Insights into the effect of various supports on hydrothermal liquefaction of food waste over iron-oxide nano-catalysts

This work investigates the effect of supported iron-oxide nano-catalysts for hydrothermal conversion of food waste. The studied supports were Vulcan carbon (VC), CeO₂, ZSM-5 and amorphous SiO₂-Al₂O₃. Catalytic hydrothermal liquefaction experiments were carried out in a batch reactor at 16 MPa and 300 °C maintained for 1 h. Different fractions of Fe(0), Fe²⁺ and Fe³⁺ alter its tendency toward deoxygenation, hydrogenations and condensation reactions, which influence the bio-crude yield, elemental compositions, and energy recoveries. The fresh and spent catalysts were characterized using X-ray photoelectron spectroscopy, physisorption analysis, thermogravimetric analysis, transmission and scanning electron microscopy. It was found that the change in catalyst support influences HTL pathways and product compositions. The results reveal that the inclusion of FeO_x catalyst on Vulcan carbon, SiO₂-Al₂O₃ and ZSM-5 supports can increase the bio-crude yield by ~7–9 wt% compared to their FeO_x-free yields. The increase in bio-crude yield was associated with the decrease in the surface ratios of Fe³⁺/Fe²⁺ at the range of 0.8–1.6. In overall, catalysts that had higher tendencies in converting amines into oil-soluble compounds increased the bio-crude yield, while catalysts that promoted dehydration and decarboxylation route decreased the bio-crude yield. The maximum energy recovery in bio-crude was obtained using FeO_x/SiO₂-Al₂O₃ catalyst with values ~95 %. The deactivation of catalysts was associated with the increase in Ca and P poisonous elements on catalytic sites, which decreased the energy recovery of recycled FeO_x/SiO₂-Al₂O₃ to ~85 % after three cycles.