Immobilization catalysts

The concept of immobilized ionic liquids entrapped, for instance, on the surface and pores of various porous solid materials (supported ionic liquid phase, SILP) is rapidly become an attractive alternative. In addition, the SILPs can also answer other important issues, such as the difficult procedures for product purification or IL recycling, some toxicity concerns and the problems for application in fixed-bed reactors, which should be addressed for future industrial scale-up. This new class of advanced materials shares the properties of true ILs and the advantages of a solid support, in some cases with an enhanced performance for the solid material. Nevertheless, a central question for the further development of this class of materials is to understand how much the microenvironment provided by the functional surfaces is similar or not to that imparted by ILs. Recent studies carried out using the fluorescence of pyrene to evaluate the polarities of a series of SILPs based on polymeric polystyrene networks reveal an increase in polarity of polymers, whereas the polymer functional surfaces essentially maintain the same polarity as the bulk ILs. However, this is surely not a simple task, in particular if we consider that the basic knowledge of pure ILs is still in its infancy, and we are just starting to understand the fundamentals of pure ILs when used as solvents. 

An alternative sol-gel strategy has been reported for the preparation of a closely related material. Using the approach of the initial grafting of the imidazolium cation onto pre-dried silica, followed by treatment with PdCl2, non-acidic supported palladium-based ILs were also prepared and used in Suzuki-Miyaura coupling reactions.  

Analogously, oxidation catalysts were prepared by using an interesting integrated approach that combines sol-gel entrapped perruthenate as aerobic catalyst, an encapsulated ionic liquid as solubility promoter, and SCCO2 as 

In the simplest methodology, however, the immobilization approach involves the treatment of a solid with a substantial amount of ionic phase. The IL, or the IL-metal complex, is sucked into the support by capillary forces, resulting into a seemingly dry texture. In contrast to the previously described situations, this approach is based on non-bonding attachment of the IL to the support and results in the formation of multiple layers of free IL on the carrier, which then acts as an inert phase to dissolve catalysts. The resulting ionic phase behaves both as reaction medium and catalyst. 

The possibility of using the ordering effect to induce unusual properties in the supported complexes was suggested by the authors, who included among the peculiar features the enhanced metal-substrate interactions, the reorientational of substrate molecules within the solvent cage and the possibility of directing the approach of molecules to catalytically active centers. 

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These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

De Vos DE, Vankelecom IFJ, Jacobs PA (eds) (2000) Chiral catalysts immobilization and recycling. Wiley-VCH, Weinheim... 

Oehme G (1999) Catalyst immobilization two-phase systems. In Jacobsen EN, Pfaltz A, Yamamoto H (eds) Comprehensive asymmetric catalysis, vol III. Springer, Berlin Heidelberg New York, p 1377... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Fig. 1. Scheme for preparation of PM012 catalyst immobilized on PS bead. 

In this work, various Ru-BINAP catalysts immobilized on the phosphotungstic acid(PTA) modified alumina were prepared and the effects of the reaction variables (temperature, H2 pressure, solvent and content of triethylamine) on the catalytic performance of the prepared catalysts were investigated in the asymmetric hydrogenation of dimethyl itaconate (DMIT). 

At the next level of abstraction are measurements performed at a thin film of fuel cell catalyst immobilized on the surface of an inert substrate, such as glassy carbon (GC) or gold (Fig. 15.2c). Essentially, three versions of this approach have been described in the fiterature. In the first case (a porous electrode ), an ink containing catalyst and Nafion ionomer is spread onto an inert nonporous substrate [Gloaguen et al., 1994 Gamez et al., 1996 Kabbabi et al., 1994]. In the second case (a thin-fihn electrode ), the ink does not contain Nafion , but the latter is... 

Catalysis of supported metal ions is an area of interest. There are a number of advantages in depositing catalytically active metal ions on a support. The ion exchange method of catalyst immobilization is simple and the attractiveness of this method is further increased by providing stable inorganic ion exchangers of known structures as supports. 

Hu, A.G., Yee, G.T. and Lin, W.B. (2005) Magnetically recoverable chiral catalysts immobilized on magnetite nanopartides for asymmetric hydrogenation of aromatic ketones. Journal of the American Chemical Society, 127 (36), 12486-12487. 

The invention also included catalyst immobilization, which was incorporated to facilitate the separation step. Maintaining the catalytic activity while meeting the requirements of the mechanical properties clearly imposes very restrictive conditions on the immobilization support material. Where and how a fixed bed of catalyst is included in a desalting process, or even a moving phase, is not clearly defined. The patent [251] was issued in 1994, and no further development to this scheme has been published since then. 

Immobilization of catalysts is an important process design feature (see Chapter 9.9). A recent example of catalyst immobilization is the biphasic approach which seems superior to immobilization on solids, as successfully proven in the Ruhrchemie/Rhone Poulenc process for the hydro-formylation of olefins.286 Supported liquid phase catalysis was devised as a method for the immobilization of homogeneous catalysts on solids. When the liquid phase is water, a water-soluble catalyst may be physically bound to the solid. 

Thus, a very efficient catalyst immobilization can be carried out by route [6]. However, route [5] is much less succeSsful ... 

Ionic liquids have also been applied in transfer hydrogenation. Ohta et al. [110] examined the transfer hydrogenation of acetophenone derivatives with a formic acid-triethylamine azeotropic mixture in the ionic liquids [BMIM][PF6] and [BMIM][BF4]. These authors compared the TsDPEN-coordinated Ru(II) complexes (9, Fig. 41.11) with the ionic catalyst synthesized with the task-specific ionic liquid (10, Fig. 41.11) as ligand in the presence of [RuCl2(benzene)]2. The enantioselectivities of the catalyst immobilized by the task-specific ionic liquid 10 in [BMIM][PF6] were comparable with those of the TsDPEN-coordinated Ru(II) catalyst 9, and the loss of activities occurred one cycle later than with catalyst 9. 

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