Abstract

With water providing a highly favoured solution environment for industrial processes (and in biological processes), it is interesting to develop water-based electrolysis processes for the synthesis and conversion of organic and biomass-based molecules. Molecules with low solubility in aqueous media can be dispersed/solubilised (i) by physical dispersion tools (by milling, by power ultrasound, or by high shear ultra-turrax processing), (ii) in some cases by pressurising/super-saturation (e.g. for gases), (iii) by adding co-solvents or “carriers” such as chremophor EL, or (iv) by adding surfactants to generate micelles, microemulsions, and/or stabilised biphasic conditions. This review examines and compares methodologies to bring the dispersed or multi-phase system into contact with an electrode. Both the microscopic process based on the individual particle impact as well as the overall electro-organic transformation are of interest. Distinct mechanistic cases for multi-phase redox processes are considered.

Most traditional electroorganic transformations are performed in homogeneous solution with reagents, products, electrolyte, and possibly mediators or redox catalysts all in the same (usually organic) solution phase. This may lead to challenges in the product separation step and in the re-use of solvents and electrolytes. When exploiting aqueous electrolyte media, reagents and products (or even electrolyte) may be present as microdroplets or nanoparticles. Redox transformations then occur during interfacial “collisions” under multiphase conditions or within a reaction layer when a redox mediator is present. Benefits from this approach can be (i) the use of highly conducting aqueous electrolyte, (ii) simple separation of products and re-use of electrolyte, (iii) phase transfer conditions in redox catalysis, (iv) new reaction pathways, and (v) improved sustainability. In some cases, a surface phase or phase boundary processes can lead to interesting changes in reaction pathways. Controlling the reaction zone within the multi-phase redox system poses a challenge and methods based on micro-channel flow reactors have been developed to provide a higher degree of control. However, detrimental effects in micro-channel systems are also observed, in particular when considering limited current densities (which can be very low in microchannel multiphase flow) or when developing technical solutions for scale-up of multi-phase redox transformations.

This review describes physical approaches (and reactor designs) to bring multi-phase redox systems into effective contact with the electrode surface, as well as cases of important electro-organic multi-phase transformations. Mechanistic cases considered are “impacts” by microdroplets or particles at the electrode, effects from dissolved intermediates or redox mediators, as well as effects from dissolved redox catalysts. These mechanistic cases are discussed for important multiphase transformations for gaseous, liquid, and solid dispersed phases. Processes based on mesoporous membranes and based on hydrogen-permeable palladium membranes are discussed.
Original languageEnglish
Pages (from-to)3325-3338
Number of pages14
JournalAccounts of Chemical Research
Volume52
Issue number12
Early online date25 Nov 2019
DOIs
Publication statusPublished - 17 Dec 2019

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