Study On The Selective Oxidation Of Cellulose

Aldehyde and carboxyl groups can be introduced into cellulose polymer chains by selective oxidation, which is one of the most important methods to modify and explore cellulose resources. The two main selective oxidation systems of cellulose are NaIO4and NaClO/TEMPO/NaBr. The secondary hydroxyl group of cellulose can be oxidized to aldehyde group selectively by the former oxidation system, and the primary hydroxyl group of cellulose can be oxidized to carboxyl group selectively by the latter. The oxidation reactions are usually performed under heterogeneous condition by mixing oxidation systems and cellulose with slow oxidation reaction rate and distinct degradation of the products due to the special structure of cellulose. The paper improved these two oxidation systems to increase the selective oxidation rate and decrease the degradation of the products.(1) CaCl2was applied to assist NaIO4to oxidate cellulose powder. The effect of CaCl2on periodate oxidation of cellulose powder was studied under different conditions. CaCh could enhance the reacion rate and the aldehyde content of dialdehyde cellulose (DAC), the improvement was more obvious at higher temperature. FT-IR, XRD and TGA were applied to characterize cellulose powder and DAC. The mechanism of CaCl2promoting periodate oxidation of cellulose was discussed based on the characterization results. CaCl2could break the hydrogen bond of inter-and intra-polymer chains of cellulose, forming coordination bond between Ca2+and06of cellulose, leading to the decrease of the degree of cristallinity of cellulose, then the oxidant could access hydroxyl group in the inter part of cellulose so that the reaction rate was increased.(2) The selective oxidation system NaClO/TEMPO/NaBr can oxidize primary hydroxyl group of cellulose to carboxyl group with good selectivity. However, the degradation of oxidation products is obvious and the reaction system must be maintained at a high alkalinity of pH10.5. TCCA substituting for NaClO as the main oxidant, could selectively oxidize primary hydroxyl group of cellulose to carboxyl group under the catalysis of TEMPO and NaBr, in a acetone-water mixed solvent as well as NaHCO3 buffer solution to maintain the pH. It was found that the reaction rate was faster and the carboxyl group content of the product was higher than that by NaCIO under the same condition, meanwhile the weight loss of the product decreased and the degree of polymerization of the product increased.(3) NHPI/Anthraquinone was used to replace TEMPO because TEMPO was expensive. It was found that TCCA/NHPI/anthraquinone/NaBr could also oxidize cellulose to uronic acid. Compared to NaClO/TEMPO/NaBr and TCCA/TEMPO/NaBr, TCCA/NHPI/anthraquinone/NaBr oxidation product had lower degradation degree and higher polymerization degree.