Study On Preparation Of Core-shell Double Acid Catalyst And The Reaction Of Furfural From Xylose

Furfural is one of the most potential value-added chemicals among furan derivatives,which is often used in the synthesis of furfuryl alcohol,tetrahydrofuran and nylon 66.Compared with other processes,the preparation of furfural from xylose is considered to have great application potential because of its abundant raw materials,short path and simple operation.In recent years,researchers began to develop supported solid acid catalysts to replace corrosion-prone equipment,difficult recovery and environmental pollution of liquid acid catalysts.However,supported catalysts still have problems such as easy loss of active sites,low catalytic efficiency and poor recycling.In addition,for the multistep reaction of furfural preparation from xylose,the synergistic catalysis of double acid sites is required to improve the selectivity of furfural.The core-shell structure provides a strategy to fully exploit the synergistic effect of the two acid sites.Therefore,in this paper,core-shell double acid catalyst was designed and prepared from the view of high activity,high stability and high synergistic effect,and the structure-activity relationship between core-shell structure and catalytic performance was revealed.The research content and results of this paper are as follows:In the second chapter,WO₃@mSiO₂-SO₃H catalyst was synthesized by sol-gel method.The core WO₃ provided Lewis acid active sites and the shell mSiO₂-SO₃H provided Br（?）nsted acid sites.It was used to synthesize furfural from xylose,and under the optimal reaction conditions（0.05 g catalyst,0.1 g xylose,5 m L toluene,2.5 m L water,170℃,4 h,500 r/min）,the conversion of xylose was 98.7%and the furfural yield was70.1%.An interface with internal and external spatial relationships was constructed by the core-shell structure,improving the synergistic effect of the two acid sites and reducing the repeated diffusion of intermediates xylulose between active sites.Compared with supported catalysts,the catalytic efficiency increased from 17.7-23.3mmol L^-1 h^-1.In addition,the catalyst was used to supplement Br（?）nsted acid sites with H₂SO₄ impregnation.After five cycles,the furfural yield remained at 67.1%.In order to solve the problems in chapter 2 that the Br（?）nsted/Lewis of catalyst was not easy to adjust and the sulfonation method was complex,the third chapter designed and synthesized the mZSM-5@SO₄^2-/mSiO₂ core-shell catalyst by changing the kernel and sulfonation method.The effect of Br（?）nsted/Lewis on the properties of furfural was studied in detail by using core-shell structure coupled with"top-down"alkali etching method and direct acid introduction method.The mZSM-5@SO₄^2-/mSiO₂ double acid core-shell catalyst was used to synthesize furfural from xylose under the reaction conditions:0.05 g catalyst,0.1 g xylose,5 m L dichloromethane,2.5 m L water,140℃,1 h,100 r/min.The conversion of xylose was 95.8%and the furfural yield was 72.1%.The results showed that the catalytic effect was great when the catalyst Br（?）nsted/Lewis=0.142,because the Br（?）nsted/Lewis too high or too low would reduce the selectivity of furfural,resulting in the condensation of furfural with intermediate products or reactants to produce humus and other byproducts.In addition,the reaction path and possible reaction mechanism were studied by in-situ infrared characterization.It was proved that xylose was firstly isomerized to xylulose under the action of Lewis acid and then was dehydrated to furfural under the action of Br（?）nsted acid.In order to improve the acid density of the catalyst and reduce the use of sulfonating agents such as sulfuric acid,the mZSM-5@SO₄^2-/mC-xN core-shell catalyst with SO₄^2-/mC-xN coating mZSM-5 was designed and prepared in chapter 4 on the basis of chapter3,using phenolic resin as carbon source.The effect of nitric acid pretreatment on the structure and performance of core-shell catalyst was systematically studied,and the relationship between oxygen-containing functional groups and catalytic performance was discussed.The conversion of xylose was 97.4%and the furfural yield was 75.4%under the reaction conditions of 0.04 g catalyst,0.1 g xylose,5 m L methylene chloride,2.5 m L water,140℃,1 h,500 r/min.The-OH and-COOH groups on the surface of carbon shell were increased after pretreatment with nitric acid,which not only provided a number of BA sites,but also improved the hydrophilicity of materials and increased the amount of-SO₃H groups on the catalyst surface.In addition,the improved hydrophilicity of catalysts was conducive to the adsorption of catalysts with the xylose in the aqueous phase,avoiding the over reaction of furfural in the organic phase,and thus improving the selectivity of furfural.The reaction kinetics calculation showed that the pretreatment with HNO₃ could significantly reduce the reaction activation energy of55.42 k J/mol,which was benefical to improve the catalytic activity.In summary,three catalysts,WO₃@mSiO₂-SO₃H,mZSM-5@SO₄^2-/mSiO₂ and mZSM-5@SO₄^2-/mC-xN,were prepared respectively in this paper for the reaction of furfural from xylose.The physicochemical properties and structure-activity relationship of core-shell catalysts were studied.The effects of B-L biacid interface,B/L and oxygen-containing functional groups of core-shell catalysts on furfural yield were revealed,and the reaction mechanism of core-shell catalysts for preparing furfural from xylose was described.This study provided a broad prospect for the application of core-shell catalyst in the synthesis of furfural from xylose.