Supported chromium oxide 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). 

Alumina supported chromium oxide catalyst is highly selective in paraxylene... 

Polymerization with Complex Catalysts. High density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silica-alumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system ... 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

As with peracids, hydroxylation is not the only useful oxidation possible. Dihydroxyarenes are readily converted to quinones using hydrogen peroxide in combination with a wide range of metal species 486 An interesting example is the use of supported chromium oxide-bis(tributyltin) oxide catalyst in the manufacture of vitamin Ki (Figure 3.115).478... 

Weckhuysen, B.M., Verberckmoes, A.A., Debaere, J., Ooms, K., Langhans, 1. and Schoonheydt, R.A. (2000) In situ UV-Vis diffuse reflectance spectroscopy-on line activity measurements of supported chromium oxide catalysts relating isobutane dehydrogenation activity with Cr-spedation via experimental design. Journal of Molecular Catalysis A Chemical, 151 (1-2), 115-31. 

If a copolymer such as VLDPE or LLDPE is the target resin, satisfactory comonomer incorporation must be achieved. This is manifested by the amount of comonomer incorporated (evidenced by density) and the distribution of comonomer in the polymer (evidenced by composition distribution). In general, supported chromium oxide catalysts incorporate comonomer more easily than Ziegler-Natta catalysts. 

Jehng, J.M. et al., Surface chemistry of silica-titania-supported chromium oxide catalysts,, 7 Chem. Soc. Faraday Trans., 91, 953, 1995,... 

The most active catalyst is chromium oxide [7]. Silica (Si02) or aluminosilicates (mixed Si02/Al203) are used as the support material. The support is sometimes modified with titania (Ti02). The chromium oxide (Cr Os) catalyst was originally developed by Phillips Petroleum Company and is referred to as Phillips catalyst. Other metal oxide catalysts were developed primarily at Standard Oil of Indiana, the best known among them being the molybdenum oxide (Mo Os) catalyst. 

The supported chromium oxide catalysts can be prepared by impregnating a silica-alumina support with a solution of chromium ions or by coprecipitating the oxides. The preferred impregnating solutions contain dissolved Cr(N03)s.9H20 or CrOs in nitric acid because catalysts made from chromium chlorides or sulfates retain some of the anions after calcination. The solid mixture of chromium-silicon-aluminum compounds is calcined in dry air at 400-700° C or higher to obtain the desired oxide. This probably results in the reaction of surface hydroxy groups in the support material with CrOs to form chromate (IV) and dichromate (V) species ... 

The supported chromium oxide catalysts can be activated by carrying out the heat treatment of the catalyst in a reducing atmosphere of CO, H2, or metal hydride or treatment with AIR3 or Al(OR)s. Poisoning of the catalyst occurs in the presence of such materials as water, oxygen, or acetylene. 

In the literature, most of the early discussion of the "active" valence is in reference to silica-supported chromium oxide catalysts. However, many organochromium compounds of widely differing valence are also known to be active upon contact with a support and subsequent exposure to ethylene. For example, as early as 1961, Walker et al. showed that diare-nechromium(O) compounds polymerize ethylene when deposited onto silica or another support [280,281]. The Cr(0) is probably oxidized by silanol groups to Cr(I), consistent with the inference that it too can be an active precursor. 

The characterization of the surface of supported metal oxide catalysts is vital to the understanding of many catalytic reactions. Supported chromium oxide catalysts are used for many industrial catalytic processes. Chromium oxide supported on alumina is used as a catalyst for propane and butane dehydrogenation. " Determination of the surface structure under reaction conditions is important for a complete understanding of the catalyst system. 

The supported chromium oxide catalysts were prepared by the incipient wetness impregnation method with aqueous solution of chromium nitrate [Cr(N03)3 9H2O, Aldrich]. The supports employed in the present study were Ti02 (Hombikat UV-100) and AI2O3 (Aldrich), which are commonly regarded as the best catalysts for the present reaction system [2]. After the impregnation of chromium oxide on the supports, the catalysts were subsequently dried at 110 °C for 12 h, and calcined in air at 450 C for 5 h. 

