Catalytic Hydrothermal Liquefaction of Grape Pomace Using Ni–ZrO₂–MSS and Ni–HZSM5 in a Water–Crude Glycerol Cosolvent

This study focused on the catalytic hydrothermal liquefaction (HTL) of grape pomace using Ni-HZSM5 and Ni-ZrO₂-modified steel slag (MSS) catalysts, employing a water-crude glycerol cosolvent. The research aimed to understand how temperature, solvent ratios, and crude glycerol composition affect biocrude yield and properties, and to evaluate the stability of regenerated catalysts. The results revealed that when fatty acids/glycerides-rich crude glycerol was used, the highest biocrude yield of 76 wt %, HHV of 41 MJ/kg, H/C ratio of 1.81, and energy recovery of 90.9% was achieved at 320 °C with 75% crude glycerol concentration in the cosolvent. Catalytic HTL with Ni-HZSM5 significantly reduced the acid content of the biocrude by 44%. Although Ni-ZrO₂-MSS increased the biocrude yield from 44.07 to 49.97 wt %, it promoted the production of acids and reduced the esters in biocrude. TGA refinery cut results showed that both catalysts enhance diesel production, with Ni-HZSM5 yielding the highest diesel fraction (41.24%) compared to Ni-ZrO₂-MSS (37.51%) and the noncatalytic process (33.56%). Moreover, both catalysts significantly reduced the production of heavier fractions, such as residual fuel oil and bitumen. While the modification significantly enhanced the BET surface area of raw steel slag from 4.04 to 49.61 m²/g in Ni-ZrO₂-MSS, the surface area of the regenerated catalyst after HTL dropped to 15.88 m²/g, aligning with decreased H₂ uptake, indicating a loss of active sites. Similarly, the surface area of Ni-HZSM5 decreased from 522.25 to 387.37 m²/g after HTL, while the pore volume increased from 0.2095 to 0.3425 cm³/g. However, the spent Ni-HZSM5 catalyst displayed an increase in H₂ uptake (269.49 μmol/g) with a shift in the reduction peaks to higher temperatures, suggesting the creation of new active sites or changes in the dispersion of NiO species during the reaction. The TPO graphs confirm the presence of coke with an intermediate structure between amorphous and graphitic carbon.