Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

Abstract

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 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 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.

Shedding New Light on Nanostructured Catalysts with Positron Annihilation Spectroscopy

“…Thus although the effects of acid centers appeared significant, they remained unclear. To gain further insight, recent work examined a series of well‐crystallized MFI‐type zeolites with tailored Brønsted (by varying the Si/Al ratio) and Lewis (by substitution with tin or by sodium exchange) acidity . In agreement with the previous findings, the total amount of o Ps measured was found to reduce with increasing concentration of Brønsted acid sites ( Figure a).…”

“…Experimentally, the effects of the type, strength, and location of the acid site are still poorly understood. For example, although showing similar trends, the magnitude of the effect of Lewis acid sites in sodium‐exchanged and tin‐substituted MFI zeolites differed . An improved theoretical description of the interaction of o Ps with acid centers of distinct character will undoubtedly facilitate the interpretation will undoubtedly be facilitated by.…”

“…Beyond the study of pore networks, positron annihilation measurements can provide valuable information about various other important properties of nanostructured catalysts. Prominent examples include the analysis of acidic centers in aluminosilicate‐based compounds like zeolites, of defect structures such as missing building blocks in microporous organic materials, vacancies in strategically engineered metal (hydr)oxides, substitutional defects, and interfacial properties in composite materials …”

Analysis of the Chemical State in Y-zeolite Pores by Positron Annihilation Lifetime Spectroscopy

“…Moreover, if Ps is formed, Ps oxidation by these acid sites also strongly affects the amount of o-Ps annihilation. 53 Both these effects result in the net suppression of the Ps pick-off component. These results confirm the role of Brønsted acid sites as inhibitors of Ps formation which was suggested by previous studies 12,51 although over more limited Si/Al ranges.…”

“…19,36,39,40 As o-Ps diffusion provides non-destructive insight into the surface chemical activity with higher sensitivity than other techniques, 41 it was naturally used to characterize the surface acidity. 12,[42][43][44][45][46][47][48][49][50][51][52][53][54] Those studies highlighted a potential Ps oxidizing effect of the acid sites that decreased the net amount of o-Ps formation. Nonetheless, the interaction of o-Ps with these acid centers remains relatively poorly understood as earlier measurements cover a rather limited range of Si/Al ratios.…”