Methanol synthesis copper oxide catalysts

Supported copper-based catalysts are active for a great variety of reactions and there have been many fundamental studies of their catalytic and solid state properties. Among them, the oxidation of hydrocarbons and CO (1), alkanes (2) and alcohols (3) dehydrogenation, hydrogenation of ketones (4), allyl alcohols and a- and 6-unsaturated aldehydes and ketones (5), alcohol amination (6), low temperature water gas shift (7). methanol synthesis (8), oxidative condensation of methanol (9), hydrolysis of acrylonitrile to acrylamide (10), and removal of NOx pollutants (11). 

Methanol is a common substance used both as a solvent and as a reactant in the synthesis of many other substances. Methanol can be made from synthesis gas, a mixture of CO g) and Vi2 g)- When a mixture of these gases is allowed to come to equilibrium over a copper oxide catalyst at 200 °C, the partial pressures of the gases are 0.96 atm for CO(j ), 1.92 atm for H2(j ), and 0.11 atm for CH30H(j ). What is Kp for this reaction at 200 °C ... 

The top reaction, between carbon monoxide and hydrogen to form methanol, is the basis for all conunercial methanol synthesis plants, and is desirable. This reaction is carried out using a heterogeneous catalyst containing copper and zinc oxide, and is quite reversible at conunercial reaction conditions. The bottom, undesirable reaction is referred to as methanation. It is relatively slow with today s methanol synthesis processes and catalysts. However, methanation can be important if die catalyst becomes contaminated with elements such as nickel and iron, which catalyze the methanation reaction. 

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. 

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. 

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. 

Several other important commercial processes need to be mentioned. They are (not necessarily in the order of importance) the low pressure methanol process, using a copper-containing catalyst which was introduced in 1972 the production of acetic add from methanol over RhI catalysts, which has cornered the market the methanol-to-gasoline processes (MTG) over ZSM-5 zeolite, which opened a new route to gasoline from syngas and ammoxidation of propene over mixed-oxide catalysts. In 1962, catalytic steam reforming for the production of synthesis gas and/or hydrogen over nickel potassium alumina catalysts was commercialized. 

Figure 6.20 Quick EXAFS and XRD measurements recorded during the temperature programmed reduction of copper in a Cu/Zn0/Al203 methanol synthesis catalyst. The disappearance and appearance of peaks with increasing temperature in the series of EXAFS spectra corresponds to the conversion of oxidic to metallic copper. The intensity of the relatively sharp peak around 9040 eV, indicative of Cu metal, clearly illustrates the kinetics of the reduction process, as does the intensity of the (111) reflection of Cu metal in the XRD spectra (adapted from Clausen 44J).

Figure 6.20 shows an example in which QEXAFS has been used in combination with XRD to study the temperature programmed reduction of copper oxide in a Cu/ZnO/Al203 catalyst for the synthesis of methanol [43,44]. Reduction to copper metal takes place in a narrow temperature window of 430-440 K, and is clearly revealed by both the EXAFS pattern and the appearance of the (111) reflection of metallic copper in the XRD spectra. Note that the QEXAFS detects the metallic copper at a slightly lower temperature than the XRD does, indicating that the first copper metal particles that form are too small to be detected by XRD, which requires a certain extent of long range order [43,44],... 

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. 

Skeletal Cu-Zn catalysts show great potential as alternatives to coprecipitated Cu0-Zn0-Al203 catalysts used commercially for low temperature methanol synthesis and water gas shift (WGS) reactions. They can also be used for other reactions such as steam reforming of methanol, methyl formate production by dehydrogenation of methanol, and hydrogenolysis of alkyl formates to produce alcohols. In all these reactions zinc oxide-promoted skeletal copper catalysts have been found to have high activity and selectivity. 

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... 

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

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. 

Li JL, Inui T. Characterization of precursors of methanol synthesis catalysts, copper zinc aluminum oxides, precipitated at different pHs and temperatures. Appl Catal A Gen. 1996 137(1) 105 17. 

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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