natural/waste oils and their free fatty acid components into engine-compatible diesel-range alkanes. Industrially, sulfided NiMo or CoMo catalysts are typically used, leading to total hydrocarbon yields of 70-80 wt%. However, their inevitable sulfur leaching often contaminates the products and requires the addition of sulfur-containing agents. Here, we developed a highly selective sulfur-free Ni-Cu alloy catalyst by disentangling the electronic and size effects, to enable green diesel synthesis through the decarbonylation of palmitic acid—the most common fatty acid in these feedstocks. An unprecedented decarbonylation product (pentadecane) yield of 98% (on a molar basis) was achieved using a Ni-Cu alloy catalyst with a Ni/Cu ratio of 1:1 under mild reaction conditions (2 MPa H 2 , 250 °C). Minimal metal sintering or leaching was observed after multiple reuses. Kinetic studies show that incorporating Cu into Ni markedly reduces both the apparent activation energy and the reaction order for the conversion of palmitic acid. These results, combined with catalyst characterization, reveal that introducing Cu into Ni not only facilitates the adsorption and activation of the carboxylic hydroxyl group to promote the reductive deoxygenation of palmitic acid into aldehyde/alcohol intermediates—the slow step in decarbonylation—but also weakens the adsorption of C–C bonds on Ni by enhancing its electron density.