Monte Carlo simulation for methanol synthesis on metal catalyst in supercritical n-hexane

A computer model based on the Monte Carlo (MC) method for methanol synthesis from CO and H2 in supercritical n-hexane was investigated. The lattice model was adopted to describe surface processes, which included adsorption of reactants, surface reaction, surface diffusion of adsorbed species and re-adsorption of the product. The model took into account the effects of methanol aggregation and concentration of supercritical solvent on the synthesis reaction and converted MC steps to real time according to Botzmann equation. Computational characteristics such as dependence of the system behavior on the size of the lattice and the sensibility of sticking coefficients were presented. The results indicated that TOF (turnover frequency) and conversion number (defined as the ratio of the total number of methanol molecules produced by reaction over the total number of carbon monoxide impinging on the surface) increased with methanol aggregation number (MAN) at the same reaction condition. We simulated the influence of mole fraction of n-hexane on the reaction in different MAN, and found that conversion number increased monotonously with the mole fraction of n-hexane. Each TOF versus n-hexane mole fraction curve had a maximum, and the maximum shifted towards higher n-hexane concentration as the MAN increased. The simulation also showed an interesting phenomenon, TOF achieved maximum when the concentrations of H and CO on the surface were the same, i.e. catalyst activities depended not only on the number of activity sites, but also on the ratio of different adsorbed species on the catalyst surface.