Insights into the Mechanism of Zeolite Detemplation by Positron Annihilation Lifetime Spectroscopy

Abstract

Templating approaches play a key role in expanding the structural diversity of many important classes of porous materials. Understanding the interaction of the templating agent with the resulting porous framework and the mechanism of detemplation is critical to ensure their effective application. This study explores the scope of positron annihilation lifetime spectroscopy (PALS) to gain insight into the location of tetrapropylammonium (TPA+) industrially employed as a structure directing agent (SDA) in the synthesis of MFI-type zeolites. ortho-Positronium (o-Ps) formed upon positron implantation are initially confined within the zeolite crystals. Simulations using a modified Tao potential confirm that o-Ps atoms localize between the propyl chains of adjacent TPA+ molecules within the sinusoidal channels of the micropore network. Controlled removal of the SDA leads to a linear increase in the micropore volume determined by nitrogen sorption. In contrast, the out-diffusion of o-Ps is found to be crystal size dependent. Analysis of the expected percolation threshold shows that the experimental data is best represented by an isotropic detemplation mechanism favoring the formation of small isolated SDA-free volumes. The restricted accessibility of the zeolite micropores due to the presence of the SDA is supported by confocal fluorescence microscopy. These findings emphasize the unique sensitivity of PALS to the presence of guest species within the micropore network opening new doors to study their impact on the textural properties of other porous materials.

Diffusion and catalyst efficiency in hierarchical zeolite catalysts

“…Positron annihilation lifetime spectroscopy (PALS) recently developed, was used for quantitative evaluation of mesopore connectivity [ 101 , 102 ]. The positron is the anti-particle of the electron, and when a targeted material is bombarded by positron beam, most of the positrons are directly annihilated and form gamma rays.…”

Creation of Hierarchical Titanosilicate TS‐1 Zeolites

“…PALS has emerged as an effective nondestructive technology in probing the porosity of hierarchical zeolites. [ 119,126 ] Ortho ‐positronium ( o ‐Ps) formed by the implanted positron has a long enough lifetime to diffuse through the porous media, but loses energy rapidly as colliding with the channel surface, thus decays with a lifetime proportional to the aperture. As such, PALS provides an indirect yet effective way to evaluate the degree of interconnectivity of mesopore networks in zeolite crystals.…”

Shedding New Light on Nanostructured Catalysts with Positron Annihilation Spectroscopy

“…Yet, despite having been studied by numerous techniques, the development of the micropore network upon template removal had not been spatially resolved. Using a combination of beam‐PALS measurements, modelling, and simulations coupled to the study of a series of partially detemplated zeolites derived from a single well‐crystallized MFI zeolite sample showed that the detemplation mechanism was best described by an isotropic model favoring the formation of small, isolated template free volumes (Figure b) . Using bulk PALS, a later study identified that the presence of aluminum in the sample could influence the detemplation mechanism .…”

“…This is consistent with the need for improved models to predict o Ps behavior identified in Section . In another study, PALS was applied to derive insights into the detemplation mechanism of MFI‐type zeolites . Templating strategies continue to play an important role in zeolite synthesis due to the structural flexibility they afford.…”