| Summary: | 博士 === 國立成功大學 === 化學工程學系碩博士班 === 95 === In a general electrochemical system, the products are produced at working electrode, the counter electrode is not availability. To improve the disadvantage, the electro-organic synthesis process is upgraded when the same product is obtained on both anode and cathode electro-organic synthesis being invented. In this way, the power consumption and cost of equipment are reduced. D-arabinose is a monosacharides, which plays an important role in the synthesis of vitaminB2 and vitaminD. D-arabinose is prepared by degradation oxidation, in which the carbon chain length of sodium gluconate or its derivatives is reduced. Production of D-arabinose via anodic oxidation, cathodic oxidation and paired electro-oxidation were studied in this dissertation. The purpose of this study is to research and development of electro-organic synthesis process which will be less power consumption and high economy.
In anodic oxidative system, using Cl-/OCl- as redox medicator and direct electro-oxidation in divided cell are studied. Effect of operation condition on the reaction rate, reaction kinetics and rate determining step are systematically investigated. The experimental results show that direct electro-oxidation has better than indirect electro-oxidation. The current efficiency are 86% and 59% for direct and indirect electro-oxidation, respectively. The reaction mechanism of sodium gluconate anodic oxidation was proposed. The theoretical analysis correlated with the experimental results well. The rate determining step was found to be the anodic oxidation of gluconate anion to form gluconate free radical.
R1s -->�� R2s + e- (1)
And the current of the anodic oxidation of sodium gluconate can be expressed as
i=((0.89[NaC6H11O7])/(0.7357+[NaC6H11O7]))*exp(0.37Fn2/RT) (2)
where F is faraday’s constant and is the overpotantial of forming gluconate free radical from gluconate anion.
In cathodic oxidative system, using Ce4+/Ce3+，Fe3+/Fe2+and V3+/V2+ as redox medicators in divided cell are studied. The oxygen is reduced to form hydrogen peroxide at cathode. The OH free radical decomposed form hydrogen peroxide. The OH free radical oxidize sodium gluconate to from D-arabinose. The experimental results indicated that the mediator couple enhanced current efficiency relative to and at 60 mM sodium gluconate, 25℃ with a current density of 1.32 in cathodic chamber. But reaction has many side reactions and the current efficiency of D-arabinose is low, about 19%.
In paired electro-oxidative system, by combining the cathodic and anodic oxidation in both undivided and divided cells are studied. In undivided cell, applying Fe2+/Fe3+as redox mediator for anodic and cathodic paired electro-oxidation of sodium gluconate to D-arabinose, the experimental results show at a 200ml/min oxygen flow rate, 0.2N acetic butter solution contained 10mM Fe3+,25℃, both anodic and cathodic current density are 0.94mA/cm2, the total current efficiency is 75%, in the paired electro-oxidation system.
In divided cell, the experimental results indicate that the current efficiency of anodic/cathodic oxidation in the paired electro-oxidation is the same as that of anodic/cathodic oxidation only. So the maximum total current efficiency for the paired electro-oxidation of sodium gluconate to D-arabinose was 127% with a charge of 68.22 C being passed. The best current efficiency for anodic oxidation was 85% in this system which was found in a single anode as the working electrode. The mediator in the catholyte generates the best current efficiency with the maximum value is 41% in the a cathodic system. The current efficiency for the electro-oxidation of sodium gluconate to D-arabinose in cathodic chamber was unsatisfied. In this study, the highest current efficiency of D-arabinose is 75% in undivided cell and 127% in divided cell. The production of D-arabinsoe via paired electro-oxidation in dived cell is better than in undivided cell. The current efficiency in paired electro-oxidative divided cell’s system is 1.5 times of that of anodic oxidation. Accordingly, the operation in divided cell is better. The paired electro-oxidation of sodium gluconate to form D-arabinsoe is a new process and a potential for commercialization.
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