Continuous Flow Upgrading of Glycerol towards Epichlorohydrin

Abstract

We report herein on the valorization of glycerol, a “bio-based platform” molecule with a large availability, towards epichlorohydrin, a major building block in the production of epoxy resins. Epichlorohydrin production from glycerol has become economically viable with the rise of the bio-fuel market. Glycerol is obtained as a by-product of the biofuel industry, making it available in large quantities and at low price. Epichlorohydrin is traditionally produced from propene, a petro-based compound whose price tends to fluctuate a lot. The classical process towards epichlorhydrin suffers from several drawbacks such as a low atom economy, the use of hazardous materials and the production of toxic by-products. Our research is focused on the development of a safe and efficient continuous-flow process that allows the production of epichlorohydrin from glycerol. The first step of the synthesis involves the conversion of glycerol into monochlorohydrins and then dichlorohydrins with a chlorination agent (here aqueous HCl) catalyzed by carboxylic acids. The temperature and residence time were optimized in a microfluidic setup. Then, a catalyst screening was undertaken, highlighting the high efficiency of common lactones such as γ-butyrolactone and ε-caprolactone, as well of aliphatic dicarboxylic acids such as adipic and pimelic acids. The latter showed an exceptional catalytic activity (>99% conversion, 81% cumulated yields towards chlorohydrins) and high selectivity for 1,3-dichloro-2-propanol. The second step of the synthesis is the epoxidation of dichlohydrins towards epichlorohydrin with the help of a base such as NaOH. The epoxidation reaction was firstly studied in conventional batch reactors and then, transposed to continuous flow operation under microfluidic conditions. This step was very fast and showed >90% of conversion in less than 5 minutes at room temperature. Finally, the two steps of the reaction were concatenated into a single microfluidic system allowing the direct use “on the spot” of the toxic chlorohydrins generated in the first step. The process showed very promising results with 98% of conversion and 96% of cumulated yields towards a ca. 1:1 mixture of epichlorohydrin and glycidol, another value-added compound. Moreover, the conditions reported herein have a low environmental footprint since the reaction occurs in water, and the chlorination agent is HCl. The inherent high heat and mass transfer capacities related to microfluidic reactors coupled with the use of an excellent catalyst significantly reduced the spaceframe for this process, with total reaction times of 25 minutes under continuous flow conditions.