Study On The Mechanism Of Holocellulose Pyrolysis Catalyzed By Acidic Catalysts

With the decline of fossil energy reserves and the continuous deterioration of climate and environment,the substitution of traditional fossil resources with renewable biomass resources has become one of the important research directions,and has attracted widespread attention from scholars at home and abroad.Biomass pyrolysis liquefaction technology can selectively convert biomass into high value-added chemicals.Due to the low content of target products in the bio-oil obtained by conventional fast pyrolysis and the difficulty in purification,the use of acidic catalysts to regulate the production of target products has become one of the mainstream research directions.Acidic catalysts that catalyze the pyrolysis of biomass can either come from the primary pyrolysis of biomass or be added externally.In this study,density functional theory(DFT)was used to study the secondary interaction mechanism and ZnCl2 catalytic mechanism during the pyrolysis of holocellulose.The specific research details are as follows:(1)Biomass pyrolysis involves complex primary and secondary decomposition reactions,which together determine the final pyrolytic product distribution.Carboxylic acids produced by primary pyrolysis have significant influence on the secondary reactions,whereas the specific manifestation has not been determined.In this work,acetic acid(AA)and levoglucosan(LG)were selected as the research objects to explore the influence of carboxylic acids on secondary pyrolysis,considering AA is usually the most abundant carboxylic acid product,while LG is the major depolymerization product of cellulose as well as the representative of polyhydroxy compounds.The interaction mechanism between AA and LG were investigated by DFT calculation.The results indicate that there are four types of important interaction reactions between AA and LG,namely esterification reactions,organic redox reactions,AA-catalyzed LG dehydration reactions,LG-catalyzed AA decomposition reactions.These interaction reactions are more competitive than the unimolecular decomposition reactions of AA and LG.Moreover,AA-catalyzed dehydration reactions dominate the interaction reactions.(2)ZnCl2 can catalyze the pyrolysis of holocellulose to selectively produce furfural(FF)with an excellent catalytic effect,but the specific manifestation is unknown.Considering that xylan is one of the typical components of hemicellulose,β-D-xylopyranose was selected as the model compound of hemicellulose to study the formation mechanism of FF under the non-catalytic condition and ZnCl2 catalytic condition.The results indicate that β-D-xylopyranose can generate FF through the acyclic D-xylose decomposition mechanism and the oxygen ring intermediates decomposition mechanism,which involve 34 reaction pathways.The catalytic action of ZnCl2 can reduce the reaction energy barriers of most elementary reactions(except for a couple of reactions)in the above paths,especially the dehydration reactions where ZnCl2 exists in the reaction center.In addition,the catalytic action of ZnCl2 can also change the type and the competitiveness of the speed-determining step in some pathways.However,under the ZnCl2 catalytic condition,the advantageous FF formation pathways remain unchanged,as well as their speed-determining steps,which indicate that β-D-xylopyranose always tends to undergo the ring-opening reaction to generate acyclic D-xylose.Subsequently,acyclic D-xylose preferentially generates xylulose through the isomerization,while xylulose tends to undergo different acetal reactions to generate two important hemiacetal products h-6 and h-7.The former can undergo the pinacol rearrangement and two 1,2-dehydration reactions on the furan ring to generate FF,while the latter can undergo the 1,2-dehydration at 2-OH+1-H site,the electrocyclic dehydration and the 1,2-dehydration at 4-OH+5-H site to generate FF or undergo the reaction process similar to h-6 to form FF.