Chromia reactions catalyzed

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

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).

This chapter will concern itself with unsupported chromia as a catalyst for relatively low temperature reactions under reducing conditions. It would seem desirable, however, to give an outline of the wide variety of reactions catalyzed by chromia or supported chromia. This is done in Table I. Some of these reactions were carried out on chromia... 

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

The above is formally equivalent to the picture of a coordinatively unsaturated surface (CUS) put forward by Burwell et al. (8) in their discussion of chromia. The acid-base formalism does have the advantage of drawing attention to the analogy of acid and base catalyzed reactions. If a hydrocarbon undergoes reaction at these sites via loss of a proton to the oxide site, the reaction should be analogous to a base catalyzed reaction if it undergoes reaction via the loss of a hydride to the zinc site or addition of a proton from the oxide site, the reaction should be analogous to an acid catalyzed reaction. This view, which we find useful, is implicit in the discussion that follows. 

Pines and Csicsery reported on the formation of diolefins in chromia catalyzed dehydrocyclization of Cj-Cg hydrocarbons 49). The kinetic behavior of heptadienes and heptatrienes in chromia and molybdena catalyzed aromatization of unsaturated n-Cj hydrocarbons 22, 49a) indicated that they were intermediates of the reaction. 

Another method to hydrogenate butadiene occurs during an oxidation—reduction reaction in which an alcohol is oxidized and butadiene is reduced. Thus copper—chromia or copper—zinc oxide catalyzes the transfer of hydrogen from 2-butanol or 2-propanol to butadiene at 90—130°C (87,88). 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

The chromia-promoted iron oxide catalyzed water-gas shift reaction ... 

Fluorinated chromia used to catalyze the isomerization reaction of CHF2CHF2 to CF3CH2F, also exhibited evidence for the importance of chromium in higher oxidation states. FTIR spectroscopic measurements of CO adsorption confirmed the occurrence of Cr4+ and Cr5+ on the surface of chromia catalysts before being used [52]. During the activation, Cr4+ and Cr5+ sites were reduced and enhanced activity of the catalyst was observed. The reaction pathway proposed for isomerization involves the formation of hydrogen fluoride due to the degradation reaction of the fluoroalkane. 

A comparison of the UV Raman spectrum measured for coke deposited during the MTH reaction with that deposited during butane dehydrogenation catalyzed by chromia on alumina (66) shows clear differences in the spectral intensity distribution (Fig. 11). In particular, the intensity of the features in the regions 1340-1440cm and 1560 1630 cm are nearly equal for the MTH reaction. 

The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

Examples include hydrogenation of propanal over nickel, dehydrogenation of ethanol over copper-cobalt, dehydrogenation of methylcyclohexane to toluene over platinum, hydroformylation of olefins catalyzed by cobalt hydrocarbonyls on solid polymers, hydrogen-ion catalyzed hydration of olefins on ion exchangers, dehydrogenation of 1-butene to butadiene over chromia-alumina, and various hypothetical reactions. 

Because of the corrosive nature of HF, there are few reports of fluorination reactions in the liquid phase using solid catalysts. The use of zeolites in fluorination was reviewed in our earlier publication [1]. An unusual, room-temperature, solid-catalyzed fluorination of CH3CCI3 and related species by anhydrous HF has been reported by Thomson et al. [92-94]. He found that fluorination at ambient temperatures is possible in the presence of catalysts derived from fluorinated y-alu-mina, chromia, Fc304, and C03O4 and conditioned before reaction by treatment... 

The specific effect shown by alumina as a support or promoter for chromia is due to its existence as a stable, high-area oxide which is isomorphous with, and thus able to stabilize, i -chromia and which does not catalyze undesirable side reactions. 

Neither substance catalyzes hydrogen disproportionation reaction of cycloolefins. An essential difference was found in the action of these catalysts on alcohols. In the presence of vanadia, alcohols are hydro-genolyzed to the corresponding paraffins. At comparable conditions in the presence of chromia, alcohols undergo a dehydrogenation-condensation reaction with production of ketones. 

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. 

In order to substantiate this measure of chromia area, the rates of carbon monoxide oxidation over the various catalysts were measured. It was found that the alumina portion of the surface could be rendered inactive by selective poisoning with water and, under these conditions, the reaction was catalyzed exclusively by the ehromia surface. Since the activation energy was independent of the chromium content, it was reasonable to expect a linear variation of specific activity (i.e., activity per unit total surface area) with the fraction 0 (Table I) of the total surface contributed by the chromia phase. In Fig. 3 the specific rate is... 

Pretreatment involving the partial reduction of the oxide with hydrogen can similarly produce significant effects that vary with the metal oxide used. The effect that prereduction has on supported chromia and molybdenum oxides is widely different. On a chromia catalyst, the reduction step leads to the formation of Br0nsted sites, which then catalyze the isomerization reaction via a cationic intermediate. On a reduced molybdena catalyst, metathesis-type mechanisms dominate, with the cationic mechanism proceeding only on a fully oxidized molybdena surface. 

Moore and Massey [61] detailed a reaction in which 1,1,2,2-tetrafluoroethane was isomerized to 1,1,1,2-tetrafluoroethane over chromia catalyst at 400°C, indicating that fluorine shifts may be catalyzed. The same authors also indicated that AIF3 and NaF may also be used as catalyst. Similar work for TFE has not been reported, though it stands to reason that such a shift would be beneficial to the formation of HFP (See Scheme 5.7). 

Equations (1 )-(3) represent a synthesis scheme from the patent literature, using various catalysts including fluorinated chromia. Equation (4) is a fluorinated chromia-catalyzed reaction receiving... 

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