Adsorption methanol synthesis

Zn/AljOj catalysts, 31 249 -Zn/Cr Oj catalysts, 31 250 -ZnO/AljO, 31 276, 292-295 -ZnO binary catalyst, 31 257-287 activity patterns, 31 271-274 BET argon surface areas, 31 259 calcination, 31 261-262 catalytic testing, 31 272 chemisorption, 31 268-271 CO2 effects, selectivity, 31 284-285 color spectra, 31 259-261 component comparison, 31 258-259 methanol synthesis, 31 246-247 modifiers, weakening of adsorption energy, 31 283... 

Another interesting example of reactive adsorption is the so-called gas-solid-solid trickle flow reactor, in which adsorbent trickles through the fixed bed of catalyst, removing selectively in situ one or more of the products from the reaction zone. In the case of methanol synthesis this led to conversions significantly exceeding the equilibrium conversions under the given conditions (67). 

In situ spectroscopic studies have identified a variety of species, such as formate, dioxymethylene, carbonate, and methoxide, to coexist under methanol synthesis conditions on Cu/ZnO-based catalysts [22, 23], Fourier transform infrared spectroscopy studies of CuZn-based catalysts under H2/C02 identified the presence of formate bound to both Cu and ZnO, whereas methoxide was found on ZnO only. Carbonates were found to form via C02 adsorption on ZnO [24] and partially oxidized Cu [23], and were quickly converted into formate via Cu-activated hydrogen. Upon exposure to CO mixtures, only zinc-bound formate was observed [22], The hydrogenation of these formates to methoxide is thought to be rate determining in methanol synthesis. 

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],... 

The question as to what is the active site of Cu-based catalysts in MSR is still unclear and debated in the literature. Similar to the methanol synthesis reaction, either metallic Cu° sites, oxidized Cu+ sites dispersed on the oxide component or at the Cu-oxide interface, or a combination of both kinds of sites are thought to contribute to the active ensembles at the Cu surface. Furthermore, the oxidic surface of the refractory component may take part in the catalytic reaction and provide adsorption sites for the oxygenate-bonded species [126], whereas hydrogen is probably adsorbed at the metallic Cu surface. Similar to methanol synthesis, factors intrinsic to the Cu phase also contribute to the MSR activity in addition to SACu- There are two major views discussed in the literature relating these intrinsic factors either to the variable oxidation state of Cu, in particular to the in situ adjustment of the Cu°/Cu+ ratio at the catalyst s surface [102, 107, 127 132], or to the defect structure and varying... 

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... 

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. 

Values of Activation Energies of Methanol Synthesis from Carbon Monoxide, Efk), and from Carbon Dioxide, E,(k ), and Adsorption Enthalpies AH and Entropies AS Derived from the Kinetic Model Utilizing Constants in Table IX ... 

Coadsorption of reactants and subsequent thermal decomposition of the surface complexes formed have been used to resolve the mechanisms in several studies (81-84). Mutual enhancement of the adsorbed amounts of the reactants is indicative of their interaction, and if the adsorption of separately admitted components is negligible, the stoichiometry of the adsorbed complex can be determined. Further evidence for the formation of an adsorbed complex, employed in a mechanistic study of methanol synthesis over ZnO (84), is obtained by thermal decomposition of the adsorbed complex if the reactants appear simultaneously at one temperature upon thermal desorption from a coadsorbed layer, but if each reactant adsorbed separately gives a thermal desorption peak at a different temperature, the existence, although not necessarily the structure or com-... 

Many studies of simultaneous adsorption of hydrogen or water and CO or C02 have been carried out on the high-pressure methanol synthesis catalysts based on zinc oxide and one or several other oxides, but only three investigations (104, 113, 114) dealt with catalysts containing copper, and two of these were made in reference to the mechanism of the low-temperature shift reaction. 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

Binary and ternary systems containing copper, zinc oxide and aluminum are widely used for industrial methanol synthesis. The adsorption of CO on a fully reduced and clean Cu surface is fully reversible at room temperature, with heats of adsorption ranging between 70 kj mol at low coverage and 45 kj mol" at high coverage [88]. 

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... 

Table VII presents the estimated investments. The methanol case investment reflects the same gasifier type as used for the IBG and SNG cases. A conceptual Chem Systems methanol synthesis step is used. EPRI is sponsoring the development of the Chem Systems technology (5 ). The ammonia case investment reflects the same wood gasification concepts, employs pressure swing adsorption for hydrogen gas purification (based on information provided by the Linde Division, Union Carbide Corporation), and uses a conventional high-pressure ammonia synthesis loop.

Then the product gas is fed to a low-temperature reactor where a Cu/Zn-Al2O3 particulate WGS catalyst works at about 200°C. Outlet CO concentration is decreased to <0.5%, while the remaining CO, which can poison downstream ammonia or methanol synthesis catalysts, is removed by pressure swing adsorption (PSA) unit. This method exploits the adsorption capacity of different molecular sieves or active carbon, which selectively permit the crossover of hydrogen but not of the other compounds present in the effluents. This technology has been... 

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