Homogeneous titanium catalyst

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

CycloeiMBmerizatioii of etkyUite and 1,3-bmUulieae. This homogeneous titanium catalyst promotes the cyclocodimerization of ethylene and 1,3-butadicnc to vinylcyclo-butane. The usual product of codimerizalion is 1,4-hexadienc. 

Proto, A. Capacchione, C. Venditto, V. Okuda, J. Synthesis of isotactie poly-l,2-(4-methyl-l,3-pentadiene) hy a homogeneous titanium catalyst. Macromolecules 2003,36,9249-9251. 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

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-... 

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]. 

Table VI summarizes important homogeneous Ziegler catalysts. The best known are the systems based on bis(cyclopentadienyl)titanium(IV), titanium alcoholates, vanadium chloride, or chromium acetylacetonate with trialkylaluminum or alkylaluminum halides.

Whereas these solid catalysts tolerate water to some extent, or even use aqueous H2O2 as the oxidant, the use of homogeneous Ti catalysts in epoxi-dation reactions often demands strictly anhydrous conditions. The homogeneous catalysts are often titanium alkoxides, possibly in combination with chiral modifiers, as in the Sharpless asymmetric epoxidation of allylic alcohols (15). There has recently been an increase in interest in supporting this enantioselective Ti catalyst. 

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. 

Atactic polypropene has been synthesized with homogeneous catalytic systems based on mono-Cp trialkoxo titanium complexes activated by MAO.951 Syndiotactic polystyrene has been synthesized with different mono-Gp trialkoxo titanium derivatives activated by MAO and AlMe3, and the catalytic efficiency has been compared with bis-Cp titanium catalysts.952 The titanium ligands affect both catalytic activity and stereoregularity of the polypropylene obtained. For the CpTi(OPrn)3/MAO system, factors influencing the propylene polymerization, such as temperature, Al/Ti molar ratio, and monomer pressure, have been studied. 

Gel-type poly(styrene-co-divinylbenzene) beads have been used as a carrier to encapsulate bis-Cp titanium catalysts through a simple swelling-shrinking procedure. These catalytic species are homogeneously distributed in the PS bead particle and exhibit high and stable ethylene polymerization and ethylene/1-hexene co-polymerization activity.1236... 

The IFP-SABIC Alphabutol process utilizes a proprietary homogeneous titanium based catalyst which demonstrates high dimerization activity coupled with excellent selectivity to butene-1 at moderate pressures and temperatures. This performance is influenced by the catalyst composition and reaction parameters. 

J. M. Thomas, G. Sankar, M. C. Klunduk, M. P. Attfield, T. Maschmeyer, B. F. G. Johnson, R. G. Bell, The identity in atomic structure and performance of active sites in heterogeneous and homogeneous, titanium-silica epoxidation catalysts, J. Phys. Chem. B 103 (1999) 8809. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

A close and well-researched homogeneous analogue of the epoxidation catalyst is the so-called Sharpless catalyst, a homogeneous titanium (TV) catalyst for fhe asymmefric epoxidation of allylic alcohols with f-butyl hydroperoxide (TBHP) [14]. A mechanism has been proposed for this type of epoxidation [15], which can be expressed in a generic form as shown in Fig. 13.4. 

In Figure 2 it is shown that the reaction continues unabated in the absence of the solid catalyst, whereas the recovered catalyst has lost the major part of its activity. The leaching of the titanium was further investigated by ICP-OES analysis. The silicium/titanium ratio of the Ti-MCM-41 as-synthesised is 230, while after the reaction this ratio was increased till 4720. We found that the native catalyst was hydrolytically stable under aqueous conditions, whereas in the presence of hydrogen peroxide rapid leaching was observed. Apparently the titanium hydroperoxide is more sensitive to hydrolysis than the native catalyst. The homogeneous titanium species is apparently an oxidation catalyst. A recent paper on Ti-MCM-41 also reports Ti-leaching in the liquid phase . 

In conclusion this system can be used for the formation of the uronic acids if a method could be developed for removal of the products before they are further oxidised. Homogeneous titanium is apparently a good oxidation catalyst, this is under further investigation. 

An important development, especially with respect to mechanistic and stereochemical investigations of alkene polymerization, was the use of homogeneous polymerization catalysts, such as metallocene dihalides of titanium, zirconium and hafnium (Cp2MX2), usually activated by alumoxanes as cocatalysts10. 

Use of Br-containing catalysts poses challenges such as high rates of corrosion. As such, commercial oxidation reactors and equipment that come in contact with the MC catalyst solution must be cladded with titanium in order to minimize corrosion. This desire to discover an alternative halogen-free homogeneous oxidation catalyst to the MC catalyst has been a significant motivation for many. 

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