SILP Catalysis

Supported Ionic Liquids Fundamentals and Applications, First Edition. 

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann. 

As examples, Mehnert et al. [5] showed how SILP systems had enhanced activity with comparable selectivity to the biphasic analogs in the batch hydroformylation reaction of 1-hexene and in the batch hydrogenation of 1-hexene, cyclohexene, and 2,3-dimethyl-2-butene [6]. Furthermore, in the hydrogenation reactions the catalyst could be reused for 18 batch runs without any significant loss of activity and with catalyst leaching below the detection limit. 

Wolfson et al. [7] also compared the activity between biphasic IL-organic solvent and SILP system in batch hydrogenation reactions. In different examples, the SILP system provided better activity and selectivity than the biphasic system. In both systems, the catalyst phase could be recycled without loss of activity. Breitenlechner et al. [8] showed how SILP catalysts combined the enhanced catalytic activity with improved selectivity over biphasic and homogeneous systems for batch hydroamination reactions. 

Furthermore, Heck [9], aldol [10], epoxidation [11], hydrogenation [12], cyclo-propanation [13], Mukaiyama aldol condensation [14], and oxidative kinetic resolution [15] batch reactions have also been successfully performed with SILP systems. 

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Rhodium Catalysed Hydroformylation Using Supported Ionic Liquid Phase SILP) Catalysis... 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

Figure 7.10. Supported ionic liquid phase (SILP) catalysis exemplified for the Rh-catalysed hydroformylation reaction...

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

The few examples where SILP catalysis has been tested so far showed highly encouraging results. It is very likely that other applications where ionic catalyst solutions were tested in liquid-liquid biphasic reactions could be reinvestigated under SILP conditions. If very high catalyst stability over time can be realised or simple catalyst regeneration protocols can be developed than SILP catalysis can be expected to make its way into industrial processes. 

TABLE 7.6. Process characteristics for optimised nonanal production (using liquid-liquid biphasic catalysis with ionic liquids) and butanal production (using SILP catalysis) on a 100.000 tons/year scale... 

Silica-supported ionic liquid-phase (SILP) catalysis has been developed as an alternative approach to address the problem of product isolation, a methodology well known from aqueous catalysis, and an overview of SILP-hydroformylation reactions in ionic liquids is given in Table 4.4.[631... 

Reactions are run for a time which depends on catalyst half-life. A benchmark process for SILP catalysis is the hydroformylation of alkenes. Wasserscheid and coworkers reported the hydroformylation of propene catalysed by a silica-supported phosphane 35-Rh complex in [bmim] [zj-CgHiyO-SOs] (Figure 39). TOF values range from 16 to 46 h under different reaction conditions (reagent partial pressures, support pre-treatment, etc.), while selectivity in favour of the linear aldehyde was constantly around 94-95%. 

It must be also noted that supported ionic liquid phase (SILP) catalysis can also be successfully combined with supercritical fluids. Cole-Hamilton et al. [127] have reported recently high activity (rates up to 800 h ), stable performances (>40 h) and minimum rhodium leaching (0.5 ppm) in the hydroformylation of 1-octene using a system that involves flowing the substrate, reacting gases and products dissolved in... 

The reactants in SILP catalysis are preferentially processed in gaseous form. Processing of solid SILP catalysts in a liquid reaction phase as a slurry requires extremely low solubility of the ionic liquid film in the liquid reaction mixture and affords special constraints upon the mechanical stability of the liquid film. In contrast, for the - by r more attractive - gas-phase applications of SILP catalysts the extremely low volatility of the ionic liquids is the key success fector. It is noteworthy that earlier attempts to apply supported liquid catalysts in continuous gas phase reactions - using organic liquid phases [32] or water [33] as the immobilized liquid phase - resulted in catalyst systems too unstable for technical use due to evaporation of the liquid film with time. In contrast, SILP systems have been... 

