Supported ionic liquid phase catalysis advantages

This modern method, commonly known supported ionic liquid phase catalysis, is based on the simple chemical attachment of ILs to the support surface or the simple deposition of the catalytically active speciesmain idea for the preparation of these alternative materials is to avoid or at least decrease the deactivation of the catalyst after reactions as well as to minimize the amount of IL used in each process. In addition, the SILP method provides some advantages compared to other catalytic systems. For example, SILP catalytic systems offer the elimination/reduction of mass transfer limitations and give access to more robust/recyclable catalysts with an easy separation after reactions. In other words,... 

Structured supported ionic liquid-phase (SSILP) catalysis is a new concept that combines the advantages of ionic liquids (ILs) as solvents for homogeneous catalysts with the benefits of structured solid catalysts. In an attempt to prepare a homogeneous IL film on a microstructured support, SMFs were coated by a layer of carbon nanofibers as described above. An IL thin film was then immobilized on the CNF/SMF support. The high interfacial area of the IL film enabled the efficient use of a transition metal catalyst for the selective gas-phase hydrogenation of acetylenic compounds [267,268]. 

Another highly efficient protocol for the hydroformylation consists in the combination of an ionic liquid with a solid support material (Figure 6.1). This process denominated supported ionic liquid phase (SILP) catalysis is a concept that combines the advantages of ionic liquids with those of heterogeneous support materials and allows the use of fixed-bed reactors for continuous reactions. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

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. 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

Normally, catalytic systems that include an IL phase require large amounts of these neoteric solvents in most cases, which are often costly and may affect the economic viability of a chemical process. Even though ILs have become commercially available, they are still relatively expensive compared to most of the conventional solvents. Furthermore, I Ls are usually viscous and have low diffusion coefficients for chemical reactions. In this regard, a new concept of a supported ionic liquid (SIL) phase has been adopted for immobihzation of catalysts [3]. SIL phases, which are much easier to separate, are advantageous for chemical reactions and have great potential in catalysis. This strategy helps to immobilize catalysts... 

Initial works in the area of ionosilica materials were motivated by the enormous potential of ionic liquids in catalysis and the desire to immobilize ionic liquid phases on solid support. Chemical fixation of chemical functionality on solid support is highly desirable as (i) it prevents from leaching, (ii) it facilitates the separation of reactants and catalyst, and finally (iii) it facilitates the reuse of the functional solid [75]. Immobilization of ionic liquids is attractive as these compounds are still expensive, and the limitation of the quantity of used ionic liquid and its reuse are highly advantageous from both economical and ecological viewpoints. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

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