Physically Entrapped Catalysts

In the 1990s a large variety of organometallic catalysts were physically entrapped in leach-proof silica gels showing enhanced activity and 

The emulsion which contains the substrate (1) spills its content into the catalyst material (2), the catalytic process takes place (3) and then the adsorbed surfactant carries the product back into solution (4). (Reproduced from ref. 18, with permission.) 

Upon reaction, the heterogenized catalyst can be easily separated from the reaction mixture by filtration and then recycled. The hydro-phobic substrate is microemulsified in water and subjected to an orga-nometallic catalyst, which is entrapped within a partially hydrophobized sol-gel matrix. The surfactant molecules, which carry the hydrophobic substrate, adsorb/desorb reversibly on the surface of the sol-gel matrix breaking the micellar structure, spilling their substrate load into the porous medium that contains the catalyst. A catalytic reaction then takes place within the ceramic material to form the desired products that are extracted by the desorbing surfactant, carrying the emulsified product back into the solution. 

Along with hydrophobicity, large amounts of both water (to promote hydrolysis) and methanol employed as co-solvent in the catalyst preparation (to promote homogeneity) are needed to ensure optimal reactivity, showing the number of experimental parameters of the sol-gel synthesis which can be controlled independently to optimize the performance of the resulting catalyst. Finally, in contrast to zeolites and other crystalline porous materials, amorphous sol-gel materials show a distribution of porosity which does not restrict the scope of application of sol gel catalysts to substrates under a threshold molecular size. 

a single TPAP-doped ORMOSIL can be efficiently employed for the oxidative dehydrogenation of very different alcohol substrates. 

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Rh(CO)2 (3), [h -CsH4(CH2)2Si(OMe)3]Rh(COD) (4), and [h -CsH4(CH2)2Si (OMe)3]Rh(CO)(PPh3) (5). Air-stable, leach-proof, and recyclable catalysts were obtained, which were used for C=C double bond (styrene) and nitro group (nitrobenzene) hydrogenations. Comparison with physically entrapped catalysts was carried out, and it was found that the recyclability in the case of the covalent entrapment was better. 

Under certain conditions, one key advantage of sol-gel entrapped catalysts identified by Blum et al. in the early studies in the 1990s is the leach-proof nature of many physically entrapped catalysts [6]. Then, the same sol-gel-entrapped catalyst can be employed in incompatible solvents such as in the case of encapsulated Ru-BINAP and other water-insoluble chiral transition metal catalysts utilized for the enantioselective hydrogenation of itaconic acid in water instead of in organic solvent [32]. 

The first two methods have the advantage that no modification of the homogeneous catalyst is needed. Surface hydrogen-bonded catalysts are limited to cationic complexes, while physical entrapment is more widely applicable. However, both methods are very sensitive to the solvent properties of the reaction medium. The chemical methods of immobilization require modification of the ligand, and this may be quite laborious. In the case of irreversible catalyst deacti-... 

Several sol-gel entrapped catalysts are likely to soon find commercial applications. A variety of transition metal catalysts physically entrapped in silica matrices as ion pairs generated from the metal halides and quaternary ammonium or phosphonium salts developed in the mid-1990s by Avnir and Blum resulted in truly heterogeneous, stable and... 

The chloromethylated polystyrene is usually obtained by chloromethylation of polystyrene (Eq. 9-38), although polymerization of p-vinyl benzyl chloride has also been used [Arshady et al., 1976]. A less desirable variation of this approach is the physical entrapment of a catalyst, substrate, or reagent within a polymer. 

The mode of immobilization, as well as the source and extent of purification of the enzyme, are important factors in determining the lifetime of the bio-catalyst. Generally, the lifetime of a soluble enzyme electrode is about one week or 25-50 assays, and the physically entrapped polyacrylamide electrodes are satisfactory for about 50-100 assays, depending primarily on the degree of care exercised in the preparation of the polymer. The chemically attached enzyme can be kept for years, if used infrequendy. In frequent use, the GOD electrode has a lifetime of over one year and can be used for over 1000 assays. For 1-amino acid oxidase or uricase (100) biosensors, about 200-1000 assays per electrode can be obtained, depending on the immobilization technique. 

One characteristic of in situ NMR experiments is that there is typically a wide range of correlation times characterizing molecular motion. Some species will be essentially immobile as a result of strong chemisorption to the catalyst surface or physical entrapment, as in the case of a coke molecule. Other species may reside exclusively in the gas phase or else be partitioned into adsorbed and gas phase populations in slow exchange on the NMR time scale owing to diffusional constraints. Figure 7 shows an example. At high temperatures, methanol and dimethyl ether are partitioned between the gas phase and adsorbed phase on zeolite HZSM-5 [31 ]. For many adsorbates on zeolites, especially at reaction... 

Sol-gel physically entrapped chiral rhodium and ruthenium complexes are used as recyclable and effective catalysts in enantioselective hydrogenation of itaconic acid up to 78% chiral methylsuccinic acid is obtained [65]. 

The preparations and performance of molecular catalysts successfully heterogenized by physical entrapment for a wide range of reactions are reviewed in detail in [86,93]. Two synthetic protocols were used, depending on the chemical properties of entrapped molecules addition of dopant solution to prehydrolyzed Si(OR)4 at pH close to 7, or together with ammonium hydroxide to tetraalkoxysilane precondensed in acidic solution. Both, followed by gelation. 

Figure 5.7 Steps in forming physically entrapped (adsorbed) molecular catalysts by sol-gel processing. (Adapted from [81].)...

With a different architecture of advanced carbon materials, graphene or graphene sheet is very attractive material to accelerate fast electron transfer process on the cholesterol sensor. As shown in Fig. 6, the sensor was composed of Pt catalyst supported by chitosan-graphene sheet. Both enzyme ChE and ChOx were physically entrapped layer by layer with Nafion polymer. A close look of morphology on the film could be observed by field emission SEM images. 

A variety of olefin metatheses were carried out by sol-gel entrapped Grubbs catalysts (e.g.. Scheme 24-21) (Kingsbury, 2001). Heck coupling of bromo- and iodoarenes with various alkenes by (physically) entrapped PdCl2(PPh3)2 has recently been carried out (Scheme 24-22). ... 

Apart Ifom the aforementioned metal complexes there were entrapped in sol-gel matrices also several acids, bases and salts. Examples are the physically entrapped p-toluenesulfonic acid which has been used as an esterfication catalyst (Scheme 24-23) (Sarussi, 2000) the entrapped poly(vinylbenzyltrimetnyl)ammonium hydroxide which catalyzes aldol condensation (Scheme 24-24) (Gehnan, 2003a) and some entrapped sulfates and chlorides salts that promote dehydration of alcohols (Scheme 24-25) (Nishiguchi, 1989) and Friedel-Crafts alkylation (Scheme 24-26) (Miller, 1998), respectively. 

The approach is general, and was demonstrated quite widely. For instance, Talhami et al. [5] have physically entrapped dichlorobis(triphenylphosphine)pal-ladium(II) within a silica sol-gel matrix, resulting in efficient catalysts for crosscoupling of aryl iodides, bromides, and triflates with intramolecularly stabilized... 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

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