Supported transition metal complex catalysts hydrogenation

It is very clear that the field of supported transition metal complex catalysts is a rapidly expanding field. Indeed, only their application to hydrogenation, hydrosilylation, and hydroformylation reactions have received more than a preliminary skirmish. Already a number of points are becoming clear. 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Supported Early Transition-Metal Complexes as Heterogeneous Hydrogenation Catalysts... 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

The catalytic activity of zeolites in alkane to olefin reactions, photochemical conversion reactions, Fischer-Tropsch hydrogenation, isocyana-tion, carbonylation, and related chemistry make up the last theme. An important focus of this is to explore the utility of zeolites as selective heterogeneous catalysts for reactions that involve Group VIII metals. The mechanistic nature of some of this chemistry is presented, along with the characterization of supported organometallic transition metal complexes. 

The topic of this chapter is enantioselective hydrogenation over chiral or chirally modified solid catalysts. Diastereoselective hydrogenation of chiral compounds and asymmetric hydrogenation with heterogenized (supported, embedded) homogeneous transition metal complexes will not be discussed. 

Adsorption on silica gel surfaces or silica gels coated with water or thin layers of ionic liquids has been used to immobilize transition metal complexes % ionic interactions and hydrogen bonding. Reversed-phase silica gels were used to retain catalysts by hydrophobic interactions. Support of catalysts on fluorous reversed-phase silica gel by the solvophobic nature of perfluoroalkyl chains is a new and promising approach with potential in catalysis and combinatorial chemistry. 

Methods of synthesizing polymers with carbonyl groups as supports of transition metal complexes have, moreover, been analyzed [104]. The activity of these catalysts in polymerization, oligomerization and hydrogenation reactions as a function of the polymer support structure has been discussed. 

Collman and co-workers (25) first reported the use of 2 to support transition metal species in 1972 when they prepared complexes of Rh, Co, and Ir. They found that complexes of 2 when exposed to oxygen or 1 atmosphere of decomposed to metal particles and that such catalysts were active for typical "heterogeneous" reactions such as the hydrogenation of arenas. They compared the activity of Rhg(C0)j5 on 2 to that of 5Z Rh on AI2O3. 

An illustration of a ship-in-a-bottle synthesis is given below. Other examples that follow illustrate (a) transition metal complexes bonded in nearly uniform sites in a zeolite, constituting one of the best-defined supported catalysts, and (b) transition metal clusters in a zeolite that are simple and uniform enough to allow precise characterization by x-ray absorption spectroscopy and density functional theory, with results that provide fundamental new, and apparently general, understanding of the metal-support interface in supported metals and the chemistry of hydrogen spillover. 

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