Pervaporation of (R)/(S)-methyl 3-hydroxybutyrate (Sigma 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 (Sigma 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 Sigma 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 Sigma methyl 3-hydroxybutyrate, methanol, ionic liquid and/or the catalyst. The technique was applied at various temperatures, using the industrial composite PDMS membrane, PERVAP (TM) 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 Sigma MHB with respect to the retentate was 1.24 and the Sigma 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.