Chemically Entrapped Catalysts

The concept is general and one-pot acid/base and enzyme/catalyst enantioselective solid-state syntheses are easily achieved by entrapment of the mutually destructive reagents in two different sol-gel silicas. It is worth pointing out that while acids and bases adsorbed at the surface of polymers are left partly exposed and consequently require acid/base solid-state 

In other cases, organic modification of the sol gel cages markedly protects the entrapped molecular dopant from degradation by external reactants, as shown for instance by the entrapment of the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). This is a highly active catalyst which in the NaOCl oxidation of alcohols to carbonyls in a CH2CI2-H20 biphasic system becomes highly stabilized upon sol gel entrapment in an ORMOSIL matrix it progressively loses it activity when entrapped at the external surface of commercial silica.25 

Encapsulation starting from the readily available triacetonamine derivative 4-oxo-TEMPO and propylaminetrimethoxysilane, in fact, prevents the intermolecular quenching of the radicals bound at the silica surface that has been found to be responsible for the loss of activity of TEMPO tethered at the surface of commercial silica. 

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Human life, furthermore, certainly benefits from a less polluted world, and here, again, sol-gel entrapped catalysts are, literally, able to have transferred to within their large inner porosity the whole chemistry of fine chemicals production. Think for instance of an innocuous easily handled orange powder called SiliaCat TEMPO (Chapter 5) that added to a mixture of alcohols at 0 °C with a modest excess of aqueous, cold bleach rapidly converts them into all those fragrances, vitamins, hormones and drugs made of carbonyl compounds. 

In all experiments in which the separately entrapped chemicals are catalysts rather than expendable reagents, the ceramic materials are recyclable and can be reused in further runs after washing. 

Similarly an entrapped acid and an entrapped base were placed in one and the same reaction flask for execution of the two-step process shown in Scheme 24-46. In the first step, phethyl iodide was dehydroiodinated either by the chemically entrapped diamine used in Scheme 24-41 or by entrapped modified TBD (vide supra). The resulting styrene reacted then in the second step in an acid-catalyzed reaction with anisole to form a mixture of l-(4-and 1 -(2-methoxyphenyl)ethylbenzene. Either sol-gel entrapped Nafion or immobilized molybdosilic acid have been used as catalysts (Gehnan, 2001b). 

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. 

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-... 

Biological catalysts in the form of enzymes, cells, organelles, or synzymes that are tethered to a fixed bed, polymer, or other insoluble carrier or entrapped by a semi-impermeable membrane . Immobilization often confers added stability, permits reuse of the biocatalyst, and allows the development of flow reactors. The mode of immobilization may produce distinct populations of biocatalyst, each exhibiting different activities within the same sample. The study of immobilized enzymes can also provide insights into the chemical basis of enzyme latency, a well-known phenomenon characterized by the limited availability of active enzyme as a consequence of immobilization and/or encapsulization. 

Atlas Chemical Industries, Inc Nitric Acid Sensitized Cap Sensitive Explosives with Gelation Catalyst and Entrapped Air... 

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. 

A variety of metal cluster compounds have been chemically bound on amorphous metal oxides and entrapped inside zeolite cages by new preparative tools such as surface organometallic chemistry and the so-called ship-in-bottle technique. They oflier much promise as molecular precursors for rational preparation of tailored metal catalysts having a uniform distribution of discrete metal-bimetallic ensembles, namely, organometallics which are active for catalytic reactions. They also provide advantages as metal precursors to achieve higher metal dispersions and well-managed metal... 

In this context, a functionalized ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate [hemim][BF4], is reported as an efficient and recyclable reaction medium for the palladium catalyzed Heck reaction. The olefination of iodoarenes and bromoarenes with olefins generates the corresponding products in good to excellent yields under phosphine-ffee reaction conditions. After separation of the product, fresh starting materials are charged into the recovered ionic liquid which entraps the palladium catalyst. The reactions still proceed quantitatively for six cycles, without significant loss of catalytic activity. " The effect of both the cation and the anion on the chemical yield is shown in Figure 28. 

The metal catalysts derived from the zeolite-entrapped metal cluster complexes have been studied because of the interest in a uniform distribution and a high degree of metal dispersion through the zeolite frameworks. Nevertheless, little information is available on the structural and chemical behavior of the entrapped metal cluster complexes, particularly on the retention of the cluster character under the reaction conditions, e.g., CO + H2, alkane hydrogenolysis and methane homologation re-... 

However, the above-mentioned reaction media represent only a small part of a large variety of colloidal particle preparation. In particular, one could cite, among others, physical vapor deposition, chemical vapor deposition, Langmuir-Blodgett films, polymer films [6-9], zeolite-entrapped nanoparticles [10], and supported catalysts [11]. 

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