Catalysts based on other metals, such as gallium and vanadium oxides, can be also employed in DH processes [8, 9]. For example, silica-supported gallium oxide catalyst has been found to be moderately active, but quite selective in propane dehydrogenation (up to 80%) and results in much less coking, 1/10 of that using a silica-supported chromium oxide [8], There is an extensive research aimed to find new DH catalysts that will perform well at moderate temperatures, suffer less from coke deposition and maintain catalytic activity for long periods of time without regeneration. 

Weckhuysen BM, Schoonheydt RA (1999) Alkane dehydrogenation over supported chromium oxide catalysts. Catal Today 51 223-232... 

The loop reactors, which are recycled tubular reactors, are used by the Phillips Petroleum Co. and Solvay et Cie. The Phillips process is characterized by the use of a light hydrocarbon diluent such as isopentane or isobutane in loop reactors which consist of four jacketed vertical pipes. Figure 1 shows the schematic flow diagram for the loop reactor polyethylene process. The use of high-activity supported chromium oxide catalyst eliminates the need to deash the product. This reactor is operated at about 35 atm and 85-110° C with an average polymer residence time of 1.5 hr. Solid concentrations in the reactor and effluent are reported as 18 and 50 wt %, respectively. The reactor diameter is 30 in. (O.D.) and the length of the reactor loop is about 450 ft. 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

Before the discovery of the Pt/Al203-Cl catalyst (ie, before 1950s), the catalysts were chromium oxide or molybdenum oxide supported on alumina. In this... 

Supported chromium oxide catalysts. At promotion of a nonmodified support with 7% of Cr at 773 K, a solid chromium solution forms, which is proved by variations of the corresponding lines intensities. After the thermal treatment at 973 K, X-ray patterns of the samples change insignificantly, while at 1273 K, both 0- and a-Al203 forms coexist. The increased parameter of a-Al203 elementary cell testifies to the formation of a solid chromiiun solution with the content not exceeding 10%. 

At about the same time (1951) it was discovered that supported chromium oxide catalysts would also polsrmerize ethylene at low pressures to produce high molecular weight polymers (17). Reaction temperatures were in the range of 60-190°C. Polymer characteristics, particidarly, molecular weight and molecular weight distribution, could be varied by reactor temperature, pressure, and activation temperature of the Phillips catalyst. ... 

Other Early Developments. In addition to the breakthrough by Ziegler, two other discoveries of ethylene polymerization catalysts were made in the early 1950s. A patent by Standard Oil of Indiana, filed in 1951, disclosed reduced molybdenum oxide or cobalt molybdate on alumina (13). At the same time, Phillips discovered supported chromium oxide catalysts, prepared by impregnation of a silica-alumina support with Cr03 (14 16). Both the Phillips catalyst and titanium chloride based Ziegler catalysts are widely used in the production of high density polyethylene (HDPE). 

Yim SD, Chang K, Koh DJ, Nam I, Kim YG. Catalytic Removal of Perchloroethylene (PCE) over Supported Chromium Oxide Catalysts. Catal Today, 2000 63 215-222. 

Kang M, Lee C. Methylene Chloride Oxidation on Oxidative Carbon-supported Chromium Oxide Catalyst. Appl Catal A Gen 2004 266 163-172. 

Yim, S., Koh, D. and Nam, I. (2002). A pilot plant study for catalytic decomposition of PCDDs/PCDFs over supported chromium oxide catalysts, Catal. Today, 75, pp. 269-276. 

Ji, M., Hong, D., Chang, J., et al. (2004). Oxidative dehydrogenation of ethane with carbon dioxide over supported chromium oxide catalysts, in S. Park, J. Chang and K. Lee (eds). Carbon Dioxide Utilisation for Global Sustainability (Studies in Surface Science and Catalysis, 153), Elsevier, Amsterdam, pp. 339-342. 

Michorczyk P, Pietrzyk P, Ogonowski J (2012) Preparation and characterization of SBA-1-supported chromium oxide catalysts for CO2 assisted dehydrogenation of propane. Micropo-rous Mesoporous Mater 161 56-66... 

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