Screening of alternative catalysts for upgrading pyrolysis vapors from Chlorella sp. to hydrocarbon-rich bio-oil as a strategy for valuing naturally occurring materials and industrial wastes

The present study introduces an innovative and cost-effective approach for producing hydrocarbon-rich biofuels from fast-growing microalgae (Chlorella sp.) by upgrading pyrolysis vapors using low-cost catalysts derived from naturally occurring minerals and industrial wastes. Seven candidate materials were evaluated: three industrial wastes, cement kiln dust (CKD), granite and marble polishing waste (GPW), and kaolin processing waste (KPW); two clay minerals, attapulgite (APG) and palygorskite (PGK); and two naturally occurring zeolites, heulandite (HLD) and clinoptilolite (CPL). Catalysts were characterized and tested in a pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) setup to assess their activity and selectivity. Catalytic upgrading significantly improved bio-oil quality compared to non-catalytic pyrolysis, increasing hydrocarbon content by at least 1.5-fold and reducing oxygenated compounds by over 2-fold. CKD and GPW were the most active catalysts, promoting decarbonylation, decarboxylation, and aromatization reactions to generate deoxygenated bio-oil precursors. CKD showed superior selectivity toward aromatic hydrocarbons, achieving a 2.5-fold increase, while GPW favored aliphatic hydrocarbons, with a 1.9-fold enhancement. Overall, the catalysts demonstrated high selectivity for monoaromatics over polyaromatics and produced hydrocarbons predominantly in the gasoline range (C5–C12). Considering both catalytic performance and economic feasibility, GPW, APG, and CPL emerged as the most promising materials for large-scale applications. The findings highlight a viable pathway to produce drop-in biofuels from microalgae-derived pyrolysis vapors, providing deoxygenated, hydrocarbon-rich intermediates suitable for the transportation sector. This approach addresses key challenges in biofuel production by combining low-cost, abundant materials with efficient upgrading strategies, thereby contributing to the transition toward low-carbon energy systems.