Carbon Based Solid Acid Catalyzed Targeted Conversion Of Glucose To Methyl Levulinate

In the face of the inevitable depletion of fossil resources and the deterioration of the climate,the use of modern science and technology to produce clean and renewable energy is one of the effective ways to solve these problems.As nature’s only renewable carbon resource,biomass is a very promising energy feedstock that can be catalytically converted into a range of high value-added products.Among them,levulinic acid esters have received wide and extensive attention due to their good reactivity and economic value.As an important chemical,methyl levulinate can be widely used in pharmaceuticals,coatings,flavours,fuel additives and other industries,and can also be used as a platform compound to prepare fine chemicals with high added value such asγ-valerolactone.Currently,the targeted catalytic alcoholysis of biomass to methyl levulinate is one of the most promising synthetic routes.In this paper,we designed and synthesized a variety of bifunctional carbon-based solid acid catalysts for the conversion of biomass and its carbohydrates into methyl levulinate by alcoholysis.By studying the conversion pattern and bond-breaking mechanism of the intermediate compounds in the conversion process,we reveal the process mechanism of the targeted alcoholysis of biomass carbohydrates and provide some theoretical basis and application basis for the high value-added utilization of forest biomass.The main research components and findings are summarised below:（1）A series of bifunctional carbon-based solid acid catalysts were prepared by high temperature carbonisation,hydrothermal sulfonation and metal salt impregnation using cellulose as the carbon material precursor for the preparation of methyl levulinate by alcoholysis of glucose.The structure and properties of the prepared catalysts were characterized by XRD,Raman,SEM-EDX,BET,FT-IR and XPS,and the total acid density of the catalysts was determined by acid-base titration.The results showed that the sulfonic acid group was successfully loaded on the carbon carrier by hydrothermal sulfonation and the metal Cr was uniformly dispersed on the catalyst,while the catalyst 300-AC-Cr-SO₃H had a suitable pore size（4.79 nm）,specific surface area(56.7 m²·g^-1)and acidity（1.04 mmol/g）.Examination of the experimental factors such as catalyst preparation conditions,catalyst dosage,reaction temperature and time on the process of targeted alcoholysis of glucose revealed that the conversion of glucose was 99.6%and the total yield of methyl levulinate and levulinic acid was 43.1%at a dosage of 300-AC-Cr-SO₃H of 0.15g,a reaction temperature of 200℃and a reaction time of 5 h.In addition the catalyst maintained good stability after three cycles.（2）In order to enhance the reaction activity of the catalysts,the effect of different carriers on the catalyst performance was investigated.Chitosan was used as the raw material,impregnated with metal salts and then carbonised at high temperature to obtain metal-loaded carbon-based materials,which were hydrothermally sulfonated to obtain metal-loaded bifunctional solid acid catalysts.The prepared chitosan-based solid acid catalysts were characterised physically and structurally to study the physical phase,degree of graphitization,surface functional groups,morphology,structural properties and chemical valence states of each element of the catalysts,while they were used for the The targeted conversion of glucose to methyl levulinate was investigated and the mechanism of the conversion process was studied.The effect of calcination temperature and metal loading on the performance of the catalysts was investigated,and it was found that 800-ACN-Al-SO₃H had the best catalytic performance,with a yield of 48.7%of levulinic acid and methyl levulinate at 200℃and 5 h reaction conditions.The conversion of other biomass carbohydrates（fructose,cellulose disaccharide and sucrose）under the action of the catalyst 800-ACN-Al-SO₃H was also investigated and found to have excellent catalytic activity,and the catalyst maintained good performance after 5 cycles.（3）In order to achieve efficient separation and recovery of catalysts,the introduction of magnetic nanoparticles into carbon materials is considered to be an effective strategy.Therefore,firstly,chitosan and Fe Cl₃·6H₂O were mixed evenly by magnetic stirring and ultrasonic means,and then carbonized at high temperature under nitrogen protection,and then prepared a series of bifunctional magnetic carbon-based solid acid catalysts for fiber disoglycolysis by hydrothermal sulfonation.The catalysts were characterized by XPS,Raman,FT-IR,N₂ adsorption and desorption,XRD and acid-base titration.The results showed that the acidity（1.65 mmol/g）and specific surface area(85.3 m² g^-1)of the catalyst 800-ACN-Fe-SO₃H were the highest,and the catalytic performance was the best.Under the optimal reaction conditions（catalyst dosage 0.20g,reaction temperature 200℃,reaction time 5 h）,the conversion rate of cellobiose was 99.8%,and the yield of levulinic acid and methyl levulinic acid was 77.7%.The cyclic experiments showed that the catalyst had good stability.In addition,the reaction mechanism of Cellobiose alcohol hydrolysis was preliminatively explored.The cyclic experiments showed that the catalyst had good stability,and in addition,the reaction mechanism of cellobiose alcoholysis was initially explored,in which the L acid trivalent iron ions favoured the conversion of glucose to fructose isomerisation reactions,and the B acid sulfonate group facilitates subsequent reactions such as dehydration.