Species surface concentrations on a SAPO-34 catalyst exposed to a gas mixture

Abstract

The use of the Langmuir isotherm for competitive adsorption and the use of ideal adsorbed solution theory (IAST) for calculating the surface concentrations of adsorbed species on a SAPO-34 catalyst exposed to a gas mixture were compared with experimental measurements. Two binary mixtures, namely, methanol and dimethyl ether (DME) at 80 °C and DME and ethene at 25 °C were used. IAST gave good agreement in the whole range of experimental concentrations, while Langmuir theory only gave good agreement up to when one-third of the acid sites were covered and then showed increasingly large deviations with increased pressure, and it failed to capture the phenomenon that the concentration of an adsorbed species with a smaller saturation adsorption capacity would decrease with increased pressure. Thus, for reactions of gas mixtures whose components have different saturation adsorption capacities, Langmuir theory can only be used for the reaction kinetics when the surface concentrations are quite low, and IAST should be used when the surface concentrations are high. Methanol has a larger adsorption capacity than dimethyl ether (DME) on SAPO-34, which caused DME coverage to decrease with increasing pressure of a gas mixture. Ideal adsorbed solution theory calculations showed this but the Langmuir isotherm for competitive adsorption did not.