influence of alternative cations distribution in agxli96 x lsx on dehydration kinetics and its selective adsorption performance for n2 and o2

Influence of alternative cations distribution inAgxLi96-x-LSX on dehydration kinetics and its Hamida Panezai, Jihong Sun,aand Xiaoqi Jin Beijing Key Laboratory for Green Catalysis and Se

kinetics and its selective adsorption performance for N2 and O2

Hamida Panezai, Jihong Sun, and Xiaoqi Jin

Citation: AIP Advances 6, 125115 (2016); doi: 10.1063/1.4973337

View online: http://dx.doi.org/10.1063/1.4973337

View Table of Contents: http://aip.scitation.org/toc/adv/6/12

Published by the American Institute of Physics

Influence of alternative cations distribution in

AgxLi96-x-LSX on dehydration kinetics and its

Hamida Panezai, Jihong Sun,aand Xiaoqi Jin

Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry

and Chemical Engineering, Beijing University of Technology, Beijing 100124,

People’s Republic of China

(Received 26 October 2016; accepted 15 December 2016; published online 23 December 2016)

Adsorption characteristics of pure gases N2and O2on 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 stud-ied 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 proba-ble 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 N2and Ag+clusters due to strong adsorption of N2 at low pressure, whereas O2 adsorption is affected to a negligible extent In addition, the N2/O2adsorption 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 adsorp-tion strongly depends on temperature as higher adsorpadsorp-tion occurs at low temperature

273 K as compared to 298 K © 2016 Author(s) All article content, except where

otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4973337]

I INTRODUCTION

In industry, oxygen and nitrogen are considered as large volume commodities and have been produced by cryogenic distillation of air However, cryogenic distillation methods need high energy requirements and cost.1Therefore production of oxygen from air up to 95 % purity has become more economical using pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) processes

as compared to the conventional cryogenic separation.2 , 3In these processes, the selection and com-bination of efficient adsorbent materials are of primary importance The large quadrupole moment

of nitrogen relative to oxygen is responsible for the selective adsorption characteristics of nitrogen

on zeolites,4The ion-exchanged synthetic zeolites particularly, 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 PSA process.4 6 There-fore, the concept of binary exchanged LSX was introduced to attain enhanced nitrogen adsorption capacity and high thermal stability over those of Na-X, Li-X, and Ca-X zeolites Earlier in 1964, Habgood7found that silver has very strong influence on the adsorption characteristics of bi-metallic zeolites due to its stronger polarizing force than other alkali and alkaline earth cations Sun and

a Author to whom correspondence should be addressed Electronic mail: jhsun@bjut.edu.cn Tel.: +86 10-67396118 Fax: +86 10-67391983.

2158-3226/2016/6(12)/125115/14 6, 125115-1 © Author(s) 2016

Seff8 further demonstrated that Ag+ can be exchanged completely into zeolites, and its reversible oxidation-reduction properties provide an excellent model system not only for studying the mech-anism of formation of noble metal clusters in zeolite channels and cavities but also the catalytic mechanism of hydrocarbons for the dehydrogenation Since the good N2/O2selectivity and higher

N2 adsorption capacity of bi-metallic LixAly-X zeolite have been reported, the LixNa96-x-LSX and

AgxLi96-x-LSX have become the best sorbents for use in air separation via PSA or VSA processes.9

In addition, Yang et al.10proposed that nitrogen adsorption on a bi-metallic AgxLi96-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 AgxLi96-x-LSX also provides a significantly higher (10 %) oxygen throughput.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,15–17 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, 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 obtained N2storage 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 N2 adsorp-tion capacity and N2/O2 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 sec-ond element The present investigation also explains the influence of presorbed water as well as zeolite activation procedure to promote the formation of intra-crystalline silver clusters in bimetal-lic AgxLi96-x-LSX on the nitrogen and oxygen adsorption capacity and selectivity with respect to the percent of silver exchanged In addition, 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)

II EXPERIMENTAL SECTION

A Ion-exchange of Li in Na-LSX

Na-LSX in the powder form was supplied by the LuoYangJianLong Co., Ltd Ion exchange was carried out by refluxing the zeolite samples at 363 K for 2 h with a 0.4 mol/L lithium chloride solution (obtained from Sinoharm Chemical Reagent Co., Ltd, 97.0%, A R grade) followed by filtration and thorough washing with hot distilled water The ion exchange procedure was repeated for a total of eight times to prepare 99 % LixNa96-x-LSX The samples were dried overnight at

