Cu/ZnO-containing methanol synthesis catalysts

B. Gas-Induced Shape Changes in Cu/ZnO-Containing Methanol Synthesis Catalysts... 

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. 

A practical methanol synthesis process greatly requires a high performance catalyst, which must be highly active and selective for methanol synthesis and also stable for a long period in a continuous operation. RITE and NIRE have been jointly implicated in the development of high performance methanol synthesis catalysts since 1990. We developed the Cu/ZnO-based multicomponent catalyst on the basis of the role of metal oxides contained in a Cu/ZnO-based catalyst, which were found to be highly active and selective for methanol synthesis, and also stable for a long period in continuous methanol synthesis. 

As described here, it is understandable that the activity and selectivity of methanol synthesis over Cu-based catalysts depend upon not only the Cu dispersion but also the starting salts for preparation and the residual ions on the catalysts and their effects on the structure of active sites. As the candidate of new catalyst system for methanol synthesis, the following catalysts have been vigorously studied in recent years (1) Cu-ZnO containing new components (2) precious metal such as Pd, Pt, Au, and multicomponent catalysts containing precious metals (3) transition metal except for Cu and their complex oxides (4) homogeneous catalysts consisting of Ru, Ni, etc. 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

There are several metal-oxide combinations other than the Cu/ZnO that have been reported active in methanol synthesis at low temperatures and pressures. These can be divided into catalysts containing copper and an oxide and catalysts containing a transition metal and an oxide. 

Copper and zinc containing mesoporous molecular sieves AIMCM-41 have been studied by MAS NMR, electron spin resonance, nitrogen and carbon monoxide adsorption and temperature programmed reduction. AlMCM-41 materials with ns,/nAi = 15, 30 and x have been synthesized in the presence of copper and zinc Carbon monoxide adsorption shows the presence of Cu ions after mild activation, but Zn ions have not been detected indicating that only a ZnO phase is formed Temperature programmed reduction reveals the presence of CuO clusters of various size depending on the on the ns./n ij ratio and the zinc concentration The results of this study allow the preparation of mesoporous molecular sieves with remarkable redox properties, which are potential model catalyst for methanol synthesis... 

In previous studies the authors have reported that metals oxides such as GaaOa, AI2O3, Zr02 and Cr203 contained in Cu/ZnO-based catalysts have an important role to improve simultaneously the activity and the selectivity[1, 2]. Unlike Cu/ZnO-based catalysts, Raney copper catalysts have not been widely reported in the literature as practical catalysts for methanol synthesis. However, 20 years ago Wainwright and co-workers have been the first to report the potentiel use of Raney Cu and Raney Cu-Zn as catalysts to produce methanol from syngas to use as synthetic liquid fuel [3]. Recent works of Wainwright et al. on methanol synthesis... 

Since classical Cu/ZnO catalysts exhibited a poor stability while the addition of alumina resulted in much better systems, it was tempting to add alumina to Cu-Ce intermetallic compounds. Jennings et al. (1992a), prepared ternary Cu-Ce-Al alloys of various compositions and also tried a variety of other metals (Ca, Cr, Mn, Pd, Zn). Among these ternary alloys aluminum-containing catalysts were the best. In spite of lower initial activities as compared to binary alloys, they exhibited a much better long-term stability. It is believed that the role of aluminum is to stabilize the disperse copper-ceria phases responsible for methanol synthesis activity, although the mechanism for such a process remains unclear. 

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