Research On Technology And Mechanism Of Selective Hydrolysis Of Cellulose By Heteropoly Acid

Microcrystalline cellulose (MCC) is a kind of functional product prepared by cellulose hydrolysis with level off degree of polymerization. It is widely used in medicine, food, cosmetic, leather, and other industrial fields because of many characteristics such as high crystallinity, small and uniform particle size, water insoluble. However, the existing technology of cellulose hydrolysis does poorly in selectiving the amorphous region of cellulose, so that makes the crystalline region degrade severely, eventually leads to the low performance of MCC products, moreover, there are problems in acid recycle and serious environmental pollution caused by the traditional acid catalysts, which are used in the cellulose hydrolysis. In order to solve these problems, some research were conducted in this paper as follows:Firstly, different kinds of heteropoly acids were used in the research of cellulose hydrolysis:the crystallinity, yield, degree of polymerization and average particle size (fines) of hydrocellulose products were 75.41%、67.40%,91.22%、92.30%,84、89,31μm、34μm, respectively, by phosphotungstic acid (HPW) and silicotungstic acid (HSiW) under the condition of reaction temperature 100℃, reaction time 2h, solid-liquid ratio 1:40, concentration of H+ 1.50mol·L-1. This result showed that the product hydrolyzed by HPW demonstrated a better performance due to the strong acidity and highly active of HPW. Compared with traditional HC1, the particle size of product could decrease to 31μm (based on 63μm by HCl) by HPW hydrolysis with similar crystallinity. This result showed that HPW was more advantageous to reduce the size of the hydrocellulose. Furthermore, the cycles of HPW could be up to 4 times and still remained high activity under the premise of high performance of product.Secondly, the reaction process and mechanism of heteropoly acid hydrolysis of cellulose were discussed in this paper:not only H+ was involved in cutting off the glycosidic bond in cellulose hydrolysis, but also a stable complexes could be formed by heteropoly anion and cellulose intermediates, which could reduce the reaction activation energy and improve the reaction efficiency; in addition, the special space structure of heteropoly acid was more conducive to promoting selectivie hydrolysis of amorphous region of cellulose. The chemical structure of cellulose did not change and still kept the crystal type of cellulose Iβ, and presented a more complete crystal shape by XRD observations; FT-IR indicated that the amorphous region of cellulose could be effectively destroyed by HPW, which led the breaking of hydrogen bonds, made the selectivity of degradation reaction, eventually increased the crystallinity of cellulose; TGA showed a relative higher thermal stability of hydrocellulose compared to the raw materials.Finally, response surface method (RSM) and microwave processing method were used to optimize the selectivity of acid hydrolysis:the crystallinity of hydrocellulose could be reach up to 77.43% under the optimal conditions with concentration of H+ 1.63mol·L-1, reaction temperature 93℃, reaction time 2h, solid-liquid ratio 1:38 after being optimized by RSM; due to the joint action of microwave-HPW could effectively improve the absorption performance of microwave energy by the system, the reaction temperature of hydrolysis could be reduced (6℃) and the reaction time could be shorten (0.5h), the selectivity of HPW hydrolysis of cellulose was improved and the crystallinity of products could be increased to 78.81%, the crystal size of 002 surface were 6.10nm at the same time.In general, by the use of HPW with the characteristics of high activity, strong acid, easy to recycle, combined with the use of microwave coordination treatment technology, the selectivity and reaction efficiency of cellulose hydrolysis. could be improved effectively, and the products were obtained with good performance such as high crystallinity, small and uniform particle size, low degree of polymerization. Eventually the goal of the high value of utilization of biomass resources was realized as well as the pollution of the environment was reduced.