Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

Positron annihilation lifetime spectroscopy (PALS) is a tool used to study pore networks in zeolites, as the diffusion of metastable ortho-positronium (o-Ps, an electron positron bound pair) provides insights into connectivity that cannot be obtained by other techniques. The accurate assessment of porosity requires knowledge of the interaction of o-Ps with acid centers commonly present in these materials. Although previous studies have highlighted a potential effect, the specific impact of the nature and concentration of the acid sites remains unclear. By preparing a series of well-crystallized aluminum- and tin-containing MFI-type zeolites and with the study of commercial samples, we map the effects of Brensted and Lewis acidity on the PALS response. The results reveal that both types of acid sites decrease the amount of o-Ps detected but through different mechanisms. Brensted acid sites strongly affect the amount of o-Ps being annihilated in the micropore network but only nominally influence the amount out-diffusing from the crystal, which is attributed to an energy threshold for the interaction. Lewis acid sites originating from the incorporation of framework tin have a more substantial but uniform impact ascribed to the suppressed formation of o-Ps. A similar effect is observed due to the Lewis acid sites in sodium exchanged aluminum-containing MFI, but the introduction of larger extraframework cations also hinders the diffusion of o-Ps. A preliminary model is put forward to describe the changes in the PALS response in the presence of acid centers.