Simultaneous existence of defects and mesopores in nanosized ZSM-5 zeolite studied by positron annihilation and X-ray diffraction spectroscopies

Abstract

Crystallization, formation, and accumulation of defects and mesopores in the ZSM-5 zeolite samples, which are synthesized from the gel composition of 1.2Na2O 0.1Al2O3 0.8 tetra-propylammonium hydroxide (TPAOH) 6SiO2 400H2O at a temperature of 140 degree Celsius (°C) in 10, 15, and 18 h, are studied by using the Positron annihilation lifetime (PALS) and X-ray diffraction (XRD) spectroscopies. The XRD is used for investigating the crystalline concentration and nano-crystal size of ZSM-5 during the crystallizing process, whereas the PALS is performed in order to determine the presence of templates, defects, and mesopores in the zeolite samples. The latter are calcined in air during 1, 2, and 3 h at a temperature of 600 °C before being measured. The results obtained indicate that there exist clusters of small crystals in the early crystalline stages of the samples. The size of these crystals increases with time and reaches approximately 100 nm after 18 h of reaction. In addition, the template (TPAOH) is found to exist not only in the channels inside the framework but also in the mesopores outside it. Finally, by analyzing the Positron lifetime spectra, we have found for the first time the simultaneous existence of defects and mesopores, which are formatted and accumulated during the crystallization of ZSM-5. Those important results contribute significantly to our understanding of the internal structure of the synthetic zeolite ZSM-5 as well as the synthetic processes for producing zeolites with special features.

Exploring the Sub-nanoscale Structure of Cobalt Molybdenum Sulfide and the Role of a Cobalt Promoter in Catalytic Hydrogen Evolution

“…The DB and EMD measurements were performed using a HPGe detector (Ortec, USA) with an energy resolution of 1.2 keV at the 511 keV energy peak of positron annihilation. Details on the positron experiments can be found in our previous works. ,,− …”

A hybrid model for estimation of pore size from ortho-positronium lifetimes in porous materials

“…could affect the local electron distributions and consequently lead to a significant change of the positron lifetimes [11,15]. As a result, the localized o-Ps lifetime is sensitive to the trap size, that is, the larger the trap is, the longer lifetime of the localized o-Ps can be achieved [14]. For the spherical pores, the first and simplest model describing the correlation of o-Ps lifetime (1/λ) and pore radius (R) via the pick-off annihilation was proposed long time ago by Tao and Eldrup (TE model) by using a spherically symmetric infinite potential well [7,10].…”

“…In the structural research of above materials, the applicable limitations of adsorbed/desorbed methods at the nanometer scale lead to participation of the nuclear techniques such as the small angle neutron scattering (SANS) [2], small angle X-ray scattering (SAXS) [3], positron annihilation lifetime spectroscopy (PALS) [4,5], ect. Among these techniques, PALS has been commonly and widely applied for the structure study of various materials and matters in many decades, including solids [6], molecular substances [7], electron liquid [8] and gas [9], solid pivalic acid [10], thin films [11], semiconductors [12], zeolites [13,14], etc. One of the most obvious applications of PALS arises from the fact that in materials, the structural traps such as micro/mesopores (channels, cavities or cages) and defects (vacancies, voids, etc.)…”