Thermal and Kinetic Performance of Water Desorption for N₂ Adsorption in Li-LSX Zeolite

Abstract

The effect of small amounts of water in inhibiting the adsorption of N2 gas on different cationic forms of LSX zeolite was investigated using thermogravimetric and derivative thermogravimetry (TG-DTG) techniques at different heating rates, which is shown to be a very effective way of studying the influence of strongly adsorbed water on adsorption of less-strongly adsorbed N2 molecules. According to DTG profiles, the apparent activation energies (E) relating to the thermal desorption of water molecules existing in the skeleton of Na- and Li-LSX zeolites were estimated through both the Kissinger and Flynn–Wall–Ozawa methods. The E values calculated from these two equations were in close agreement, and the thermal dehydration mechanism was further studied on the basis of the Coats–Redfern method. Moreover, the observations for the exponential declination of N2 adsorption amount with the loading of water revealed that the adsorption capacity of cationic zeolites was significantly influenced by water through reducing heterogeneity and strength of the electric field. More specifically, the E value at high temperature was higher than that at low temperature, implying that at high temperature, decomposition of bonded water belongs to dynamic-based control, whereas the dehydration process of physisorbed water belongs to diffusion-based control at low temperature.

Influence of alternative cations distribution in AgxLi96-x-LSX on dehydration kinetics and its selective adsorption performance for N2 and O2

“…In bi-metallic Ag x Li 96-x -LSX zeolites a significant amount of most tenaciously held (chemisorbed and crystalline) water removed at higher temperatures (620-780 K) as compared to parent Li x Na 96-x -LSX (620-800 K) zeolite and is found in good agreement with our previously reported results and literature. 29 Furthermore, as it has also been reported in our previous studies that sodalite-cage and double six-ring (hexagonal prism) are sterically inaccessible to nitrogen molecules, therefore initial physisorption of water is not expected to influence the nitrogen adsorption, whereas chemisorbed water largely occupies SIII sites in α-cages (supercages) strongly influence the adsorption capacity 16 capacity after dehydration at 573 K and this decrease in N 2 adsorption capacity cannot only be attributed to the low content of crystalline water but also to the formation of charged silver clusters in zeolite at high temperature. Therefore complete suppression of N 2 and O 2 adsorption on bi-metallic Ag x Li 96-x -LSX zeolites by presorbed water molecules might be an evident cause of molecular sieving effect caused by the blocking of twelve ring windows of supercages.…”

“…26 SEM image of Ag 3.87 Li 88.82 Na 3.31 -LSX is shown in Fig. 1B 16 While the SEM results of Ag x Li 96-x -LSX also confirmed that the crystallite morphology after silver exchange were remained intact and sizes varied between 5.35-6.8 µm as shown in Fig. 1B (inset), were found in agreement with the XRD results and further confirmed that no major changes were occurred in the structure.…”

“…The electrostatic interaction energy of Li + is higher than that of Na + due to its charge density, which tends to enrich nitrogen adsorption leading to higher N 2 /O 2 selectivity. 12,16 That is why; strengths of adsorption for both N 2 and O 2 are dependent upon the charge densities (charge/ionic radius) of Li + and Ag + cations and it is quite clear that charge density of Li + (1.47), is also higher than that of Ag + ion (0.79). 10 As far as cation size is concerned, from Figs.…”

“…13 The location of the extraframework silver cations in relation to the aluminosilicate framework plays a key role for elucidating the influence of silver cations or clusters on the adsorptive characteristics of zeolite. 14 In our preliminary work, [15][16][17] we have already measured the nitrogen adsorption capacity on 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, N 2 -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 obtained N 2 storage capacity corresponds to the following order Ca-LSX>Li-LSX>Na-LSX.…”

“…The obtained N 2 storage capacity corresponds to the following order Ca-LSX>Li-LSX>Na-LSX. 16,17 The basic idea of this study is to synthesize new bi-metallic adsorbents to enhance N 2 adsorption capacity and N 2 /O 2 selectivity for the production of pure oxygen from air by exploiting the nature and amount of cations. Ag + ion is the obvious first choice after Li + and exchanged as a second element.…”