Scale-up of the solid polymer electrolyte reactor for electro-organic synthesis

Abstract

Electro-organic reactions are often complicated by the need to add supporting electrolytes and co-solvents. In many cases these additives take part in side reactions causing low yields and hinder the purification stages. The solid polymer electrolyte (SPE) reactor uses an ion exchange membrane to transfer charged species between the electrodes and so eliminates the need for any additives. In this way improvements in electrochemical processing can be achieved. The SPE reactor has only been studied for model organic and aqueous based electrochemical reactions. The aims of this project were to develop the reactor for use as a suitable means of synthesising alcohols and acids based on substituted toluenes. This involved selection of suitable electrode material, polymer electrolyte pre-treatment and reactor modelling. According to published reports the direct electro-oxidation of toluene takes place with maximum yields of 19% with an acetic acid co-solvent and a nitric acid supporting electrolyte. Higher yields are possible with inorganic mediators such as Mn³⁺ and Cr⁶⁺. 30% yields of methoxylated products are possible from electrolysis in methanol although many non volatile by-products are formed. Initial research was spent investigating the oxidation of toluene in sulphuric acid at a lead dioxide rotating disk electrode. It was found that the reaction is mass transfer limited in the potential region below gas evolution. The order of reaction with respect to toluene was 0.5. Electrolysis of toluene on platinum mesh in nitric acid with and without acetic acid was found to produce benzyl alcohol and benzaldehyde with low current efficiencies. Without co-solvent the maximum current efficiency was 10% at 2S0Alm². An SPE reactor fabricated from glass with an active electrode area of Scm2 was used to perform electrode tests. Highest yields of benzaldehyde were obtained using nickel foam, graphite felt and palladium coated mesh electrodes. The current efficiencies were 52.4%, 20.3% and 10.7% respectively. This work highlighted the need for a good membrane-electrode contact. The oxidation of benzyl alcohol in the same reactor using nickel foam Abstract was accomplished with a current efficiency of 85.4% showing that the difficult step in the oxidation of toluene was the first one to benzyl alcohol. Pre-treatment of the membrane by swelling in solvents was considered to be an important factor in the performance of the SPE reactor. Several ion exchange membranes were pre-treated in a variety of aqueous and organic solvents including methanol, toluene, DMF, water and sulphuric acid. Nafion® 117 was found to increase in size more than the other tested membranes in all solvents except water and sulphuric acid. Many of the pre-treated membranes were tested in an SPE reactor made from steel with an active electrode area of 2lcm2 for the oxidation of toluene in methanol. The anode-membrane potential was measured as a function of time and current density with Nafion® 117 having the lowest values of potential. Selection of the pre-treatment method for future use was determined by assessing the performance in the reactor, contamination of products and chemical hazards. Swelling in aqueous solvents was the chosen procedure. The steel SPE reactor was operated in continuous mode with recycle for the oxidation of toluene in methanol. Galvanostatic electrolysis took place at several current densities, temperatures and feed concentrations. Two products were identified as ⍺-methoxytoluene and ⍺,⍺-dimethoxytoluene and these were formed at low current efficiencies between 1.4% and 9%. The main product was thought to be an oligomer of toluene. The gas generated was found to be mainly hydrogen with a small amount of oxygen thought to come from residual water in the pre-treated membrane. A computer simulation of the SPE reactor for toluene oxidation in methanol was based on two series and one parallel reaction. These were first order in reactant species and followed Tafel type kinetics. Mass transfer of dilute reactants was based on Fickian diffusion. Parameters not available in the literature such as membrane potential and electro-osmotic flow were correlated to applied variables using experimental data and multiple linear regression. The importance of electro-osmotic flow in the SPE reactor was demonstrated by considering its effect on product distribution. The model showed that the oligomerisation of toluene was the dominant reaction making the SPE reactor unsuitable for the oxidation of toluene.