Cellulose Degradation In High Temperature And High Pressure Solvent

Biomass energy is the type of energy utilized earliest by human and now is also the important energy exploited to solve the dual crisis of energy and environment. Biomass liquefaction technology, which can convert low-quality biomass to high-quality liquid fuel or valuable chemicals, has attracted extensive attention these years. As the most important constituent in biomass, a deep research on liquefaction of cellulose will provide basic data for conversion of biomass and therefore improve the development and utilization of biomass energy. So, researches were conducted on degradation of microcrystalline cellulose (avicel) in three hot-compressed aprotic solvents & mixture of protic and aprotic solvent, and degradation of amorphous cellulose in subcritical water.Microcrystalline cellulose were treated in hot-compressed aprotic solvents, acetone, 1,4-dioxane and sulfolane, using a batch-type of high pressure vessel with a molten tin bath as heat resource. The effects of reaction conditions such as heat resource temperature, reaction time, ratio of cellulose to solvent and solvent density on conversion rate and distribution of typical products were investigated. As a result, in all solvents, more than 90% of cellulose was found to be decomposed to the solvent-soluble products mainly by pyrolysis in a temperature range from 290 to 390℃without any catalyst. The degradation rate was affected highly on solvent, the order of which from lowest to fastest was 1,4-dioxane, acetone and sulfolane. The products were levoglucosan, glycoaldehyde, furfural and 5-HMF etc., of which levoglucosan was the main product. When the ratio of cellulose mass to solvent volume was 20 mg/ml, the highest yield of levoglucosan on original cellulose basis from cellulose treated in sulfolane (360℃/12s/0.50g/cm3),1,4-dioxane (370℃/360s/0.41g/cm3) and acetone (350℃/240s/0.63g/cm3) were 35.4%,34.8% and 11%, respectively. Levoglucosan was more stable in acetone and 1,4-dioxane than in sulfoane. Hydrolysis reaction was also partly happened with glucose, fructose and others as products due to water produced through pyrolysis of cellulose.In order to further investigate the mechanism of cellulose degradation in aprotic solvents, the chemical structure of solvent-insoluble residues from cellulose treated in 1,4-dioxane and sulfoane were analyzed by using a Fourier Transform Infrared spectrophotometer. When cellulose was treated in sulfolane, the spectra of the residue were almost the same as that of untreated cellulose, indicating that the structure of residue was basically same as cellulose. No absorption peaks corresponding to carbonization appeared in the residues from cellulose treated in sulfolane, so sulfolane can dissolve levoglucosan well and protect it from repolymerization and resulting carbonization. On the other hand, cellulose treated in 1,4-dioxane revealed a spectrum of carbonization reaction with the emergence of a band at 1720cm-1 for carbonyl stretch, and formation of unsaturated double bond at 1600cm-1. Combined with the change of selectivity of typical products with cellulose conversion rate, different mechanism of cellulose degradation in sulfolane and 1,4-dioxane were proposed. When cellulose is degraded in sulfolane, it is inheritance relationship between levoglucosan and glycoaldehyde, and no dehydration and carbonization reaction happen. When cellulose is degraded in 1,4-dioxane, it is parallel reaction for the production of levoglucosan and glycoaldehyde, and dehydration reaction is also happen with charcoal produced. The relative crystallite size and crystallinity of cellulose treated in acetone evaluated from X-ray diffraction did not change a lot with degradation, indicating heterogeneous decomposition of cellulose.On the basis of above experiments, decomposition kinetics of microcrystalline cellulose in three protic solvents at high pressure and temperature were investigated by model fit method and isoconversional method respectively. It was shown from the results that there are two stages for cellulose degradation, first is the degradation of cellulose itself, and second is the degradation of residue. The apparent activation energy for the first stage in acetone, 1,4-dioxane and sulfolane is 184.56,198.04 and 33.77 kJ/mol respectively. It is much lower in sulfolane than in other two solvents.Taking sulfolane and water as representatives, a series of experiments were done on cellulose decomposition in mixture of protic and aprotic solvents. Results showed that cellulose pyrolysis and hydrolysis reaction could happen simultaneously in mixture, giving levoglucosan and glucose as main products respectively. When the volume ratio of water was low (such as 5%), the selectivity of pyrolysis and hydrolysis was equivalent, resulting almost same yield of main product. But with higher ratio of water, the selectivity of pyrolysis path decreased while the selectivity of hydrolysis path increased. Under these conditions, the yields of glucose were high, but the reaction conditions (such as temperature and pressure) were mild compared to that of supercritical water hydrolysis of cellulose. So it is feasible to control the reaction path and products distribution by simply adjusting the ratio of mixture solvents with different properties.In order to explore the way of decreasing the cost of biomass conversion, decomposition behavior of amorphous cellulose in subcritical water was investigated. It was shown from the results that amorphous cellulose could be hydrolyzed in subcritical water at much lower temperatures (220～280℃) compared to microcrystalline cellulose, giving glucose and fructose as main products. The maximum yield of glucose is 40% at temperature of 280℃, reaction time of 30 s, which is higher than that of microcrystalline cellulose hydrolysis in supercritical water.