Metallocenes soluble/supported catalyst

The ansa-sandwich complexes, sometimes called constrained-geometry catalysts, exhibit a good ability to incorporate co-monomers. The catalysts (a) to (g) are soluble in solvents such as toluene. In the catalyst (h) the metallocene is supported on silica. Such solid-supported catalysts can be suspended in hydrocarbons for metering into the polymerization reactor [4],... 

Supported metallocene catalyst systems are preferred to soluble versions in conventional polyolefin plants, which were designed to use supported Ziegler-Natta or Cr203-based catalysts. Metallocenes can be supported on a number of substrates, such as Si02, MgC or AI2O3. Supported catalysts also provide polypropylene with fewer stereochemical defects. 

Soluble metallocene catalysts can be prepared by using hydrocarbon-soluble methylaluminoxane or organoborate activators, or their insoluble counterparts can be made by deposition of the metallocene onto silica or alumina solid acid activators [496]. In our experience, the differences in activity, and in the polymer, between soluble nonsupported and insoluble supported catalysts, are not great, which is another indication that under normal conditions the mass transport is not a major issue. 

Furthermore, the solvent used in many leaching experiments has been toluene, in which many metallocene—MAO catalysts have some solubility, especially at high Al/M ratios. However, to the best of the author s knowledge, few if any commercial polymerization processes use toluene as a diluent. Aliphatic hydrocarbons, bulk monomers, and fluidizing gas streams are used in large-scale plants. Especially at the lower excess of aluminum used in many supported catalysts (50—200 1), metallocene— MAO catalysts are insoluble in hydrocarbon solvents. The heptane extracts from a Cp2ZrCl2—MAO catalyst supported on silica showed no activity in ethylene polymerization, even when additional MAO was added. 

The most notable difference between the two families is catalyst activity. With few excep-tions,2 225 229 myriad cases, the activity of the supported catalyst is half to a tenth that of the soluble catalyst. This is widely ascribed to diminished diffusion of monomer into the interior pores of the supported catalyst, but may also be the result of fewer active centers present in the heterogeneous variant. Catalyst centers could be deactivated when supported, or may not be generated in the metal— cocatalyst interaction. Tait and co-workers determined that 91% of zirconocene centers were activated by MAO in solution, but when the metallocene was supported on silica, the concentration of active sites generated by MAO was only 9% of the total zirconium supported. 

Unfortunately, the catalytic activity of supported metallocenes is usually inferior to that of the equivalent soluble catalysts, probably due to deactivation of catalytic sites during the supporting process. Broadening of the MWD and CCD for supported catalysts can also occur under certain supporting conditions. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

In only a few polymerization processes are metallocene catalysts used in a soluble form. Supported metallocene catalysts are preferred for the production of polyethylene or isotactic polypropylene on an industrial scale, especially in the slurry and gas-phase processes. To use them in existing technological processes (drop-in technology) as replacements for the conventional Ziegler-Natta catalysts, the metallocenes have to be anchored to an insoluble powder support, including silica, alumina, and magnesium dichloride (208-217). Various methods of anchoring catalysts to supports are possible (Fig. 25) ... 

Metallocenes immobilized on solid support materials have been successfully introduced in industry as polymerization catalysts for the production of new application-oriented polymer materials. Industrial polymerizations, which are carried out either as a slurry process in liquid propylene or as a gas-phase process (Section 7.2.3), require that catalysts are in the form of solid grains or pellets soluble metallocene catalysts thus have to be supported on a solid carrier. 

Supported metallocene catalysts are an important field of research for the polymer industry and shall be mentioned here only briefly. Single-site olefin polymerization catalysts have been extensively investigated and are now achieving wide acceptance in the polyolefin industry. In order to achieve commercial significance, these soluble metallocene catalysts have to be immobilized on a carrier. The challenges of supporting these catalysts have been addressed in many creative ways and this topic has been taken up in an excellent review by Carnahan et al. [87]. This paper mainly describes anchoring techniques to silica, but these concepts may also be applied for other purposes. 

One area which continues to attract interest is increasing the activity of single-site catalysts when supported, making up some of the deficit from which they suffer compared to their solution-soluble cousins. In this vein, Collins recently described in broad terms the synthesis of and polymerization using tethered ansa-metallocenes, the activity of which is comparable to a homogeneous analogue we await full details of catalyst structures and methods of preparation. ... 

Typical polydispersity index 1. Stereorigid metallocene catalysts 2. Heterogeneous MgCl2 supported TiCls catalyst 3. Soluble magnesium-titanium catalyst 2-3 5-11 2.0-2.7 (4) (3) (2)... 

Supporting can also affect the stereochemical control of the metallocene. Depending on the supporting technique employed, catalysts that are syndiospecific when soluble might produce mainly isotactic PP when supported on SiOj. This inversion of stereoselectivity has also been observed when the type of cocatalyst is changed. 

Additionally, supported metallocenes usually require smaller aluminum to transition metal ratios than the equivalent soluble systems and some can be activated in the absence of aluminoxanes by common alkylaluminums. This reduced dependence on the presence of aluminoxanes and on high ratios of aluminum to transition metal has been related to a reduction in catalyst deactivation by bimolecular processes due to the immobility of the active sites on the surface of the support. 

There are relatively many different catalysts that are usually used for specific chemical reactions. Types include Z-N, metallocene, and others including their combinations. These different systems are available worldwide from different companies. Terms and information are used to identify the behavior of catalysts. An autocatalyst is a catalytic reaction induced by a product of the same reaction. This action occurs in some types of thermal decomposition. The catalyst benzoyl peroxide is a white, granular, crystalline solid, tasteless, faint odor of benzaldehyde, has active oxygen, and soluble in almost all-organic solvents. Use includes polymerization catalyst with different plastics such as thermoset polyester, rubber vulcanization without sulfur, embossed vinyl floor covering, etc. A catalyst carrier is a neutral material used to support a catalyst, such as activated carbon, diatomaceous earth, or activated alumina. There are fluid catalysts that are finely divided solid particles utilized as a catalyst in a fluid bed process using certain thermoset plastics. 

All heterogeneous Ziegler-Natta and Phillips catalysts have two or more active-site types and many soluble Ziegler-Natta and metallocene catalysts may also show multiple-site behavior [36, 37]. In addition, several metallocene catalysts, when supported on organic and inorganic carriers, may behave like multiple-site catalysts even if they behaved as single-site catalysts in solution polymerization. Therefore, several of the catalysts used industrially for polyolefin manufacturing have in fact two or more active-site types. 

This is not as important for Phillips catalysts, where molecular weight control is achieved via support treatment, but can become a limiting factor with Ziegler-Natta catalysts. Metallocenes are generally very sensitive to the presence of hydrogen and therefore less influenced by this reduced solubility that Ziegler-Natta catalysts. 

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