Chemoselective decarboxylation of higher aliphatic esters to diesel-range alkanes over the NiCu/Al2O3 bifunctional catalyst under mild reaction conditions

An efficient and highly selective decarboxylation catalyst has been developed for the production of diesel-range alkanes from the conversion of higher aliphatic esters under mild reaction conditions. Using methyl stearate as a model substrate, the NiCu/Al2O3 bifunctional catalysts derived from layer double hydroxide (LDHs) precursors showed excellent catalytic performance with 100% conversion and 97.5% selectivity to heptadecane at 260 °C and 3.0 MPa H2. On the other hand, the catalytic performance of NiCu/Al2O3 catalyst was also tested in the conversion of soybean and waste cooking oils under the mild conditions. The results showed that a high yields of diesel-range alkanes (>80 wt%) were obtained from the conversion of soybean oil and waste cooking oil, respectively. Detailed characterization (XRD, H2-TPR, HRTEM, XPS, and NH3-TPD) indicates, after reduction at 500 °C in H2 atmosphere, the NiCu alloy phase is the predominant phase and the Ni, Cu, Al species exhibited homogeneous distribution in the NiCu/Al2O3 catalyst. The remarkable decarboxylation performance mainly results from the synergistic effect between the metal active sites of NiCu alloy and Lewis acid sites of support Al2O3. The Lewis acid sites (Al3+) of support serve as active sites for the activation of carbonyl group of higher aliphatic esters, while H2 dissociation and decarbonylation of intermediate fatty aldehyde are achieved on the NiCu alloy sites. Compared with the monometallic Ni/Al2O3 catalyst (183.8 kJ/mol), the introduction of Cu species in the bimetallic NiCu/Al2O3 catalyst greatly reduced the activation energy (138.1 kJ/mol) of fatty acid ester conversion via promoting the dispersion of Ni species, reducing the dissociation energy of hydrogen on Ni, and increasing the electron density of Ni, which results in high decarboxylation performance of NiCu/Al2O3 under mild reaction conditions.