NiCo/SiO2 nanospheres for efficient synergetic decarboxylation of fatty acids and upgrading of municipal sludge HTL bio-crude to biofuels

Municipal sludge (MS) and derived HTL (hydrothermal liquefaction) bio-crude contains a large quantity of organics, including fatty acids, which may be a potential resource for producing biofuels via catalytic treatment. Herein, decarboxylation of fatty acids is chosen as the model reaction to screen a series of nickel-based bimetallic catalysts supported on homemade SiO2 nanospheres to investigate the synergetic effect. NiCo/SiO2 turned out with the best performance compared to NiCu-, NiZn-, NiMo- and NiFe/SiO2. An 83% conversion of stearic acid and an 84% selectivity of heptadecane via hydrothermal decarboxylation were achieved at 280 °C under 1 MPa H2. XPS results revealed the electron transfer from Co to Ni, leading to the formation of Niδ-, which is more accessible to bond with Cδ+ in the carboxyl group of fatty acids, thus facilitating the decarboxylation reaction. Compared to commercial SiO2, our homemade SiO2 showed a stronger carbon accumulation resistance, indicating a medium specific surface area was essential. Both the synergetic effect of NiCo bimetal and the carbon accumulation resistance of homemade SiO2 nanospheres contributed to the efficient hydrothermal decarboxylation. The NiCo/SiO2 catalyst was further applied for upgrading of MS HTL bio-crude to biofuels, and the theoretical higher heating value (HHV) was significantly improved from 34.4 to 37.2 MJ/kg, suggesting the potential application of converting MS bio-crude to biofuels.