Ni dopped MgAl hydrotalcite catalyzed hydrothermal liquefaction of microalgae for low N, O bio-oil production

Catalytic hydrothermal deoxygenation and denitrogenation of microalgae for bio-oil production can potentially avoid energy-intensive upgrading of bio-oil, while tandem amidation of deoxygenation products and reesterification of NH3 with free fatty acids make simultaneous removal of N, O a dilemma to achieve. Taking advances of Ni in catalytic hydrogenation of biofuel, Ni dopped MgAl layered double hydroxides (NixMg2AlLDHs, x = 0, 0.5, 0.7, 0.9, 1.1) with tunable acidic/basic property are synthesized to simultaneously catalyze deoxygenation and denitrogenation for low N, O biofuel production. The results indicate that Ni doping is favorable to hydrothermal liquefaction (HTL) of microalgae with maximum yield of 31.4 wt% obtained via Ni0.9Mg2Al-LDH catalyzed HTL of microalgae at temperature of 300 degrees C, time of 1 h, and catalyst loading of 5 wt %. Ni doping initially reduces basicity of NixMg2Al-LDH, subsequently leading to increase of O/C ratio of bio-oil, while a continuous increase of Ni2+ provides more active species for deoxygenation with a final 13.49 % decrease of O/C ratio. Simultaneously, Ni doping of NixMg2Al-LDHs weakens denitrogenation of amino acids. However, tandem reactions of NH3 with reducing sugars, deoxygenation fatty acids radicals, and free fatty acids through Maillard reaction, amidation, and re-esterification are initially enhanced, leading to increase N/C ratio of bio-oil. Nevertheless, reducing property of NixMg2Al-LDH is continuously enhanced with increase of (Ni + Mg)/Al ratio from 0 to 2.9, finally resulting in 9.09 % decrease of N/C of biocrude. This work reports a methodology for simultaneous removal of N, O from biocrude, possibly providing a strategy for low N, O bio-oil production without further hydrogenation.