Liquefaction of wheat straw catalyzed by acidic ionic liquid and analysis of liquefied products

Abstract: With the increasing energy requirements and pollution problems worldwide, energy from renewable resources has received global attention in recent decades. Biomass is one of the most abundant renewable resources. It mainly includes forest residues, agricultural wastes, industrial residues, municipal solid wastes, bagasse, aquatic plants, and algae animal wastes. Due to the advantages of abundance, non-polluting, being renewable and easy to obtain, biomass is considered as the most promising energy feedstock to replace the traditional energy. Meanwhile, it is the only resource that can be converted into solid, liquid, and gaseous products for use of fuels. Thermo-chemical conversion is an effective technology of biomass conversion. Liquefaction is the typical thermo-chemical technology for the conversion of biomass to obtain liquid biofuels and valuable chemicals, such as bio-oil and fuel additives. The conversion process is not only influenced by organic reagents but also by different catalysts. Effective catalyst is an essential factor to improve liquefaction efficiency. As catalyst, sulfuric acid has very strong corrosion and needs high-quality liquefaction equipment, and its recycling is difficult. Solid acid is used to overcome shortcomings of organic acid in the liquefaction, but it is easy to form coke to cause the deactivation of catalyst, and needs to be calcined before re-use, which increases the reaction cost. Now, it is found that the ionic liquid has non-corrosiveness, low melting point, high thermal stability and low vapor pressure, and some other merits. It has a broad application prospect used as solvents and catalysts in catalytic reactions. Ionic liquid, especially sulfonated bisulfate ionic liquid can dissolve cellulose, because it has higher acidic sites that can easily break the hydrogen bonds of biomass material, and promote the degradation and conversion of cellulose. At present, ionic liquid is usually used to catalyze carbohydrate, which is changed into 5-hydroxymethylfurfural (HMF). The report about producing levulinate from agriculture waste catalyzed by ionic liquids is rare. Therefore, we choose wheat straw as raw material and ionic liquids as catalyst in the liquefaction process. In this study, 1-methyl-3-(4-Sulfobutyl)-imidazolium hydrosulfate is synthesized and used as catalyst for the liquefaction of wheat straw in ethanol. Fourier transform infrared spectrometer (FT-IR), nuclear magnetic resonance carbon-13 spectrum (13C NMR), thermo gravimetric analyzer (TG) and gas chromatography-mass spectrometry (GC-MS) characterized the structure of ionic liquid and liquid products. The synthesized ionic liquid is confirmed to be 1-methyl-3-(4-Sulfobutyl)-imidazolium hydrosulfate. Results of the experiments show that the ionic liquid has the optimum catalytic properties for the liquefaction of wheat straw. Under the conditions that wheat the straw mass is 5 g, the mass fraction of catalyst is 26%, the reaction temperature is 200 ℃, and the reaction time is 60 min, a high conversion rate of 85.5% is obtained; under the conditions, the yield of ethyl levulinate is 9.97%, and the relative percentage content of ethyl levulinate is 29.9% in liquid products. The liquid products include aldehydes, ketones, esters, carboxylic acids, phenols and other oxygenated chemicals; among them, phenols are from the degradation of lignin mainly, and the other compounds are from the degradation of hemicellulose and cellulose principally. The results can provide theoretical basis for the development and utilization of low corrosive and environment-friendly catalysts, which will be used on liquefaction to prepare high grade chemicals.