Solid immobilised catalysts

Some degree of success in supported enantioselective catalysis was accomplished by using functionalisation of mineral support. Due to their unique textural and surface properties, mesoporous micelle-templated silicas are able to bring new interesting properties for the preparation of optically active solids. Many successfully examples have been reported for enantioselective hydrogenation, epoxidation and alkylation. However, the stability of the immobilised catalysts still deserves efforts to allow industrial development of such attractive materials. 

Although Cinchona alkaloids are easily separated from products by acid-base extractions and recycled, immobilisation of the catalyst on polymers was investigated by Oda. In parallel with new catalyst synthesis, their immobilisation to various solid supports was also studied. Immobilised catalysts are easily isolated by filtration and reused several times, although their initial enantioselectivity is slightly lower compared with homogeneous catalysis. 

Since 1978, several papers have examined the potential of using immobilised cells in fuel production. Microbial cells are used advantageously for industrial purposes, such as Escherichia coli for the continuous production of L-aspartic acid from ammonium fur-marate.5,6 Enzymes from microorganisms are classified as extracellular and intracellular. If whole microbial cells can be immobilised directly, procedures for extraction and purification can be omitted and the loss of intracellular enzyme activity can be kept to a minimum. Whole cells are used as a solid catalyst when they are immobilised onto a solid support. 

Another successful approach to catalyst immobilisation involves attachment of the carbene precursor to a peptide on solid support. Treatment with base generates the corresponding carbenes that undergo in situ complexation to Pd(ll) centres (Scheme 6.33). Again, the main drawback of this approach was the low reactivity of the catalytic system that only allowed the coupling of aryl iodides and bromides [116], The reasons for this outcome are in need of further studies. 

Scheme 6.33 Pre-catalyst immobilisation by attachment to a peptide on solid support...

Arya et al. used solid phase synthesis to prepare immobilised dendritic catalysts with the rhodium centre in a shielded environment to mimic nature s approach of protecting active sites in a macromolecular environment (e.g. catalytic sites inside enzymes) [51], Two generations PS immobilised rhodium-complexed dendrimers, 6 and the more shielded 7, were synthesised.The PS resin immobilised rhodium-complexed dendrimers were used in the hydroformylation of styrene, p-methoxystyrene, vinyl acetate and vinyl benzoate using a total pressure of 70 bar 1 1 CO/H2 at 45 °C in CH2C12. 

The lesson learned from the catalysis in interphase approach is that the spacer between the insoluble support and the actual catalyst should be sufficiently long and soluble in the solvent of interest to obtain active catalysts. The use of supercritical fluids can also be very beneficial for the activity. Upon using Xantphos immobilised on silica in scC02 for example, the rates are only half of those of the homogeneous catalyst. Expressed as space-time yields the solid catalysts are almost an order of... 

The basic idea of immobilising a catalytically active solution on a solid support to obtain a solid catalyst with homogeneous type selectivity and efficiency is not new. In the late seventies, Scholten [69] and Hjortkjaer [70] described supported liquid phase... 

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]. 

Attempts to achieve optical induction during the reduction of aromatic ketones to the secondary alcohol by immobilising a single layer of chiral catalyst on a solid electrode surface have been much less successful. The preparation of such coatings... 

The alkylation reactions of toluene and benzene with dodecene were used as test reactions in order to evaluate the catalyst activity. It could be observed that the IL is catalytically activity after being immobilised. Moreover, due to the better dispersion of the solid catalyst in the reaction media, even the conversions obtained for the supported IL were better than for the pure IL. In the alkylation of toluene conversions reached at standard conditions were twice as high for the immobilised ILs, for benzene the difference was even bigger. 

An area in which functionalised ionic liquids are already playing an important role in catalysis is heterogenisation on solid supports. The general concept involves the immobilisation of imidazolium and other cationic fragments onto solid supports using appropriate functional groups attached to the cation. An ionic catalyst then resides within the ionic matrix and several examples of such supported ionic liquid phase catalysts are provided in the subsequent chapters of this book. The concept is illustrated in Figure... 

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