Zeolites Containing Mixed Cations for Air Separation by Weak Chemisorption-Assisted Adsorption

Abstract

NaX zeolite was ion-exchanged to obtain LiX and AgX zeolites. The LiX form was further exchanged to replace 20% of the Li+ cations by Ag+, to obtain a LiAgX zeolite. Equilibrium adsorption isotherms of pure-component N2 and O2 were measured at 25 and 50 °C on these four zeolites. AgX was stable since the N2 isotherm was not affected after prolonged exposure of the zeolite to air at 350 °C. Bonding of N2 was substantially stronger on AgX than on the other zeolites. The high isosteric heat of adsorption (8.4 kcal/mol) and the relatively slow desorption of N2 on AgX indicated some degree of weak π-complexation, which was substantiated by molecular orbital calculation results using model systems. Binary N2/O2 selectivity (or separation factor, α) was calculated by using the ideal adsorbed solution theory. The high N2/O2 selectivities at low total pressures for AgX will result in difficult N2 desorption; therefore, AgX is not suitable for air separation. LiX is presently employed in industry as the sorbent for air separation by pressure-swing adsorption. Comparing LiX with LiAgX, the N2/O2 selectivities were higher for LiAgX at high total pressures and lower for LiAgX at lower pressures, due to a (relative) selectivity reversal. This result, combined with the higher N2 capacity for LiAgX, led to the conclusion that LiAgX can be superior to LiX for air separation.

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

“…31 In the case of lithium cations, the curve shows highest nitrogen adsorption capacity with small cation size. Yang et al reported, 10 the ionic radius of Ag + is (1.26 Å) considerably larger than that of Li + (0.68 Å). Therefore, N 2 adsorbs more strongly on Li x Na 96-x -LSX, as the distance between the nucleus of Li + and the center of N 2 molecule is shortest as compared to Ag + cations.…”

“…This decrease is attributed to the ionic radius and atomic mass of Ag + ion (1.26 Å and 107.86) which is greater than the sodium (1.02 Å and 22.98) and lithium (0.68 Å and 6.94) cations, and causes hindrance to N 2 molecules into the cavities of zeolite. 10,15 In addition, all the isotherms of Ag x Li 96-x -LSX samples displayed hysteresis loop of type H4 similar to parent Li x Na 96-x -LSX and two inflections can be seen in each isotherm: first one is found in a relative pressure range of 0.01-0.2, while the second is observed at P/P 0 of 0.95-1.0, which is more steeper than the first one and confirms the presence of mesoporous channels with uniform distribution. The decrease in crystallinity observed in XRD patterns (Fig.…”

“…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. 4A-B (b-f), it is clearly indicated that the adsorption capacity of nitrogen in highly cation exchanged Ag x Li 96-x -LSX decreases sharply with an increase in the size of cations that agrees well with the literature.…”

“…Since the good N 2 /O 2 selectivity and higher N 2 adsorption capacity of bi-metallic Li x Al y -X zeolite have been reported, the Li x Na 96-x -LSX and Ag x Li 96-x -LSX have become the best sorbents for use in air separation via PSA or VSA processes. 9 In addition, Yang et al 10 proposed that nitrogen adsorption on a bi-metallic Ag x Li 96-x -LSX (80/20) zeolite (Si/Al 1.25) could be enhanced through a weak chemical interaction, particularly, with the Ag + cation located in the zeolite framework as compared to almost fully exchanged Li + -zeolites. 11,12 These bi-metallic Ag x Li 96-x -LSX also provides a significantly higher (10 %) oxygen throughput.…”

Hydrophobic dipeptide crystals: a promising Ag-free class of ultramicroporous materials showing argon/oxygen adsorption selectivity

“…The potential for PSA separation of Ar/O2 mixtures of Ag-mordenite was immediately recognised 23 . In the next two decades, several articles and patents followed, describing adsorbents with greater selectivities and capacities, as well as more efficient O2-generating PSA processes 20,22,24,27,[29][30][31][32] .…”