Application Of Bipolar Membrane Electrodialysis In Non-aqueous Systems

Bipolar membrane electrodialysis (BMED), due to its special water/alcohol-splitting, can realize a plenty of reactions in-situ and eleminate the generation of pollutants, so it becomes an important electro-membrane technology for green chemistry and separations. Theorectically, alcol-splitting in bipolar membranes can be used for any condensation reactions by introducing alkoxy groups from alkoxide ions, which will derive many new routes of organic synthesis and is thus of significance to green organic synthesis. However, much fewer alchol-splitting applications were reported for cleaner industrial production than water-splitting applications. This is mainly because there is a lack of research on the mechanism of alcohol-splitting, mass transport in organic systems, and industrial opearations.To achieve cleaner production in chemical industry, this research focuses on the application of BMED in non-aqueous systems by using methnol-splitting or ethanol-splitting. The contents include the methods of membrane characterization in non-aqueous systems and green routes of organic synthesis. The conclusions are as follows:For investigation on the alcohol-splitting of bipolar membranes in methanol or ethanol, current-voltage curves and impedance spectra can also be employed to measure the thickness of the intermediate layer of bipolar membrane and provide guidiance for improving membrane performances.Better than FT-BP and FBM, Neosepta BP-1 swells in methanol or ethanol but still functions well, so it can be counted as a commercial bipolar membrane suitable for application in alcohol-splitting.When bipolar membranes split ethanol,1H NMR analysis can be employed to identify ethoxide ions (CH3CH2O-:δ3.736-3.666 (q,2H) andδ1.255-1.209 (t,3H)) and thus gives direct evidence for the alcohol-splitting in bipolar membranes.Methyl methoxyacetate can be prepared from methyl chloroacetate in a safe and environmentally-friendly manner by running BMED in methanol. The experimental results indicate that as current density increases, the product yield increases, but the current efficiency decreases. Besides, as methyl chloroacetate concentration increases, both the product yield and current efficiency increase; i.e., the product concentration increases from 1.08 mmol dm-3 to 6.59 mmol dm-3, and the current efficiency increases from 2.6% to 15.7%.The electrical resistance of a non-aqueous system can be reduced to a great extent by integrating BMED with ion-exchange (i.e., filling ion-exchange resins in a BMED stack), and the obstacle to the industrial application of BMED due to high energy-consumption can be removed. This integration is adopted for production of methyl methoxyacetate in methanol, and the performance of 4 kinds of ion-exchange resins are assessed in terms of the voltage drop, product yield, and current efficiency. Under the experimental conditions, the lowest voltage drop was achieved by using D201 macrorecticular anion-exchange resin, and the voltage drop decreased by 44.3%-61.4%. However, there was a slight decrease in the product yield and current efficiency due to the adsorption of methyl methoxyacetate onto the resins. As a compromise,201*7 gel-type anion-exchange resin is the best choice.In summary, this work further extends the application of BMED in non-aqueous systems. Particulary, the method of introducing aloxy groups via alcohol-splitting is not only a new strategy for organic synthesis but also is of great significance to the sustainable development in organic industry.