363 K

B Ion-exchange of Ag in Li, Na-LSX

Prior to Ag+ exchange, all the LixNa96-x-LSX samples were dried at 363 K for overnight A total of 4.00 g of ion exchanged form of LixNa96-x-LSX zeolite was used in two exchanges by using 400 mL of different molar solutions of (0.001, 0.005, 0.01, 0.05 and 0.1M) AgNO3at room temperature and stirred for 15 h The samples were vacuum filtered and washed with enormous amount of distilled/deionized water to remove all salts, and until pH of the last washings get to pH 9.5 to minimize any hydrolysis

Calcination was conducted sequentially for 1h at 573 K in muffle furnace and then dried at 573 K for 4∼6hrs prior to N2 adsorption at room temperature (298 K) The whole experiment was carried out in dark area due to the sensitivity of Ag+to light and also flasks were shielded from bright light during transfer operations

Degree of lithium and silver exchange in these samples was determined by chemical analy-sis using an inductively coupled plasma-optical emission spectrometry (ICP-OES) The percent of lithium and silver exchange of the samples presented in unit cell structural formulas are given in TableI Li-exchanged zeolite samples obtained have lithium content in the range 95.95-95.75 %, which are approximately the same The first bi-metallic silver sample (Ag3.87Li88.82Na3.31-LSX) has the lowest silver content (3.87) and last one (Ag87.08, Li7.94, Na0.98-LSX) has the highest silver content equal to 87.08

C Kinetics of dehydration of bi-metallic zeolites

The isothermal and non-isothermal thermogravimetric data were used to evaluate kinetic param-eters of the thermal dehydration and decomposition of parent LixNa96-x-LSX and AgxLi96-x–LSX zeolites.18 , 19Kinetics are basically related to the decomposition mechanisms and used as a starting tool to suggest mechanisms for the thermal dehydration and decomposition; therefore it is strongly recommended that the selection of correct model is a critical point in kinetic analysis to justify exper-imental data.19Consequently, a number of methods have been developed for scientific and practical reasons to analyze solid-state kinetic data since they provide reliable information on the thermal behavior and character of solids transformation during the isothermal or non-isothermal heating Among which, mathematical approaches were employed that can be classified into model-fitting and model-free (isoconversional) methods.20

1 Model-free method

a Kissinger method.21 The equation used for the calculation of E value is:

ln β

T P2

!

= − E

RT P + ln AR

E

!

(1)

where, β is the heating rate; T P is the maximum peak temperature; E is the apparent activation energy;

A is the pre-exponential factor and R is the gas constant.

b Flynn-Wall-Ozawa method.22 The integral formula for the calculation of E value is given by the

following equation:

ln ( β)= ln AE

Rg (a)−5.331 − 1.052

E

where, β is the heating rate; E is the apparent activation energy; A represents the pre-exponential factor; g(α) is integral form of mechanism function; T P is the maximum peak temperature; and R is

the gas constant

TABLE I The physisorbed, chemisorbed and total water weight loss (wt%) calculated from TG profiles and BET surface area (m 2 /g), and pore volume of Li 95.95 Na 0.05 -LSX and Ag x Li 96-x -LSX zeolites.

Weight Loss (wt %) Total SaBET Vbpore

Samples 300-460 460-620 620-780 (wt %) (m 2 /g) (cm 3 /g) Mean pore size c (nm)

Ag 3.87 , Li 88.82 , Na 3.31 -LSX 18.66 5.40 1.90 25.96 800.9 1.33 6.80

Ag 18.67 , Li 72.84 , Na 4.48 -LSX 18.94 4.98 1.88 25.80 785.8 1.15 5.65

Ag 32.54 , Li 71.6 , Na 2.53 -LSX 17.93 4.61 1.67 24.21 670.2 1.14 4.70

Ag 85.62 , Li 8.77 , Na 1.61 -LSX 12.76 2.51 1.07 16.34 499.5 0.95 5.35

Ag 87.08 , Li 7.94 , Na 0.98 -LSX 12.57 2.39 1.01 15.97 493.2 0.34 5.35

a BET surface area.

b pore volume.

c

slope=d(ln β)

d( TP1 ) = −1.052E

2 Model-fitting method

a Coats and Redfern method.23 The following equation is used for the calculation of E value is:

ln g(α)

T2

!

