Deoxygenation of esters over sulfur-free Ni–W/Al₂O₃ catalysts for production of biofuel components

Sulfur-free Ni–W/Al₂O₃ catalysts synthesized through incipient wetness impregnation technique were investigated in deoxygenation of methyl palmitate and ethyl caprate blend at 300 °C and P = 1 MPa (PH₂ = 0.25 MPa) in a fixed-bed continuous flow reactor. Modification of Ni/Al₂O₃ catalysts by W (0.4–4.4 wt%) was found to give rise to higher both conversions of the feedstock and deoxygenation degree. In addition, this also resulted in a change in the selectivity of deoxygenation towards alkanes formed via direct hydrodeoxygenation route, compared to alkanes formed through decarboxylation/decarbonylation. Selectivity by alkanes with carbon chain length equal to those of an initial fatty acid formed via hydrodeoxygenation was found to increase with W content increasing. The synthesized catalysts were investigated by XRD, XPS, TEM, BET and H₂-TPR techniques. According to XPS study of the reduced samples, tungsten was in 6+ oxidation state on the sample surface and W⁶⁺ content increased in the near-surface layer of samples with tungsten content increasing in catalysts.