Entrapped catalysts

Figure 3.1. Several approaches that lead to supported catalysts catalyst entrapment, anchoring via an interaction between the metal and the support, anchoring a ligand to a support, and anchoring via ion pairing...

The trend was studied and verified, for instance, for reactions catalysed by transition metal, organo- and enzyme catalysts entrapped in ORMOSIL prepared by copolymerization of tetramethoxysilane (TMOS) and the modifying co-precursor methyltrimethoxysilane (MTMS). It has been correlated with the encapsulation itself but also with the structure of the sol-gel matrix, namely the hydrophobicity-lipophilicity balance (HLB) and the textural properties of the materials.9... 

IR and UV-vis spectroscopy show that the majority of the encapsulated complexes still have structural integrity. As observed for many other oxidation catalysts, entrapment shows significantly enhanced activity. This effect is more pronounced when the degree of ligand substitution with Cl is higher. The simultaneous occurrence of two mechanisms is confirmed again, the selectivity of the encapsulated Mn(salen) complexes for the formation styrene epoxide is only 30%, the dominant product being benzaldehyde. 

Yang HQ, Li J, Yang J, Liu ZM, Yang QH, Li C (2007) Asymmetric reactions on chiral catalysts entrapped within a mesoporous cage. Chem Commun 1086... 

Liu, C., Tang, T, Wang, D., and Huang, B. 2003. In situ ethylene homopolymerization and copolymerization catalyzed by zirconocene catalysts entrapped inside functionalized mont-morillonite. Journal of Polymer Science, Part A Polymer Chemistry 41 2187-21%. 

The sol-gel approach has been used in catalysis in four main directions preparation of inorganic oxide catalysts " entrapment of metal microcrystallites polymerization of trialkoxysilyl derivatives of metal ligands and by direct, straightforward entrapment of organometallic catalysts in sol-gel matrices ". In this Section we concentrate on the latter and compare it with covalent attachment of trialkoxy derivatives, which is fully described in Section IV. As a representative example we consider the case of ion-pair catalysts " and, in particular, RhCls/Aliquat 336 and RhCl3/[Me3N(CH2)3... 

Although most studies on catalysts entrapment were carried out with silica as the main model material, it should be emphasized that practically all other sol-gel oxides can serve for that purpose. An example is the entrapment of Ru3(CO)i2, Rh2Co2(CO)i2, and Os3(CO)i2 within alumina sol-gel matrices [7], which were used as recyclable catalysts for hydrogen transfer, for isomerization of allylbenzene, or for the disproportionation of dihydroarenes. In feet, this study is an example for a situation where moving to another sol-gel matrk can solve problems of inactivity Whereas Rh2Co2(CO)i2 silica neither promotes the isomerization nor the disproportionation, Rh2Co2(CO)i2 alumina is an efficient catalyst for these reactions. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Temperature control is also reasonably simple. An important advantage in the case of a rapidly deactivating catalyst is the possibility of continuous catalyst replacement. There are, however, a number of problems associated with handling fine catalyst particles. They have to be separated from the products, which is usually troublesome, plugging of lines and valves can occur, and pyrophoric catalysts may also require special procedures. This is less important if the product can be removed from the reaction mixture (e.g. products are volatile and are stripped during the operation). In case of excessive gas flow rates, however, small catalyst particles can be entrapped and deposited in downstream equipment. The catalyst load is limited to what can be kept in. suspension with a reasonable power input. 

Immobilized spores Entrapment in Caalginate/ Methyl ketone from Isoparaffin-water Improvement in catalyst 20... 

To assess the utility of this resin, we chose to employ it in the evaluation of the heterogeneity of a commercial polymer-entrapped Pd(OAc)2 precatalyst, Pd-EnCat, also sold by Reaxa. This precatalyst was designed with the goal of providing a heterogeneous catalyst that would allow simple removal of palladium from reactions (24-26). PVPy and QTU were first used as poisons in the Heck reaction of iodobenzene and n-butyl acrylate in DMF using PdfC as the palladium... 

When only the enzyme was entrapped in the sol-gel, the rhodium complexes poisoned the enzyme, resulting in low yields, whereas when both enzyme and complex were trapped in separate sol-gel mixtures, good yields were achieved. In one particular case, this procedure resulted in an increase in yield of 1-pentyl undecano-ate from 1 -undecenoic acid and 1 -pentanol by a factor of 6.5, indicating successful site isolation of the mutually interfering catalysts. 

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

Sol-gel-entrapped catalysts provide a generic method for the encapsulation of a wide variety of catalysts. Comprehensive reviews are available [59-61]. The essence ofthe concept is captured in Figure 5.6. 

Figure 5.6 Sol-gel nanocomposites are composed of an entrapped catalyst and an organically modified Si02 based matrix. Reproduced with permission from [60].

The exact procedure for entrapment will depend on the nature of the catalyst and the required sol-gel, but for enzyme entrapment it generally involves the hydrolysis or partial hydrolysis of an alkoxysilane precursor, giving a precursor sol-gel solution, followed by addition of more solvent and of the catalyst. Figure 5.7 provides a simple... 

Similar methods of encapsulation are also observed in pillared clays, which were also introduced as catalysts as long ago as the early 1980s. The field has been thoroughly reviewed up to 2000 [65], Layered double hydroxide structures have also been used for the entrapment of metal coordination compounds [66],... 

Vorlop et al. described a novel cross-linked and subsequently poly(vinyl alcohol-entrapped PaHNL for synthesis of (//(-cyanohydrins. These immobilized lens-shaped biocatalysts have a well-defined macroscopic size in the millimeter range, show no catalyst leaching, and can be recycled efficiently. Furthermore, this immobilization method is cheap and the entrapped (/ )-oxynitrilases gave similar good results compared with those of free enzymes. The (//(-cyanohydrin was obtained in good yields and with high enantioselectivity of up to >99% ee [55],... 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

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