Initial pyrolysis mechanism of cellulose revealed by in-situ DRIFT analysis and theoretical calculation

Abstract

Cellulose is one of the major components of biomass. The study on its pyrolysis process will be beneficial to the in-depth understanding of biomass pyrolysis mechanism. In this work, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) was first used to characterize the evolution process of the functional groups in cellulose during pyrolysis. The results showed that the degradation of carbon skeleton was prior to the dehydration of free hydroxyls after the destruction of hydrogen bond networks during pyrolysis. The thermal stability of CO in cellulose followed by the order of glycosidic bond <CO in glucopyranose ring <CO between glucopyranose ring and hydroxyl. Followingly, micro pyrolysis experiment was performed to analyze the pyrolysis products of cellulose at various temperatures. It was found that the rupture of glucopyranose rings to form 2C and 4C products was more difficult than the dissociation of C6 hydroxymethyls, and required higher pyrolysis temperature. Quantum chemistry calculation was further carried out to study the key reaction pathways including dehydration, cleavage of glycosidic bond, ring opening and fragmentation in the initial stage of cellulose pyrolysis. The result showed that the concerted cleavage of glycosidic bond to form LG-end short chain was the most favored with the lowest activation energy. The ring opening of the glucose unit in the chain occurred via the cleavage of C1-O. The formed ring opening product was more likely to degrade via the dissociation of C6 hydroxymethyl compared with the breakage of C2–C3 to form 2C and 4C products. Besides, the dehydration of hydroxyls in glucose units required high energy barriers and was difficult to occur.