Methanol synthesis mechanism

Struis RPWJ, Quintilii M, and Stuck S. Feasibility of Li-Nafion hollow fiber membranes in methanol synthesis Mechanical and thermal stability at elevated temperature and pressure. J Membr Sci 2000 177(l-2) 215-223. 

Although methanol synthesis from COj hydrogenation over supported copper catalysts has been widely investigated, there are still controversies concerning the methanol synthesis mechanism and the effect of copper on the catalytic activity[l-6]. In this work, the influence of the copper dispersion in Cu/ZrO catalyst on the catalytic activity in COj hydrogenation was investigated. In order to understand the reaction mechanism, FT-IR spectroscopy under reaction conditions and TPD of adsorbed methanol were performed. 

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. 

Methanol synthesis served as the model for the true mechanism. Stoichiometry, thermodynamics, physical properties, and industrial production rates were all taken from the methanol literature. Only the reaction mechanism and the kinetics of methanol synthesis were discarded. For the mechanism a four step scheme was assumed and from this the... 

Here a four-step mechanism is described on the framework of methanol synthesis without any claim to represent the real methanol mechanism. The aim here was to create a mechanism, and the kinetics derived from it, that has an exact mathematical solution. This was needed to perform kinetic studies with the true, or exact solution and compare the results with various kinetic model predictions developed by statistical or other mehods. The final aim was to find out how good or approximate our modeling skill was. 

A.1 Model for Mechanism of Methanol Synthesis Assumed for the Test Problem... 

Remarks The aim here was not the description of the mechanism of the real methanol synthesis, where CO2 may have a significant role. Here we created the simplest mechanistic scheme requiring only that it should represent the known laws of thermodynamics, kinetics in general, and mathematics in exact form without approximations. This was done for the purpose of testing our own skills in kinetic modeling and reactor design on an exact mathematical description of a reaction rate that does not even invoke the rate-limiting step assumption. 

To facilitate the use of methanol synthesis in examples, the UCKRON and VEKRON test problems (Berty et al 1989, Arva and Szeifert 1989) will be applied. In the development of the test problem, methanol synthesis served as an example. The physical properties, thermodynamic conditions, technology and average rate of reaction were taken from the literature of methanol synthesis. For the kinetics, however, an artificial mechanism was created that had a known and rigorous mathematical solution. It was fundamentally important to create a fixed basis of comparison with various approximate mathematical models for kinetics. These were derived by simulated experiments from the test problems with added random error. See Appendix A and B, Berty et al, 1989. 

The UCKRON AND VEKRON kinetics are not models for methanol synthesis. These test problems represent assumed four and six elementary step mechanisms, which are thermodynamically consistent and for which the rate expression could be expressed by rigorous analytical solution and without the assumption of rate limiting steps. The exact solution was more important for the test problems in engineering, than it was to match the presently preferred theory on mechanism. 

The reaction mechanism of methanol synthesis is complex since two processes are involved and coupled. Formally, the reaction can be written as the hydrogenation of CO by the overall reaction ... 

Alternative mechanisms for carbon-oxygen double bond hydrogenation and methanol synthesis are shown in Schemes 4 and 5. 

Figure 8.3 Proposed reaction mechanism for methanol synthesis on Pd and comparison with gas-phase mechanism surface intermediates are speculative and associated energies are estimates...

Campbell and coworkers287,297 also published work on methanol synthesis. In that case, they also found that the active site is metallic copper, but they suggest that the mechanism in that case does involve formate intermediates, and that the ratedetermining step is likely the hydrogenation of the formate species to methanol. 

Wainwright, Tomsett, Trimm, and coworkers/Mellor, Copperthwaite, and coworkers—Raney copper catalysts for WGS and methanol synthesis. In 1995, Wainwright and Trimm295 reviewed Raney178 copper catalysts for both water-gas shift and methanol synthesis applications and discussed the possibility of either a redox mechanism or a formate mechanism for Raney copper catalysts. Formates, they indicated, rapidly decompose to C02 and H2 over metallic copper surface. They... 

Mechanisms for both reactions are proposed, which are supported by the identification of several intermediate reactions. The role of various organic and inorganic reaction promoters is discussed. Amongst these, phosphine oxides are exceptionally efficient in that they induce high reaction rates combined with high selectivities. Reactions a) and b) potentially allow two-step, methanol/ /synthesis gas-based routes to ethyl acetate and proprio-nic acid, respectively. 

This suggests that formyl species are key intermediates both to hydrocarbons and to oxygenates. The first part of the mechanism for oxygenates formation would be the same as in methanol synthesis. 

The mechanism for methanol synthesis from mixtures of H2, CO and CO2 over supported Cu catalysts have been studied extensively in the literature using a number of chemical methods (Bell et al 1995). One of the key issues has been the role of CO and CO2 in maintaining the carbon supply for methanol synthesis or interconversion betwen the two via the water-gas shift reaction ... 

Although numerous investigations have been performed on methanol synthesis catalysts, the structure of the active catalysts, the nature of the active sites, and the reaction mechanism are still subjects of considerable controversy. 

The experimental results on conjugated methane oxidation under pressure with hydrogen peroxide displayed the most probable mechanism of methanol synthesis. 

The mechanism of CH4 oxidation with hydrogen peroxide shown in Figure 5.4 assumes proceeding of several radical-chain reactions of methanol synthesis. In analyzing kinetic equations (5.31)—(5.35) the following inequalities were deduced ... 

The oxidation mechanism should be interpreted in the context of the conjugated reaction theory. For this purpose, the overall equation of the methanol synthesis secondary reaction was deduced by combining the elementary stage of PPFe3+OOH/AlSiMg intermediate formation (7.13) and the elementary stage (7.14) ... 

A comparison between a liquid entrained slurry reactor and a mechanically agitated slurry reactor for methanol synthesis was made by Vijayaraghavan et al. [11]. 

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