Catalytic Performance of AAEM-Loaded Biochars for Regulating Anhydrosugar Formation During Cellulose Pyrolysis

Biochar has attracted increasing attention as a low-cost catalyst for biomass pyrolysis due to its developed pore structure, abundant surface functional groups and tunable physicochemical properties. In this study, biochars loaded with alkali and alkaline earth metal (AAEM) species were prepared by pyrolyzing cellulose impregnated with different chloride and acetate salts, including NaCl, KCl, CaCl₂, MgCl₂, CH₃COONa, CH₃COOK, (CH₃COO)₂Ca and (CH₃COO)₂Mg. The resulting AAEM-loaded biochars were subsequently employed as catalysts for cellulose pyrolysis to investigate their effects on product distribution, particularly levoglucosan (LG) and levoglucosenone (LGO) formation. SEM and XRD analyses revealed that the AAEM precursor significantly affected the morphology and phase composition of the biochars. Chloride-derived biochars retained crystalline salt phases or formed corresponding metal oxides, whereas acetate-derived biochars exhibited more dispersed metal-containing species. The introduction of AAEM-loaded biochars generally decreased bio-oil and LG yields while increasing char production, indicating enhanced secondary cracking and repolymerization reactions. Among the investigated catalysts, alkali metal-loaded biochars exhibited stronger inhibition toward LG formation than alkaline earth metal-loaded biochars. The catalytic effects followed the order of C-KCl ≈ C-NaCl > C-MgCl₂ > C-CaCl₂ for chloride-derived biochars and C-CH₃COOK ≈ C-CH₃COONa > C-(CH₃COO)₂Mg > C-(CH₃COO)₂Ca for acetate-derived biochars. Notably, C-(CH₃COO)₂Ca and C-(CH₃COO)₂Mg slightly promoted LGO formation, which was attributed to the synergistic effects of alkaline earth metal species, surface oxygen-containing functional groups and acetate-derived intermediates on dehydration reactions. Thermogravimetric and kinetic analyses further demonstrated that AAEM-loaded biochars reduced the apparent activation energy of cellulose pyrolysis and facilitated thermal decomposition. These findings provide new insights into the catalytic role of AAEM-loaded biochars and suggest a promising strategy for regulating anhydrosugar selectivity, particularly for the production of high-value LGO from biomass pyrolysis.