Quantum Chemical Study On The Mechanism Of Release Of Small Molecular Gases By Pyrolysis Of Cellulose

Biomass pyrolysis can be used to obtain clean fuels such as liquids or gases,and is a promising approach to biomass energy utilization.Cellulose is the most important component in biomass.The research on its pyrolysis mechanism can reflect the pyrolysis law of the whole material.In the past,the research on the mechanism of cellulose pyrolysis was mainly based on experiments,and there were few reports on the chemical reactions involved in the pyrolysis process of cellulose,the formation mechanism of small molecule compounds and the theoretical study of the evolution process of intermediates.Levoglucosan?six-membered heterocyclic compound?and furan compound?five-membered heterocyclic compound?account for a large proportion in the pyrolysis process of cellulose.They are an important component of secondary pyrolysis of cellulose.Moreover,as the pyrolysis temperature increases,both types of materials continue to undergo pyrolysis.Therefore,in this paper,levoglucosan and furan compounds were selected as model compounds of cellulose.The molecular model was established using the DMol³ module of Materials Studio software to study the evolution of oxygen-containing functional groups during pyrolysis.Moreover,combined with the pyrolysis experiment of levoglucosan,the law of CO and CO₂ formation was analyzed.The main research contents are as follows:First,levoglucosan was thermally cracked at different temperatures?700,750,800,850??on a microfluidized bed to analyze the formation characteristics of CO and CO₂ gas.The isothermal kinetics calculation was carried out,and the three most suitable mechanism functions were selected from 21 typical gas-solid reaction mechanism functions,namely three-dimensional diffusion equation,random nucleation?n=2/3?and reaction order?n=1?.The calculation results show that the activation energy of CO is much smaller than that of CO₂,which indicates that the pyrolysis of levoglucosan is more likely to generate CO gas.Second,levoglucosan was selected as a molding of cellulose.Twenty-three possible reaction pathways were designed and the 51 compounds involved in the pathway?including reactants,intermediates and products?and 47 transition states were quantified.The optimal production path of CO₂ is ring-opening?decarboxylation with an energy span of 301 kJ/mol.The optimal reaction pathway for CO is dehydration?alcohol-ketone tautomerization?ring-opening?decarbonylation with an energy span of 286 kJ/mol.Therefore,it is theoretically simple to produce CO from levoglucosan than to generate CO₂.Moreover,by analyzing the dehydration reaction in the path,it can be found that dehydration is beneficial to the production of CO by levoglucosan,which is not conducive to the formation of CO₂.Finally,furan,furfural,furfuryl alcohol and 5-hydroxymethylfurfural were selected as cellulose moldings,and nine possible pyrolysis reaction pathways were designed and calculated,including 21 compounds and 17 transition states.The optimal path for furan pyrolysis is that the furan ring is opened to form an aldehyde group,and then the aldehyde group is decarbonylated to form CO.The optimal route for the formation of CO by aldehyde-containing compounds such as furfural and 5-hydroxymethylfurfural is direct decarbonylation.Thermodynamic calculations show that the heat absorption of direct decarbonylation of furan species is lower than that of decarbonylation after ring opening,and the direct decarbonylation reaction becomes spontaneous with increasing temperature.The presence of a side chain functional group reduces the bond energy of C-O in the attached furan ring,making it easier to initiate a ring opening reaction from this position in the furan ring and promoting the opening of the furan ring.In this paper,by calculating the process of pyrolysis of cellulose molding to form small molecular gas,the understanding of the mechanism of secondary pyrolysis of cellulose is deepened from the molecular level,which is an important supplement and enrichment of the mechanism of cellulose pyrolysis.