Basic Research And Thermodynamic Of Relative Solution For Direct Electrosynthesis Process Of Niacin

Niacin is an important drug, food and feed additive, dyeing and medical intermediate with wide uses and optimum application prospects. With the development of medicine, food and feed industry, the need for niacin was yearly increased both in China and abroad, however the supply is insufficient. The electrochemical synthesis of niacin is characterized by mild reaction conditions, high product purity and reduced waste, and is called as "Green Technology". So the study to fluid phase equilibria, volumetric and viscosity properties, the analysis method and technological conditions optimization has a great science and applied significance for electrochemical synthesis of niacin using 3-picoline as raw material. The research contents of this paper are one of subjects funded by Henan Key Research Projects (No.0323024900) in 2003, and by Henan Natural Science Foundation (No.0411022600) in 2004.In the synthesis process of niacin, supporting electrolytes were sulfuric acid aqueous solution, and the reaction solution were mixtures of niacin, 3-picolin (3-MP), water and H2SO4. The panoramic analysis for this mixture was not found in literature. A new analysis method was proposed by combining nonaqueous solution titration with UV spectrophotometry.The anode is the catalyst and a reaction field for electro-catalytic reaction. Base on the properties of the electro-synthesis process of niacin, the catalytic activity and lifetime of Ti/I_rO₂-Ta₂O₅, Ti/RuO₂-SnO₂, Ti/I_rO₂-Ta₂O₅/PbO₂ and Ti/SnO₂-Sb₂O₃/PbO₂ anodes in oxidizing 3-picoline process were investigated by steady-state polarization curve. It was found that the Ti/SnO₂-Sb₂O₃/PbO₂ anode exhibited higher overpotential to oxygen evolution, better electro-catalytic activity and electrochemical stability, and can be applied in the electro-synthesis of niacin. The active phases of Ti/SnO₂-Sb₂O₃/PbO₂ electrode was characterized by XRD and SEM. The kinetic parameters and apparent activation energy in oxidizing 3-picoline process were determined.In this study, using Nafion 427 cation-exchange membrane as membrane, Ti/SnO₂-Sb₂O₃/PbO₂ as anode and Pb - Sb alloy as cathode, the technology conditions for synthesizing niacin were optimized in the plate and frame electrolyzer. Under the conditions of catholyte sulfuric acid concentration 20wt%, flow-rate 10L/min and distance between anode and cathode 3mm, by using orthogonal design and mono-factor experiment, the optimum technological conditions were found that current density 200A/m2, solution temperature 60℃, anolyte sulfuric acid concentration 20wt% and 3-MP concentration 15wt%. Under the optimum technological conditions, the five times repeat experiments were tested, the results show that the current efficiency is about 70% and the yield of niacin is 90% or more.The direct electro-synthesis of niacin is a heterogeneous catalytic reaction between the electrolyte solution and electrode surface. The electrolyte solutions were not only an important part of electro-catalytic reactor, but also supplier of raw material, and necessary condition for electro-catalytic reaction. The research on composition, phase state, density and volumetric properties, viscosity and transport properties for electrolyte solution is very important for the design of electro-catalytic reactor andreaction process. There are quite a few researches on solubility of niacin in electrolyte solution, density and volumetric properties, viscosity and transport properties for containing niacin electrolyte solution. So in this study, using a laser monitoring observation technique, the solubilities of niacin in water, 3-MP, 3-MP aqueous, sulfuric acid aqueous and 3-MP + sulfuric acid aqueous have been determined experimentally from 292.65 K to 361.35 K. According to Scatchard-Hildebrand theory and experimental results, the synergistic effect of binary solvents for niacin + 3-MP aqueous and niacin + sulfuric acid aqueous systems were discussed. The experimental data are correlated with the M equation and modified Apelblat; the constants of equations were obtained. The total standard deviations between the measured and fitted values were 5.1*10"^ and 2.67* 10"4, respectively for M equation and modified Apelblat. Based on relationship between the parameter h in M equation and excess enthalpy ff, the excess enthalpy was calculated using experimental data for every system. The variation of Z/6 with content of 3-MP in niacin + 3-MP aqueous was discussed in terms of molecular interaction.The densities of 3-MP + water, niacin + 3-MP, niacin + 3-MP + water and niacin + 3-MP + H2SO4+water mixtures have been measured from 293.15 K to 343.15 K. The experimental data are correlated with the VTF equation; the constants of equation were obtained. From measurements of densities, the excess molar volume, the partial excess molar volume and the apparent molar volumes were calculated. The variation of volumetric properties was discussed based on molecular interactions.The viscosities of 3-MP + water, niacin + 3-MP, niacin + 3-MP + water and niacin + 3-MP + H2SO4+water mixtures have been measured from 293.15 K to 343.15 K. For the containing niacin binary, ternary and quaternary systems, a molecular thermodynamic model of viscosity was developed based on molecular thermodynamic model of excess enthalpy. The parameters of model were obtained, the relationship between the parameters and temperature were studied. For 3-MP + water binary system, the active free energy, active free enthalpies, active free entropies and excess Gibbs energy of activation were calculated according to Eyring's rate process theory.Through the study of solubility, density, volumetric properties, viscosity and appropriate model, this thesis not only enriches the basic research of chemical engineering and molecular thermodynamic, but also provides basic data and information for industrial production and farther research to the direct electro-synthesis technology of niacin.