Micellar Solutions In Catalysis and Separation: Systematic Approach to Select the Surfactant System

Micellar systems have been successfully applied as reaction media for hydrogenation reactions [1]. They offer an advantage to solubilize reactants and catalysts which do not dissolve well in the aqueous phase. Moreover, catalyst recovery as well as product isolation can be achieved simultaneously by using ultrafiltration. Since the partitioning of reactants, products, and catalyst between the aqueous and the micellar phase has a substantial influence on the reaction kinetics as well as on the separation process, optimization of reaction conditions in micellar systems can be done by taking into account the partition coefficient.

Micelle/water partition coefficients can be either measured [2] or adequately predicted using thermodynamic models such as the group-contribution method UNIFAC and the a priori Conductor-like Screening Model for Realistic Solvation (COSMO-RS) based on quantum mechanics [3]. Both models have shown their applicability in the field of chemical engineering for more than 10 years [4]. The main goal of the present study is to develop a systematic method to select surfactants (by size and nature) to optimize the separation step of reactions taking place in micellar systems based on the partitioning.

The catalytic hydrogenation of itaconic acid and its derivatives, such as itaconic acid-dimethylester and –diethylester, in micellar solutions is used as a model system. As a first step, partition coefficients of all reactants are predicted in the system non-ionic ethoxylated alcohols C_nE_m surfactants –water (n=number of carbon atoms in alkyl chain; m=number of ethoxygroups) by UNIFAC. As a consequence, the head size E_m and tail length C_n preferred for the separation of products in ultrafiltration are found. Furthermore, ionic surfactants such as cationic alkyltrimethylammonium bromide (C_nTAB) and anionic sodium alkylsulphate (NaC_nS) are studied by COSMO-RS. The variation of the tail length of anionic and cationic surfactants leads to diverse effects in anionic NaC_nS and cationic C_nTAB. Based thereon, an optimal surfactant can be chosen for the separation step.

The predictions of both thermodynamic models are then compared regarding the optimization of reaction and separation conditions. As a result, a systematic approach is developed to select a surfactant system. This approach will minimize the experimental effort significantly.

We thank DFG (grant AR 236/32-1 and SCHO 687/7-1) for the financial support.

[1] G. Oehme et al, Angew. Chem. Int. Ed. 44 (2005) 7174-7199

[2] M. Schwarze, R.Schomäcker, CIT 7 (2006) 931-936

[3] L. Mokrushina, M. Buggert, I. Smirnova, W. Arlt, R. Schomäcker, Ind. Eng. Chem. Res. 46 (2007), 6501-6509

[4] S. Maaßen, W. Arlt, A. Klamt; Chem.-Ing.-Tech. 67 (1995), 476; J. Gmehling. 20th ICCT, 3-8 August 2008