Density Functional Theory Study On The Degradation And Conversion Mechanism Of Cellulose Model Under Subcritical Hydrothermal Condition

As the most widely distributed and most productive biomass resource,it is of great significance to convert cellulose into high value-added chemicals to replace fossil fuels or industrial chemicals.Due to the high structural similarity of C-C and C-OH bonds in cellulose,the same chemical bonds in cellulose are prone to similar chemical reactions under the acid-base dual-function catalysis of subcritical water,resulting in a variety of complex products with low selectivity.In order to reveal the essential driving force of chemical bond fracture and clarify the formation mechanism and formation competition mechanism of different products under acid-base dual catalysis,glucose,the basic structural unit of cellulose,was selected as the cellulose model and dispersion modified density functional theory was adopted.With cellulose,glucose,subcritical water thermal degradation experiment measured the product of fructose,glucose,5-hydroxymethyl furfural,levulinic acid,and lactic acid,1,6-anhydroglucose,furfural,1,2,4-Benzenetriol,erythrose,glyceraldehyde,pyruvaldehyde,dihydroxyacetone and glycolaldehyde as the goal,building a network of hydrothermal degradation of cellulose research subcritical water hydrothermal degradation reaction mechanism of cellulose.（1）Based on the dominant solvent model,the reaction mechanisms and their activation energies for the degradation and conversion of glucose to 1,6-anhydroglucose,fructose,5-HMF,levulinic acid,1,2,4-benzenetriol,lactic acid,furfural and erythrose were calculated,and a model for the degradation and conversion of cellulose in neutral subcritical water described by the dominant solvent model was established.which predicts the protonation of glucose O（5）site first by the Fukui function,which readily leads to the formation of1,6-dehydrated glucose or fructose with similar apparent activation energies（29.441 vs29.305 kcal/mol）.The cyclic pathway through the dehydration of fructose to 5-HMF is more favorable than the acyclic pathway through the dehydration of glucose to prepare 5-HMF with an apparent activation energy of 12.537 kcal/mol.5-MF to levulinic acid requires an apparent activation energy of 33.321 kcal/mol and the reaction rate is limited by numerous reaction steps.the formation of 1,2,4-Benzenetriol requires a high apparent activation energy of76.682 kcal/mol.The inverse hydroxyl aldol condensation of C4 compounds is more likely to produce C2 compounds.The inverse hydroxyl aldol condensation of the open-chain C6intermediate produces C2,C3 and C4 compounds.（2）Based on the dominant solvent model,the reaction mechanism of glucose degradation and conversion products and their activation energies were calculated,and a model of cellulose degradation and conversion reaction in neutral subcritical water described by the dominant solvent model was established.glucose undergoes a series of ketoaldol-enol isomerization and enol-ketoaldol isomerization reactions with the assistance of solvent H₂O molecules to generate a variety of open-chain C6 intermediates.Open-chain fructose G6preferentially generates-fructose F with an activation energy of 27.86 kcal/mol.the1,6-anhydroglucose generation reaction has an activation energy of 34.14 kcal/mol.the open-chain glucose G2 generates the 5-HMF pathway thermodynamically more favorably compared to the dehydration of fructose F to prepare 5-HMF with an activation energy of53.53 kcal/mol.Due to the stability of the conjugated large bond structure of 5-HMF,the hydration reaction of 5-HMF remains a rate-controlling step for the levulinic acid and1,2,4-Benzenetriol generation reactions.the C6 intermediate,C4 intermediate is more inclined to undergo an acidic inverse hydroxyl aldol condensation reaction.（3）Based on the dominant solvent model,the reaction mechanism of glucose degradation conversion products and their activation energies were calculated,and a model of hydrothermal degradation reaction of cellulose in acidic subcritical water described by the dominant solvent model was established.the presence of H⁺decreases the activation energy of each radical reaction during the isomerization of glucose,the 1,6-anhydroglucose generation reaction,the generation of lactic acid from C3 compounds,the isomerization of ethanolic aldehydes,and the acidic inverse hydroxyl aldol condensation reaction of C4 and C6 compounds to different degrees.In contrast to neutral conditions,the rate control step for the preparation of 5-HMF by dehydration of fructose under acidic conditions is the first dehydration.the presence of H⁺reduces the activation energy of the 5-HMF hydration reaction,but the hydration reaction is still the rate control step for the levulinic acid generation reaction.some of the radical reactions during the 1,2,4-Benzenetriol generation reaction are inhibited to varying degrees by the presence of H⁺,resulting in an increase in the corresponding activation energy.The essential effect of H⁺（Bronsted acid catalyst）on each radical reaction during the hydrothermal degradation reaction of cellulose is explained in detail.