Study On The Complexation Of Vanadium Ions With The Tannin Extract

The tannin extract desulfurization technology belongs to the binary wet catalytic oxidation desulfurization method. At present, it is widely applied to remove H2S in the field of fertilizer, coking and oil refining in our country. In the process of removing H2S, vanadium(Ⅴ) ion is reduced to vanadium(Ⅳ) ion by HS", and then the vanadium(Ⅳ) ion is oxidized to vanadium(Ⅴ) ion by air in the presence of oxygen-carrying tannin extract. So the vanadium ion can be circled. In this process, the tannin extract also acts as a ligand agent, which can avoid vanadium(Ⅳ) to form deposition and keep its concentration stable. In this paper, pyrocatechol, pyrogallol and gallic acid have been used as model compounds for tannin extract, tannic acid as a substitute for tannin extract. By using the pH potential method and UV-vis spectrophotometry, the complexation of the tannin extract with vanadium ion has been investigated comprehensively and systematicly.The reaction between pentavalent vanadium V (Ⅴ) and disulfide HS-and its influencing factors were investigated. The results show that the pH value of de-sulfurization liquid is the main influencing factor for the pH value of desulfuri-zation liquid, temperature, concentration of reactants. The tannin extract is a tannic acid-containing polyphone mixture which contains phonehydroxyl and carboxyl groups. In order to define which groups form complexation, the coordination reaction of tetravalent vanadium (Ⅳ) ion with such organic compounds as phenol, hydroquinone, resorcinol, pyrocatechol, m-hydroxybenzoic acid, potassium sodium tartrate and gallic acid were investigated by using UV-visible spectrophotometry. The results show that tetravalent vanadium ion only reacts with these organic compounds containing ortho hydroxyl and carboxyl functional groups, and not with those containing a hydroxyl, or meta-hydroxyl and carboxyl functions.The protonation constants of such model compounds as pyrocatechol, pyrogallol and gallic acid were determined at25℃,30℃,35℃,40℃,45℃by using pH potential method. The protonation constants decrease with the increase of temperatures. The distribution of various species of these model compounds with the pH value change can be mapped by using these protonation constants. The figure shows that the pH value of the solution markedly impacts the distribution fraction of various species. The species greatly affects the stability of the metal ions complexes with the model compounds. The apparent stability constants of tetravalent vanadium ion with these model compounds were determined. All the values are larger. These complexes are stable from the point of view of thermodynamics. The thermodynamic functions of the complexes were calculated by the stability constants. The standard molar Gibbs functions of these reactions are negative, which figure that these coordination reactions are spontaneous. The standard molar reaction enthalpies of these coordination reactions are negative, which figure these reactions are exothermic reactions and low temperature is favorable to the reactions. Moreover the enthalpy changes are the main driving force of the reactions.Through the analysis of the infrared spectra of tannic acid, valonia tannin extract, oxidant of tannic acid and the one of valonia tannin extract, tannic acid appears more characteristic peaks than valonia tannin extract. But they contain the similar main functional groups. The infrared spectra of the oxidant of valonia tannin extract and the oxidant of tannic acid are very similar. After oxicidization, partial phenolic hydroxyl group turned into the quinone. Therefore, the tannic acid can be used as a substitute of valonia tannin extract. Then we can quantitatively research the complexation between valonia tannin extract and vanadium ions.The protonation constants of tannic acid at defferent temperatures were determined by using the pH potential method. The protonation constants decrease with the temperature increasing, which is not markedly. The distribution of of various species of tannic acid with the pH value change can be mapped by using these protonation constants. The pH values have a great influence on the existence form of different body. At the pH value8.5～9.5of the tannin extract desulfurization system, the main form of tannic acid in solution is H2L". The apparent stability constants of V (Ⅳ) complex with tannic acid at different temperatures were determined. These values show that the tannic acid and V (Ⅳ) can form stable complex.Using UV-vis spectrophotometry, the coordination reaction of tannic acid with vanadium ions was studied in Na2CO3/NaHCO3buffer solution. The results show that the tetravalent vanadium ion and tannin can form complex in this buffer system. The ligand ratio of the complex is1:1, the stability constant is K=5.34×104.