Chromia on silica

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. 

Jozwiak WK, DaHa Lana IG Interactions between the chromium oxide phase and support surface redispersion of alpha-chromia on silica, alumina and magnesia, J Chem Soc Faraday Trans 93(15) 2583-2589, 1997. 

Nickel oxide/cobalt oxide/chromia on silica alumina. Cracked gas streams containing hydrogen. 

Aside from the recently described Cu/Th02 catalysts, copper on chromia and copper on silica have been reported to catalyze methanol synthesis at low temperatures and pressures in various communications that are neither patents nor refereed publications. It is not feasible to critically review statements unsupported by published data or verifiable examples. However, physical and chemical interactions similar to those documented in the copper-zinc oxide catalysts are possible in several copper-metal oxide systems and the active form of copper may be stabilized by oxides of zinc, thorium, chromium, silicon, and many other elements. At the same time it is doubtful that more active and selective binary copper-based catalysts than... 

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... 

A series of papers was published by Vansant and coworkers dealing with the gas-phase deposition and thermal transformation of Cr(acac)3 to chromia on the surface of alumina and silica supports. Cr(acac)3 binds to the hydroxyl-terminated alumina surface by hydrogen bonding and/or a donor-acceptor interaction with coordinatively unsaturated Al + ions as outlined in Figure 23 . ... 

Several adsorption experiments have been done under irradiation. Such experiments are simpler than radiolysis in the presence of a solid, and may assist us in understanding the more complex systems. Two general types of behavior have been reported, one no different from adsorption after preirradiation and the other different. Several gases were adsorbed on silica gel during irradiation to the same extent as when the irradiation preceded the adsorption (202), while the adsorption of oxygen on chromia gel was different for the two procedures (203). It is not clear that such results have any bearing on our main subject, but adsorption is so central to catalysis that it seems useful to indicate the scope of the work available by some leading references where examples of the various results and ideas can be found ... 

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. 

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. 

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). 

Active sites for oligomerization and polymerization of olefins over chromia supported on silica-alumina are believed to be Cr(II) and/or Cr(III). Dehydrogenation of alcohols proceeds on chromia. Formate ion detected by IR has been suggested to be the reaction intermediate for conversion of methanol to H2, CO and CO2. ... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

C -C, on platinum-silica, 30 352 chain termination bed residence time, 39 255-256 probability, CO pressure effects, 39 258 chemical mechanisms for reactions with deuterium, on chromia, 20 73-84 chemisorption, carbon atom complexes, 32 167-167, 175-176 coupling, 27 235-238 double, 27 238, 239 cracking, 39 283 cyclic, catalysis of, 20 309-311 cyclization, 28 295 degree of strain, 25 135 dehydrogenation of, 19 88, 89 deuteration of, 25 140, 141 dimerization, 20 304... 

Gas-phase epoxidation of propylene with 02/H2 mixtures was accomplished over Ag1267 or Au1268 catalysts dispersed on TS-1 or other Ti-containing supports and Ti-modified high-silica zeolites.1269 Sodium ions were shown to be beneficial on the selectivity of propylene epoxidation with H202 over titanium silicalite.1270 A chromia-silica catalyst is active in the visible light-induced photoepoxidation of propylene by molecular oxygen.1271... 

A common feature of the two models is that the metal centers should have at least two coordination vacancies prior to the interaction with ethene—one for the alkyl or the carbene species and one for the coordination of ethylene. On the basis of the results discussed so far (which have demonstrated that a significant fraction of Cr2+ centers is highly coordinatively unsaturated), it can be understood why Cr2+/silica is such a good catalyst, whereas Cr3+ ions on chromia/silica or exposed on extended faces of cy-C CF are not. 

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