Study On Several Fundamental Problems On The Preparation Of Ultrahigh-Purity Gallic Acid

Gallic acid(GA,3,4,5-trihydroxybenzoic acid) and several polyhydroxy-compounds synthesized by it are industrially important chemicals widely used in many fields such as organic synthesis, pharmaceutical and food.Recently;ultrahigh-purity gallic acid and propyl gallate have been used in electronics industry for replacing catechol(1,2-dihydroxybenzene) to remove the photoresist,other polymeric material or residue from the substrate.With the development of economics and the enhancement of environmental protection consciousness,gallic acid will certainly replace 1,2-dihydroxybenzene used currently in China.Due to the international competition and the protection of intellectual property rights,literature about the preparation process of ultrahigh-purity gallic acid is quite scarce. Accordingly,systematical exploration and theoretical study are currently unavailable as well.In the present study,several fundamental problems related to the preparation of ultrahigh-purity gallic acid were investigated.In addition,the preparation process combining the resin absorption and crystallization has been studied particularly.The main contents are summarized as follows.Firstly,in the non-catalyzed hydrolysis of gallotannin and tara tannin in high temperature liquid water(HTLW),after establishing a new high-performance liquid chromatography(HPLC) method for the analysis of tannin hydrolysis products,the decomposition reaction kinetics of tannins and their intermediate gallic acid were systematically studied for the first time in the temperature range from 413.15 K to 463.15 K. It was found that gallotannin and tara tannin could be hydrolyzed easily without catalysts and the reaction was indicated to be a typical consecutive first-order one in which gallic acid formed as a main intermediate product and pyrogallol as the final product.Additionally, selective decomposition of tannin in HTLW was proved to be possible by adjusting reaction temperature and time.Based on the assumption of first order kinetics equation,the apparent activation energies were evaluated.Next,the solubilities of several polyhydroxy-compounds,including gallic acid,methyl gallate,propyl gallate,octyl gallate, 2,4-dihydroxybenzophenone,2,3,4-trihydroxybenzophenone, 2,3,4,4'-tetrahydroxybenzophenone and 2,2',4,4'-tetrahydroxybenzophenone,in ethanol+water mixture were determined by a static analytical method in the temperature range from 273.15 K to 363.15 K.As shown in the results,the solubilities increase as the temperature and the concentration of ethanol in the mixture solvent increase.In addition,the solubilities increase with the increasing molecular polarity generally.The simplifiedλ-h equation and two empirical equations were proposed to correlate the experimental data with good agreement.Then,the complexation of gallic acid with several metal ions was studied by means of acid-base potentiometric titration.The stability constants between gallic acid and Fe(Ⅱ),Al(Ⅲ),Ni(Ⅱ) and Pb(Ⅱ) were determined for the first time.Results showed that gallic acid can form stable complexes with metal ions involved in this study,and the sequence of stability constants is:Ca(Ⅱ)＜Mg(Ⅱ)＜Mn(Ⅱ)＜Fe(Ⅱ)＜Ni(Ⅱ)＜Zn(Ⅱ)＜Cu(Ⅱ)＜Fe(Ⅲ)＜Pb(Ⅱ)＜Al(Ⅲ).Simultaneously,the complexation between gallic acid and several metal ions was investigated by using UV-vis spectroscopy.Also, theoretical and experimental study on the complexation reaction of gallic acid with ferrous ion was performed systematically.The results showed that gallic acid can form stable complexes with metal ions investigated in this study,which makes the red shift of gallic acid in the presence of metal ions.The stoichiometric composition of GA-Fe(Ⅱ) complex was 1:1 under the adopted experimental conditions.The complexation reaction of gallic acid with ferrous ion was found to be a second order reaction. According to the Arrhenius equation and the Absolute-rate theory,the activation energy and free energy of activation of the complexation reaction were calculated.To understand the kinetic results obtained,a three step reaction mechanism was proposed,which can satisfactorily explain the dependence of the rate constant on the temperature,ionic strength and permittivity of the reaction medium.Finally,the preparationprocess of ultrahigh-purity gallic acid combining resin absorbance and crystallization was investigated particularly.As shown in the results,crystallization with ultrahigh-purity water can not only improve the purity of gallic acid but also decrease the concentration of metal ions effectively.After three times of crystallization,the purity of gallic acid increased from 96.05% to 99.77%,while the concentration of calcium,iron,sodium and kalium ions were all reduced to around 1 ppm.Different resins have different effects on the removement of metal ions.The three ultra-cleaning resins with high purity all have good effects,especially ZGC-ER80.Treating with several resins with common purity,such as XSC-800 and 001×10,the concentration of metal ions in gallic acid can also be reduced effectively.The information yielded from this work will provide essential basic data and theoretical instruction for the preparation process of ultrahigh-purity gallic acid.