Study On Reaction Mechanism Of Cellulose Pyrolysis To Produce Small Molecule Compounds

Cellulose as a renewable biomass energy,its pyrolytic small molecular compounds are highly available.In recent years,the mechanism of cellulose pyrolysis and dehydration has become one of the hottest topics in theoretical research.In this paper,we mainly focus on the theoretical research of two systems which are the dehydration and the pyrolysis of cellulose.The possible mechanism of the reaction is calculated by using density functional theory（DFT）,at the theoretical level of M062X/6-311g**,the reactants,products and transition states involved in the optimization process were determined by vibration analysis,and it is determined that there is only one virtual frequency in the structure of transition state.In order to obtained more reliable data,two methods of M062X/def2-tZVP and MP2/cc-PVTZ,were used to calculate the optimization structure.1.In the system of dehydration mechanism of cellulose,1,4-dimethoxy glucopyranose was used as the model molecule.to cyclic olefinic bond which formed by dehydration of the pyran rings’hydroxyl group with itβ-hydrogen,to ring structures which formed by dehydration between two adjacent hydroxyl groups,six possible reaction paths have been designed.In addition,the effect of water as catalyst on the activation energy of the above reaction paths was considered,and we discussed the reaction mechanism of cellulose aging dehydration from two aspects of reaction thermodynamics and kinetics.The calculated results show that the above six dehydration reactions are all exothermic,and the changes of the reactions are about-200^-260KJ/mol,and the standard Gibbs free energies all change into negative.Due to these reactions were all entropy increasing processes,the absolute value of the standard Gibbs free energy change is larger about 50KJ/mol compared with the corresponding enthalpy variation.Therefore,from the thermodynamic point of view,these dehydration reactions could occur spontaneously.Under the condition of anhydrous catalysis,the activation energies of the above dehydration reactions were calculated by M062X/def2-tZVP and MP2/cc-PVTZ methods are large about 300-360KJ/mol.The activation energies of the reactions calculated at the M062X/def2-tZVP level decrease about 70-90 KJ/mol after adding one molecule of water as catalyst,indicating that the water molecule has obvious catalytic effect on the dehydration of cellulose.The transition state structure of reaction is changed from the anhydrous four-member ring or the five-member ring structure to the six-member ring or the seven-member ring structure.The water molecules play the role of stabilizing the transition state structure,transferring protons and reducing the molecular tension during the whole reaction process.The activation energies of the formation of double bonds and the formation of furan ring are comparable.2.In the system of cellulose to small molecular compounds,arabinol was used as model molecule to study seven possible reaction channels the initial reaction of C=C double bond formation by intramolecular dehydration and the C-C bond breakage caused by intramolecular hydrogen migration then through intramolecular rearrangement,intramolecular hydrogen migration,dehydration and other reactions to decompose into small molecules such as alcohols,aldehydes and ketones.At the same time,the influence of water on the activation energy of the reaction was investigated by adding one molecule water and two molecule water as catalyst,respectively.Under the condition of anhydrous catalysis,the activation energy of each step of the seven reaction channels is relatively large,about 200 KJ/mol to 400 KJ/mol.The activation energy decreases about 70-150 KJ/mol after adding a molecule water.In the range of decrease of activation energy,the most obvious one is the enol rearrangement reaction and the activation energy decreases by about 150 KJ/mol,and the activation energy could be maximumly reduced to about 200 KJ/mol after catalyzed by two water molecules.That is to say,the catalytic effect of two molecules water was stronger than that of one molecule water,and the catalytic effect of water molecule on cellulose pyrolysis is obvious.Comparatively,the activation energy of C-C bond fracture induced by intramolecular hydrogen migration is much higher than intramolecular dehydration.Therefore,intramolecular dehydration was the main factor in the previous stage of cellulose pyrolysis,while the C-C bond breakage by hydrogen migration play a part in the later stage of pyrolysis.Furthermore,HCHO,CH₃OH,ethylene glycol and HAA（ethanolic aldehyde）are the main small compounds.