Switchable Product Selectivity in Biodiesel Synthesis

Deoxygenation of biomass-derived fatty acids is a renewable and effective route for the production of diesel-range alkanes and fine chemicals; however, a high selectivity toward a single-target product is rather challenging. Herein, a Ni4Fe1 alloy catalyst was prepared via topological transformation of hydrotalcite precursor, which was featured with an interface structure (Niδ+–Feδ−) originating from electron transfer from Ni to Fe. The Ni4Fe1 catalyst shows a switchable product selectivity in hexadecanoic acid hydrogenation reaction by using different solvents: hexadecanol (yield: 97%) is obtained with dioxane as a solvent, whereas pentadecane (yield: 98%) is attained with dodecane as a solvent. In addition, in the presence of a physical mixture catalyst containing Ni4Fe1 and ZSM-5 (1:9, m/m) with dodecane as a solvent, hexadecane (yield: 94%) is obtained. A comprehensive study including temperature-programmed surface reaction with a mass spectrometer (TPSR-MS), in situ Fourier transform infrared (FT-IR), molecular dynamics simulations, and density functional theory (DFT) calculations reveals that hexadecanol serves as the key intermediate that determines the final product through different transformation pathways. High-binding energy between hexadecanol and dioxane solvent induces the desorption of hexadecanol from the Ni4Fe1 catalyst surface as the final product; in contrast, weak interaction between hexadecanol and dodecane solvent results in the subsequent decarbonylation (C–CO* bond cleavage) of intermediate on the surface of Ni4Fe1 with pentadecane as the final product (a cascade reaction on the same catalyst). In the case of the mixture catalyst, a cascade reaction on different catalysts is established: the hexadecanol intermediate undergoes desorption from Ni4Fe1, experiences activation adsorption, and C–O bond cleavage at the Brønsted acid site on ZSM-5, which leads to the final product hexadecane. This work provides an efficient product-switching strategy in biodiesel synthesis that is enabled by a subtle control over the pathway of key intermediate, which is expected to be applied in the catalytic conversion of biomass resources.