Catalysis silica-supported catalysts

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

Organically modified silica gel finds a number of applications as stationary phases in chromatography or as anchors for transition metal complexes used as inunobilized catalysts. The metal complexes are attached to the silica gel by coordinating them with phosphine (PR2-), mercapto (-SH) or other groups, which are linked to the support over an alkyl chain ( spacer ) [1-3]. To understand the mechanism of catalysis by supported metal complexes it is necessary to know more about the coordination of the silica-supported catalyst on the surface and the influence of the spacer length on the catalytic activity [4]. 

ATP apparatus equipped with a mass spectrometer. Right-hand side TPR patterns of silica-supported Rh, Fe, and Fe-Rh catalysts, which had been previously calcined to ensure that all metals are oxidized at the start of the measurement. [Adapted from J.W. Niemantsverdriet, Spectroscopy in Catalysis, An Introduction (2000), Wiley-VCH, Weinheim, and H.F.J. van t Blik and J.W. Niemantsverdriet, Appl. Cota/. 10 (1984) 155.]... 

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

Davda, R. R. Shabaker, J. W. Huber, G. W. Cortright, R. D. Dumesic, J. A., Aqueous-phase reforming of ethylene glycol on silica-supported metal catalysts. Applied Catalysis B 2003,43,13-26. 

As for heterogeneous olefin polymerization catalysis, the activity of rare-earth metal catalysts may be also enhanced in organic transformations by the use of silica supports or other carriers [7]. Indeed, several catalytic C-C and C-X (with X = H/D, Si, O) bond formation reactions as weU as functional group transformations witness to the potential of SOLn/AnC-based heterogeneous catalysts for fine chemical synthesis. 

Titanium Alkoxides Silica-supported titanium(IV) alkoxides and Ti-silicalite are industrial epoxidation catalysts [53-56] and have been applied in deperoxidation reactions [57]. Computational and EXAFS data [53, 54] as well as spectroscopic investigations on the surface species [58] have indicated that the dominant active surface species is a four-coordinate trisUoxy complex [(=SiO)3TiOH] [59] whose coordination shell expands to six-coordinate during catalysis [60]. 

There has been an enormous technological interest in tertfa/j-butanol (tBA) dehydration during the past thirty years, first as a primary route to methyl te/f-butyl ether (MTBE) (1) and more recently for the production of isooctane and polyisobutylene (2). A number of commercializable processes have been developed for isobutylene manufacture (eq 1) in both the USA and Japan (3,4). These processes typically involve either vapor-phase tBA dehydration over a silica-alumina catalyst at 260-370°C, or liquid-phase processing utilizing either homogenous (sulfonic acid), or solid acid catalysis (e.g. acidic cationic resins). More recently, tBA dehydration has been examined using silica-supported heteropoly acids (5), montmorillonite clays (6), titanosilicates (7), as well as the use of compressed liquid water (8). 

The effect of the basicity of aldol condensation catalysts on their activity was thoroughly investigated by Malinowski et al. [372—379]. The observed linear dependence of the rate coefficients of several condensation reactions on the amount of sodium hydroxide contained in silica gel (Figs. 12 and 13) supported the view that the basic properties of this type of catalyst were actually the cause of its catalytic activity, though the alkali-free catalyst was not completely inactive. The amphoteric nature of the catalysis by silica gel, which can act also as an acid catalyst, was demonstrated [380]. By a stepwise addition of sodium acetate to a HN03-pretreated silica gel catalyst the original activity for acetaldehyde self-condensation was decreased to a minimum (when an equivalent amount of the base was added) by further addition of sodium acetate, the activity increased again because of the transition to a base... 

Hamilton N, Wolfram T, Tzolova Muller G, Havecker M, Krohnert J, Carrero C, Schomacker R, Trunschke A, Schlogl R. Topology of silica supported vanadium-titanium oxide catalysts for oxidative dehydrogenation of propane. Catalysis Science Technology. 2012 2(7) 1346—1359. 

Shore, S. G., Ding, E. R., Park, C., and Keane, M. A., Vapor phase hydrogenation of phenol over silica supported Pd and Pd-Yb catalysts. Catalysis Commun 2002, 3 (2), 77-84. 

The supramolecular binding motifs described here were also used to attach catalysts to solid (silica) supports (128). The active metal complex could be switched from palladium to rhodium by using a polar solvent that breaks up the binding of the supramolecular motif. Allylic alkylation and hydroformylation catalysis could be carried out by using the same support "receptor" material and different "guest" ligands for the two metals a... 

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