Adsorption isotherms of methanol and dimethyl ether on SAPO-34 measured together with differential adsorption heat measurement

Adsorption isotherms of methanol and dimethyl ether on a SAPO-34 zeolite were measured at 25, 60, and 100 °C. The curves of differential heats of adsorption versus coverage (calorimetric curves) were simultaneously measured with a microcalorimeter, which showed that a simple interpretation of the adsorption isotherms would be incorrect. The calorimetric curves showed sharp drops, and the coverages at which these occurred were taken as the saturation amounts of adsorbed methanol and dimethyl ether on regular sites. However, beyond the saturation coverage, the adsorption isotherms still showed increasing coverages, which were attributed to adsorption on weaker sites, and which should not be used during curve fitting to get the Langmuir parameters of the adsorption isotherm for the regular adsorption sites. In the absence of the calorimetric data, these regions would have been assumed to be part of the adsorption isotherm for the regular adsorption sites, which would be erroneous. To account for the existence of another weaker adsorption site which continued to be populated at high pressures, in addition to the regular adsorption site, we used the dual-site Langmuir equation with the saturation amount on the regular adsorption site obtained from the calorimetric curves. This gave good fits also in the high pressure region where adsorption mainly occurred on the weaker sites.

Graphical AbstractAdsorption of methanol and dimethyl ether on SAPO-34 needs to be fitted by the dual-site Langmuir isotherms for acid sites (site 1) and physisorption sites (site 2).Figure optionsDownload as PowerPoint slide