Quantum Chemical Computational Study Of Hydrothermal-catalyzed Conversion Of Glucose To Soluble Polymers

Hydrothermal carbonization is a promising thermochemical technology that can well convert sludge into hydrochar,which is a clean solid fuel.The introduction of Lewis acid and Br?nsted acid in the hydrothermal carbonization process can speed up the reaction process and reduce the generation of by-products,thus increasing the yield of hydrothermal carbon.The reaction mechanism of Lewis acid and Br?nsted acid catalyzing the conversion of small molecule carbohydrates into hydrochar is not yet clear.Since glucose is an important intermediate in hydrothermal conversion of sugars in biomass.In this study,the monosaccharide glucose will be used as a small molecule modulo the hydrothermal process of sludge,and Lewis acid CuCl₂and Br?nsted acid NH₄⁺will be selected as catalysts to investigate the reaction mechanism in the hydrothermal catalytic process of sludge using density flooding theory.The study and main findings are as follows:Firstly,glucose was isomerized to fructose catalyzed by CuCl₂,HCl,and Cl^-.three reaction pathways of CuCl₂-catalyzed glucose conversion to fructose decreased the reaction energy barrier by 6.71,1.21,and 5.36 kcal/mol compared to the corresponding non-catalyzed pathways.For the two reaction pathways of HCl and Cl^-catalyzed glucose conversion to fructose,the reaction energy barriers were reduced by46.78 and 23.19 kcal/mol,respectively,compared to the corresponding non-catalytic pathways.Next,fructose was dehydrated three times in succession under NH₄⁺to produce5-hydroxymethylfurfural（5-HMF）,which reduced the reaction energy barrier by15.14 kcal/mol compared to its corresponding non-catalytic pathway,while 5-HMF underwent multiple hydration and dehydration reactions under NH₄⁺to produce small molecules such as levulinic acid（LA）,1,2,4-benzenetriol（BTO）,2,5-dioxo-6-hexanal（DHH）and other small organic molecules,which have reduced reaction energy bases of 25.4,9.05 and 11.92 kcal/mol,respectively,compared with the corresponding non-catalytic pathway.Finally,NH₄⁺-catalyzed polymerization occurs between small molecules to produce hydrothermal carbon precursor polymers.The NH₄⁺-catalyzed polymerization of HMF and HMF by intermolecular dehydration,and the polymerization of HMF and BTO by intermolecular hydrogen transfer,reduce the reaction energy barriers by 28.04 and 0.72 kcal/mol,respectively,compared with the corresponding non-catalyzed pathways.In the conversion of glucose to hydrochar precursor polymer,the reaction energy barrier was reduced by adding CuCl₂and NH₄⁺.Cu²⁺in CuCl₂easily combined with oxygen in glucose to form a stable structure and thus promoted the reaction,while Cl^-mainly promoted the hydrogen transfer process.H⁺in NH₄⁺promoted the hydrogen transfer,dehydration and hydration reactions.In the whole catalytic process,the best catalytic step is the intermolecular hydrogen transfer in the conversion of NH₄⁺-catalyzed 5-HMF to LA compared with the non-catalytic process,and the reaction energy base is reduced by 60.58 kcal/mol,which is due to the change of the hydrogen transfer reaction from the original intra-molecular hydrogen transfer to the intermolecular hydrogen transfer.The least effective catalytic step was the intermolecular hydrogen transfer in the polymerization reaction of BTO and DHH catalyzed by NH₄⁺,and the reaction energy barrier decreased by 0.72 kcal/mol,which was due to the fact that the type of hydrogen transfer reaction did not change after the addition of NH₄⁺,and all of them were intermolecular hydrogen transfer reactions,and the number of reaction steps did not change.So NH₄⁺has the best catalytic effect for the intermolecular hydrogen transfer step.In this study,the conversion mechanism of Lewis acid CuCl₂and Br?nsted acid NH₄⁺-catalyzed hydrothermal formation of soluble polymers from glucose was thoroughly investigated at the atomic level based on quantum chemical theory and verified by comparison with experimental results,providing theoretical support for the intrinsic mechanism of sludge hydrothermal carbonization technology.