Titanium, silica-supported

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). 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

The highest ethylene polymerization activity for a tetradentate salen-type group 4 complex was reported for silica supported (64) (600gmmol-1h bar ).193 Activities for a range of related zirconium and titanium complexes such as (65)-(67) are typically an order of magnitude lower.194-196... 

A combination of DAT and a metal alkoxide other than titanium alkoxide serves as a poor catalyst for the epoxidation of allylic alcohols. However, the combination of DAT and silica-supported tantalum alkoxides (2a) and (2b) prepared from Ta(=CHCMe3)(CH2Cme3)3 and silica(5oo) shows high enantioselectivity in the epoxidation of E-allylic alcohols, though chemical yields are not very great (Scheme 4).3... 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

SMPO [styrene monomer propylene oxide] A process for making propylene oxide by the catalytic epoxidation of propylene. The catalyst contains a compound of vanadium, tungsten, molybdenum, or titanium on a silica support. Developed by Shell and operated in The Netherlands since 1978. 

Tetraneopentyl zirconium reacts in the same way as tetraneopentyl titanium to give, on a silica (soo), a tris(neopentyl) monografted species [32]. Treatment under H2 of this surface species yields silica-supported zirconium hydrides [33], which have been characterized as a mixture of mono- (65-70%) and bis- (35-30%) hydrides based on double quanta NMR experiments (Scheme 2.11) [34]. Interestingly, the double quantum experiment allows us to prove not only the presence of the two hydrides and the monohydride of zirconium by the presence or the absence of the double quanta correlation but also to detect the through space magnetic interaction between the zirconium monohydride and the silicon di-hydride, proving thus the spatial arrangement on the surface. This confirms the mechanism by which these hydrides have been formed on the surface. 

Olefin epoxidation is an important industrial domain. The general approach of SOMC in this large area was to understand better the elementary steps of this reaction catalyzed by silica-supported titanium complexes, to identify precisely reaction intermediates and to explain catalyst deachvahon and titanium lixiviation that take place in the industrial Shell SMPO (styrene monomer propylene oxide) process [73]. (=SiO) Ti(OCap)4 (OCap=OR, OSiRs, OR R = hydrocarbyl) supported on MCM-41 have been evaluated as catalysts for 1-octene epoxidation by tert-butyl hydroperoxide (TBHP). Initial activity, selechvity and chemical evolution have been followed. In all cases the major product is 1,2-epoxyoctane, the diol corresponding to hydrolysis never being detected. 

Figure 3.28 Activity (after 0.5 h) of various silica-supported titanium complexes, monopodal (=SiOTi(OCap)3 ...

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]. 

Transition metals and their complexes can be immobilized in the mesopores or incorporated in the structure to make silica-supported metal catalysts. For instance, titanium catalysts for selective oxidation can be formed by modifying the mesoporous structure with either Ti grafted on the surface (Tif MCM-41) or Ti substituted into the framework (Ti->MCM-41). The grafted version makes the better catalyst for the epoxidation of alkenes using peroxides, and has good resistance to leaching of the metal. 

Finally, the concept was broadened by supporting these titanium silsesquiox-anes on silica remarkably, the heterogeneous silica-supported catalysts displayed an epoxidation activity per mole of titanium (94% TBHP conversion after 3 h) similar to that of the homogeneous titanium-silsesquioxane a2b4 complexes, although with a lower selectivity towards 1,2-epoxyoctane (92%). These heterogeneous catalysts did not leach active species and proved to be recyclable [45]. 

Union Carbide (95, 96) developed chromium catalysts that need activation by aluminum alkyls, but which have the advantage that molecular weight can be controlled with hydrogen, as in the case of the titanium-based catalysts. To prepare the catalysts, organosilanols are combined with chromium trioxide to afford silylchromate. The silylchromate is then deposited on a silica support and activated with an aluminum alkyl (see Fig. 13). 

The Michael addition of methyl a-acetamidoacrylate (196) with pyrrole (1) under silica-supported Lewis acid (Si(M) Si(Zn), Si(Al) and Si(Ti)) assisted by microwave irradiation (MW) afforded the alanine derivatives 395 and 396 dependent on the reaction conditions (Scheme 81) [153]. Both MW and thermal activation for pyrrole gave only Michael product 396, whereas alanine derivatives 395, which are the a-Michael addition product, and 396 were observed with A1 and Ti-catalyst. This behavior shows that aluminium and titanium Lewis acids can form a new acceptor in an irreversible way. The Si(M) or p-TsOH catalyzed reactions of N-benzylpyrrolc 397 with the acrylate 196 under MW gave the product 398 as sole product. The reaction yield has been increased by using a catalytic amount of p-TsOH (Scheme 82). 

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. 

The earliest Ziegler-Natta catalysts were insoluble bimetallic complexes of titanium and aluminum. Other combinations of transition and Group I-III metals have been used. Most of the current processes for production of high-density polyethene in the United States employ chromium complexes bound to silica supports. Soluble Ziegler-Natta catalysts have been prepared, but have so far not found their way into industrial processes. With respect to stereo-specificity they cannot match their solid counterparts. 

The heart of the process is formed by the catalytic epoxidation of propylene with ethylbenzene hydroperoxide using a silica-supported titanium catalyst. Fundamental studies have provided models for the active sites, the reaction mechanism, and the cause of catalyst deactivation. Other process elements have been investigated and improved as well. Minor by-products... 

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