Chromia-silica-alumina catalyst

The reactor equipment used for solution polymerizations is typically glass-lined stainless steel. An example of solution polymerization is the reaction of ethylene in isooctane with a chromia silica alumina catalyst initiator (see Figure 3.23) to form polyethylene. Typical reaction conditions for this polymerization are 150-180°C and 2.1-4.8 MPa (300-700 psi). 

Insoluble initiator. An interesting, though not commercially important, example has been described in which the catalyst is in the form of a fixed bed and the polymer that is formed remains in solution [14]. A chromia-silica-alumina catalyst (an initiator that can be regenerated) will convert ethylene to linear polymer at moderate pressmes. In this case, a dilute (2%-4%) solution of ethylene in an inert carrier (saturated hydrocarbon) is passed over a fixed bed of catalyst at 150°C-180°C and 300-700 psi (2.1-4.8 MPa) (Figme 5.8). Conversion may be high, but there is considerable volume of solvent to recover and recycle. Periodic reactivation of catalyst at about 50-h intervals is required. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

The 8-phase chromium resonance in chromia-alumina has been observed by several workers. It is dependent upon the alumina support since it does not appear in samples of chromia-silica, although it is observed in the case of chromia-silica-alumina. The isolated Cr + ions could conceivably be situated either in the bulk of the support, or on its surface. There is some evidence that both situations prevail. Nuclear magnetic resonance studies to be discussed presently (121) indicate that for impregnated chromia-alumina catalysts calcined at 500 the majority of these ions are on the surface. However, when such catalysts are heated above 600°, the 8 phase is greatly enhanced because of diffusion of chromium ions from the -phase into interior sites in the alumina lattice. The same situation arises with coprecipitated chromia-alumina catalysts (34). The S-phase resonance intensity of coprecipitated chromia-alumina was found to increase with calcination temperature, indicating an increasing three-dimensional dispersion of Cr + ions. In general, the S-phase resonance dominates the ESR spectra of chromia-alumina catalysts at low chromium concentrations, and therefore it... 

Udomsak, S., Anthony, R.G., 1996. Isobutane dehydrogenation on chromia/silica—titania mixed oxide and chromia/Y-alumina catalysts. Industrial Engineering Chemistry Research 35... 

Phillips (1) A process for polymerizing ethylene and other linear olefins and di-olefins to make linear polymers. This is a liquid-phase process, operated in a hydrocarbon solvent at an intermediate pressure, using a heterogeneous oxide catalyst such as chromia on silica/ alumina. Developed in the 1950s by the Phillips Petroleum Company, Bartlesville, OK, and first commercialized at its plant in Pasadena, TX. In 1991, 77 reaction fines were either operating or under construction worldwide, accounting for 34 percent of worldwide capacity for linear polyethylene. 

Tretyakov and Filimonov (219) describe a coordinative interaction between benzonitrile and aprotic sites on magnesium oxide, and Zecchina et al. (256) came to the same conclusion for the adsorption of propionitrile, benzonitrile, and acrylonitrile on a chromia-silica catalyst. Chapman and Hair (257) observed an additional chemical transformation of benzonitrile on alumina-containing surfaces, which they describe as an oxidation. Knozinger and Krietenbrink (255) have shown that acetonitrile is hydrolyzed on alumina by basic OH- ions, even at temperatures below 100°C. This reaction may be described as shown in Scheme 2. The surface acetamide (V) is subsequently transformed into a surface acetate at higher temperatures. Additional reactions on alumina are a dissociative adsorption and polymerizations (255) analogous to those observed for hydrogen cyanide by Low and Ramamurthy (258), and a dissociative adsorption. Thus, acetonitrile must certainly be refused as a probe molecule and specific poison. 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed. 

An investigation of chromia supported on alumina or silica as catalyst for partial oxidation of paraxylene [144] revealed influence of the oxidation states and degree of oligomerization of ehannium on the catalytic activity. Superior catalytic properties were obtained when chromium was supported on alumina. 

