Chromia supported

Nickel. As a methanation catalyst, nickel is presently preeminent. It is relatively cheap, it is very active, and it is the most selective to methane of all the metals. Its main drawback is that it is easily poisoned by sulfur, a fault common to all the known active methanation catalysts. The nickel content of commercial nickel catalysts is 25-77 wt %. Nickel is dispersed on a high-surface-area, refractory support such as alumina or kieselguhr. Some supports inhibit the formation of carbon by Reaction 4. Chromia-supported nickel has been studied by Czechoslovakian and Russian investigators. 

In contrast with chromia supported on alumina, pure chromium oxide is a poor catalyst for the nitroxidation of hydrocarbons as it deactivated rapidly with time on stream and favoured deep oxidation at the steady state (ref. 3), althouoh it exhibits good dehydrogenation properties (ref. 2). It was concluded that alumina prevents the segregation of chromia phase and thus favours the formation of... 

Chromia supported on powdered sapphire (a-Al2Oa) of surface 2 m2 g 1 has also been studied (22). Results for ka and /c7 5 on a sample containing 0.17 atom% Cr203 are shown in Fig. 18. Figure 19 shows AkH as a function of field at 173 and 298 K. 

The similarities in catalytic reactivity between Cr3 53-montmorillonite and chromia supported on alumina suggest that the structure of the intercalated chromia particles may resemble the structure of the bulk oxide. In order to obtain structural information for the chromia aggregates in pillared clays, we have initiated structural studies of these materials. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is being recognized as an effective tool for determining the local structure of a variety of materials. The basic principles and utility of this technique have been discussed elsewhere (18.). ... 

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... 

Fig. 11. UV Raman spectra of coke formed during the methanol-to-hydrocarbons reaction catalyzed by zeolite H-MFI and during propane dehydrogenation catalyzed by chromia supported on alumina (66).

Magnetic methods are, like x-ray diffraction, a tool for gaining structural information. These methods have been used to measure the effective dispersion of a paramagnetic oxide such as chromia gel or chromia supported on alumina and to determine oxidation states and bonding types under conditions where other procedures are difficult or inapplicable. Magnetic methods are useful also in the identification and estimation of ferromagnetic components such as iron carbide in Pischer-Tropsch or synthetic ammonia catalysts. 

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. 

It may be objected that manganese nitrate would give +4 manganese even if ignited on low-area rutile. This is, of course, correct, but in that case the normal dispersion effect would be absent, the susceptibility of the manganese would be independent of concentration, and the situation would resemble that previously described for chromia supported on low-area alumina. 

The literature contains many other studies of supported oxides by adsorption microcalorimetry, and in particular oxides used for propane or isobutane dehydrogenation such as chromia supported on Z1O2 [121] or Y-AI2O3 [122], or Ca-doped chromium oxide catalysts supported on Y-AI2O3 [123]. 

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. ... 

Chemical exchange between hydrogen and steam (catalyzed by nickel—chromia, platinum, or supported nickel catalysts) has served as a pre-enrichment step in an electrolytic separation plant (10,70). If the exchange could be operated as a dual-temperature process, it very likely... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

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. 

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... 

Pines and Csicsery (90, 90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over non-acidic chromia-alumina. A similar, 1,3-methyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (90b). Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. 

The first instance of successful application was in the examination of some supported chromia catalysts. A pronounced variation in catalyst performance, caused by certain oxidative pretreatments, was found to correlate with variations in absorption edge spectra. Spectra involved were... 

Additional information about the reactivity was obtained by determining the kinetic parameters during methanol oxidation for vanadia, molybdena, rhenia, and chromia on different oxide supports. For all these systems the activation energy is approximately the same, 18-22 kcal/mol. The activation energy corresponds to that expected for the breaking of the C-H bond of a surface methoxide intermediate, and should be... 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

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... 

G-41 A chromia-alumina catalyst, used for hydrodealkylation and dehydrogenation reactions G-S8 Palladium-on-alumina catalyst, for selective hydrogenation of acetylene in ethylene G-52 Approximately 33 wt % nickel cm a refractory oxide support, prereduced. Used for oxygen removal from hydrogen and inert gas streams... 

The selectivity of catalysts in COj is always lower than 3 % in all the temperature range investigated (410 - 490°C). An excess of chromia deposited on the support reduces the activity of the catalysts which become more oxidant and less selective in nitriles. Sample containing 50 % of chromium tends to behave as pure Cr O which favours deep oxidation and yields 71.5 % of CO2 at 440°C (see Table 2). 

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