The Catalyst Review June 2020 - 9

SPECIAL FEATURE
a Ziegler catalyst supported on silica comprising an organoaluminum component and a titanium trichloride component which has
been prepared by reducing TiCl4 with magnesium metal (Mg) in suspension of anhydrous tetrahydrofuran (THF). This suspension
is spray dried and the solid catalyst is partially activated in slurry of mineral oil by tri-n-hexylaluminum (THA) and diethylaluminum
chloride (DEAC). PE Ziegler catalysts usually made on-site are used for the production of bimodal HDPE in slurry process. According
to a patent assigned to Hoechst (Franke Patent 1989), the one pot ZN PE catalyst is prepared at the PE plant site as the reaction
product of a magnesium alcoholate - Mg(OCH3)2 with TiCl4 and an organoaluminium compound. Being Hoechst a predecessor of
LyondellBasell in HDPE, such kind of catalysts are still used for the production of basic HDPE types in the Hostalen slurry process
using a cascade of agitated vessel reactors. PE chromium based catalysts are the most important catalyst systems for the production
of broad MW HDPE. The chrome catalysts are used in different process technologies, like slurry loop reactor process technology, and
GP fluidized bed reactors process technologies. Historically the chrome catalysts were first used in the Phillips loop reactor process
technology. The Phillips chrome catalyst is typically prepared from an amorphous silica gel and chromium oxide. The Cr oxide is
dissolved in water and the resulting solution contacted with a silica support. The silica surface rapidly adsorbs the Cr oxide and a dry
solid catalyst is obtained removing the free water. The addition prior of the precipitation of a salt of Ti or Al could increase the melt
index values and modify favorably the MWD, if needed. In addition to free water, the silica contains bound water that is removed
heating the catalyst at temperatures up to 900C for several hours; this step is called the catalyst activation and is usually carried out
at the polymerization plant site for safety reasons. (Seymour, 1993). An aluminum alkyl as THA is usually fed to the polymerization
reactor to function as scavenger of possible poison's to catalyst in the streams feeding the reactor.
Metallocene catalysts are a class of single-site catalysts, resulting from the reaction of metallocenes, complexes of biscyclopentadienyl (Cp) ligands or other Cp-type ligands on a transition metal of group 4 such as titanium, zirconium or hafnium, with an
organoaluminum activator compound suitable to polymerize both propylene and ethylene to PP or PE. Early metallocene catalysts
showed poor activity toward ethylene and did not polymerize propylene, although they polymerized the higher alpha-olefins. This
changed with the discovery of the activating effect of small amounts of water on the metallocene, and the subsequent synthesis of
methylaluminoxane (MAO) as a potent co-catalyst able to activate group 4 metallocenes toward the polymerization of olefins. The
metallocenes catalysts supported usually on silica (SiO2) produce innovative PP, HDPE, LLDPE homopolymers and/or copolymers
and EPR/EPDM rubbers. Modification of the ligand can control the catalytic properties of single-site catalysts. While the general
structures of Group 4 metallocene catalysts have a metal center sandwiched by two Cp ligands, the so-called post-metallocene
catalysts have a metal center with no or just one Cp ligand. Initially ligand modifications focused on various cyclopentadienyl
derivatives but great a diversity was covered through high throughput screening and now post-metallocene catalysts employ a range
of chelating ligands (Ali, 2019), often including pyridine and amido (R2N−).
A fascinating innovation
Table 3: Performance development of Ziegler-Natta catalysts for polypropylene.
story regards the
developments along
the years of the diverse
generations of innovative
ZN PP catalysts since
the inception of the
industry until today. In
the following, find a
brief description of main
advances characterizing
these catalyst generations
(see a summary in Table 3).
Some crucial discoveries as
the "activated" MgCl2, its
use as catalyst support and
the addition of appropriate
Lewis bases (electron
donors), made it possible
to obtain modern highly
active and stereospecific
Source: Wang 2019 modified by the Author
PP catalyst systems. More
specifically in the early 1980s was discovered a combination of electron donors, namely alkyl-phthalates as internal donor and
alkoxysilanes as external donors, able to afford a much better productivity (yields up to 40 tons PP/Kg catalysts and more) and
isotacticity balance (II about 99%). These catalyst systems of the 4-th PP catalyst generation are currently used in most of the
The Catalyst Review 										

	

June 2020

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The Catalyst Review June 2020

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