Effect of solvent on the solvolysis liquefaction of sawdust with phosphotungstic acid under supercritical condition

Phosphotungstic acid is an efficient, green and highly acidic heterogeneous catalyst. Different from other solid heteropholy acids, phosphotungstic acid is dissoluble in alcohols. This research makes the best use of the physicochemical properties of phosphotungstic acid combining with high solubility of supercritical alcohols. Sawdust was liquefied in the supercritical solvent using phosphotungstic acid as catalyst. The effects of different solvents on the solvolysis liquefaction and the compositions of bio-oil were investigated. Each solvolysis experiment was conducted at 260℃ for 30min in a stainless-steel autoclave, in which 1g of fir sawdust, 0.5g of phosphotungsic acid, 150g of alcohol (such as methanol, ethanol, n-propanol or iso-propanol) were added. The liquefaction products were separated by filtration after quenching the reaction. Then, the filtrate was extracted with n-hexane and separated into light bio-oil and heavy bio-oil after removing the solvent. Simultaneously, the residue, heavy bio-oil and light bio-oil were characterized by Fourier transform infrared (FT-IR) and Agilent 7890A/5975C gas chromatography-mass spectrometry (GC-MS). The results show that the reaction pressure and polarity of alcoholic solvents significantly impact the liquefaction efficiency and liquefaction products. The liquefaction yield using methanol, ethanol, n-propanol and iso-propanol is 54.75%, 90.29%, 85.90% and 89.15%, respectively, while the relative content of main compound esters in liquefaction products is 43.759%, 23.531%, 41.761% and 28.619%, respectively. Especially, the relative content of methyl levulinate in methanol system is 33.374%. The main compounds in liquefaction products using methanol, ethanol, iso-propanol as solvent are esters and phenols, while n-propanol system are esters, ketones and alcohols. Meanwhile, levulinate erster, which is produced through the reaction of cellulose/hemicellulose and alcohols, is detected in the liquefaction products of all four solvents. Phenols is determined in the liquefaction products in iso-propanol system with relative content of 24.342%. The aldehyde compounds only exist in methanol system. The absence of phenols in n-propanol system indicates that its weakest polarity provides less hydrogen radical and may lead to difficult degradation of lignin.