Catalysts Phillips

Phetharbital [357-67-5] pH Glass electrode Philips 2P Process Phillips catalysts... 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

Molecular Weight Distribution. In industry, the MWD of PE resins is often represented by the value of the melt flow ratio (MER) as defined in Table 2. The MER value of PE is primarilly a function of catalyst type. Phillips catalysts produce PE resins with a broad MWD and their MER usually exceeds 100 Ziegler catalysts provide resins with a MWD of a medium width (MFR = 25-50) and metallocene catalysts produce PE resins with a narrow MWD (MFR = 15-25). IfPE resins with especially broad molecular weight distributions are needed, they can be produced either by using special mixed catalysts or in a series of coimected polymerization reactors operating under different reaction conditions. 

HDPE resias are produced ia industry with several classes of catalysts, ie, catalysts based on chromium oxides (Phillips), catalysts utilising organochromium compounds, catalysts based on titanium or vanadium compounds (Ziegler), and metallocene catalysts (33—35). A large number of additional catalysts have been developed by utilising transition metals such as scandium, cobalt, nickel, niobium, molybdenum, tungsten, palladium, rhodium, mthenium, lanthanides, and actinides (33—35) none of these, however, are commercially significant. 

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

Keywords Chromium Ethylene polymerization Phillips catalyst... 

The Cr/Si02 Phillips catalysts, patented in 1958 by Hogan and Banks [2], are nowadays responsible for the commercial production of more than one third of all the polyethylene sold world-wide [7,16]. 

The Phillips catalyst has attracted a great deal of academic and industrial research over the last 50 years. Despite continuous efforts, however, the structure of active sites on the Phillips-type polymerization systems remains controversial and the same questions have been asked since their discovery. In the 1950s, Hogan and Banks [2] claimed that the Phillips catalyst is one of the most studied and yet controversial systems . In 1985 McDaniel, in a review entitled Chromium catalysts for ethylene polymerization [4], stated we seem to be debating the same questions posed over 30 years ago, being no nearer to a common view . Nowadays, it is interesting to underline that, despite the efforts of two decades of continuous research, no unifying picture has yet been achieved. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

With the exception of LDPE, polyolefins like other polyethylenes and polypropylene, which represent the largest amount of vinyl-type polymers produced in the world, are neither synthesized by radical nor by classical ionic polymerisation processes. Different types of polymerisation catalysts are in use for these purposes. The Cr-based Phillips catalyst, Ziegler-Natta type catalysts, metallocene or other more recently discovered catalysts, including late transition metal catalysts, are all characterized by their propagation step where the olefin monomer inserts into a carbon-transition metal link. ... 

Phillips catalysts, 17 702 20 151-152 active chromium species on, 20 156 Phillips chromium catalyst, 20 152 Phillips Petroleum loop slurry process, 20 168... 

Chromate, in Phillips catalysts, 33 48-49 Chromatography, gas phase, of evaporated metal film catalysts, 23 18 Chromia, see Chromium oxide a-Chromia... 

Dichromate, in Phillips catalyst, 33 48-49 Dicoordination, versus monocoordination, BOC-MP, 37 125-127... 

Phenyl group, as hydrocarbon surface species, vibrational spectra, 42 228 Phenylindanyl cation NMR spectra, 42 146 Phenylnaphthalene, cyclization, 28 318 2-Phenylpentane cyclization, 28 299 isomerization, 30 67-68 reactions on different catalysts, 28 296,301 Phillips catalyst, 31 29... 

A. Damin, F. Bonino, S. Bordiga, E. Groppo, C. Lamberti and A. Zecchina, Vibrational properties of Cr centers on reduced Phillips catalysts highlighted by resonant Raman spectroscopy, ChemPhysChem, 1, 342-344... 

Initiation hy the Phillips catalyst is not well understood. Both Cr(II) and Cr(III) have been proposed as the active oxidation state of chromium. Initiation involves the formation... 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Infrared spectroscopic studies show that hdpe produced by Ziegler-Natta catalysis has about 10 to 14 ethyl groups per 1000 repeating units. However, hope produced by the use of the Phillips catalyst has less branching and fewer ethyl groups. Most of the commercial hope used in the United States is produced by use of the Phillips catalyst. 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

Supported organochromium complexes exhibit catalytic behavior very similar to that of the traditional Phillips catalyst, and both systems are very sensitive to minute experimental variables.322 Modification of the catalyst allows for control of... 

Since the significance of this transformation increases with increasing temperature, it provides an easy way of controlling molecular weight in polymerization with the Phillips catalyst. 

The Phillips catalyst contains hexavalent chromium after calcining, but the early discoverers quickly realized that reduction takes place in the reactor on contact with ethylene, leaving chromium in a lower oxidation state as the active species. A worldwide debate has continued to this day about the valence of this reduced species. Chromium in every valence state from Cr(II) to Cr(VI) has been proposed as the active site, either alone or in combination with another valence. The question has received far more attention than it probably deserves, undoubtedly at the expense of more fundamental issues, like the polymerization mechanism. 

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