Monitoring the Crystallization Stages of Silicalite by Positron Lifetime Spectroscopy

Abstract

In porous media, positron annihilation techniques (PAT) revealed the existence of annihilation modi with long lifetimes connected to structural and extrastructural free volumes acting as trapping centres for o-Ps. Our aim in this work was to study the influence of the synthesis applied for the production of the zeolite framework and the correlation between the trapping pattern and crystallization progress. PAT lifetime (LT) measurements were performed in silicalite-1 samples in various stages of crystallization and a large change was observed in the long components. The results are discussed in connection to the crystallization and the availability of free volume.

Acidity Effects in Positron Annihilation Lifetime Spectroscopy of Zeolites

“…Positron annihilation lifetime spectroscopy (PALS) has attracted interest due to its ability to dynamically probe open volumes, 1−3 providing complementary insights into more widespread equilibrium techniques, such as gas sorption. This has been demonstrated for silica powders 4−6 and films, 4,7,8 aluminosilicate gels, 9 metal−organic frameworks, 10,11 zeolites, 12−19 and layered materials. 20,21 The high sensitivity of PALS stems from the possibility of positrons binding with electrons in solids to form metastable ortho-positronium (o-Ps) atoms, which are sufficiently long-lived to diffuse through open pore networks and be annihilated with a lifetime characteristic of the pore size.…”

“…Positron annihilation lifetime spectroscopy (PALS) has attracted interest due to its ability to dynamically probe open volumes, − providing complementary insights into more widespread equilibrium techniques, such as gas sorption. This has been demonstrated for silica powders − and films, ,, aluminosilicate gels, metal–organic frameworks, , zeolites, − and layered materials. , The high sensitivity of PALS stems from the possibility of positrons binding with electrons in solids to form metastable ortho -positronium ( o -Ps) atoms, which are sufficiently long-lived to diffuse through open pore networks and be annihilated with a lifetime characteristic of the pore size . This property has recently been exploited in the characterization of zeolitic materials in discriminating the effectiveness of hierarchical pore networks in enhancing transport properties, − in studying coking and detemplation mechanisms, , in assessing the micropore topology, , and in monitoring pore evolution during structural transformations …”

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

“…In this regard PALS can provide a unique level of insight into the micropore network of zeolites containing SDA materials. PALS studies of MFI zeolites have readily identified o -Ps annihilating in micropores (Ps micro , 1–15 ns), mesopores (Ps meso , 15–100 ns), and in vacuum (Ps vac , > 100 ns). ,, Indeed, the out-diffusion of o -Ps, i.e., the fraction that is able to diffuse out from the microporous zeolite framework to intragranular spaces or vacuum, has been correlated with the pore connectivity in zeolites. − Several studies have examined the variation of Ps micro with the inclusion of water and counterions within zeolite frameworks, ,, but studies exploring the systematic blocking of the micropore structure have not been reported.…”

“…PALS has been utilized to study different zeolite frameworks, 34−36 porous silica powders 37−39 and films, 37,40,41 and the crystallization of zeolites from aluminosilicate gels. 42 Recently, it also yielded key insights into the connectivity of hierarchical pore networks in MFI 43,44 and FAU 45 zeolites. This manuscript studies the porosity evolution upon SDA removal in MFI-type zeolites by PALS coupling measurements with simulation and modeling studies on the behavior of o-Ps within the micropores.…”

“…The method exploits ortho -positronium, a bound state between a positron and an electron in the solid (herein denoted as o -Ps) as a diffusional probe that actively localizes in pores within a material before annihilating. − Unique facets of PALS are that the formation of o -Ps occurs within the material and that the lifetime of o -Ps depends on the pore size. , This enables the study of isolated pore volumes that would be inaccessible to standard diffusion-based techniques as gas sorption, while the dynamic response directly reflects the pore architecture. PALS has been utilized to study different zeolite frameworks, − porous silica powders − and films, ,, and the crystallization of zeolites from aluminosilicate gels . Recently, it also yielded key insights into the connectivity of hierarchical pore networks in MFI , and FAU zeolites.…”