| At present, fossil fuels, represented by coal, oil and natural gas, are the main resources of fuel and chemical products. However, these fossil fuels are unrenewable and their reserves are limited, and human society demand on fuel and chemical products is sharply increasing with the development of economy and technology as well as the surge of population, which may result in a serious influence on the human development. Biomass is considered as the most potential alternatives for fossil energy due to its abundance, widespread distribution and renewability, and one of the most promising approaches for use of biomass is to convert biomass to high value chemicals. Subcritical water owns some particular properties, such as acid and base properties, which can act as catalyst as well as solvent. Therefore, some chemical reactions depended on acid or alkali catalyst can be performed in subcritical water system, on the one hand reducing the cost of production and on the other hand avoiding the environmental pollution. In this work, we mainly investigated the preparation of formic acid and acetic acid from cellulose in subcritical water. Besides, the catalytic oxidation of sodium ligninsulfonate and lignite were studied preliminarily. The results are summarized as follows.(1) The preparation of formic acid and acetic acid by oxidation of cellulose in subcritical water was studied. The results show that the conversion of cellulose reaches 92.65% and the yield of formic acid and acetic acid attain 17.2% and 10.45% respectively under the optimum conditions with active carbon (AC) catalyst. Co(Ac)2, V2O5 Mn7Ce3Oz, and AC have been demonstrated to be efficient catalysts, which can adjust the distribution of products and improve the yield and selectivity of formic acid and acetic acid, and the yields of acetic acid can be improved by 69.0%,55.5%,57.2% and 12.6% for Co(Ac)2, V2O5, Mn7Ce3Oz, and AC, respectively. As the temperature increases from 210℃to 250℃, the conversion of cellulose and the yields of formic acid and acetic acid increase first and then decrease. The conversion of cellulose is increased to 100% at 60 min, the yield of formic acid increases first and then decreases with time, and the yield of acetic acid increases within 60 min and then keeps constant. Under the initial oxygen pressures ranging from 1 MPa to 6 MPa, the conversion of cellulose increases gradually with increasing pressure, and reaches 100% at 4 MPa and higher pressures. The yield of formic acid increases with the increase of pressure among the whole pressures, the yield of acetic acid increases first and then decreases, and the maximal yield is reached at 4 MPa. When the concentration of cellulose ranges from 13 g/L to 33 g/L, the conversion of cellulose is 100%, then it decreases gradually when the concentration of cellulose is from 33 g/L to 53 g/L, and the yields of formic acid and acetic acid increase first and then decrease, both of them reaching a maximum at 33 g/L. There are also small amount of by-products, such as 5-HMF, furfural, glucose, fructose and some other oligosaccharides, besides formic acid and acetic acid in the liquid phase. Moreover, there is a large amount of CO2 in the gas phase because of degradation and peroxidation of liquid products.(2) The catalytic oxidation of sodium ligninsulfonate to prepare vanillin has been primarily explored. The results reveal that the addition of catalysts can largely improve the yield of vanillin. Moreover, the presence of alkali plays an essential role in the yield of vanillin, and the yield of vanillin shows a linear relationship with the concentration of alkali to some extent; while no vanillin is detected when alkali is absent.(3) The catalytic oxidation of lignite to prepare aromatic carboxylic acids has been also investigated. The results show that the oxidation of lignite by alkali and oxygen is feasible theoretically, but its application is prohibited due to the vast consumption of alkali. The catalyst MnBr2, FeCl3, CuBr2 and CoBr2 show little catalytic property in this process. |
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