Preparation of catalytic deep eutectic solvent-based silica microreactors using a non-aqueous sol–gel route

Abstract

This study describes the microencapsulation of deep eutectic solvents (DESs) via a non-aqueous sol–gel technique. The microencapsulation was performed using oil in oil emulsions as templates for the interfacial polycondensation of silane monomers such as tetraethoxyorthosilicate and polydimethoxysiloxane. The non-aqueous polycondensation of the silane monomers was achieved using formic acid, which acted as both a catalyst and a reagent. The process also required the presence of cetyl polyethyleneglycol/polypropyleneglycol-10/1 dimethicone as an emulsifier to stabilize the emulsion droplets. The silica-enclosed compounds were separated by centrifugation and dried to provide powders that were much easier to handle compared to the original, highly viscous DES bulks. These microcapsules were then used as hosts for palladium nanoparticles and applied in the hydrogenation of different unsaturated compounds. In addition, microcapsules containing choline chloride:L-tartaric acid (1 : 0.5) were used as heterogeneous Brønsted acidic catalysts for the preparation of N-substituted pyrroles. In both cases, the capsules were recycled several times without significant decrease in conversion. This work demonstrates: (1) the efficiency of combining O/O emulsions and formic acid-mediated non-aqueous sol–gel as a technique for microencapsulation, and (2) the benefits of using microencapsulated DESs as heterogeneous catalysts.