Masked Lewis sites in proton-exchanged zeolites: A computational and microcalorimetric investigation

Lewis sites, in both proton-exchanged zeolites and silico-alumina systems, exhibit different coordination states, depending on the thermal and/or chemical treatments undergone by the material as well as on the experimental conditions. A new modulating effect of the Lewis acidity, similar to that reported for sulfated zirconia, vanadia, and tungstena materials, is addressed here for alumino-silicates, showing that the Lewis acidity of a formal Al(III) site may become "masked" when the local topology around the Al(III) ions allows the latter to expand their coordination by making an extra bond to the aluminosilicate-framework oxygen atoms. Their intrinsic Lewis acidic character is, however, not irreversibly lost because adsorbed molecules of sufficiently high basicity can unhook the Al atom from the framework. Although the "masking" effect has been discovered by ab initio modeling on the edingtonite framework, microcalorimetric data for the adsorption of N-2, CO, and NH3 on a Lewis-rich H-BEA zeolite provides experimental evidence that the "demasking" is an endothermic process occurring only with strong bases like NH3. The resulting enthalpy of adsorption is thus a compromise between the endothermic demasking process and the exothermic interaction with the restored Al(III) site.