Supported Ionic Liquid Phase Catalysis

Continuous flow hydroformylation of long chain alkenes (1-octene) has been carried out in a supported homogeneous system using a SILP (Supported Ionic

Liquid Phase) catalyst, where a thin layer of ionic liquid catalyst is immobilized at the surface of an inert solid support, like for instance, silica gel.

SILP catalysts have the ability to combine the advantages of both homogeneous and heterogeneous catalysts. By using a fixed-bed reactor in a homogeneous system, it is possible to achieve a better catalyst/product separation and still expect the same performance in terms of selectivity as a “normal” homogeneous catalyst.29, 33

SILP catalysis can be carried out in the liquid29, 34-37 or gas28, 37-40 phases. When using liquid phase substrates, there is a possibility that the catalyst and ionic liquid may leach from the support, especially if the ionic liquid has been designed to dissolve the substrate so as to reduce problems with mass transport. One way of getting around this problem is chemically to anchor the ionic liquid to the support. Mehnert and co-workers29 investigated the hydroformylation of 1-hexene using Rh/[BDMIM]3[P(3-C4H6SO3)3] (BDMIM = 1-butyl-2,3-dimethylimidazolium)

and a series of ionic liquids supported in the pores of silica, from which the surface hydroxyl groups had been removed by reaction with an imidazolium modified triethoxysilane. This gives a layer of imidazolium salt anchored to the

silica, which may aid in the immobilisation of the ionic liquid. In general, it was found that turnover frequencies were low (< 55 - 65 h-1) compared with the ones for an analogous homogeneous system using Rh/PPh3 (400 h-1), but higher than

the ones for the bulk biphasic system (23 h-1). Leaching of the ionic liquid and the catalyst was significant but less when very polar ionic liquids, such as [BMIM]PF6, were used or if the conversion of the non-polar alkenes to the more

polar aldehydes was kept to a minimum. Despite the leaching, the SILP process provides a very elegant solution to the catalyst product separation problem.

Hydrogenation of alkenes does not produce a polar product, so a similar system to the one described above for the hydroformylation of liquid alkenes was used in the hydrogenation of 1-hexene to give enhanced activity compared with the homogeneous or bulk biphasic systems without any measurable leaching. The catalyst could be reused 18 times without any loss of activity.41 Continuous reactions in a flow reactor were not reported.

One other problem that can arise when using SILP catalysts for liquid phase substrates in reactions which also involve gases, is severe gas depletion. The aspect ratio of the pores within the support is high, so that gases have to diffuse a long way to arrive at the catalytically active centres. Once the gas that is initially dissolved in the substrate has been consumed, the reaction rate will drop significantly as it becomes gas transport limited.

None of the problems occur if the reaction is carried out in the gas phase. Here, diffusion to the catalytic sites is fast and, being essentially involatile, neither the catalyst nor the ionic liquid is transported out of the reactor. Thus, continuous

flow propene33 or butene hydroformylations have been performed over a SILP catalyst for several hundred hours using, for example, a Rh/sulpho-xantphos catalyst in supported [BMIM][n-C8H17OSO3].42 The support must first be treated

to remove acid sites which protonate the ligand, but under optimised conditions, the system works very well. There is a small fall off in rate at longer reaction times because aldol condensation products of the C4 aldehydes formed block the

pores and/or catalyst, but these products can be removed by evacuating the catalyst at elevated temperature. Following this treatment, the catalyst returns to its initial activity and selectivity.33, 37, 38, 40, 43-45

The main problem with using all gas phase reactions is their limited scope. There are rather few reactions, methanol carbonylation being another that have been investigated36 where the substrates and products are all in the gas phase at the reaction temperature. An alternative is to use high temperatures and very low flow rates of the less volatile substrates. However, the total throughput of such a system will be rather low.

One other alternative is to carry out the SILP catalysis using relatively low volatility substrates transported over the catalyst bed dissolved in a supercritical fluid.46This type of system has a number of potential advantages. Gas diffusion is fast in the supercritical phase. With the substrates and gaseous reagents all being contained in the one phase, they all have very good access to the pores of the support and hence the catalytic centres. By using ionic liquids and ionic catalysts that are insoluble in the supercritical fluid, leaching should be minimised. Supercritical carbon dioxide (scCO2) has been shown to increase the solubility of

the ionic liquid should be improved. Finally, any heavy products that form may be soluble in the supercritical fluid, so that fouling of the catalyst can be avoided.

A system for the hydroformylation of 1-octene using a SILP catalyst consisting of a rhodium complex formed in situ from [Rh(acac)(CO)2] (acacH = 2,4-

pentanedione) and [PrMIM][Ph2P(3-C6H4SO3)] (PrMIM = 1-propyl-3-

methylimidazolium), dissolved in a thin film of [OctMIM]Tf2N (OctMIM = 1-

octyl-3-methyimidazolium; Tf = CF3SO2) with CO2 flow has been demonstrated

and is shown schematically in Figure 1.9.46 The reaction works best close to the critical point of the mobile phase producing over 500 catalyst turnover h-1 continuously for 40 h, with Rh leaching of only 0.5 ppm. Aldol condensation products from the C9 aldehydes were also detected in the collected products,

Figure 1.9 Schematic diagram of SILP catalyst with supercritical flow for the hydroformylation of 1-octene.46