Olefin Polymerization Over Supported Chromium Oxide Catalysts
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Catalysis Today 51 (1999) 215±221 Ole®n polymerization over supported chromium oxide catalysts Bert M. Weckhuysen*, Robert A. Schoonheydt Centrum voor Oppervlaktechemie en Katalyse, Departement Interfasechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001, Heverlee, Belgium Abstract Cr/SiO2 or Phillips-type catalysts are nowadays responsible for a large fraction of all polyethylene (HDPE and LLDPE) worldwide produced. In this review, several key-properties of Cr/SiO2 catalysts will be discussed in relation to their polymerization characteristics. It will be shown how the polyole®n properties can be controlled via rational catalyst design. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ole®n polymerization; Chromium oxide catalysts; Cr/SiO2 1. Introduction 2. Ziegler±Natta catalysts, which consist of a combi- nation of titanium chloride and an alkylaluminum The production of polyole®ns such as high density chloride, mostly supported on MgCl2 (e.g. TiCl3± polyethylene (HDPE), low density polyethylene Al(C2H5)3/MgCl2); and (LDPE), linear low density polyethylene (LLDPE), 3. the recently commercialized single site metallo- and polypropylene (PP) is a multi-billion industrial cene catalysts, which are bis-cyclopentadienyl activity. With the exception of LDPE, which is made derivatives of either Zr, Hf or Ti, in combina- by radical combination reactions at high temperature tion with methylaluminoxane (e.g. Cp2ZrCl2± (250±3008C) and pressure (>50 bar), all these poly- [MeAlO]n). ole®ns are produced by using either homogeneous or heterogeneous catalysts [1]. These catalysts always Phillips-type catalysts, named after the company operate at relatively low temperature (80±1808C) and name of the inventors J.P. Hogan and R.L. Banks, are pressure (<50 bar). Three classes of polymerization nowadays responsible for the commercial production catalysts can be distinguished: of more than one third of all polyethylene sold world- wide. It is certainly this industrial importance, which 1. Phillips-type catalysts, which are composed of a has attracted the great deal of academic and industrial chromium oxide supported on an amorphous research [2±10]. Despite all these efforts, however, material such as silica (e.g. Cr/SiO ); 2 Phillips-type polymerization catalysts remain contro- versial, and still the same questions ± often truly *Corresponding author. Fax: +32-16-32-19-98; e-mail: academic in nature ± are asked since their discovery [email protected] in the early 1950s [7]. These questions are: 0920-5861/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0920-5861(99)00046-2 216 B.M. Weckhuysen, R.A. Schoonheydt / Catalysis Today 51 (1999) 215±221 1. What is the oxidation state of the active site? 2. General description of the Phillips 2. What is the exact polymerization mechanism? and polymerization process 3. What is the molecular structure of the catalyst? The heart of the Phillips polymerization process is a Although in the last two decades a lot of progress has supported chromium oxide catalyst, which is usually been made in resolving these issues by applying prepared by impregnation of a chromium oxide on a advanced spectroscopic techniques, no unifying pic- wide pore silica [2,5]. The impregnated material is ture has yet been achieved [3]. This lack of agreement subsequently heated in air at high temperature, cooled must be attributed to the following reasons: down, ¯ushed with nitrogen, and stored under dry 1. Only a small fraction of the chromium centers are nitrogen until loaded in the polymerization reactor. considered to be active. As a consequence, almost all Chromium-based catalysts are able to polymerize the reported characterization results are not directly ole®ns which have no branching closer than the 4- related to the active centers of the catalysts. position to the double bond and which contain no more 2. Phillips-type catalysts are often regarded as than about eight carbon atoms. Thus, propylene, 1- ``monolithic''; i.e., treated if only one catalyst type/ butene, 1-pentene, and 1-hexene are polymerized to composition is used. However, the catalyst structure is branched, high molecular weight polymers ranging extremely sensitive towards different preparation and from solids to viscous liquids. From ethylene, a broad treatment conditions. As a consequence, the results of range of solid polymers are produced, which are different research groups are dif®cult to compare. characterized by a linear backbone without long-chain Rather, a whole battery of spectroscopic techniques branching (HDPE). Copolymerization of ethylene have to be applied on the same set of catalysts to with mostly C4±C8 ole®ns gives a branched polymer, gather relevant information. In this respect, it is in which the number of branches per molecule and the important to note the differences in sensitivity of number of carbon atoms per branch depend on the different spectroscopic techniques towards