Synergistic catalytic upgrading of spirulina bio-oil via Ni @ alkali-modified HZSM-5 in microwave-assisted two-stage hydrothermal liquefaction

The urgent need to address global energy shortages and environmental pollution has accelerated the development of renewable algal bio-oils. Spirulina stands out as an ideal feedstock for hydrothermal liquefaction (HTL) owing to its high carbon sequestration rate, non-competition with arable land, and rich protein-lipid content. However, Spirulina-derived bio-oil typically suffers from high oxygen content and low heating value. To overcome these limitations, this study developed a microwave-assisted two-stage HTL process integrated with a Ni-loaded, alkali-modified HZSM-5 catalyst. Unlike conventional catalysts that often rely on costly noble metals or suffer from diffusion limitations in microporous structures, this system employs earth-abundant nickel on a hierarchically structured zeolite to enhance both cost-effectiveness and catalytic efficiency. HZSM-5 was treated with NaOH to create mesoporosity, followed by Ni impregnation to introduce hydrogenation functionality. The effects of NaOH concentration and Ni loading on catalyst properties were systematically investigated. Process optimization was performed via response surface methodology using a 3 % sulfuric acid medium, and bio-oil composition was analyzed by GC/MS. Modification with 1 mol/L NaOH increased hydrocarbon content to 32.9 %, while subsequent loading with 5 wt% Ni synergistically boosted the yield to 48.1 %. Molecular dynamics simulations provided mechanistic insights into bond cleavage and conversion pathways of Spirulina-derived model compounds in subcritical water. Compared with existing catalytic systems, this study significantly improves the quality of biofuels, providing an efficient catalytic strategy and fundamental insights for the production of high-quality bio-oil from spirulina.