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Effects of reaction conditions (temperature, retention time, and cellulose/ ethanol ratio) on biomass liquefaction in sub-and super-critical ethanol were investigated in this work. The liquefaction system was divided into the following fractions: a volatile organic compounds fraction, a gas fraction, a heavy oil fraction, a water-soluble oil fraction, and a solid residue fraction. Results showed that for three samples, the SR yield of microcrystalline cellulose was highest compared with corn stalk cellulose and rice straw cellulose at the same temperature, while the HO yield was lowest in the liquefaction process. At the same retention time in supercritical ethanol, the SR yield of microcrystalline cellulose was highest, suggesting that the microcrystalline cellulose was difficult to liquefy. The effect of different samples on liquefaction in ethanol with various cellulose/ethanol ratios can be clearly seen from the distribution yields. The FT-IR analysis of the solid residues showed that the structure of celluloses changed after liquefaction. The GC-MS analysis showed that the volatile organic compounds, water-soluble oil, and heavy oil comprised a mixture of organic compounds, which mainly included furfural, acids, furans, esters, and their derivatives. XRD analysis revealed that the decomposing reaction primarily occurred within amorphous zones of the celluloses at the low temperatures. Keywords: Liquefaction; Cellulose; FT-IR; XRD; GC-MS INTRODUCTONThe demand for energy has been increasing dramatically due to the rapid increase in the world's population and developing technologies. Meanwhile the current energy resources have limited reserves and are decreasing (Ozcimen and Karaosmanoglu 2004). Today, biomass is considered a renewable resource with high potential for energy production. Biomass can be converted to various forms of energy through numerous thermo-chemical conversion processes, depending upon the type of energy desired (Yanik et al. 2007).Among the many thermo-chemical procedures, biomass liquefaction into liquid fuel is a promising one, during which the common products are gas, liquid, and char. Liquefaction has many advantages such as, (1) The presence of solvent dilutes the concentration of the products, thus tending to minimize cross-linked reactions and reverse reactions, and (2) The processing temperature is relatively low (less energy consumption) in comparison with other thermo-chemical processes (such as pyrolysis and gasification) (Liu and Zhang 2008). Some articles have reported on the liquefaction of biomass; the presence of solvents has been shown to effectively lower the viscosity of heavy oil derived from biomass liquefaction (Demirbas 2000).

Cornstalk cellulose was liquefied in sub-and supercritical ethanol using an autoclave at 320 °C with 160 mL of ethanol. The effects of reaction time on esters formation during cellulose liquefaction were investigated. The yield of esters was 10.0% at 30 min, increasing to 19.1% after 60 min. Ethanol favored esters formation from cellulose liquefaction. The liquid products at different reaction time were analyzed by FT-IR and GC/MS. The results showed that many free radicals were produced in sub-/supercritical ethanol interactions. Cellulose was converted to active cellulose, which was transformed into large molecular acids by dehydration, decomposition, ring-opening reactions, isomerization, and aldol condensation, and then formed ethyl esters such as ethyl lactate by esterification. In addition, ethyl esters were decomposed to acids, alcohols, and other compounds with increasing reaction time in the presence of ethanol free radicals. Using these results, a reaction network for the formation of ethyl esters from cellulose in sub-and supercritical ethanol was proposed.

Liquefaction of cornstalk in sub‐ and supercritical cyclohexane with a zeolite catalyst was performed in a stainless‐steel reactor. The reaction system was divided into four lumps: gas, residue, water‐soluble fraction (WSF), and ethanol‐soluble fraction (ESF). The effects of the cyclohexane‐to‐cornstalk ratio, reaction temperature, reaction time, and zeolite amount on the lump yields were studied. The results show that reaction time and temperature had significant effects on the distribution of the products. The amounts of organic acids, acetic ester and aldehyde in the ESF obtained from cornstalk liquefaction with zeolite were higher than without zeolite. The distributions of the WSF changed with zeolite use. The main secondary reaction pathways of bio‐oil generation are proposed. Part of the unstable WSF was converted into the ESF and gas. The ESF further formed a residue through condensation, cyclization and repolymerization in sub‐/supercritical cyclohexane. GC/MS analysis showed that the WSF and the ESF comprised a mixture of organic compounds, which mainly included organic acids, aldehydes, ketones and phenolic compounds.