Process Characterizations And Kinetics Of Solid Acid-catalyzed Preparation Of Levulinic Acid From Rice Husk In Gamma-valerolactone/water Co-solvent

Levulinic acid is a promising platform compound that can be used in the synthesis of chemicals and pharmaceuticals,as well as an important precursor for the biofuel production.Rice husk is a common agricultural waste with a high cellulose content,making it an ideal raw material for the preparation of levulinic acid.There are still some challenges to realize the production of levulinic acid from rice husk resources,such as the complex cell-wall structure of the biomass itself as a barrier to its catalytic conversion,the complexity of the biomass fraction as a limitation to its efficient,and corrosion of the reactor and the high cost of extracting levulinic acid from the acid solution associated with the use of liquid acid as a catalyst.Hence,in view of the above problems,we proposed a"two-step"solid acid catalytic method for the preparation of levulinic acid from rice husk inγ-valerolactone/water system,that is,using the prepared solid acid to replace the conventional liquid acid catalyst,to destroy the recalcitrant structure of the rice husk and to separate the hemicellulose and lignin from the rice husk,then,the solid residue（cellulose from the rice husk and catalyst）was collected and continued to be converted to levulinic acid using the solid acid.At the same time,the mechanism and kinetics of levulinic acid preparation from rice husk were studied.The results are as follows.（1）A series of solid acids were synthesized by hydrothermal method using P123 surfactant as template agent,tetraethyl orthosilicate as silicon source and 3-mercaptopropyltrimethoxysilane as sulfonic acid precursor.The solid acid CH₃-SBA-15-SO₃H has a two-dimensional ordered mesoporous structure with an average pore size and specific surface area of 4.75 nm and 587.16 m²·g^-1respectively,and the solid acid is loaded with 0.65 mmol·g^-1 of sulfonic acid groups.The increase of the aging temperature of the solid acid CH₃-SBA-15-SO₃H is beneficial to increase the polycondensation degree of the silicon carrier and improve the stability of the solid acid.However,the aging temperature should not exceed 180°C,and the template decomposes at high temperature,resulting in a partially disordered silicon carrier of the solid acid and the decline of its stability.The high polycondensation degree of silicon carrier and hydrophobic group of the solid acid CH₃-SBA-15-SO₃H results in superior stability compared to the solid acid 100-SBA-15-SO₃H prepared by conventional methods,and the solid acid CH₃-SBA-15-SO₃H is stable inγ-valerolactone/water co-solvent at 180°C for more than 12 h without the loss of sulfonic acid group,the collapse of the ordered structure and the change rate of specific surface area was 5.45%.Excellent solvent thermal stability and mesoporous structure conducive to mass transfer make solid acids have high potential application value in the field of acid catalyzed biomass conversion.（2）The solid acid CH₃-SBA-15-SO₃H was used as a substitute for catalysts such as ionic liquids or inorganic acids in the pre-treatment process in aγ-valerolactone/water co-solvent system to greatly retain the cellulose in the rice husk while removing the hemicellulose and lignin.The higher the pretreatment temperature,the better the removal of hemicellulose and lignin from the rice husk,but more cellulose is lost when the temperature exceeds 150°C.The higher the content ofγ-valerolactone,the more beneficial to the removal of lignin,but not more than 70%（v/v）,the reduction of water in the system is not conducive to the hydrolysis of hemicellulose,resulting in the depolymerization and dissolution of lignin affected.The Br（?）nsted acid provided by the solid acid catalyst can effectively hydrolyze the glycosidic bond in hemicellulose,break the aryl etherβ-O-4 bond in lignin and the covalent bond linking hemicellulose to lignin,so that the dense structure of the pretreated rice husk is destroyed,and the crystallinity of cellulose in rice husk is reduced,thus facilitating the catalytic conversion of cellulose to levulinic acid.The removal rates of lignin,hemicellulose and cellulose were 100%,84%and 4.8%respectively,and the removal rates of hemicellulose and lignin from rice husk in the system with the catalyst were 17.4and 8.6 times higher than in the system without the catalyst,respectively.（3）The solid acid CH₃-SBA-15-SO₃H was used to catalyze the preparation of levulinic acid from glucose,a model compound of rice husk.The formation mechanism and kinetics of levulinic acid were studied while the solvent system for levulinic acid preparation was optimized.The yield of levulinic acid in theγ-valerolactone/water co-solvent system was five times higher than that in the pure water system when the reaction was carried out at 160°C for 120 min under the same conditions.Theγ-valerolactone/water co-solvent system favoured the production of levulinic acid from glucose than pure water system.The addition ofγ-valerolactone solvent not only reduces the competitiveness of hydrogen ions between glucose and water molecules,also induces a break in the C2-O2 bond of glucose and directly dehydrate into 5-hydroxymethylfurfural rather than isomerisation to fructose due to the solvent-induced energy barrier of glucose reduction reaction.In the co-solvent system,the kinetic analysis of the production of levulinic acid from glucose with the range of 140℃to 180℃showed,that the process conformed to the first-order reaction kinetics,the activation energies for the dehydration of glucose to 5-hydroxymethylfurfural and rehydration of 5-hydroxymethylfurfural to levulinic acid were 73.73 k J·mol^-1 and 64.53 k J·mol^-1,respectively.Theγ-valerolactone/water co-solvent system using a recyclable solid acid catalyst achieved a high yield of levulinic acid in a short reaction time and low activation energies for the main reactions,indicating that the system is favourable for the catalytic conversion of biomass to levulinic acid.（4）A"two-step"method for the preparation of levulinic acid from rice husk catalyzed by solid acids is proposed,the first step is to greatly retain the cellulose in the rice husk while removing the hemicellulose and lignin;The second step is to convert the remaining residue of the first step into levulinic acid.The maximum yield of levulinic acid from rice husk using the"two-step"process was 65.89%,more than 1.77 times that of the"one-step"process.The rate constants of the reactions in the preparation of levulinic acid from rice husk were solved based on the four differential equations,indicating that increasing the temperature was favourable to the production of levulinic acid,but the increase in the rate constant for the conversion of 5-hydroxymethylfurfural to levulinic acid at a reaction temperature of 200°C was less than the increase in the rate constant for the side reaction,instead of increasing the yield of levulinic acid.The activation energies for the formation of sugars,5-hydroxymethylfurfural and levulinic acid in the system were 97.52 k J·mol^-1,110.64 k J·mol^-1 and 51.26 k J·mol^-1,respectively.The"two-step"method has lower activation energies for the main reactions and gives a higher yield of levulinic acid in a shorter time compared to the"one-step"method.The"two-step"method for the preparation of levulinic acid from lignocellulosic biomass catalyzed by solid acids in aγ-valerolactone/water co-solvent system has promising applications.