Metal-catalyzed hydrogenations supported materials

Asymmetric hydrogenations catalyzed by supported transition metal complexes have included use of both chiral support materials (poly-imines, polysaccharides, and polyalcohols), and bonded chiral phosphines, although there have been only a few reports in this area. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

As shown in Table III, supported Mo catalysts may be derived not only from the traditional Mo02(acac)2, Mo naphthenate, or Mo(CO)6 precursors. A yellow Mo peroxide complex, synthesized from Mo metal and hydrogen peroxide, was immobilized on a cross-linked polystyrene functionalized with triethylenetetramine (229). This Mo-containing material was capable of catalyzing the epoxidation of various olefins with t-BuOOH as the oxidant in the temperature range of 298-333 K. For example, cyclohexene oxide was obtained in 90% yield after 5 h of reaction at 333 K in benzene. 

However, there is a lot of discussion on the relations between support material and electronic properties on one hand and the catalytic properties on the other. Thus, the need for a good test reaction is obvious. An important requirement for a proper test reaction is that it should be metal-catalyzed only. Among the possible reactions are the conversion of neopentane , neohexane23 25 and the isotopic exchange of hydrogen with deuterium in hydrocarbons (H/D exchange)26 29. 

Supported nickel catalysts catalyze steam-methane reforming and the concurrent shift reaction. The catalyst contains 15-25 wt% nickel oxide on a mineral carrier. Carrier materials are alumina, aluminosilicates, cement, and magnesia. Before start-up, nickel oxide must be reduced to metallic nickel with hydrogen but also with natural gas or even with the feed gas itself. 

The addition of hydrogen is catalyzed by a large variety of materials, but synthetically useful procedures generally employ nickel or the platinum metals, whose surface provides the reaction centers. To increase the fraction of the metal which is exposed to the fluid phase, the metal is finely divided or distributed over the surface of an appropriate, presumedly inert, support. Although the properties of a catalyst are affected by the method of preparation, the characteristics of the metal are dominant. - ... 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

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