Quantum Chemical Computational Study On Hydrothermal Conversion Of Glucose To Soluble Polymers

Sewage sludge has high moisture content and contains a large amount of biorefractory substances and heavy metals and other harmful substances,which are harmful to human body and the environment.Hydrothermal carbonization has attracted much attention as a new treatment method for converting biomass into biochar by wet process.Sewage sludge contains a large amount of carbohydrates,proteins and other organic components,of which carbohydrates are the main components for the formation of hydrochar.Therefore,it is of great significance to study the transformation mechanism of carbohydrates in sewage sludge.Most of the current research is mainly based on experiments,and the theoretical calculation of the conversion mechanism of small molecular carbohydrates from a microscopic perspective is not complete.Therefore,in this paper,glucose was selected as the model of carbohydrate hydrolysate in sludge,and based on density functional theory,quantum chemical calculation was carried out to study the mechanism of hydrothermal conversion of glucose into soluble polymer（hydrochar precursor）.The main research contents are as follows:First,the Condensed Fukui Function（CFF）and the Condensed Dual Descriptor（CDD）were calculated for glucose,and it was clear that the reactive site of glucose was the O atom on the six-membered ring,and then the subsequent reaction calculation was carried out purposefully.In a hydrothermal environment,the C-O bond on the six-membered ring of glucose is first broken,and fructose is formed through ring-opening,isomerization,and ring-closing reactions.There are two different reaction paths in this stage,and the four step reaction is the optimal path（energy span is83.12 kcal/mol）.Next,fructose undergoes three dehydration reactions to form 5-hydroxymethylfurfural（HMF）,where the first dehydration reaction has the highest energy barrier（59.59 kcal/mol）.As an important intermediate product,HMF plays an important role in the subsequent transformation and polymerization.Next,HMF can be converted into a variety of small organic molecules:formic acid（FA）,levulinic acid（LA）,1,2,4-benzenetriol（BTO）,2,5-dioxo-6-hexanal（DHH）.HMF forms FA and LA through multiple hydration,dehydration,cracking,and isomerization reactions.The reaction is divided into three stages:（1）HMF due to the existence of the conjugatedπbond on the furan ring makes the reaction energy barrier higher in the hydration and dehydration reaction stages（75.38 kcal/mol）;（2）The chain structure hydration and dehydration stage after five-membered ring cleavage The reaction energy barrier was lowered（43.60 kcal/mol）;（3）LA was rapidly exothermic from the chain structure after FA was dissociated（35.64 kcal/mol）.Likewise,HMF undergoes multiple hydration and dehydration reactions to form BTO and DHH,which are exothermic and endothermic reactions,respectively.Finally,these small organic molecules are polymerized to form soluble polymers,which are important components of hydrochar precursors.The reactive sites were predicted by calculating the CFF and CDD values of HMF,DHH,BTO molecules.Then,the formation paths and structures of six soluble polymers were calculated:（1）C3 of HMF₁ forms a bond with C6 of HMF₂,and forms a soluble polymer through intermolecular dehydration,and the energy barrier is 75.52kcal/mol;（2）DHH is isomerized to form four kinds of alcohols containing both C=O and C=C bond structures.C1 forms bonds with C1,C3,C4,and C6 of four alcohols,respectively,and forms four soluble polymers through hydrogen transfer/intermolecular dehydration,and the reaction energy barriers were 72.33 kcal/mol,60.73 kcal/mol,68.62 kca/mol and 65.47 kcal/mol,respectively;（3）C3 of HMF forms bonds with C4 of BTO,and forms soluble polymers through hydrogen transfer.thing.Among them,HMF is the most easily polymerized with 2-3 enol,and its reaction energy barrier is the lowest（60.73 kcal/mol）.Water not only acts as a solvent and reactant in this process,but also catalyzes the reaction.Explicit water molecules were added to the system,and the results showed that water molecules catalyzed the entire reaction stage by assisting the hydrothermal conversion of glucose and the hydrogen transfer process of each step of DHH isomerization.The addition of explicit water molecules significantly reduced the chemical reaction energy barrier（10.38～52.31 kcal/mol）,which promoted the process of hydrothermal carbonization.In this study,the conversion mechanism of glucose hydrothermally forming soluble polymers was clearly stated at the atomic level,and the comparison and verification were carried out with the experimental results,which provided a certain theoretical support for the research on the hydrothermal conversion of sludge into hydrochar.