Heterogeneous epoxidation titanium catalyst

Among the large bulk of work regarding titanium(IV) silsesquioxanes, specific studies have been focused on synthesizing close molecular models of the active sites of titanium heterogeneous epoxidation catalysts in siUcaUte and in mesostruc-tured MCM-41 silica (Figure 14.3 gives some selected examples) [54, 61-66]. 

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]. 

A shortcoming of these catalysts, in common with supported titanium catalysts, is their hydrophilicity which precludes effective catalysis with hydrogen peroxide. Hence, much effort has been devoted to the synthesis of hydrophobic analogs of these materials [54] but this has not yet resulted in the synthesis of truly effective catalysts for epoxidations with hydrogen peroxide. Apparently, hydrophobicity is not the only requirement for an effective heterogeneous Ti catalyst for epoxidations with hydrogen peroxide. 

Various methods for anchoring Gp titanium derivatives onto organic or inorganic supports have been developed. Heterogeneous epoxidation catalysts have been prepared by the co-polycondensation of Cp2TiCl2 and (CpTiCl2)20 with tetraethoxysilane by a modified sol-gel procedure, and the use in epoxidations of non-activated olefins have been described.1936... 

The Sharpless epoxidation of allylic alcohols with tert-butylhydroperoxide and titanium(IV) isoproxide has been carried out with a polymeric tartrate made from tartaric acid and 1,8-octanediol.114 The yield of epoxide (92%) was comparable with that when dimethyl tartrate was used (91%), but the 79% ee was lower than the 98% ee found with the dimethyl tartrate. It may be possible to raise the ee by further variations in the structure of the polymer. The heterogenization of a catalyst for a homogeneous reaction often requires optimization to obtain comparable or higher yields and stereospecificity. 

High-valent early transition metals like titanium(IV) and vanadium(V) have been shown to efficiently catalyze the epoxidation of alkenes. The preferred oxidants using these catalysts are various alkyl hydroperoxides, typically tert-butyl hydroperoxide (TBHP) or ethylbenzene hydroperoxide (EBH P). One of the routes for the industrial production of propylene oxide is based on a heterogeneous Ti ySi02 catalyst, which employs EBHP as the terminal oxidant [6]. 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

It is carried out in the Hquid phase at 100—130°C and catalyzed by a soluble molybdenum naphthenate catalyst, also in a series of reactors with interreactor coolers. The dehydration of a-phenylethanol to styrene takes place over an acidic catalyst at about 225°C. A commercial plant (50,51) was commissioned in Spain in 1973 by Halcon International in a joint venture with Enpetrol based on these reactions, in a process that became known as the Oxirane process, owned by Oxirane Corporation, a joint venture of ARCO and Halcon International. Oxirane Corporation merged into ARCO in 1980 and this process is now generally known as the ARCO process. It is used by ARCO at its Channelview, Texas, plant and in Japan and Korea in joint ventures with local companies. A similar process was developed by Shell (52—55) and commercialized in 1979 at its Moerdijk plant in the Netherlands. The Shell process uses a heterogeneous catalyst of titanium oxide on siHca support in the epoxidation step. Another plant by Shell is under constmction in Singapore (ca 1996). 

The original epoxidation with titanium-tartrate is homogeneous, but it can be carried out heterogeneously without diminishing enantioselectivity by using titanium-pillared montmorillonite catalyst (Ti-PILC) prepared from titanium isopropoxide, (+)-DAT, and Na+-montmorillonite.38 Due to the limited spacing of Ti-PILC, the epoxidation becomes slower as the allylic alcohol gets bulkier. 

A number of heterogeneous systems have been developed for oxidation reactions using H2O2 as oxygen source . In 1981, Taramasso, Notari and collaborators at Enichem opened new perspectives in this field with the discovery of the Ti-silicalite (TS-1) ° , a new synthetic zeolite of the ZSM family. In the TS-1 zeolite, titanium atoms are located in vicariant positions in the place of Si atoms in the crystalline framework . The remarkable reactivity of TS-1 is likely ascribable to the site-isolation of tetrahedral Ti(IV) in a hydrophobic environment. TS-1 has proved to be an efficient catalyst for the epoxidation of unfunctionalized short-chain olefins, especially terminal ones (equation 28). In addition, polyunsaturated compounds are mainly converted into the mono epoxides (equation 29). 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

Titanium-pillared montmorillonite may be used as a heterogeneous catalyst for the Sharpless asymmetric epoxidation of allylic alcohols (Scheme 20) (46). The enantiomeric purities of the epoxy products are comparable with those achieved using homogeneous Ti isopropoxide with molecular sieves as water scavengers (Chapter 4). Since basal spacing of the recovered catalyst after the reaction is unaltered, the catalyst can be recycled. 

Finally, a titanium(IV) pillared clay (Ti-PILC) catalyst has been prepared.71 In the presence of tartaric acid esters as chiral ligands Ti-PILC is an effective, heterogeneous catalyst for the asymmetric epoxidation of allylic alcohols. Enantioselectivities were comparable to those observed in the homogeneous system - and reactions could be carried out at concentrations up to 2M with a simple work-up via filtration of the catalyst. 

APTMS-modified MCM-41 surface. In a last step, titanium tetra-wo-propoxide reacted with the chiral organic-inorganic hybrid material, to give the heterogeneous variant of the asymmetric epoxidation catalyst of allylic alcohols, proposed by Katsuki and Sharpless.312... 

Rrijnen, S., Mojet, B. L., Abbenhuis, H. C. L., Van Hooff, J. H. C. and Van Santen, R. A. MCM-41 heterogenized titanium silsesquioxane epoxidation catalysts a spectroscopic investigation ofthe adsorption characteristics, Phys. Chem. Chem. Phys., 1999,1,361-365. 

Two variations of the process are used, the only essential difference being the catalyst employed in the epoxidation step. In the Arco (Atlantic Richfield) process a homogeneous molybdenum catalyst is used. The Shell process employs a heterogeneous titanium/silica catalyst. 

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