Supported, ionic liquid catalysis

Supporting ionic liquids in the pores of solid materials offers the advantage of high surface areas between the reactant phase and that containing the supported liquid catalyst. This approach is particularly useful for reactants with less than desired solubility in the bulk liquid phase. Another incentive for using such catalysts is that they can be used in continuous processes with fixed-bed reactors (26S). The use of an ionic liquid in the supported phase in addition to an active catalyst can help to improve product selectivity, with the benefit being similar to what was shown for biphasic systems. However, care has to be taken to avoid leaching the supported liquids, particularly when the reactants are concentrated in a liquid phase. 

Leaching of the ionic liquid from the support was negligible. However, deactivation was observed, caused by moisture present during product separation (which can be minimized with proper care) and by oligomerization of dodecene on the catalyst surface. Unfortunately, both factors cause irreversible loss of catalytic activity. 

It is also remarkable that the rhodium complex catalyst in the supported ionic liquid showed long-term stability, being used for 18 batches without any significant loss of activity. It was confirmed that no rhodium metal clusters were formed. However, acetone could be used to remove the ionic layer and its presence led to a total loss of activity. 

The first fixed-bed application of a supported ionic liquid-phase catalyst was hydroformylation of propylene, with the reactants concentrated in the gas phase (265). The catalyst was a rhodium-sulfoxantphos complex in two ionic liquids on a silica support. The supported ionic liquid phase catalysts were conveniently prepared by impregnation of a silica gel with Rh(acac)(CO) and ligands in a mixture of methanol and ionic liquids, [BMIMJPFg and [BMIM][h-C8Hi70S03], under an argon atmosphere. 

With this preparation method, a high ligand/Rh ratio (10 or 20) was shown to be essential for catalyst activity and for high nji ratios in the product aldehyde. In the presence of ionic liquid, the nji ratio was as much as 23.7, compared to a value of 16.9 observed in the absence of ionic liquids. Surprisingly, an L/Rh ratio of 2.5 resulted in an inactive catalyst. The catalytic activity and selectivity decreased steadily during the 5-h test, independent of the type of ionic liquid and the L/Rh ratio. 

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Yonker, C. R. and Linehan, J. C., A high-pressure NMR investigation of reaction chemistries in a simple salt hydrate,. Supercrit. Fluids, 29, 257 2004. Mehnert, C. R, Supported ionic liquid catalysis, Chem. Eur. ]., 11,50,2005. Giernoth, R. and Bankmann, D., Transition-metal free synthesis of perdeuter-ated imidazolium ionic liquidsby alkylation and H/D exchange, Eur. J. Org. Chem., 2008 (in print). DOT 10.1002/ejoc.200700784. 

The hydroformylation of 1-hexene by supported ionic liquid catalysis (SILC) was recently reported by researchers at ExxonMobil. In this system, the active catalyst HRh(CO)(tppti)3 (tppti = tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium)) is contained within the ionic liquid phase while excess tppti ligand is immobilized in the support material. TOP values of 65 min" were obtained with silc while an unsupported biphasic ionic liquid medium gave TOP values of 23 min. ... 

Supported ionic liquid catalysis is one of the main examples of SLPC adopted [120] to take advantage of ionic liquid properties without the drawbacks evidenced in Section 2.3.6. The viability of this concept has been confirmed by several studies that have successfully confined various ionic phases to the surface of support materials and explored their potential catalytic applications. Although most of the evaluated supports were silica based, several studies have focused on polymeric materials, including membranes. These materials were prepared by using two different immobilization approaches. The first involves the covalent attachment of ionic liquids to the support surface whereas the second simply deposits the ionic liquid phases containing catalytically active species on the surface of the support. 

Gruttadauria et al. [71a] utilized the previously described supported ionic liquid catalysis (SILC) concept for the L-proline-catalyzed aldol reaction (Scheme 2.33). 

Mehnert CP (2005) Supported ionic liquid catalysis. Chem Eur J 11 50-56... 

Catalyst leaching was not observed and the same catalyst was used for 18 batch runs without significant loss of activity. Both hydroformylations and hydrogenations clearly showed that supported ionic liquid catalysis is a very useful and efficient principle. 

In summary, the few initial studies conducted so r on the inter cial structure between ionic liquids and solid sur ces clearly suggest - in a more general context - that confinement of ionic liquids on solid sur ces definitely induces some distinctive, structural control of the molecular kyering of the ionic liquid and of the ionic hquid distribution (i.e. wetting ability). However, further investigations are surely needed to elucidate these effects in the context of supported ionic liquid catalysis in more detail. 

Afeworki, Supported ionic liquid catalysis for hydroformylation and hydrt enation reactions, 2003, 226th ACS National Meeting, New York. USA. 

Mehnert, C.P., Mozeleski, E.J., and Cook, R.A. (2002) Supported ionic liquid catalysis investigated for hydrogenation reactions. Chem. Common., 8 (24), 3010-3011. 

Attachment to the Support via SILC or SILP Catalysis Supported, ionic liquid catalysis (SILC) or supported, ionic hquid-phase (SILP) catalysis involves the incorporation of an organometalhc complex in an IE and the subsequent immobilization or confinement within the pores of an inorganic support [83]. This is an attractive... 

Figure 7.18 Supported ionic liquid catalysis applied in the hydroformylation of 1-hexene (tppti = tri(m-sulfonyl)triphenyl phosphine tris(1-butyl-3-methyl-imidazolium) salt. (Taken and modified from Ref. [108].)...

The concept of supported ionic liquid catalysis involves the surface of a support material that is modified with a monolayer of covalently attached ionic liquid fragments. Treating this surface with additional ionic liquid results in the formation of multiple layers of free ionic liquid on the support material. These layers serve as the reaction phase in which a homogeneous hydroformylation catalyst was dissolved. The concept of supported ionic liquid catalysis has successfully been used for hydroformylation reactions ]81]. 

C.P. Mehnert, R.A. Cook, N.C. Dispenziere, M. Afeworki, Supported ionic liquid catalysis - a new concept for homogeneous hydrofor-... 

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