Unravelling the catalytic influence of naturally occurring salts on biomass pyrolysis chemistry using glucose as a model compound: a combined experimental and DFT study

Fast pyrolysis is an efficient thermochemical decomposition process to produce bio-oil and renewable chemicals from lignocellulosic biomass. It has been suggested that alkali- and alkaline-earth metal (AAEM) ions in biomass alter the yield and composition of bio-oil, but little is known about the intrinsic chemistry of metal-catalyzed biomass pyrolysis. In this study, we combined thin-film pyrolysis experiments and density functional theory (DFT) calculations to obtain insights into AAEM-catalyzed glucose decomposition reactions, especially forming major bio-oil components and char. Experiments reveal the difference in the yield and composition of bio-oil of metal-free and AAEM complexed glucose. Metal-free glucose produced 2,3-dihydro-3,5-dihydroxy-6-methyl-4 H -pyran-4-one (DHMDHP) as the predominant compound in bio-oil, while 1,6-anhydroglucofuranose (AGF) was dominant in Na( i )/glucose, levoglucosan (LGA) in K( i )/glucose, levoglucosenone (LGO) in Ca( ii )/glucose and furfural in Mg( ii )/glucose. To evaluate the stereoelectronic basis of metal ions in altering pyrolysis reaction kinetics, the reaction mechanisms of AGF, LGA, 5-hydroxymethylfurfural (5-HMF), furfural, 1,5-anhydro-4-deoxy- d -glycerohex-1-en-3-ulose (ADGH), LGO, and char formation were investigated using DFT calculations. DFT results showed that the presence of Ca( ii ) and Mg( ii ) ions catalyzed furfural and LGO formation, while alkali ions decatalyzed the formation of these products. Conversely, Na( i ) and K( i ) ions catalyzed the concerted dehydrative ring closure of glucofuranose during AGF formation. For ADGH, AAEMs showed an anti-catalytic effect. We also described a novel route for char formation via coupling between 1,2-anhydroglucopyranose and a carbonyl compound. The presence of alkali ions catalyzed char formation. Thus, the atomistic insights obtained from DFT calculations assist in understanding the observed change in experimental yields of individual bio-oil compounds governing their composition. Alkali and alkaline-earth metal loaded biomass pyrolysis highlights that different metal ions have different effects on bio-oil composition.