surface amount and nature of the comonomer (LLDPE). It is Cr species. important to note that chromium-based catalysts 3. Most published polymerization studies have been always produce a resin with a broad distribution of conducted on a vacuum line at 258C and pressures individual polymer molecules, each of which contri- lower than 1 bar. Such experiments, although inter- butes to the overall properties of the resin. It is this esting, are often far away from real industrial condi- molecular weight distribution (MWD) which deter- tions. Therefore, such experiments are not always mines the properties, such as melt viscosity and reliable indicators of the performances of a particular elasticity, impact resistance, and environmental stress catalyst. There is clearly a need for in situ character- crack resistance. Besides molecular weight distribu- ization studies which allow measurements as close as tion, the amount, type and pattern of branching also possible to real catalytic ones. in¯uences the properties of the resin. The aim of this review paper is to touch on the most The success of the Phillips polymerization process important aspects of Phillips-type polymerization cat- originates from its diversity. Phillips catalysts are able alysts. However, no attempt will be given to cover the to make more than 50 different types of polyethylene, whole ®eld, and in this respect, we refer to several and a whole battery of chromium-based catalysts are excellent review papers, most notably the review developed, each of which are able to produce a paper of McDaniel published in 1985 [2]. For back- different type of HDPE or LLDPE [4]. Thus, the ground, a general description of the Phillips polymer- critical resin parameters are determined by the catalyst ization process will be given. In the second section, the performances, and the different industrial companies elementary steps in catalyst synthesis will be are able to control their resin properties via catalyst reviewed, while the third section gives an overview design. of the recent literature on catalyst characterization. In The polymerization of ethylene can be done over a the ®nal section, it will be shown how the polyole®n relatively broad range of temperatures, however, the properties can be controlled via rational catalyst commercial temperatures range between 658C and design. 1808C. The relative rate of termination of the poly- B.M. Weckhuysen, R.A. Schoonheydt / Catalysis Today 51 (1999) 215±221 217 ethylene chain determines the average chain length; UNIPOL1 process, and is licensed by union carbide. i.e., the molecular weight (MW) of the polyethylene. Gaseous ethylene, eventually together with a como- Another important indicator is the melt index (MI). nomer, such as 1-hexene, and the catalyst are reacted The MI is a measure of the molten polymer ¯uidity, in a ¯uidized bed reactor at temperatures and pressures and is inversely related to the MW. In the case of the at around 1008C and 20 bar, respectively. The gas reaction temperature, it is found that a higher tem- phase process has the advantage of eliminating the perature results in a higher relative termination rate, cost of supplying and recycling the solvent or dis- and as a consequence, in a higher MI and lower MW. persant. However, heat transfer rates are lower than in The ethylene pressure is also an important factor, and the particle form process, resulting in lower produc- is usually kept between 20 and 30 bar. In general, the tion rates per unit volume of reactor. higher the ethylene pressure in the polymerization reactor, the higher the MW of the produced polyethy- lene. 3. Catalyst synthesis There are three different industrial modes of operation for the Phillips polymerization process Phillips polymerization catalysts can be based [5,10]: either on organochromium compounds or on chro- 1. The solution process uses an inert hydrocarbon mium oxides. The ®rst type is made by calcining a that dissolves the polymer as it is formed. The solvent high surface area support, and then depositing an may consist of a cycloparaf®n, such as cyclohexane. organochromium compound, such as chromocene, Both monomer and polymer remain in the solution onto it anhydrously. The calcination treatment par- during the reaction while the ®nely-divided catalyst is tially dehydroxylates the surface, leaving mostly iso- maintained in suspension by agitation. Reaction tem- lated hydroxyl groups, which can then react with the peratures range from 1258C to 1758C, and reaction organochromium compound to attach the Cr to the pressures from 20 to 30 bar. The reactor product is carrier through an oxide bond. It is important to notice withdrawn and the monomer evaporated. The sus- that one or more ligands may still remain attached to pended catalyst is then removed by ®ltration, and the Cr. In the second family of catalysts, a chromium the solvent is evaporated from the ®ltrate. compound is impregnated from aqueous or non-aqu- 2. In the slurry process a liquid dispersant is used. eous solutions onto a support.