Chromium promoter

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

CO conversion is carried out over chromium-promoted iron oxide catalyst employing two stages of catalytic conversion the plant also incorporates a saturator and desaturator operating with a hot water circuit. 

Promoted skeletal copper was also imaged with the FIB. In particular, both zinc- and chromium-promoted skeletal copper have a structure similar to that of un-promoted skeletal copper, but on a much finer scale [110,111], This observation agrees with the increased measured surface areas for these promoted catalysts. Figure 5.2a shows the fine uniform ligaments in a zinc-promoted skeletal copper catalyst. 

In this process, propane, and a small amount of hydrogen to control coking, are fed to either a fixed bed or moving bed reactor at 950—1300° F and near atmospheric pressure. Once again the catalyst, this time platinum on activated alumina impregnated with 20% chromium, promotes the reaction. In either design, the catalyst has to be regenerated continuously to maintain its activity. 

Adjustment of the C0 H2 ratio is effected by the shift reaction (iv) which proceeds over a chromium-promoted iron catalyst at 700-800°F (370-425°C) or over a reduced copper/zinc catalyst at 375" 50°F (190-230 C) and the fraction of crude gas sent through the shift reactor is calculated from the initial gas composition and specific downstream requirements. The latter are i1 lustrated by... 

Selective catalytic hydrogenation with chromium-promoted Raney nickel is reported (e.g. citral and citronellal to citronellol) NaHCr2(CO)io and KHFe(CO)4 reduction of a/3-unsaturated ketones (e.g. citral to citronellal) has been described (cf. Vol. 7, p. 7). The full paper on selective carbonyl reductions on alumina (Vol. 7, p. 7) has been published." Dehydrogenation of monoterpenoid alcohols over liquid-metal catalysts gives aldehydes and ketones in useful yields. ... 

The undoped catalyst was prepared from the monophasic crystallized Ni Alj alloy (ref. 7). The molybdenum and chromium promoted catalysts were prepared from alloys with the composition N -x x where M = Mo (0.05 x 0.4) and M = Or (x - 0.07 or 0.11) (ref. 8). The catalysts were then prepared as described previously (ref. 9), by leaching the crushed alloys in a 6N sodium hydroxide solution at boiling temperature. The catalysts were kept under a molar solution of NaOH. 

Sometimes, an alcohol via the corresponding chromate ester may direct a chromium-promoted epoxidation of an aJkcne. This side reaction, which can happen with other chromium-based oxidants,83 depends on very exacting stereoelectronic factors to occur. 

The axial allylic alcohol directs the chromium-promoted epoxidation of the alkene. 

The copper surface areas of fresh (S ) and used (S ) catalysts are demonstrated in Table l. The ratio of S1/S0 exhibits the extent of copper surface area reduced after reaction. The copper surface areas reduce after dehydrogenation reaction. This indicates that sintering occurs in reaction process for all of the catalysts. Chromium promoted catalysts have higher fresh copper surface areas than the unpromoted one as shown in Table 1. The previous results [5] indicated that the catalyst with Cr/Cu molar ratio of 1/10 had the highest stability for unsupported catalyst nevertheless, the catalyst with Cr to Cu molar ratio of 1/40 is the most stable one in Si02-supported case. The stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases. 

Plots of various catalysts are shown in Pig. 4-5 Straight lines go through the origin for all catalysts, thus a second-order deactivation which is concentration independent applied in this study The values of k for various catalysts are illustrated in Table 2. For chromium-promoted catalyst, k value increases with increasing Cr/Cu molar ratio. The promoted catalysts with Cr/Cu=l/40 and 1/10 are more stable than the unpromoted one For alkaline earth metal-promoted catalysts, Mg-promoted catalyst is more stable than the unpromoted one however, the Ca-, Sr-, and Ba-promoted catalysts are poor in stabilities The stability of the alkaline earth metal promoted catalyst is in the order Mg> Ca> Sr> Ba. 

Sintering occurs in reaction process for all of the catalysts, For Cr-promoted catalysts, the catalyst with Cr/Cu=i/4Q has the highest activity and stability. The activity and stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases from 1/40 to 1/4. 

Activated skeletal catalysts including nickel, copper, cobalt and molybdenum or chromium-promoted nickel are available commercially. 

The addition of other metals to promote skeletal catalysts has been the subject of a number of investigations including the use of V, Cr, Mn, and Cd for hydrogenation of nitro compounds [23], Cd in the hydrogenation of unsaturated esters to unsaturated alcohols [24], and Ni and Zn for the dehydrogenation of cyclo-hcxanol to cyclohexanone. The use of Cr as a promoter is particularly attractive as copper chromite catalysts arc used in a wide range of industrial applications. Lainc and co-workers [25] have made a detailed study of the structure of chromium promoted skeletal copper catalysts. 

Citronellal, an aldehyde with a trisubstituted double bond, was hydrogenated to citronellol over a ruthenium catalyst poisoned with lead acetate in 90-100% yields (eq. 5.22)46 or over chromium-promoted Raney Ni in 94% yield in methanol at 75°C and about 0.31 MPa H2.47 Court et al. studied the selective hydrogenation of citral (1, eq. 5.24) to citronellol over unsupported Nij. o catalysts, prepared by reduction of mixtures of metal iodides with naphthalene-sodium as reducing agent, in cyclohexane and in 2-propanol at 80°C and 1.0 MPa H2.48 Higher yields of citronellol were obtained in 2-propanol than in cyclohexane, primarily via citronellal as the predominant intermediate. The yields of citronellol for the overall hydrogenation in 2-propanol over Mo-promoted catalysts were Mo0 03 96%, Mo0 06 98%, and Mo012 96%. 

The catalysts were evaluated by exposure to a simulated automobile exhaust gas stream composed of 0.2% isopentane, 2% carbon monoxide, 4% oxygen and a balance of nitrogen. The temperature required to oxidize the isopentane and carbon monoxide was used to compare catalyst performance. The chromium-promoted catalyst oxidized isopentane at the lowest temperature, and a mixed chromium/copper-promoted catalyst proved the most efficient for oxidizing carbon monoxide and isopentane. It is interesting to note that the test rig used a stationary engine with 21 pounds of catalyst. Although the catalyst was very effective it is difficult to envisage uranium oxide catalysts employed for emission control of mobile sources. 

Here, the carbon monoxide is converted to carbon dioxide at more moderate temperatures in the presence of a chromium-promoted iron catalyst. Carbon dioxide is readily separated from the hydrogen by absorption in water under pressure (Eq. 11.18). 

Rhenium compounds such as Re2Sc7 are more reactive hydrogenation catalysts for the reduction of carbonyls than for olefins, except in conjugated systems . Unsaturated carbonyls have been selectively hydrogenated to unsaturated alcohols, catalyzed by a chromium-promoted finely divided Ni (Raney Ni) catalyst" ... 

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