Chromia-alumina catalysts exhibit certain properties which would be qualitatively predicted from a consideration of the independent properties of the parent oxides, chromia and alumina, but one also observes phenomena which can only be attributed to a synergistic interaction of the two phases. Therefore, although the principal concern of the present review is the chromia-alumina catalyst system, it will obviously be necessary to include in this discussion some mention of the intrinsic properties of alumina and chromia themselves. In addition, comparisons will be made with various other chromia catalysts such as, for example, chromia supported on silica-alumina. 

The second major aspect of the surface chemistry of chromia-alumina that has to be considered is the acidic nature of its surface. The exact chemical nature of the acid sites of solid oxide catalysts such as alumina or silica-alumina has been a subject of considerable research and speculation for a number of years, yet despite these efforts a fully satisfactory chemical description of catalyst acidity has not been obtained. Nevertheless, in the case of chromia-alumina, there is good evidence for the existence of acid sites of one kind or another on the surface. Voltz and Weller (29), for example, studied the chemisorption of quinoline on chromia-alumina, with and without potassium promotion, and at the same time measured their titrable acidities in aqueous suspensions. Both methods indicated that chromia-alumina was acidic, and that the addition of potassium decreased the acidity. This observation was supported by the fact that the double bond isomerization of 1-pentene, normally an acid-catalyzed reaction, proceeded quite readily over pure chromia-alumina, but less readily over a chromia-alumina treated with potassium. 

Ismail, H.M. Fouad, N.E., and Zaki. M.I.. Nitrogen and pyridine adsorption on chromia-coated silica and alumina catalysts Probing the chromia dispersity. Adsorpt. Sci. Technol.. 8( 1). 34-43 (1992). 

Synthetic adsorbents such as silica, chromia, nnd alumina gels are expensive, but various other metals can be introvluced for improving the catalytic, adsorptive, or regenerative properties of the gels or catalysts. Likewise, various chemicals can be added to the natural adsorbents for enhancing their efficiency. 

Derossi, S., et al., 1994. Propane dehydrogenation on chromia/silica and chromia/alumina catalysts. Journal of Catalysis 148 (1), 36—46. 

The desire to have catalysts that were uniform in composition and catalytic performance led to the development of synthetic catalysts. The first synthetic cracking catalyst, consisting of 87% silica (Si02) and 13% alumina (AI2O3), was used in pellet form and used in fixed-bed units in 1940. Catalysts of this composition were ground and sized for use in fluid catalytic cracking units. In 1944, catalysts in the form of beads about 2.5 to 5.0 mm in diameter were introduced and comprised about 90% silica and 10% alumina and were extremely durable. One version of these catalysts contained a minor amount of chromia (Cr203) to act as an oxidation promoter. 

In a series of papers from the Zelinskil laboratory (152,153,158,160,-443) a systematic study was reported of a variety of catalysts for the reaction under discussion, including its application to synthetic (Fischer-Tropsch) gasoline. Chromia-alumina or silica gel was apparently preferred. Plate reviewed the field of Russian and non-Russian work up to 1940 (304), and Shuikin published another review in 1946 in which dehydrogenation of cyclohexanes as a source of aromatics is considered to be at least equal in value to dehydrocyclization (371). 

Praliaud and Martin (77) proposed the formation of Ni-Si and Ni-Cr alloys on silica and chromia supports, respectively, under H2 at sufficiently high temperatures. They suggested that hydrogen spilt over from Ni to the Cr203 carrier and partially reduced it to Cr°, which was then alloyed with Ni as indicated by magnetic measurements. The same technique in conjunction with IR spectroscopy and volumetric adsorption of H2 was applied to partially reduced Ni-on-alumina and Ni-on-zeolite catalysts by Dalmon et al. (78). These supported Ni systems contained Ni° and Ni+. H2 was found to be activated only when the couple Ni°/Ni+ was present according to... 

Apart from the degree of reduction affecting overall performance, the nature of the support is also crucial in determining final activity. For supported molyb-dena catalyst, alumina and titania support materials provide best performance. Silica, zirconia, chromia, and zinc oxide are also good support materials, although they produce less active catalysts. Inactive catalysts can be readily synthesized by supporting molybdena upon cobalt oxide, nickel oxide, magnesium oxide, or tin oxide. To date, no correlation between the acidity of the support material and cataljdic activity has been found (304). 

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