1 Introduction
The discovery of Ziegler–Natta catalysts for olefin polymerization in 1953 was one of
the most important achievements in the field of synthetic polymer chemistry during
the past 60 years. Parallel to the discovery, another important catalyst for polyethylene production, SiO 2 -supported inorganic chromium oxide, known as Phillips catalyst, was also discovered and commercially applied on a large scale [1–4]. Nowadays,
this catalyst is used to produce over 10 million tons of high-density polyethylene
(HDPE), accounting for about half of the world’s market. Compared with the
Ziegler–Natta and metallocene catalysts, the Phillips catalyst exhibits unique polymerization behaviors and can produce PE with distinctive polymer chain
microstructures. The catalyst is known to be highly active for ethylene polymerization with or without using organometallic cocatalysts or a preliminary activation step
using organometallic cocatalysts or any other reducing agents (such as CO or H 2 ). Its
HDPE products feature an ultrabroad molecular weight distribution (MWD; the
typical polydispersity index is larger than 10), small amount of long chain branches
and a vinyl end-group for each PE chain. The products show high melt strength and
are especially applicable for blow molding products like hollow containers. In the last
few decades, the applications for exclusive Phillips HDPE products, including
gasoline tanks for the automobile industry and ultralarge plastic containers and
pipes, have experienced a successively increasing market demand.
Compared with the great success in commercial applications, academic progress
on the Phillips catalyst is lagging far behind, in spite of numerous research efforts
during the past 60 years. Aspects of the Phillips catalyst concerning the formation,
structure, oxidation state of active sites, and polymerization mechanisms, especially
the initiation mechanism, are still mysterious. The difficulties for basic studies on
this important industrial catalyst system are mainly derived from the low percentage of active Cr species, the complexity of heterogeneous catalyst systems, the
multiple valence states of Cr, the instant encapsulation of active sites by produced
polymer, and the ultrafast polymerization rate.
Application of the Phillips catalyst in ethylene polymerization includes two
important processes: catalyst preparation and catalyst activation through reduction.
The catalyst is usually prepared by impregnation of an aqueous solution of chromium compound, such as chromate acetate, on porous amorphous silica gel, and
subsequent calcination at high temperatures between 300 and 900
C in oxygen or
dry air. It is generally accepted that chromate acetate first decomposes and is
oxidized into bulk CrO 3 . This is followed by a reaction with surface hydroxyl
groups on silica gel during the calcination process, through which chromium
compound could be highly dispersed and stabilized as surface hexavalent chromate
species, i.e., monochromate, dichromate, and sometimes even polychromate, as
illustrated in Scheme 1 [2, 5–9]. As for the reductive activation process for ethylene
polymerization, the hexavalent chromate species is transferred into a lower oxidation state, i.e., divalent Cr(II) species, as the final active precursor for ethylene
polymerization by ethylene monomer (C 2 H 4 ), carbon monoxide (CO), Al-alkyl
cocatalysts (e.g., TEA), or even other reducing agents.
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R. Cheng et al.
The discovery of Ziegler–Natta catalysts for olefin polymerization in 1953 was one of
the most important achievements in the field of synthetic polymer chemistry during
the past 60 years. Parallel to the discovery, another important catalyst for polyethylene production, SiO 2 -supported inorganic chromium oxide, known as Phillips catalyst, was also discovered and commercially applied on a large scale [1–4]. Nowadays,
this catalyst is used to produce over 10 million tons of high-density polyethylene
(HDPE), accounting for about half of the world’s market. Compared with the
Ziegler–Natta and metallocene catalysts, the Phillips catalyst exhibits unique polymerization behaviors and can produce PE with distinctive polymer chain
microstructures. The catalyst is known to be highly active for ethylene polymerization with or without using organometallic cocatalysts or a preliminary activation step
using organometallic cocatalysts or any other reducing agents (such as CO or H 2 ). Its
HDPE products feature an ultrabroad molecular weight distribution (MWD; the
typical polydispersity index is larger than 10), small amount of long chain branches
and a vinyl end-group for each PE chain. The products show high melt strength and
are especially applicable for blow molding products like hollow containers. In the last
few decades, the applications for exclusive Phillips HDPE products, including
gasoline tanks for the automobile industry and ultralarge plastic containers and
pipes, have experienced a successively increasing market demand.
Compared with the great success in commercial applications, academic progress
on the Phillips catalyst is lagging far behind, in spite of numerous research efforts
during the past 60 years. Aspects of the Phillips catalyst concerning the formation,
structure, oxidation state of active sites, and polymerization mechanisms, especially
the initiation mechanism, are still mysterious. The difficulties for basic studies on
this important industrial catalyst system are mainly derived from the low percentage of active Cr species, the complexity of heterogeneous catalyst systems, the
multiple valence states of Cr, the instant encapsulation of active sites by produced
polymer, and the ultrafast polymerization rate.
Application of the Phillips catalyst in ethylene polymerization includes two
important processes: catalyst preparation and catalyst activation through reduction.
The catalyst is usually prepared by impregnation of an aqueous solution of chromium compound, such as chromate acetate, on porous amorphous silica gel, and
subsequent calcination at high temperatures between 300 and 900
C in oxygen or
dry air. It is generally accepted that chromate acetate first decomposes and is
oxidized into bulk CrO 3 . This is followed by a reaction with surface hydroxyl
groups on silica gel during the calcination process, through which chromium
compound could be highly dispersed and stabilized as surface hexavalent chromate
species, i.e., monochromate, dichromate, and sometimes even polychromate, as
illustrated in Scheme 1 [2, 5–9]. As for the reductive activation process for ethylene
polymerization, the hexavalent chromate species is transferred into a lower oxidation state, i.e., divalent Cr(II) species, as the final active precursor for ethylene
polymerization by ethylene monomer (C 2 H 4 ), carbon monoxide (CO), Al-alkyl
cocatalysts (e.g., TEA), or even other reducing agents.
138
R. Cheng et al.