Silica Ziegler-Natta catalysts

The use of supports containing hydroxyl groups such as alumina, silica, Mg(OH)Cl, etc., for chemical fixing of the transition metal compound has been widespread since the early 1960s. Heat treatment (calcination) of such supports can control the number and type of surface hydroxyl groups and indirectly the amount and distribution of transition metal atoms anchored to the surface. The most commonly used Ziegler-Natta catalyst of this type is... 

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) ... 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

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. 

Diluents must be inert toward the catalyst system and are usually saturated hydrocarbons such as propane, isobutane and hexane. Slurry processes typically operate at temperatures from about 80 to 110 °C and pressures of 200-500 psig. Polyethylene precipitates as formed resulting in a suspension of polymer in diluent. The catalysts most commonly used in slurry processes are chromium-on-silica or supported Ziegler-Natta catalysts. 

In transitioning between incompatible Ziegler— Natta catalysts and metallocene catalysts in a fluidized-bed gas-phase process, polymerization of the first catalyst must be fully stopped before introducing the second catalyst. Small amounts of a catalyst killer such as water or methanol, either injected directly into the gas stream or added as wet silica, are introduced to deactivate the catalyst in the reactor irreversibly. The second catalyst is added after adjustment of the feed streams. Addition of CO above and below the distributor plate has also been used. Leftover catalyst in addition vessels can be deactivated by heat treatment, followed by exposure to air. ... 

The catalysts for these polymerizations can be separated into two groups. To the first belong the so-called Ziegler-Natta catalysts, and to the second, transition metal oxides on special supports, like carbon black or silica-alumina, etc. Besides the two, there are related catalysts, like transition metal alkyls or metal halides that also catalyze some coordinated anionic polymerization. This group also includes transition metal-TT-allylic compounds and transition metal hydrides. 

Today, less than 5 % of polypropylene is produced using metallocene catalysts. Metallocene catalysts are mainly ZrCh catalysts supported on silica in combination with co-catalysts like methylaluminoxane (MAO). These catalysts show very specific characteristics and may also be combined with Ziegler-Natta catalysts. These catalysts are mainly used to produce specific product ranges and they influence plant configurations. 

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