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

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.