Motivation

This project is focused mainly on generating chiral membranes made of Molecularly Imprinted Polymers (MIP's). These membranes should be integrated into a membrane reactor which should resolve a racemic mixture. This might take place as a prestep in an enantioseparation by crystallisation. The first objective for this complex project is to develop an enantioselective porous material which can be molded in various forms (i.e. HPLC-columns, membranes) with controlled properties (porosity, surface, selectivity). Parallel the modeling of the membrane process should support the finding of the best properties for the membrane and suitable operation parameters.

Molecularly Imprinted Polymers

The molecular imprinting is done by radical polymerisation of a mixture of functional monomers and cross-linking monomers in the presence of a template, i.e. one of the two enantiomers. The radicals are formed by thermal decomposition of an initiator. During the polymerisation the template acts as a space holder which is surrounded by the reacting monomers. It will be removed by an extracting step after the polymerisation and will leave a cavity which fits to the shape of the template. This leads to a material that can separate the template from a mixture of molecules with a similar shape.This makes such material attractive for separating racemic mixtures.

Fig.1: schematic representation of the Molecular Imprinting Process:
a) self organising, b) polymerisation, c) desorption / extraction, d) adsorption / recognition

Porous Monolithic MIP's

To induce a porous structure inside the MIP a porogen is added. During the polymerisation the growing chains precipitate when the solubility is reached. The porous structure depends on both the ratio of porogenic to monomeric mixture and the quality of the solvent, i.e. the better the solubility the bigger are the particles. The temperature has also a wide influence on the porous properties and the flow characteristics of the monoliths but with regard to the Molecular Imprinting the lower temperature is usally preferred because the greater brownian motion is counterproductive to the self organising step during the preparation.

Fig.2: Preparation of porous monoliths by in situ polymerisation

Mass transfer through porous membranes

The transport through porous chiral membranes follows mainly three independent mechanisms. The viscous flow and the diffusion occur in the pore phase while surface diffusion occurs on the solid phase.[1] Only the surface diffusion is enantioselective therefore the other two transport mechanisms should be suppressed.

References

[1] A. Tuchlenski, P. Uchytil and A. Seidel-Morgenstern Journal of Membrane Science, Volume 140, Issue 2, 1998, Pages 165-184
[2] A. Seebach, A. Seidel-Morgenstern; Analytica Chimica Acta, 591 (2007) 57-62.