A Review of DFT Studies in Biomass Pyrolysis and Gasification

Biomass thermochemical conversion is a key route for converting renewable carbon into fuels and high-value chemicals, and density functional theory (DFT) has been used to elucidate its microscopic mechanisms. This review summarizes DFT studies of reaction pathways, key intermediates, and catalyst-surface mechanisms in four core fields of biomass pyrolysis and gasification. In biomass primary pyrolysis, main model compounds of cellulose, hemicellulose and lignin, such as cellobiose, xylan fragments and lignin β-O-4 dimers, have been used to analyze glycosidic-bond, Cβ–O and Cα–Cβ cleavage, hydrogen transfer, dehydration and transglycosylation pathways leading to levoglucosan, 5-hydroxymethylfurfural, furfural and phenolic products. For bio-oil upgrading, DFT calculations on Pt, Fe, Ru–H₂O, Mo₂C and Co@CoO systems have revealed the effects of adsorption geometry, hydrogen activation and transfer, and C–O cleavage on hydrogenation, dehydroxylation and demethoxylation routes and product selectivity. For tar removal, DFT studies have calculated initial C–H/O–H cleavage, aromatic-ring opening and carbon-removal steps of toluene and phenol on transition-metal, CaO-based and carbon-based catalysts, and analyzed the effects of surface composition and interfacial structure on cracking and reforming. In gas conditioning, DFT studies have attempted to describe competitive and synergistic radical reactions of surface carbon, CH_x, O, OH, and H intermediates linking methane reforming, carbon removal, and water-gas shift/reverse water-gas shift reactions (WGS/RWGS). In conclusion, the molecular mechanism of biomass thermal conversion could be elucidated and consistent with macroscale reaction tendencies via DFT simulations; nevertheless, the relatively simplified compound models of biomass and catalytic surface models, isolated elementary steps and limited kinetic descriptions led to considerable discrepancies between computational results that still fail to perfectly reproduce authentic reaction behaviors. Future work should integrate realistic catalytic structures, interconnected reaction networks, microkinetic modelling, and experimental validation to more accurately design chemical reactions and forecast product distributions for engineering applications of solid waste utilization.