Copper zinc oxide catalysts—methanol synthesis

3 Copper zinc oxide catalysts—methanol synthesis 

Zinc oxide is a component of a number of catalysts, for example Cu/Zn0/Al203 which catalyses the synthesis of methanol from carbon monoxide and hydrogen [70]. INS spectroscopy was used to show the presence of Zn-H bonds in ZnO dosed with hydrogen. Hydrogen was chemisorbed on zinc oxide at room temperature the INS spectrum 

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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. 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

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... 

By far the most important synthesis gas reaction is its conversion into methanol, using copper/zinc oxide catalysts under relatively mild conditions (50 bar, 100-250°C). Methanol is further carbonylated to acetic acid (see Section 22-7), so that CH3C02H, methyl acetate, and acetic anhydride can all be made from simple CO and H2 feedstocks. Possible pathways to oxygenates in cobalt catalyzed reactions are shown in Fig. 22-6. 

Only a few studies of the poisoning of copper/zinc oxide catalysts have been reported (refs. 4-6). Whether copper or zinc is most su.sceptible to attack by sulfur is still a question, Tlte literature findings on the sulfur tolerance of methanol synthesis catalyst are inconsistent with industrial experience. For example, observations from indusirinl production suggest that a... 

In this section we describe INS studies of molybdenum trioxide, a precmsor of molybdenum disulfide catalysts ( 7.5), and transition metal oxides which catalyse complete or partial oxidation of hydrocarbons, and copper zinc oxide catalysts, which catalyse methanol synthesis from carbon monoxide and dihydrogen ( 7.3.3). 

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. 

The U.S. Department of Energy has funded a research program to develop the Hquid-phase methanol process (LPMEOH) (33). This process utilizes a catalyst such as copper—zinc oxide suspended in a hydrocarbon oil. The Hquid phase is used as a heat-transfer medium and allows the reaction to be conducted at higher conversions than conventional reactor designs. In addition, the use of the LPMEOH process allows the use of a coal-derived, CO-rich synthesis gas. Typical reactor conditions for this process are 3.5—6.3 MPa (35—60 atm) and 473—563 K (see Methanol). 

Microanalysis of a Copper-Zinc Oxide Methanol Synthesis Catalyst Precursor... 

Himelfarb, P. B., Simmons, G. W., Klier, K., Herman, R. G., "Precursors of the Copper-Zinc Oxide Methanol Synthesis Catalysts," J. Catal., in press. 

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. 

Saito M, et al. Development of copper/zinc oxide-based multicomponent catalysts for methanol synthesis from carbon dioxide and hydrogen. Appl Catal A Gen. 1996 138(2) 311—18. 

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. 

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. 

Many catalytic formulation are proposed for the hydroconversion of CO2, most of them are based on promoted copper-zinc oxides given by the long industrial experience on methanol synthesis from syngas (CO+CO2+H2) [3-6]. Specific methanol catalysts working for CO2 are proposed including promoted Cu-Zn catalysts [3,6], zirconia supported systems [7] as well as copper associated with stabilized rare earth oxides [8,9]. In the last case Cu-LaZr and CuZn-Lcfer catalysts were proposed and showed interesting catalytic properties in the methanol formation. 

The best catalyst for the synthesis of methanol from CO + H2 mixtures is copper/zinc oxide/alumina. Intermetallic compounds of rare earth and copper can be used as precursors for low-temperature methanol synthesis as first reported by Wallace et al. (1982) for RCu2 compounds (R = La, Ce, Pr, Ho and Th). The catalytic reaction was performed under 50 bar of CO + H2 at 300°C, and XRD analyses revealed the decomposition of the intermetallic into lanthanide oxide, 20-30 nm copper particles and copper oxide. Owen et al. (1987) compared the catalytic activity of RCux compounds, where R stands mainly for cerium in various amounts, but La, Pr, Nd, Gd, Dy and even Ti and Zr were also studied (table 4). The intermetallic compounds were inactive and activation involved oxidation of the alloys using the synthesis gas itself. It started at low pressures (a few bars) and low temperatures (from 353 K upwards). Methane was first produced, then methanol was formed and it is believed that the activation on, for example, CeCu2, involved the following reaction, as already proposed for ThCu2 (Baglin et al. 1981) ... 

Bowker, M., Hadden, R.A., Houghton, H., Hyland, J.N.K., and Waugh, C.J. (1988) Mechanism of methanol synthesis on copper/zinc oxide/alumina catalysts. J. Catal, 109, 263-273. 

Chinchen, G.C., Deimy, P.J., Parker, D.G., Short, G.D., Spencer, M.S., Waugh, K.C., and Whan, D.A. (1984) The activity of copper-zinc oxide-aluminum oxide methanol synthesis catalysts. Am. Chem. Soc. Div. Fuel Chem., 29, 178-188. 

Methanol is made on a multibillion-pound scale from a pressurized mixture of CO and H2 called synthesis gas. The reaction involves a catalyst consisting of copper, zinc oxide, and chromium(III) oxide. 

Low-pressure methanol synthesis, first introduced commercially by ICI of England, is based on a copper oxide-zinc oxide-ahimina catalyst that operates over a much lower pressure range (510 MPa nominally) and at considerably lower temperatures (200280°C). The copper-based family of methanol synthesis catalysts is extremely active, as well as selective, and is used in vapor-phase... 

Saito, M. Fujitani, T Takeuchi, M. Watanabe, T. Development of Copper/Zinc Oxide-Based Multicomponent Catalysts for Methanol Synthesis from Carbon dioxide and Hydrogen. App/. CatalA. Gen. 1996, 138, 311-318. 

Precipitated copper oxide/zinc oxide catalysts were more active for a range of reactions than zinc chromite but lost activity as the copper was poisoned by gaseous impurities in the synthesis gas. The two oxides were found to be mutually promoting in methanol synthesis because the mixture of very small crystallites was more active than the individual oxides. 

The first industrial methanol catalysts, used from 1923, were based on zinc oxide-chromium oxide mixtures. Experimental woik during development of the process had, however, demonstrated the high activity of zinc oxide catalysts containing copper. This was particularly true if a third refractory oxide such as chromia or alumina was also added." Catalysts containing copper are particularly susceptible to poisoning. At that time the synthesis gas was produced from water gas, and contained poisons snch as sulfur and chlorine compounds so that copper catalysts were unsuitable. 

When the hydrocarbon steam reforming process conld provide poison free synthesis gas, the benefits of the more active copper catalysts were quickly reviewed. It was soon shown that the high activity of copper oxide-zinc oxide catalysts, compared with the zinc oxide-chrominm oxide types, particularly when alumina or chromia promoters were added, conld revolutionize methanol production. New, more efficient processes were nrgently needed in view of the increasing demand for methanol and the economies of high capacity units. 

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