Modified red mud catalysts for enhanced bio-crude yield and metal retention in hydrothermal liquefaction of oak wood

Hydrothermal liquefaction (HTL) is a promising route for biofuel production, but the resulting bio-crude suffers from low quality, high metal content, and poor stability. This study explores the use of red mud (RM), a metal oxide-rich industrial waste, as a low-cost catalyst and hydrogen producer to enhance bio-crude yield and stability during the HTL of oak wood. To enhance RM performance, it was modified through thermal treatment, acid washing, and reduction. The study aimed not only to improve bio-crude yield and quality but also to assess the impact of these treatments on the migration of metals into the oil and aqueous phases. The acid-washed and reduced RM showed the best catalytic activity, reaching a bio-crude yield of almost 50 %. This was attributed to its high surface area which increased from 14.9 to 49.1 m²/g and consequently, enhanced iron availability, which promoted in-situ hydrogen generation. As revealed by the characterization of the samples, acid treatment led to the formation of a stable FeTiO3 (ilmenite) phase, which enabled the complete reduction and reoxidation of Fe during the process. The presence of ilmenite also significantly minimized Fe leaching into both oil and water phases, the amount of Fe in the bio-crude decreases from 2330 to 42.5 ppm. Bio-crudes obtained with both catalysts showed high H/C ratios and carbon recoveries up to 67 %. These results demonstrate the potential of modified RM as a sustainable catalyst in HTL, offering a valuable route for waste valorization and reducing the need for expensive bio-crude upgrading.