Pore structure characterization of supported polycrystalline zeolite membranes by positron annihilation spectroscopy

Abstract

Characterizing the pore structure of supported polycrystalline zeolite membranes is important but has remained a major challenge in studying microporous inorganic membranes. This paper reports the use of positron annihilation spectroscopy including positron annihilation lifetime spectroscopy and Doppler broadening energy spectroscopy to non-destructively characterize the pore structure of four MFI zeolite membranes of different microstructure on alumina supports. Positron annihilation lifetime spectroscopy analysis reveals a bimodal pore structure consisting of intracrystalline zeolitic micropores of around 0.6 nm in diameter and irregular intercrystalline micropores of 1.4 to 1.8 nm in size for the four MFI zeolite membranes studied. Distributions of the micropores along the membrane thickness direction can be inferred from Doppler broadening energy spectroscopy results, illustrating development of intercrystalline gaps during the growth of the zeolite layer. The amount and size of the intercrystalline micropores of the zeolite membranes vary with the synthesis method, and are the smallest for the randomly oriented MFI zeolite membrane synthesized without template and largest for the c-oriented MFI zeolite membrane synthesized with template. The c-oriented membrane has an asymmetric distribution of intercrystalline pores along the film growth direction as compared to the uniform distribution of the bimodal structure for the other three membranes. The pore structure data obtained by positron annihilation spectroscopy are consistent with the xylene isomer separation performance of these membranes.

Structural effects on SAPO-34 and ZIF-8 materials exposed to seawater solutions, and their potential as desalination membranes

“…In the application of zeolites, PALS characterisation found that MFI-type zeolite powder samples had three pore sizes, representing the intrinsic zeolite crystalline micropores, intercrystalline micropores and interparticle mesopores [32,37]. The technique has since been applied to characterise intact membrane films using PALS and Doppler broadening energy spectroscopy (DBES), which revealed instead only two pore sizes of the zeolite, being the intrinsic and intercrystalline pores of an intact high quality zeolite membrane [44]. Therefore PALS is an effective technique in understanding the intrinsic zeolite pores and the intercrystalline grain boundaries, for both powder samples and membranes.…”

Xylene Vapor Permeation in MFI Zeolite Membranes Made by Templated and Template-Free Secondary Growth of Randomly Oriented Seeds: Effects of Xylene Activity and Microstructure

“…Besides xylene partial pressures, the separation performance of zeolite membrane is also influenced by the zeolitic microstructure, which could be changed in the template removal step during membrane synthesis . We recently applied positron annihilation spectroscopy (PAS) to characterize the pore structure of MFI zeolite membranes and found that template-synthesized MFI zeolite membranes contain a moderate amount of intercrystalline micropores while template-free synthesized membranes have nearly no defects or very small amounts of intercrystalline micropores . We reported that template-free synthesized MFI zeolite membranes exhibited better pervaporation separation factors of p-x over o-x than those template-synthesized. ,, However, no vapor permeation xylene separation data were reported for these randomly oriented MFI zeolite membranes of different microstructure.…”

Porosimetric membrane characterization techniques: A review

“…PALS has also been used for inorganic membranes. Ma et al [243] prepared four zeolite membrane types with different microstructures on alumina supports, with PALS showing that the pore radii were between 2.73 and 3.64 Å.…”