Homogeneous catalysts, thermal

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

The flexibility in composition of LDHs has led to an increase in interest in these materials. As a result of their relative ease of synthesis, LDHs represent an inexpensive, versatile and potentially recyclable source of a variety of catalyst supports, catalyst precursors or actual catalysts. In particular, mixed metal oxides obtained by controlled thermal decomposition of LDHs have large speciflc surface areas (100-300 m /g), basic properties, a homogeneous and thermally stable dispersion of the metal ion components, synergetic effects between the elements, and the possibility of structure reconstruction under mild conditions. In this section, attention is focused on recently reported catalytic applications in some flelds of high industrial and scientific relevance (including organic chemistry, environmental catalysis and natural gas conversion). 

The formation of carbon-carbon bonds using olefin metathesis methodology is a powerful technique in fine organic synthesis and polymer chemistry. The increasing importance of these reactions is reflected by the numerous publications over the last few years. Many of these pubhcations deal with the design and apphca-tion of polymer-supported olefin metathesis catalysts with the aim to overcome the common drawbacks of the homogeneous catalysts low thermal stability and difficulties associated with their recovery from the reaction mixtures. The modem state of art in this important field is described in chapter 11 of this volume. 

Zeolites and other mesoporous materials are excellent catalysts for industrial and laboratory applications. Favourable characteristics are their capacity to immobihze homogenous catalysts rendering them heterogeneous, their thermal stability, and the ease of separation from the reaction products and reuse in hquid- and gas-phase conditions. The pore size and Brpnsted and Lewis acidic properties are determinant for their use as catalyst in the Beckmann rearrangement. Recently, a review on the use of zeolites and mesoporous materials in the Beckmann rearrangement was published. ... 

Polyoxometalates have been used as homogenous catalysts for a wide variety of thermal organic substrate oxidations37. This involves the epoxidation of relatively electron poor terminal olefins by H2O2 and heteropoly acids, principally H3[PWi2C>4o], using PTC (equation 9). 

The choice of reactor configuration depends on the properties of the reaction system. For example, bioconversions for which the homogeneous catalyst distribution is particularly important are optimally performed in a reactor with the biocatalyst compartmentalized by the membrane in the reaction vessel. The membrane is used to retain large components, such as the enzyme and the substrate while allowing small molecules (e.g., the reaction product) to pass through. For more labile molecules, immobilization may increase the thermal, pH and storage stability of biocatalysts. 

Guanidinium salt has special properties of high thermal and chemical stability, tunable groups attached on the three N atoms in the molecular structure, and excellent catalytic activity for cycloaddition of C02 and epoxides to produce cyclic carbonate [11, 12]. A functionalized-PEG, hexaalkylguanidinium bromide (Scheme 5.3) being covalently tethered to PEG (MW = 6000) is utilized as an active homogeneous catalyst, which includes the benefits of recyclability and high catalytic activity, for the synthesis of cyclic carbonates from C02 and epoxides with almost quantitative yield and excellent selectivity [13]. 

HDN with Metallophthalocyanines. The above considerations clearly show that the catalytic site geometry is important in HDN. It is difficult to control, or even to identify, the catalyst site in conventional catalysts such as the commercial Co-Mo-alumina catalyst. On the other hand, homogeneous catalysts with transition metal complexes provide a well-defined catalytic site. Unfortunately, most homogeneous catalysts are not sufficiently stable to be used at the temperatures required for the hydrogenation of hetero compounds. A class of catalysts that are thermally stable are the metallophthalocyanines,... 

Hydrogenations using heterogeneous catalysts usually require thermal condition above room temperature and H2 pressures higher than 10 Pa. Homogeneous catalysts are often more selective with individual reactions and make the reactions possible at lower temperatures and H2 pressures. 

When the reactants and the catalyst are in the same physical state the catalyst is called a homogeneous catalyst e.g. concentrated sulfuric acid speeds up the reaction between ethanoic acid and ethanol to form the ester, ethyl ethanoate. Manganese(IV) oxide catalyses the thermal decomposition of potassium chlorate(V) to give oxygen. 

Co3RuH(CO)i2 and/or salt of [Co3Ru(CO)i2] were reported to act as homogeneous catalysts in the homologation of methanol. Co3RuH(CO)i2 has been also applied to yield, by controlled thermal decomposition, very active heterogeneous Fischer-Tropsch catalysts. ... 

A conceptual process for ethylene dimerization in the presence of tantalum or niobium based catalysts has been developed by MIT researchers [6-8]. The technology is based on a metal hydride-based homogeneous catalyst that selectively dimerizes ethylene to butene-1. The particular catalyst is neopentylidene complex of tantalum or niobium. The preparation of the homogeneous catalyst is rather a complex process the tantalum complex is prepared by reacting tri-neopentyl tantalum dichloride, Ta(CH2CMe3)3Cl2 and neopentyl lithium LiCH2CMe3 in octane solvent to yield thermally stable neopentylidene tantalum catalyst in quantitative yield. 

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