Abstract
Adsorption characteristics of pure gases N2 and O2 on various silver exchanged low silica X-type (AgxLi96-x-LSX) zeolites were investigated. The equilibrium adsorption isotherms of N2 and O2 were measured at 273 and 298 K. Textual and structural properties of parent and resultant AgxLi96-x-LSX were characterized by XRD, BET surface area, and SEM techniques. Kinetics of their thermal dehydration were studied by exploiting thermogravimetric and differential data (TG-DTG) obtained at three heating rates (5, 10 and 15 K) using two model-free (Kissinger and Flynn-Wall-Ozawa) and one model fitting (Coats-Redfern) methods. Forty one mechanism functions were used to evaluate kinetic triplet (activation energy, frequency factor, and most probable mechanism/model) for different stages of dehydration. Results revealed that the impact of very small content of silver on the adsorption of N2 is pronounced and attributed to weak chemical bonds formed between N2 and Ag+ clusters due to strong adsorption of N2 at low pressure, whereas O2 adsorption is affected to a negligible extent. In addition, the N2/O2 adsorption selectivity shows unexpected low values for Ag87.08Li7.94Na0.98-LSX with higher Ag+ content (91.00 %), which might be due to low crystalline water content as well as Ag+ clusters located at SIII sites. N2 adsorption strongly depends on temperature as higher adsorption occurs at low temperature 273 K as compared to 298 K.
Highlights
In industry, oxygen and nitrogen are considered as large volume commodities and have been produced by cryogenic distillation of air
The large quadrupole moment of nitrogen relative to oxygen is responsible for the selective adsorption characteristics of nitrogen on zeolites,[4] The ion-exchanged synthetic zeolites low silica X-type (LSX) zeolite with a large-pore aperture (7.4 Å), large-pore volume (0.489 cm3/g), and low SiO2/Al2O3 (1) ratio is one of the widely used adsorbents for selective adsorption of nitrogen/oxygen in pressure swing adsorption (PSA) process.[4,5,6]
The structural properties and textural parameters of the parent Li95.95Na0.05-LSX and obtained bi-metallic AgxLi96-x-LSX zeolites were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques, and N2-adsorption/desorption isotherms
Summary
Seff[8] further demonstrated that Ag+ can be exchanged completely into zeolites, and its reversible oxidation-reduction properties provide an excellent model system for studying the mechanism of formation of noble metal clusters in zeolite channels and cavities and the catalytic mechanism of hydrocarbons for the dehydrogenation. Li-, Ca-, and Na-LSX zeolites and found that both the Li-, and Ca-LSX zeolites as adsorbents for selective oxygen/nitrogen separation exhibit a good hydrothermal stability and strong dependence on the amount of presorbed water, N2-cation interactions, porosity, nature as well as the extent of extraframework cations distributed at SIII site in the supercage as compared to parent Na-LSX zeolite. The present investigation explains the influence of presorbed water as well as zeolite activation procedure to promote the formation of intra-crystalline silver clusters in bimetallic AgxLi96-x-LSX on the nitrogen and oxygen adsorption capacity and selectivity with respect to the percent of silver exchanged. The structural properties and textural parameters of the parent Li95.95Na0.05-LSX and obtained bi-metallic AgxLi96-x-LSX zeolites were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) techniques, and N2-adsorption/desorption isotherms. For the study of non-isothermal dehydration kinetics of bimetallic Li95.95Na0.05-LSX and AgxLi96-x-LSX zeolites, the model free methods (Kissinger and Flynn-Wall-Ozawa methods) are combined with model-fitting method (Coats and Redfern) to calculate kinetic triplet (activation energy (E), frequency factor (A), and most probable mechanism) and to select the appropriate reaction model for thermal dehydration using thermogravimetric and differential (TG-DTG) data measured at three heating rates (5, 10 and 15 K)
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