Copper-zinc-alumina catalyst

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

In the traditional plant concept, the gas from the secondary reformer, cooled by recovering the waste heat for raising and superheating steam, enters the high-temperature shift (HTS) reactor loaded with an iron - chromium catalyst at 320 - 350 °C. After a temperature increase of around 50 - 70 °C (depending on initial CO concentration) and with a residual CO content of around 3 % the gas is then cooled to 200-210 °C for the low-temperature shift (LTS), which is carried out on a copper - zinc - alumina catalyst in a downstream reaction vessel and achieves a carbon monoxide concentration of 0.1-0.3 vol%. 

Methanation as final purification for the raw gas from partial oxidation was proposed by Topsoe [739]. In this case the shift conversion is carried out in two stages with a special sulfur-tolerant shift catalyst followed by removal of hydrogen sulfide and carbon dioxide in an acid gas removal unit. Because of the potential danger of a sulfur break-through causing poisoning, the normal copper - zinc - alumina catalyst is usually not applied, which is surprising as the same risk exists in partial oxidation based methanol plants for the similarly composed methanol catalyst. 

Example 6.11.4 By-product reactions in the methanol production by use of a copper-zinc-alumina catalyst ... 

Even the industrial copper/zinc/alumina-based catalysts have been modified to achieve higher productivity or longer catalyst life. ICI recently announced its third-generation copper/zinc/alumina catalyst, described as a "step change" over the previous catalysts [16, 17]. This development was made through optimized formulation and particle and pellet size. Researchers at the University of New South Wales, Australia claimed another new breakthrou on this type of the catalyst [16]. A 100% improvement in performance over the previous catalysts was claimed. 

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. ... 

High pressure processes P > 150 atm) are catalyzed by copper chromite catalysts. The most widely used process, however, is the low pressure methanol process that is conducted at 503—523 K, 5—10 MPa (50—100 atm), space velocities of 20, 000-60,000 h , and H2-to-CO ratios of 3. The reaction is catalyzed by a copper—zinc oxide catalyst using promoters such as alumina (31,32). This catalyst is more easily poisoned than the older copper chromite catalysts and requites the use of sulfiir-free synthesis gas. 

The performance of these novel catalysts was compared with that of a conventional copper-zinc-alumina methanol synthesis catalyst under the same conditions (250°C, 40,000 h, 72 /28 CO, 5 MPa). The methanol yield was only 8.0 g mol/l/hr which was more than an order of magnitude less than that obtained with the catalyst produced from the Cu-Ce-Al alloy. The methanol yields obtained using the Cu-Ce catalysts and the 72 /28 CO syngas were comparable to those obtained using conventional catalysts with CO2 present in the feed. 

Copper-zinc-alumina mixed oxide catalysts has been investigated in the low-temperature methanol synthesis. Three different copper-containing species were identified in the spent catalysts (i) metallic copper, (ii) CuO, and (iii) copper not detectable by XRD analysis, the latter being probably related to the ZnO matrix. While no correlation existed between the catalytic activity and only one of these... 

Lattner and Harold [56] performed autothermal reforming of methanol in a relatively big fixed-bed reactor carrying 380 g BASF alumina-supported copper/zinc oxide catalyst modified with zirconia. The 01C ratio was set to 0.22 while the SIC ratio varied from 0.8 to 1.5. The axial temperature profile of the reactor, which had a length of 50 cm, was rather flat, the hot spot temperature did not exceed 280° C which was achieved by the air distribution system through porous ceramic membrane tubes. More than 95% conversion was achieved. Very low carbon dioxide formation was observed for this reactor only 0.4 vol.% was found in the reformate. However, the WHSV calculated from the data of Lattner and Harold [56] reveals a low value of only 6 l/(h gcat) for the highest CHSV of 10 000 h reported. 

Bravo et al. described a catalyst coating technique, which was applied for coating of commercial copper/zinc oxide catalyst onto quartz and fused silica capillaries [138]. The catalyst was milled with boehmite alumina and deionised water in a mass ratio of 44 11 100. The thickness of the coating, which was only 1 pm for this gel formulation, could be increased to some 25 pm by addition of hydrochloric acid. The catalyst was still active after the coating procedure. The development work was moving towards coating of ready-made microreactors in the future [138]. 

Wiese et al. reported full conversion of methanol over a commercial copper/zinc oxide/alumina catalyst, with no further specifications given, at 280 °C and a maximum weight hourly space velocity of 5 LH2 (h gcat) in a tubular fixed bed [154]. This is within the usual range of space velocities for this type of catalyst. The power density of the fixed catalyst bed was calculated to be 12.5 kW L . The increase in carbon monoxide concentration at partial load, which is typical over copper/zinc oxide catalysts, is illustrated in Figure 4.6. Naturally, the lower the space velocity, the higher the conversion becomes. As soon as the space velocity is considerably below the value required to achieve full conversion, selectivity of the catalyst towards water-gas shift... 

Chen et al. prepared a hybrid copper/zinc oxide/alumina/palladium/zinc oxide catalyst by wash-coating a copper/zinc oxide catalyst supported by alumina into microchannels [192]. Palladium/zinc oxide powder was then coated onto this catalyst. 

The reformate left the reformer with a temperature of 814 °C and entered a zinc oxide trap. However, this would he not feasible in a practical system, because zinc oxide adsorbent materials cannot tolerate temperatures exceeding 450 °C. The reformate, which was cooled to 440 °C in heat-exchanger E-2 was then passed to the water-gas shift reactor. This reactor was cooled by steam generation at 15-bar pressure and a temperature of200 °C in a counternoble metal based rhenium/alumina catalyst at the inlet section followed by a copper/zinc oxide catalyst at the outlet section. Despite the fact that a water-gas shift catalyst of fairly low activity had been chosen for the... 

Miscellaneous Reactions. Ahyl alcohol can be isomerized to propionaldehyde [123-38-6] in the presence of sohd acid catalyst at 200—300°C. When copper or alumina is used as the catalyst, only propionaldehyde is obtained, because of intramolecular hydrogen transfer. On the other hand, acrolein and hydrogen are produced by a zinc oxide catalyst. In this case, it is considered that propionaldehyde is obtained mainly by intermolecular hydrogen transfer between ahyl alcohol and acrolein (31). 

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. 

Low-pressure methanol synthesis relies almost exclusively on catalysts based on copper, zinc oxide, and alumina. The catalysts are produced by ICI (now Johnson Matthay), Siidchemie (now Clariant), Haldor Topsoe, in the past also by BASF, and other chemical enterprises and contain 50-70 atomic % CuO, 20%-50% ZnO, and 5%-20% Al203. Instead of alumina, chromium oxide and rare earth oxides have also been used. The mixed oxide catalysts are usually shipped as 4-6 mm cylindrical pellets with specific surface area of 60-100 m2/g. The catalysts are activated in situ with dilute hydrogen, often derived from off-gases from synthesis gas... 

Chinchen GC, et al. Mechanism of methanol synthesis from CO2/CO/H2 mixtures over copper/zinc oxide/alumina catalysts - use of 14C-labeled reactants. Appl Catal. 1987 30(2) 333-8. 

The modern methanol synthesis catalyst consists of copper, zinc oxide, and alumina. Copper metal is seen as the catalytically active phase, and ZnO as the promoter. It is well known that the interaction between the two components is essential for achieving a high activity, but the nature of the promoting effect is still a matter of debate. Loss of activity is caused by sintering of the Cu crystallites, and, if the feed gas contains impurities such as chlorine and sulfur, by poisoning. 

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