Alumina ethylene polymerization

The electron paramagnetic resonance spectrum of transition metal ions has been widely used to interpret the state of these ions in systems of catalytic interest. Major emphasis has been placed on supported chromia because of its catalytic importance in low-pressure ethylene polymerization and other commercial reactions. Earlier work on chromia-alumina catalysts has been reviewed by Poole and Maclver 146). On alumina it appears that the chromium is present in three general forms the S phase, which is isolated Cr3+ on the surface or in the lattice the 0 phase, which is clusters of Cr3+ and the y phase, which is isolated Cr5+ on the surface. The S and 0... 

The idea that complex formation may be important in the catalytic process can be carried further. It has been found, for example, that bis-arene chromium complexes supported on silica-alumina are active for ethylene polymerization. These catalysts are prepared by activating the support alone in the usual manner and then impregnating with a hydrocarbon solution of the bis-arene compound at room temperature in the absence of air or other oxidizing agent. 

Of great industrial importance for ethylene polymerization are the derivatives of transition metal oxides. The oxides are usually anchored on silica or alumina surface, and thus they belong to the class of supported catalysts. So far we possess only rough information on the principles and mechanism of their operation. 

The actinide dimethyl complexes Cp 2MMe2 (M = Th, U) are virtnally inert in ethylene polymerization but are far more active when adsorbed on dehydroxylated alumina (see Alumina).Solid-state C NMR studies suggest that surface bound [Cp 2MMe]+ species are generated (Figure 12). 

This concept was first mathematically developed by Clark and Bailey according to an exponential distribution of active centres with respect to adsorption energy. The Langmuir-Hinshelwood mechanism for adsorption and reaction was found to fit experimental results (ethylene polymerization over chromium oxide-silica-alumina catalyst) more closely than the Rideal mechanism. 

Other chromium catalysts for ethylene polymerization employ chromo-cene [246] and bis(triphenylsilyl) chromate [247] deposited on silica-alumina. The catalyst support is essential for high activity at moderate ethylene pressures (200—600 p.s.i.). The former catalyst is activated further by organo-aluminium compounds. Polymerization rates are proportional to ethylene pressure and molecular weight is lowered by raising the temperature or with hydrogen (0.1—0.5 mole fraction) in the monomer feed wide molecular weight distributions were observed. 

However, unmodified Cr/alumina catalysts generally display only poor activity for ethylene polymerization. Alumina has only weakly acidic Al-OH groups, and it often exhibits a higher surface OH group population than silica. Both of these properties can potentially influence the electron density or ligand field of the chromium. 

Cr/alumina exhibits a "fast" kinetics profile that is quite different from that of Cr/silica. The polymerization rate develops rapidly, especially when the alumina has been acidified by treatment with silica, fluoride, phosphate, or sulfate. Cr/alumina exhibits polymerization kinetics similar to that of Cr/AlP04, a topic that is discussed in Section 15. The polymerization rate rises quickly when ethylene is added, but later it tends to decay slowly. The rapid initial rise indicates that reduction of Cr(VI), or desorption of redox by-products, and/or alkylation of the chromium, may be more facile on alumina than on silica. Alumina is known as a strong adsorbent in its own right, so that adsorption of by-products from chromium onto the neighboring surface is one possible contributing cause of the rapid development of polymerization rate. 

An example of this increase in LCB is shown in Figure 177. H3PO4 was added to Cr/alumina catalyst in varying amounts prior to activation at 650 °C, and then these catalysts were tested for ethylene polymerization activity. The LCB contents of the resultant polymers, as measured by the JC method, are plotted against the amount of phosphate added to the catalyst. Three series of polymerization runs were made. In the first series, the catalyst contained no sulfate, and BEt3 cocatalyst was used with H2 in the reactor. The LCB level in the polymer was found to increase by almost two orders of magnitude as phosphate was added to the catalyst. 

For example, data representing the kinetics of polymerization with the Cr(II) trimethylsilylmethyl compound are shown in Figure 183. The alumina support was first treated with fluoride and calcined at 600 °C, and then it was impregnated with three different loadings of the chromium compound, which actually exists as a tetramer, Cr4(TMSM)8 [660]. These catalysts were then tested for ethylene polymerization, and the observed activity was found to be roughly proportional to the chromium coverage. 

The rates of ethylene polymerization on alumina and on M-46 silica-alumina catalysts could not be rigorously compared since a rate expression could not be obtained for the latter. However, at 1 mm ethylene pressure and room temperature the catalytic activity of M-46 is about 600 times greater per gram of catalyst (350 times greater per unit surface area) than that of alumina. The apparent activation energy on M-46 at 1 mm ethylene pressure is estimated at about 7.2 kcal/mole, which is roughly twice that on alumina (3.5 kcal/mole). 

Polymerization also takes place when Cp 2ZrMe2 (Cp = Cp, Cp ) or Cp ZrMe3 are supported on dehydroxylated alumina. While only some 4% of Cp2ZrMe2 centers are active, about 12% of the Cp ZrMe3 is converted to cationic centers. The order of polymerization activity is Cp ZrMes > Cp2ZrMe2 Cp 2ZrMe2. Unlike the actinide compounds, Cp -ZrMes exhibits ethylene polymerization activity even on partially dehydroxylated alumina. 

Ethylene polymerization with conditioned alumina-molybdena catalysts Alex Zletz... 

Edmund Field, Morris Feller, Promoted Molybdenum-Alumina Catalysts in Ethylene Polymerization, Ind. Eng. Chem., 49, 1883-1884 (1957) A. Zletz, Ethylene polymerization with conditioned alumina-molybdenum catalysts,... 

Reduction with ethylene at 100 C gives blue catalysts. On alumina the spectrum is dominated by bands at 17,200 24,900 and 38,600 with a shoulder at 12,800 cm, while on silica the spectrum consists of bands at 16,400 33,800 and a weak band at 10,000 cm. In any case, the amount of Cr is higher on silica than on alumina. The better catalytic performances of Cr/SiOj for ethylene polymerization [8] can be explained by the higher amount of Cr on the silica surface. 

Standard of Indiana Catalyst. The first low pressure polyethylene catalyst invented (46), the Standard of Indiana catalyst system, saw relatively little commercial practice. Their 1951 patent discloses reduced molybdenum oxide or cobalt molybdate on alumina for ethylene polymerization, preferably in aromatic solvents. Later, work concerning the use of promoters was also disclosed. 

Although actinide complexes usually disqualify themselves from industrial utility by their radioactivity, the thorocene (C5(CH3)5)2Th(CH3)2 displays high activity for ethylene polymerization after activation (101). Like zirconocenes, this family of metallocenes tends to remain on the +4 manifold, and thus requires ionization by an appropriate cocatalyst (in this case, a highly Lewis acidic surface such as dehydroxylated alumina or MgCl2) before it is active. 

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). 

Table 3.4 Ethylene polymerization with Bis(triphenylsilyl)chromate on sihca/ alumina .

Field E, FeUer M Promoted molybdena-alumina catalysts in ethylene polymerization, Ind Eng Chem 49(11) 1883-1884, 1957. 

Polyethylene. Low pressure polymerization of ethylene produced in the Phillips process utilizes a catalyst comprised of 0.5—1.0 wt % chromium (VI) on siUca or siUca-alumina with pore diameter in the range 5—20 nanometers. In a typical catalyst preparation, the support in powder form is impregnated with an aqueous solution of a chromium salt and dried, after which it is heated at 500—600°C in fluid-bed-type operation driven with dry air. The activated catalyst is moisture sensitive and usually is stored under dry nitrogen (85). 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

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