Pervaporation of (R)/(S)-methyl 3-hydroxybutyrate (∑MHB) from a mixture containing an ionic liquid, methanol and Ru catalyst

Abstract

Complex technologies in the production of fine chemicals bring together various connected processes, each bringing specific green chemistry challenges. A valuable mixture arises in the stereoselective hydrogenation of methyl-acetoacetate (MAA) to (R)- and (S)- methyl 3-hydroxybutyrate (∑MHB) over Ru organometallic complex (R)-Ru/BINAP, and in a “green” tetra-alkylammonium bistriflimide/methanol/water solvent phase. The reaction could be carried out in various arrangements, e.g. in a batch mode, in a continuous stirred-tank reactor, or even in a tubular microfluidic chip reactor. In either case, the valuable optically pure Ru complex has been sought to be effectively separated, and potentially recycled. The ionic liquid phase (tetra-alkylammonium bistriflimide) is intended for selective accommodation of the complex in the reaction mixture and as a green alternative to other solvents. It was the main issue of this work to design a specific separation unit, to construct it, and to optimize its performance towards the efficient separation of ∑MHB from the ionic liquid phase bearing the (R)-Ru/BINAP complex. The unit was developed to be connected with the complex preparation process, employing the microreactor fluid flow platform. The separation part of the overall technology was designed as a membrane pervaporation. The membrane separation proceeded with different feed mixtures combining ∑methyl 3-hydroxybutyrate, methanol, ionic liquid and/or the catalyst. The technique was applied at various temperatures, using the industrial composite PDMS membrane, PERVAP™ 4060 from Sulzer. The feed as well as the pervaporate were analyzed by nuclear magnetic resonance spectroscopy and infrared spectroscopy. The enrichment factor of the ∑MHB with respect to the retentate was 1.24 and the ∑MHB permeate mass flux was 61 g h−1 m−2. In addition, the membrane was characterized by scanning electron microscopy and energy dispersive X-ray microanalysis after the membrane process, showing no sorption of the catalyst into the membrane.