Ziegler–Natta catalyst

With Ziegler- Natta process (formerly also Mülheim process ) refers to a process for preparing polyolefins by coordinative insertion polymerization with organometallic catalysts. This method is technological application in the synthesis of polyethylene and polypropylene.

History

From 1938 polyethylene was produced radically at high pressures and temperatures of up to 300 MPa and 300 ° C. Discovered in 1953, Karl Ziegler at the Max Planck Institute for Coal Research in Mülheim an der Ruhr, that ethylene can be polymerized with titanium catalysts even at low pressures or later even at normal pressure and room temperature for polyethylene. The available up to now because of the polyethylene had free-radical polymerization and the resulting long chain branches other mechanical properties than the new catalytically prepared linear polyethylene. This had a higher crystallinity and thus a higher hardness and stiffness and thus opened up new fields of application for polyethylene. Just a few months after the discovery of the method has been applied industrially.

The initially low activity of the catalyst, however, led to a large amount of catalyst remaining in the product, which had to be processed so awkward. Through continuous improvement in catalyst activity can with one gram of titanium in the catalyst more than 1000 kg of polymer produced and the catalyst residue remains in the product today. Second namesake of the method is Giulio Natta who successfully transferred the transition metal catalysis in the polymerization of propene. Both were jointly awarded the 1963 Nobel Prize in Chemistry for discoveries in the field of chemistry and technology of high polymers.

In 2007, the total volume of polypropylene by the Ziegler- Natta process 45.1 million tonnes, about 30 million tonnes of HDPE. The polymers are used in the manufacture of pipes, gas pipes, oil tanks, packaging materials and many other areas.

Ziegler -Natta catalysts

Classic catalysts

The catalytic systems are organometallic mixed catalysts composed of an organometallic compound of main group I, group II or III (e.g., triethylaluminum ) and a transition metal compound, especially of the groups IV-VI (for example, titanium tetrachloride).

These are also called classical Ziegler -Natta catalysts (ZN catalysts). These are homogeneous or heterogeneous multicenter (engl: multiple- site) catalysts. These have an enormous economic importance, acquired primarily for the production of polypropylene. Modern Ziegler -Natta catalysts are prepared from magnesium chloride, titanium tetrachloride, triethylaluminum and internal and external donors and achieve a turnover of 150 kg of polymer per gram of titanium. The name of Ziegler catalysts is based on a proposal by the Italian chemist Giulio Natta, which it first achieved by the developed of Karl Ziegler organometallic mixed catalysts to polymerize propene stereospecifically.

Kaminsky catalysts

In addition to the classic, heterogeneous, multiple- site ZN catalysts, homogeneous, single-site catalysts for the commercial production of polyethylene and polypropylene have been used in recent times. It is to metallocene compounds of group 4, in conjunction with the co- catalyst is methylaluminoxane, so-called Kaminsky catalyst. The most significant here zirconocene complexes which exhibit much higher catalytic activity than the corresponding titanocene or Hafniumsysteme. For procedural reasons, such are nevertheless applied to be homogeneous systems on solid porous carrier particles.

Mechanism

Under Ziegler -Natta catalysis refers to the coordinate insertion polymerization of olefins of the Lewis -acid metal complex compounds. The catalyst systems are usually of a main group metal organic component, such as triethyl aluminum, and an organometallic transition metal component of the sub-groups of four to eight.

In the first step of an octahedral complex is formed which has a vacant coordination site. Propene binds to the vacant coordination site and then inserted into the metal -carbon bond, leading to the structure of the polymer.

Low pressure processes are

The low-pressure process for the production of polyethylene is carried out in a paraffin oil in which the catalyst is dispersed. At normal pressure, but more often in Ethendrücken 2-8 bar, the reaction is carried out in a stirred tank. When using classical catalysts, the catalyst content was high in the final product and it had to be introduced for processing reasons a separation step. For this example, the catalyst was converted with alcohols into soluble compounds and separated.