Copper chromia catalysts

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

L. Ma and M.S. Wainwright, Development of skeletal copper-chromia catalysts I. Structure and activity promotion of chromia on skeletal copper catalysts for methanol synthesis. Appl. Catal. A General, 187 89-98, 1999. 

Hydrogenation activity of copper-chromia catalyst is well known. There is however, very little information whether chromia alone can serve as a hydrogenating 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. 

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). 

The reactions studied were the extensive oxidation of isooctane and of ethylene over magnesia-chromia and copper-chromia and of ethylene over tungstic oxide. The catalysts used in the oxidation of isooctane differed greatly with respect to their activities and the observed value of activation energy and frequency factor, as is indicated in Table V. 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

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... 

It is noteworthy that components other than alumina often have detrimental chemical and physical effects on the catalyst. For example, Herman et al. (77) reported that addition of ceria to the Cu/ZnO catalyst lowered methanol conversion by a factor of 5, despite the presence of a large concentration of microparticulate copper metal. This effect was explained by the ability of ceria to drive copper from the active state in zinc oxide solution to inactive metallic copper. Chromia, which had been used as a component of catalysts for methanol for a considerable period of time, is a suitable structural promoter, but some preparations result in an increase of concentration of side products such as higher alcohols (39), dimethyl ether (47), or even hydrocarbons. 

We mentioned above two copper catalysts produced by coprecipitation, viz., the Adkins catalyst (copper-chromia) and the copper-zinc oxide catalyst. The precursor of the two catalysts is produced by coprecipitation. The preparation of the catalysts involves selective removal of carbonate ions, water, and the oxygen atoms bonded to copper. The intimate mixing of the copper ions with the precursor of the supports and the strong interaction of copper with both zinc oxide and chromia furnish copper particles that are still small even after virtually complete reduction of the copper. 

Idakiev et al. [15] also explained the promotional effect of copper. Initially, they investigated the effect of temperature on the activity of 5% Cu and 15% Cu-doped Fe-Cr catalysts. The CO cmiversion increases with increasing Cu loading. Also, the degree of conversimi of carbon monoxide on the unpromoted iron-chromia catalyst sharply decreases with the increase of the space velocity. On the copper oxide promoted samples, it remains almost unchanged. However,... 

V. Idakiev, D. Mihajlova, B. Kunev, A. Andreev, Effect of copper oxide on the catalytic activity of iron chromia catalyst for water gas shift. React. Kinet. Catal. Lett. 35 (1987) 119-124. 

In the 1960s, a new catalyst revolutionized the production of methanol, which had been made by the BASF high pressure or zinc oxide-chromia catalyst process since 1923. The new catalyst—copper, zinc oxide, and chromia or other oxide— had been known as a methanol catalyst for a considerable length of time. At ICI, researchers carried out a careful and systematic program of preparing and testing mixed catalyst systems. The new process operated under much milder conditions than the old one. Pressure was reduced from 200 to 50-100 atm and temperature dropped from 350 to 250°C. Virtually all methanol plants built after 1967 employed this technology (69). 

Chromia—alumina catalysts are prepared by impregnating T-alumina shapes with a solution of chromic acid, ammonium dichromate, or chromic nitrate, followed by gentie calciaation. Ziac and copper chromites are prepared by coprecipitation and ignition, or by thermal decomposition of ziac or copper chromates, or organic amine complexes thereof. Many catalysts have spiael-like stmctures (239—242). 

The oxidative dehydrogenation of ethanolamine to sodium glycinate in 6.2 M NaOH was investigated using unpromoted and chromia promoted skeletal copper catalysts at 433 K and 0.9 MPa. The reaction was first order in ethanolamine concentration and was independent of caustic concentration, stirrer speed and particle size. Unpromoted skeletal copper lost surface area and activity with repeated cycles but a small amount of chromia (ca. 0.4 wt%) resulted in enhanced activity and stability. 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

CuCrO.Ol and CuCrO.l) there was no obvious deactivation, but due to their lower initial activities they had no advantage compared with the unpromoted skeletal copper catalyst. For the low chromia content skeletal copper catalyst (CuCr0.002), and the unpromoted skeletal copper catalyst, the deactivation in the first cycle was significant. However, CuCr0.002 had both a higher initial activity and a higher, stable residual activity than the unpromoted skeletal copper catalyst. 

Table 1 Compositions and surface areas of unpromoted and chromia-promoted skeletal copper catalysts ...

Figure 6 First order rate constants for repeated cycles of ethanolamine dehydrogenation over chromia-promoted skeletal copper catalysts under standard conditions.

The oxidative dehydrogenation of ethanolamine over skeletal copper catalysts at temperatures, pressures and catalyst concentrations that are used in industrial processes has been shown to be independent of the agitation rate and catalyst particle size over a range of conditions. A small content of chromia (ca. 0.7 wt %) provided some improvement to catalyst activity and whereas larger amounts provided stability at the expense of activity. 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

Figure 6. Theoretical dependence of Yheg on k2/ki in comparison with values observed for the reaction of DEA over unpromoted and chromia-promoted copper catalysts...

Unpromoted and chromia-promoted skeletal copper catalysts were prepared as described in detail previously (10, 11, 14, 15) by leaching a CUAI2 alloy, sieved to 106-211pm, in a large excess (500 mL) of 6.1 M NaOH, either alone or containing Na2Cr04 (0.004 M), for 24 hours at 5°C. 

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

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