Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

5 years ago

Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

Lars Gerchow, Javier Pérez-Ramírez, David Cooke, Sharon Mitchell, Richard S. Vallery, Robbie Warringham, Paolo Crivelli

Positron annihilation lifetime spectroscopy (PALS) is a complementary tool to study pore networks in zeolites as the diffusion of metastable ortho-positronium (o-Ps, an electron-positron bound pair) provides insights into connectivity unobtainable 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 acid sites remains unclear. By preparing a series of well-crystallized aluminum- and tin-containing MFI type zeolites, and the study of commercial materials, we map the effects of Brønsted 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. Brønsted acid sites strongly affect the amount of o-Ps annihilating in the micropore network, but only nominally influences 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 proposed to describe the changes in the PALS response in the presence of acid centers.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b11336

DOI: 10.1021/acs.jpcc.7b11336