A Steady-State Kinetic Model for Methanol Synthesis and the Water Gas Shift Reaction on a Commercial Cu/ZnO/Al2O3Catalyst

The main lines in the mechanism of the conversion of a CO/CO₂/H₂ feed into methanol over a Cu/ZnO/Al₂O₃ catalyst are now well established, and a large number of kinetic equations have been proposed.

Generally speaking, the mechanism can be based on three overall reactions: the hydrogenations of CO₂and CO,

CO₂+3H₂⇄CH₃OH+H₂O

CO+2H₂ ⇄CH₃OH

with equilibrium constantsK*₁and K*₂, and the water gas shift reaction，

CO+H₂O⇄H_{2 +} CO₂

with equilibrium constant K*₃.

Early kinetic models were derived for the ZnO/Cr₂O₃ catalyst of the high pressure process, which has now almost completely been abandoned in favor of the low pressure technology. A classic example from this early work is the equation proposed by Natta (1),

in which f_i denotes the fugacity of component i and A, B,C, and D are estimated constants. Apparently, Natta as-sumed that only the hydrogenation of CO occurs, in which he proposed the trimolecular reaction of CO and molecular hydrogen to be rate determining.

Bakemeier et al. (2) noted an important discrepancy between their experimental observations on ZnO/Cr₂O₃ and Natta’s predictions, particularly in the case of CO₂

rich feeds. For this reason, a CO₂ dependency was introduced in the equation in the shape of a Langmuir type isotherm. Assuming the methanol desorption to be rate determining, the authors ended up with

Whereby A, E, m, n, D, and F were determined from experimental data.

Leonov et al.(3) were the first to model methanol synthesis kinetics over a Cu/ZnO/Al2O3catalyst. Their model again assumed CO to be the source of carbon in methanol and did not account for the influence of CO2in the feed:

Andrew (4) used a power law type of equation, with an extraФ_CO2 function to account for the occurrence of a maximum in the carbon conversion to methanol when adding CO₂ to the CO/H₂ feed.

Whereas these first authors used a more or less correlative approach, a number of later contributions focused on effectively implementing detailed mechanistic considerations in the kinetic model.

Klier et al.(5) no longer considered CO to be the only, but still the most important source of carbon in methanol. Experimental variation of the P_CO/P_CO2 ratio, at a fixed total pressure and hydrogen concentration revealed a maximum in the synthesis rate. They ascribed the decrease of the re-action rate at low P_CO/P_CO2 to a strong adsorption by CO₂,while at high ratios an excessive reduction of the catalyst was thought to take place. The ratio of the number of active, oxidized sites and the inactive, reduced sites is solely determined by the P_CO/P_CO2, through a redox-like mechanism, the equilibrium of which is characterized by K_redox. They further assumed competitive adsorption of CO₂and CO or H₂, and accounted for the direct hydrogenation of CO₂by an empirical term. This led to the equation

These equations later served as a base for the work of Mc-Neil et al.(6), who expanded on the mechanism of the direct hydrogenation of CO₂ and the possible role of ZnO as a hydrogen reservoir. Despite the much larger number of parameters in the resulting model, the latter authors did not manage to show a significantly better agreement between the experimental and the simulated results than that already obtained by Klier et al.(5)

Villa et al.(7) realized that a thorough modeling of the methanol synthesis system should also involve a description of the water gas shift reaction. Assuming thereby again that the hydrogenation of CO is the only route to methanol, this resulted in the following set of equations

implying that the generation of methanol and the water gas shift occur on different types of sites.

Graaf et al.(8, 9) considered both the hydrogenation of CO and CO₂ as well as the water gas shift reaction. Inspired by the work of Herman et al.(10), the authors proposed a dual site mechanism, adsorbing CO and CO₂on an S₁type site and H2and water on a site s₂. Formation of methanol from CO and CO₂ occurs through successive hydrogenations, while the water gas shift reaction proceeds along a formate route. Assuming the ad- and desorptions to be in equilibrium and taking every elementary step in each of the three overall reactions in its turn as rate determining, the authors ended up with 48 possible models. Statistical discrimination allowed them to select the following final set of equations:

In doing this, however, the authors failed to account for the fact that some intermediates feature in two different over-all reactions. This implies that the model simultaneously predicts two different concentrations of one and the same intermediate like formyl and methoxy species.

Parallel to this evolution, Russian groups led by Rozovskii and Temkin (see references to this work in (11)) developed a number of kinetic models for the SNM type Cu/ZnO/Al₂O₃catalysts. Since neither of these groups ever succeeded in producing methanol from a dry mixture of CO and hydrogen, the models are all based on the direct

hydrogenation of CO₂to methanol, while the majority also accounts for the occurrence of the water gas shift reaction. Malinovskaya et al. (11) compared a number of these models using own experimental data and selected the following set of equations, originally presented by Mochalin et al.(12):

Unfortunately, the authors did not expand on the physical background of the model, nor did they mention the numerical value of the different parameters in the model.

In the current work, a detailed reaction scheme for the conversion of syngas over a Cu/ZnO/Al₂O₃catalyst will be proposed, serving as the backbone for the development of a

mechanistically sound kinetic model. It will account for the hydrogenation to methanol and include the reverse water gas shift.