Isolation&Purification Of Tannin And Its Acid-catalyzed Hydrolysis Kinetics

Gallnut has a long history to be used as a traditional Chinese medicine, containing Tannin as a main component. Tannin is a polyphenol compound, and the research indicates that tannin has many physicochemical properties and biological activities, such as antibacterial, anti-inflammatory, antiviral, anti-tumor and cancer, anti-oxidation, anti-aging and so on, which has been widely used in many fields including food, medicine, daily chemical, leather industry and so on. Tannin can be acid/base-or enzyme-catalytic hydrolyzed to prepare gallic acid. Gallic acid has great chemical reactivities, which is employed in organic synthesis, pharmaceuticals and intermediates, food preservation, LSI manufacturing fields and so on. It is important to produce tannin and its processed products from gallnut (and Tara).At present, the method for tannin utilization is isolation and purification for high purity food-grade tannin, and hydrolysis of tannin for gallic acid and its derivatives. There are some disadvantages in current technologies. Isolation and purification of food-grade tannin from gallnut (and Tara) need to combine a few of single-methods, and the procedure is complicated, high-cost and small-scale. The hydrolysis of tannin to prepare gallic acid consumes a large amount of acid or alkali, causing serious pollution. To solve these problems, ethyl acetate was employed to extract and purify gallnut and industrial tannin, respectively, for developing a technology for food-grade tannin. The stability of gallic acid was investigated in high temperature liquid water with hydrochloric acid, and reaction kinetics of tannin acid-catalyzed hydrolysis was studied.Ethyl acetate was employed to extract and purify gallnut and industrial tannin, respectively. The effect of extraction temperature, time, ratio of solid to liquid, additives types and contents on the extraction and purification were studied. The results showed that tannin needed to be purified in weak acid under the condition of low temperature, and tannins purity reduced as the adding amount of solvent increased. Based on the experimental results, a method to prepare high-purity tannin was obtained. The optimized process for gallnut extraction is described as follows: extraction temperature is303.15K; dosage of ethyl acetate is4ml/g gallnut; time is5h; liquid-solid ratio is3. The optimized process for industrial tannin purification tannin is: temperature is313.15K; dosage of ethyl acetate is4ml/g industrial tannin; liquid-solid ratio is8; pH=5.The stability of gallic acid in high temperature liquid water with hydrochloric acid was studied in the temperature range of393.15K.-453.15K. The effects of initial concentration of gallic acid, concentration of hydrochloric acid and reaction temperature on the stability of gallic acid were investigated. The results indicated that the stability of gallic acid decreased as the concentration of HC1increased, and the stability of gallic acid remarkably decreased as the temperature increased. Below413.15K, gallic acid was relatively stable in HCl solution. The kinetics data of gallic acid stability were fitted by a pesudo-first-order kinetics equation to obtain the rate constants at different hydrochloric acid concentrations and reaction temperatures. The apparent activation energies for the decarboxylation of gallic acid at different concentrations of hydrochloric acid were obtained by Arrhenius equation. The apparent activation energies were all close to112kJ/mol and not sensitive to HCl concentrations under the hydrochloric acid concentration in the range of0.2-2.6mol/L The results of the study provide basic data for the subsequent hydrolysis process.Reaction kinetics of hydrochloric acid and sulfuric acid catalytic tannin hydrolysis were studied in the temperature range of383.15K-413.15K. The effects of initial concentration of tannin, concentration of acid and reaction temperature on the tannin hydrolysis were investigated. The results showed that tannin hydrolysis rate rapidly increased with the increase of temperature. Increasing the concentration of acid can significantly increase tannin hydrolysis rate. Under the conditions of high temperature and acid concentration, the yield of gallic acid showed a slow downward trend as the reaction time elapsed. In the same equivalent concentration of hydrogen ion, sulfuric acid catalytic tannin hydrolysis needed longer time to achieve the highest yield than hydrochloric acid catalysis. The hydrolysis data under different acid concentration and reaction temperature were fitted by a pseudo-first-order reaction kinetics equation to obtain the hydrolysis reaction rate constants. The apparent activation energies of tannin hydrolysis at different concentrations of acid were obtained by Arrhenius equation. Compared to the results of sodium hydroxide catalytic hydrolysis of tannin, hydrochloric and sulfuric acid have the similar catalytic activities as sodium hydroxide.