Hydrothermal co-liquefaction of biomass and plastic wastes into biofuel: Study on catalyst property, product distribution and synergistic effects

This study reports an efficient conversion route for prosopis juliflora (PJ) biomass into high-quality bio-oil through catalytic hydrothermal liquefaction (HTL) process with systematically substituted hydrogen-rich plastic waste ‘polypropylene (PP)’, and using alumina supported metal oxide (Mo, Ni, W, and Nb) catalysts. The HTL treatments of PJ with PP (0-75 wt.%) were investigated in both sub and supercritical water conditions. An excellent synergy between PP and PJ was observed even in subcritical conditions (97.6% synergy at 340 °C at 25% PP to PJ), while efficient liquefaction of PP alone was observed only in the supercritical conditions. The optimum temperature, and PP substitution were found to be 420 °C and 25% respectively, with 46.5% bio-oil yield, high deoxygenation (65.1%), and carbon recovery (78.9%) when using Nb/Al₂O₃ as the catalyst. An in-depth analysis of physicochemical properties and the bio-oil product distribution with respect to each catalyst and PP/PJ substitution ratio are discussed in detail. Among all, the Nb/Al₂O₃ catalyst performed well with remarkable recyclability up to 10 cycles. The produced bio-oil mixture due to its low oxygen content is very promising to be upgraded to precursors for chemicals and transportation biofuels.