Chromia catalysts

A carrier of more or less large specific surface, often refractory to withstand high temperatures. A carrier may have some promotion action for example, sinca carrier helps chromia catalyst. 

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. 

Although antimony pentafluonde can fluorinate l,l,2-tnchloro-l,2,2-trifluo-roethane to chloropentafluoroethane, this route is not used industnally because antimony pentafluonde and hydrogen fluoride are too corrosive. Both dichloro-tetrafluoroethane and chloropentafluoroethane are produced by vapor-phase fluor-ination of tetrachloroethene with proprietary chromia catalysts at 300 to 500 °C (equation 1). 

A much more selective reaction is possible by using vapor-phase fluonnahon over a chromia" catalyst at 300 to 400 °C, but conversions are thermodynamically limited to 10-20% under acceptable operating conditions [fO 11 Despite this disadvantage, this process has been selected by ICI and Hoechst for their first plants... 

Similarly, Mitsubishi scientists developed commercial processes for the manufacture of aldehydes such as the agrochemical intermediate, m-phenoxybenzaldehyde, and the fragrance intermediate, /p-rcrt-butylbenzaldehyde, by vapour-phase hydrogenation of the corresponding carboxylic acids over zirconia or chromia catalysts (Eqn. (6)) (Yokoyama et ai, 1992). 

Catadiene [Catalytic butadiene] Also spelled Catadien. A version of the Houdry process for converting mixtures of butane isomers into butadiene by dehydrogenation over an alumina/chromia catalyst. Another version converts propane to propylene. Rapid coking of the catalyst necessitates use of several reactors in parallel, so that reactivation can be carried out continuously. Developed by Houdiy and first operated at El Segundo, CA, in 1944. By 1993, 20 plants had been built worldwide. Now licensed by ABB Lummus Crest. 

Mochida, I. Yoneda, Y. Dehydrochlorination and dechlorination of chloroethanes on chromia catalyst. 

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... 

A high temperature water-gas shift reactor 400°C) typically uses an iron oxide/chromia catalyst, while a low temperature shift reactor ( 200°C) uses a copper-based catalyst. Both low and high temperature shift reactors have superficial contact times (bas on the feed gases at STP) greater than 1 second (72). 

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... 

An industrial process to produce methanol from carbon monoxide and hydrogen was developed by BASF in 1923 using a zinc oxide-chromia catalyst.361 362 Since this catalyst exhibited relatively low specific activity, high temperature was required. The low equilibrium methanol concentration at this high temperature was compensated by using high pressures. This so-called high-pressure process was operated typically at 200 atm and 350°C. The development of the process and early results on methanol synthesis were reviewed by Natta 363... 

The complexity of the situation may be illustrated by tracing the development of our knowledge of chromia catalysts. It is now clear that Cr ions on the surface may occur in valence states from Cr11 to CrVI all intermediate valence states have been shown to occur, and all are possible in a regime of temperature and gas composition where the thermodynamically stable bulk phase is Cr2C>3. 

In the presence of chromia catalyst (3) primary alcohols of n-carbon... 

Fig. 9. Dependence of the yield of C02 upon the concentration of H3PO4 or NajSiOa added to magnesia-chromia catalyst.

Fig. 13. Dependence of the energy of activation and the frequency factor upon the concentration of H3PO4 in copper-chromia catalysts.

Table I. Analysis of C4 Products Obtained over Chromia Catalyst...

Chromia Catalyst. Sequence of Reactions. The reactions of the C4 hydrocarbons on the sulfided chromia catalyst were investigated at 415° C. and the products compared with those of thiophene (Table I). 

The effect of temperature on the percentage conversion of the thiophene was marked (Figure 3). Data from a normal flow experiment gave a typical plot with an apparent activation energy in the linear portion of 25 kcal. per mole, the figure obtained by Van Looy and Liinido (27) using a supported chromia catalyst. 

The only heat of adsorption we have found in the literature is a figure of 17 kcal. per mole (27) for thiophene on a supported chromia catalyst. This figure was obtained by analysis of reactions carried out in a static system, assuming a Langmuir mechanism for thiophene and hydrogen adsorption and neglecting the effects of H2S and butene adsorption. 

Payen et al. (1986) investigated the reduction of alumina-supported molybdena and ascribed a Raman band at 760 cm-1 to reduced supported molybdenum oxide. The transformations could be reversed by reoxidization (Payen et al., 1986). Mestl and Srinivasan (1998) described some reduced phases formed from bulk molybdena, whereas reduced dispersed vanadia and chromia catalysts do not show Raman bands (Airaksinen et al., 2005 Banares et al., 2000a Gasior et al., 1988 Weckhuysen and Wachs, 1996). 

A similar investigation combining UV-vis and Raman spectroscopy showed an equivalent effect of feed composition on the oxidation state of chromium in zirconia-supported chromia catalysts (Malleswara Rao et al., 2004). 

Bronkema and Bell (2007) analyzed the Raman bands of surface methoxy species and of supported vanadia to elucidate the mechanism of methanol oxidation to formaldehyde. In their detailed investigation, insight from Raman spectroscopy was combined with information from EXAFS and XANES spectroscopies. The authors discussed the reaction pathways in the presence and absence of 02, and identified the roles of various lattice oxygen sites. Formaldehyde was found to decompose to H2 and CO in the absence of 02 (Bronkema and Bell, 2007). Similar observations were reported by Korhonen et al. (2007) for methanol conversion on supported chromia catalysts. 

The scope of catalytic hydrogenations continues to be extended to more difficult reductions. For example, a notoriously difficult reduction in organic synthesis is the direct conversion of carboxylic acids to the corresponding aldehydes. It is usually performed indirectly via conversion to the corresponding acid chloride and Rosenmund reduction of the latter over Pd/BaS04 [65]. Rhone-Poulenc [30] and Mitsubishi [66] have developed methods for the direct hydrogenation of aromatic, aliphatic and unsaturated carboxylic acids to the corresponding aldehydes, over a Ru/Sn alloy and zirconia or chromia catalysts, respectively, in the vapor phase (Fig. 1.18). 

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