High-value Utilization Of Wheat Stalk Based On Acidic Ionic Liquids

With the continuous increase of the global population,the resource crisis and environmental impacts have become the prominent issues to be solved.Therefore,according to the concept of green and sustainable development,finding renewable resources and realizing their efficient and clean utilization have become an urgent task in the academia society.Among various resources,biomass is the only renewable organic carbon resource in nature,which has unique advantages in the production of value-added chemicals.Lignocellulosic biomass,such as crop straw,is mainly composed of cellulose（38%-50%）,hemicellulose（23%-32%）,and lignin（15%-30%）.As a large agricultural country,China possesses about 30%of crop straw output in the world.However,it has been in the state of resource utilization or low value utilization of a single component for a long time,resulting in a huge waste and environmental pollution caused by the destruction and discarding of other components.In view of this,the article takes wheat stalk as the research object and uses single or synergistic acidic ionic liquids as catalysts to construct a catalytic system for the pretreatment of straw and the efficient conversion of each component,which can improve the utilization of the whole components in biomass,obtain value-added chemicals such as ethyl levulinate（EL）,diethyl maleate（DEM）and aromatic monomers,and propose a possible reaction pathway.The specific research contents are as follows:（1）A green and economic ammonium-based Br（?）nsted acidic ionic liquid（BAIL）[BSTEA]HSO₄ was synthesized using triethylamine as a cation source,and then used as a catalyst for the extraction of organosolv lignin to achieve lignin from wheat stalk.Compared with other BAILs（[BSMIM]HSO₄ and[BSPy]HSO₄）,using N-methylimidazole and pyridine as cation sources,lignin with high extraction yield（23.72%）,good purity（97.23%）and excellent structure was obtained through[BSTEA]HSO₄.Meanwhile,the organosolv lignin extraction process usually produces a large amount of waste liquid,which is rich in carbohydrates,mainly oligosaccharide.The unreasonable discharge of the waste liquid will not only pollute the environment,but also lead to low utilization of biomass.In this study,a[BSTEA]HSO₄-USY（W/W,2/3）synergetic catalyst was designed using USY zeolite as a Lewis acid sites donor,and used for the conversion of carbohydrate-rich waste liquid under optimized conditions（50 m L ethanol,200 ^oC,60 min）,obtaining a yield of 30.23%EL.The reusability of the catalyst was also explored.Under optimal conditions,the yields of lignin and EL can respectively accomplish 83.89%and 72.28%of that catalyzed by fresh catalyst after five cycles.In short,the above methods present a green,economic and efficient route for the extraction of lignin and further treatment of liquid waste generated during the extraction process.（2）Lignin,the second largest component of biomass,is considered as an important alternative source of fossil resources for the production of fuels and chemicals.In this study,a method was developed to produce C₄ esters such as DEM by selective oxidative cleavage of lignin aromatic units in the synergetic catalytic system composed of Br（?）nsted-Lewis mixed acidic ionic liquids.The experimental results showed that under optimized conditions（1.00 MPa O₂,20 m L ethanol,160 ^oC,5 h,[BSMIM]HSO₄-[BMIM]Fe₂Cl₇（1/3,mol/mol））,lignin conversion was 73.50%,and C₄ esters were the major products,in which the yield and selectivity of DEM reached 15.85%and 44.25%,respectively.The structure and composition analysis of lignin residues and liquid products confirmed that the aromatic units in lignin were effectively and selectively oxidized.Further catalytic oxidation of other types of lignin was carried out,verifying the universal applicability of the synergistic catalytic method for lignin.Furthermore,the catalytic oxidation of lignin model compounds was explored for obtaining a possible reaction pathway of oxidative cleavage of lignin aromatic units to DEM.This study provides a promising alternative method for the production of traditional petroleum-based chemicals.（3）Lignin is an amorphous aromatic polymer,mainly composed of three phenylpropyl-guaiacyl,syringyl,and p-hydroxyphenyl,so it is an important raw material for preparing biofuels and value-added aromatic monomers.In this research,a phosphomolybdic acid-ionic liquid（PMA-IL）[BSHMIM]H₂PMo₁₂O₄₀was synthesized and used as a catalyst for further oxidative depolymerization of lignin extracted by[BSTEA]HSO₄ to prepare valuable chemicals like vanillin,vanillic acid and methyl vanillate.Under the optimized reaction conditions(180 ^oC,60 min,1.0 MPa O₂,1 mmol[BSHMIM]H₂PMo₁₂O₄₀,30 m L methanol aqueous solution(V_methanol/V_water,4/1)),the conversion of lignin reached 61.33%,and the total yield of the target products（vanillin,vanillic acid,and methyl vanillate）was 28.00 mg·g^-1,of which the selectivity of methyl vanillate reached 56.43%（based on the target products）.The liquid products and residues obtained under the optimum conditions were analyzed,further confirming the advantages of PMA-IL compared with single PMA catalyst.In addition,this method also has excellent catalytic effect for other types of lignin and has very good substrate applicability.Finally,based on the research results of model compounds,the possible reaction pathway of the formation of vanillin,vanillic acid and methyl vanillate was proposed.