(241c) Unravelling the Catalytic Effect of Naturally Occurring Inorganics on Biomass Pyrolysis Chemistry: A Combined Experimental and DFT Study

Fast pyrolysis is an economical and a promising technology for the production of renewable fuels and value-added chemicals from non-edible, second generation lignocellulosic biomass. The primary objective in designing pyrolysis process is to optimize the yield and composition of bio-oil, which in turn depends upon the intrinsic chemistry of pyrolysis. It is shown in the literature that any variation in the composition of the biomass feedstock has a direct implication on the quality and quantity of bio-oil.¹ Additionally, the presence of naturally occurring alkali- and alkaline-earth-metals (AAEMs) in biomass also change the bio-oil composition.^2-4 Hence, in this work, we present the experimental results for thin-film pyrolysis¹ (isothermal, reaction-controlled regime) of biomass to investigate the influence of alkali (Na(I) and K(I)) and alkaline-earth (Ca(II) and Mg(II)) metal ions (majorly found in biomass) on the yield and composition of bio-oil. To understand the stereoelectronic basis of these AAEMs on the key thermal decomposition reactions, we employ density functional theory (DFT) calculations. For investigating the cellulosic pyrolysis reactions, cellobiose (the simplest unit of cellulose) and β-D-glucose (monomer of cellulose and one of the intermediates formed during cellulose pyrolysis) are used as model biomass compounds.

In the absence of metal, thin-film experiments of cellobiose at 300 °C gives 10.7 ± 0.5% bio-oil and 73.6 ± 0.9% char. The presence of alkali ions and Ca(II) ions with cellobiose decrease bio-oil yield by 3.5 ± 0.6% and increase the char yield by 2.8 ± 0.5% while, Mg(II) ion increases bio-oil yield by 2 ± 0.5% and decreases char yield by 3 ± 0.5%. The bio-oil, in the absence of metal, is mainly composed of levoglucosan (LGA) and furfural, furans and levoglucosenone (LGO) predominate in presence of Mg(II) and Ca(II) ions while, alkali ions prefer furfural and pyrans. Next, the successive breakdown of intermediates/monomers (glucose) formed after glycosidic bond breaking is studied by glucose-thin film pyrolysis experiments, which shows that in metal-free conditions, pyrans are obtained in the highest yield. Anhydroglucofuranose (AGF) is selectively obtained from Na(I) loaded glucose, LGA in glucose/K(I), LGO in glucose/Ca(II), while furfural formation is maximum in glucose/Mg(II). Hence, different metal ions have different effects on bio-oil composition. Our DFT calculations show that the activation enthalpies for glycosidic bond breaking in the absence of metal are 57.5 and 71.5 kcal.mol^-1 for transglycosylation (forming LGA) and ring contraction reactions (forming (precursor to) furans), respectively. The alkali ions have a negligible effect on the activation barrier of glycosidic bond breaking by glycosylation (forming 1,2-anhydroglucopyranose – precursor to LGA), transglycosylation and ring contraction reactions, while Mg(II) and Ca(II), due to their stronger Lewis acid character, significantly lower activation barriers (about 40 kcal.mol^-1) for glycosylation and transglycosylation reactions but have a negligible effect on the activation barrier of ring contraction reaction. Further, the decrease in the LGA yield observed in the presence of alkali ion(s) is attributed to low activation enthalpies for isomerization/repolymerization reactions of LGA, favouring char (activation barrier_alkali-ions ≤ 70 kcal.mol^-1 compared to 74 kcal.mol^-1 for metal-free system), while the presence of alkaline-earth metals assist in secondary reactions of LGA forming LGO (activation barrier_AEM-ions = 72 - 74 kcal.mol^-1 compared to 97 kcal.mol^-1 for metal-free system). In glucose pyrolysis, reactions leading to AGF, LGA, furfural, 1,5-anhydro-4-deoxy-D-glycero-hex-1-en-3-ulose (ADGH) and LGO formation are investigated. The presence of Ca(II) and Mg(II) ions lower the activation barriers (catalytic effect) for LGA, furfural and LGO formation, while alkali ions have anti-catalytic effect on these products. On the other hand, alkali ions have a catalytic effect on AGF formation from glucose (activation barrier_alkali-ions = 50 kcal.mol^-1 compared to 60 kcal.mol^-1 in absence of metal), whereas for ADGH formation, all metal ions have an anti-catalytic effect (activation barrier_metal-ions = 74 - 80 kcal.mol^-1 compared to 68 kcal.mol^-1 in absence of metal). This change in the catalytic effect of metal cations on pyrolysis chemistry can be attributed to the difference in metal ion and donor atom(s) interaction within the TS, which in turn alters the reaction kinetics. Thus, the atomistic insights obtained from DFT calculations help to explain the crucial role of AAEMs on pyrolysis reaction chemistry.

References

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