Catalytic co-liquefaction of microalgae + corn straw over K₃PO₄+γ-Al₂O₃ supported Fe and Ni mono- / bimetallic in-situ composite catalysts for the production of liquid biofuel

In the present study, an array of K₃PO₄ + γ-Al₂O₃ supported Fe and Ni in-situ composite catalysts including K₃PO₄-Fe/γ-Al₂O₃, K₃PO₄-Ni/γ-Al₂O₃, K₃PO₄-Fe-Ni/γ-Al₂O₃ were prepared and characterized by Nitrogen-physisorption, XRD, XPS, NH₃-TPD, H₂-TPR, SEM, and TEM analysis. The effects of catalyst type, catalysts loading, feedstock loading and the ratio between corn straw and spirulina on the products yield and distribution from co-hydrothermal liquefaction of spirulina and corn straw were investigated. The greater specific surface area, larger amount of medium acid sites, the key roles of K and Fe-Ni bimetallic system and enhanced interaction between Fe, Ni and γ-Al₂O₃ support of K₃PO₄-Fe-Ni/γ-Al₂O₃ are mainly responsible for the greatest amount of water-insoluble (35.73 wt%) and water-soluble (2.18 wt%) biocrude. Besides, the optimal reaction condition (5 wt% catalysts loading, 10 wt% feedstock loading and spirulina: corn straw = 1:1 w/w) was confirmed. The presence of catalysts especially for K₃PO₄-Fe-Ni/γ-Al₂O₃ significantly increased the energy recovery of biocrude. Meanwhile, K₃PO₄-Fe-Ni/γ-Al₂O₃ noticeably increased the fraction of lighter molecule in biocrude up to 57.18 wt%. The esters, phenols and N-heterocyclic compounds were found to be dominated in biocrude from all co-HTL cases, and the enhanced selectivity to esters with catalysts were observed. Finally, the prospective reaction pathway and mechanism of the catalytic co-HTL of spirulina and corn straw was proposed and discussed detailly and comprehensively.