Supported ionic liquid phase catalysis in continuous supercritical flow

The separation of catalysts from the solvent and reaction products remains one of the major disadvantages of homogeneous catalytic reactions, which are otherwise advantageous because of their high activity, tuneable selectivity and ease of study. In recent years a large number of different strategies has been employed to address this problem.(1, 2) Ideally, the reactions would be carried out in continuous flow mode with the catalyst remaining in the reactor at all times, whilst the substrates and products flow over the catalyst. A variety of continuous flow reactions has been proposed,(3) but in this presentation we shall highlight the use of supported ionic liquid phase catalysts, over which the substrates flow dissolved in supercritical carbon dioxide (scCO2). The products are also removed by the flowing scCO2 stream.(4)
The catalyst is supported within a thin film of an ionic liquid supported within the pores of a microporous silica. This catalyst is then placed in a tubular flow reactor, similar to that used for heterogeneous reactions. The use of pressurised CO2 as the transport medium offers certain advantages, including:
•A wider substrate selection than is possible for all gas-phase reactions;
•Lower solubility of the ionic liquid and the catalyst in the flowing phase than when using all liquid flow;
•Better transport of gases to the catalytic centres than for liquid flow;
•Fast diffusion of all species to the catalytic centres;
Potentially, these advantages allow for high reaction rates, high rates of transport of substrate over the catalyst and low leaching of both the catalyst and the ionic liquid.
We shall describe work on various different reactions including metathesis(5), nowadays considered as one of the most powerful synthetic tools in organic chemistry, as well as discussing the effects of different reactant parameters – pressure, flow rates etc. on the reaction activity, as well as on the lifetime of the catalyst.

References
(1) D. J. Cole-Hamilton, Science, 2003, 299, 1702.
(2) D. J. Cole-Hamilton and R. P. Tooze, eds., Catalyst Separation, Recovery and Recycling; Chemistry and Process Design, Springer, Dordrecht, 2006.
(3) D. J. Cole-Hamilton, T. E. Kunene and P. B. Webb, in Multiphase Homogeneous Catalysis, ed. B. Cornils, Wiley VCH, Weinheim, 2005, vol. 2, pp. 688.
(4) U. Hintermair, G. Y. Zhao, C. C. Santini, M. J. Muldoon, D. J. Cole-Hamilton, Chem. Commun., 2007, 1462.
(5) R. Duque, E. Öchsner, H. Clavier, F. Caijo, S.P. Nolan, M. Mauduit, D.J. Cole-Hamilton, Green Chem., 2011, 13, 1187.