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 siz...