Finally, SILP catalysis has been introduced in the last three years using mainly hydroformylation as the model reaction to develop the technology [190]. Due to the general importance of SILP catalysis a separate section is devoted to this topic (see Section 5.6) in which all details of the studied SILP-hydroformylation systems can be found. 

Finally, a very interesting approach has been suggested by the same group to apply an ionic liquid to the preparation of Ru nanopartides of defined size on a meso-porous silica support [279]. The authors made use of the ionic liquid [TGA] [lactate] to customize particles obtained by reduction of dissolved RuQs on the silica support. However, prior to catalysis, the ionic liquid film was removed thermally by heating to 220 °C for 3 h. In this way a quite active catalyst for the hydrogenation of benzene to cyclohexane (TOFs up to 83 h at 10 bar H2 pressure) was obtained. Such an approach obviously combines successfully features of SILP catalysis (see Section 5.6 for details) with the use of catalytic nanopartides. 

Once a very stable ionic catalyst solution that shows all required selectivity and production rate characteristics has been identified, metal leaching into the product phase is indeed the next issue that has to be addressed. Excellent advances have been made in recent years in this field. In many applications it was possible to suppress catalyst leaching down to ppb levels using ionic hgands attached to the catalytic metal (see this section and Section 5.4 for numerous examples). Another strategy that very effectively avoids leaching problems is to isolate the reaction products from the ionic catalyst solution via the gas phase. This approach has been very effectively realized in the SILP catalysis technique (see Section 5.6 for details) and builds on the extremely low volatihty of transition metal complexes dissolved in ionic liquids. 

In Section 5.3 it was demonstrated with many examples that ionic hquids are indeed a very attractive class of solvents for catalysis in liquid-liquid biphasic operation (for some selected reviews see Refs. [16-20]). In this section, we wfll focus on a different way to apply ionic liquids in catalysis, namely the use of an ionic liquid catalyst phase supported on a solid carrier, a technology that has become known as supported ionic liquid phase (SILP) catalysis. In comparison to the conventional liquid-liquid biphasic catalysis in ionic liquid-organic liquid mixtures, the concept of SILP-catalysis combines well-defined catalyst complexes, nonvolatile ionic liquids, and porous solid supports in a manner that offers a very efficient use of the ionic liquid catalyst phase, since it is dispersed as a thin film on the surface of the high-area support. Recently, the initial applications using such supported ionic liquid catalysts have been briefly summarized [21]. In contrast to this report, where the applications were distinguished by the choice of support material, the compilation here will divide the applications using the supported ionic liquid catalysts into sections according to the nature of the interaction between the ionic liquid catalyst phase and the support. 

Riisager, A., Fehrmaim, R., Haumarm, M., and Wasserscheid, P. (2006) Supported Ionic Liquid Phase (SILP) catalysis an iimovative concept for homogeneous catalysis in continuous fixed-bed reactors. Eur. J. Inorg. Chem., 4, 695-706. 

Silica surfaces with lower surface charge are distinctly different from mica surfaces and are usually of greater importance in catalytic processes where they constitute the primary support material in supported ionic Hquid phase (SILP) catalysis. The structural arrangement of ionic liquid studied by surface analytical techniques, UHV techniques, in particular, point toward preferential cation enrichment at the innermost surface layers. 

The examples described above of the successful use of SILP catalysis were aU gas-phase reactions under conditions where no condensation of substrates or products in the gas stream was possible. (Note that capillary condensation inside the porous network of a SILP catalyst might nonetheless occur under certain circumstances.) This is an important prerequisite for effective SILP catalysis as a continuous flow of liquid could remove the thin film of IL from the support (either by dissolution or mechanical displacement), rendering the catalyst inactive in most cases. In case of propene and 1-butene hydroformylation, the formation of the so-caUed heavies 18 and 19 via aldol condensation of the respective aldehyde products (see Scheme 15.3) was reported to reduce the catalyst activity over time, whereas the selectivity remained unchanged [15]. 

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