= ln AR βE!− E

where, g(α) is the integral form of conversion or mechanism function depending on the kinetic term model; β the heating rate; E the activation energy; and T is absolute temperature; A is pre-exponential factor; and R is gas constant If the correct g(α) mechanism function is employed, a plot

of ln[g(α)/T 2 ] against 1/T should give a straight line, from the slope and intercept of which E and

A using Arrhenius equation can be calculated and the model that gives the best linear fit is selected

as the model of choice In addition, the most accurate model is supposed to be the one that produces activation energy closest to that calculated by model free methods (Kissinger and Flynn-Wall-Ozawa methods).21 , 24

3 Arrhenius equation

k = Ae (−Ea/RT )

(5)

where k is the rate constant, A is the frequency or “pre exponential factor”, Ea is the apparent activation energy, R represent ideal gas constant (8.314 J mol-1K-1) and T is the absolute temperature in K.24

The A can be calculated from the intercept of the plots of most probable g(α) function among the 41

mechanism functions determined

D Characterizations

LixNa96-x-LSX and AgxLi96-x-LSX samples were characterized by SEM analysis (Hitachi field-emission scanning electron microscope (S-4300)), which was operated at an accelerating voltage

of 15 kV, to obtain the size and morphology of crystallites In order to prevent evaporation during imaging, zeolites were oven dried at 363 K for 12 h, and then placed on one-sided sticky tape and coated using sputtered gold with C-1045 Ion Sputter Coater The XRD patterns were obtained using a Bruker-AXS D8 Advance powder X-ray diffractometer with Cu Kα-radiation, operated at 20 mA and

35 kV with a scanning speed of 2◦/min and degree step of 0.02◦ The metal contents (sodium, lithium and silver) of ion-exchanged LSX were examined by using ICP-OES (Perkin Elmer Optima 2000 DV) The water content and desorption of all the samples were examined using TG-DTG profiles measured on Perkin Elmer Pyris1 TG instrument from room temperature to 800 K at a heating rate

of 10 K min-1 in under N2 atmosphere with a flow rate of 20 mL/min N2 adsorption/desorption isotherms at 77 and 298 K, as well as N2and O2adsorption isotherms 273 and 298 K were measured using a JW-BK-300C equipment All the samples were degassed under helium at 473 K for 6 h prior

to nitrogen adsorption and data collection Brunauer-Emmett-Teller (BET) theory was employed for the measurement of surface area and pore volume Prior to N2and O2adsorption the samples were dehydrated at 573 K in the muffle furnace for 1h and then preheated at 573 K for 4-6 h Afterwards, the N2 and O2 adsorption isotherms were measured, at 273 and 298 K, using a static volumetric system Additions of the adsorbate gas were made at volumes required to achieve a targeted set of pressures ranges from 0-1.0 atm A minimum equilibrium interval of 5 s with a tolerance of 5%

of the target pressure (or 0.0066 atm) was used to determine equilibrium for each measurement point

III RESULTS AND DISCUSSION

A XRD patterns and SEM image

Fig.1Apresents the XRD patterns of parent Li95.95Na0.05-LSX and Ag+exchanged bi-metallic

Ag Li -LSX zeolites As can be seen, the diffraction patterns of Li Na -LSX and bi-metallic

FIG 1 (A) XRD patterns of Li 95.95 Na 0.05 -LSX (a), whereas bi-metallic zeolites: Ag 3.87 Li 88.82 Na 3.31 -LSX (b),

Ag 18.67 Li 72.84 Na 4.48 -LSX (c), Ag 32.54 Li 71.6 Na 2.53 -LSX (d), Ag 85.62 Li 8.77 Na 1.61 -LSX (e), and Ag 87.08 Li 7.94 Na 0.98 -LSX (f) (B) Relative crystallinity (%) and Lattice parameters (nm) of Ag x Li 96-x -LSX samples (inset: SEM image of

Ag 3.87 Li 88.82 Na 3.31 -LSX).

AgxLi96-x-LSX samples present characteristic peaks such as (111), (220), (331), (533), (553), and

(715) in the 2 theta range 5-50◦, showing a typical FAU structure.25Peaks present in Li95.95Na0.05 -LSX are intact, which confirms that the high crystallinity of Li+exchanged LSX zeolite is conserved

as shown in Fig.1A(a) However, on modification with Ag+, the relative intensity of the characteristic peaks are reduced with increasing content of silver as shown in Fig.1A (b–f)

Meanwhile, as can be seen in Fig.1B, AgxLi96-x-LSX samples show almost similar crystallinity

up to 18.34 % silver exchanged, as already identified in Na-LSX, whereas crystallinity changes

on increasing silver content Additionally, some structural modifications have been noticed such

as displacement of the 2 theta values to higher angles due to large ionic radius of Ag+ ion14 , 26and reduction of intensity in the diffraction patterns as shown in Figs.1AandB This decrease in intensity

is quite evident in the samples having 32.52-87.08 % silver content, which causes disorder in the framework structure, but it is still negligible to damage the structure These observations confirmed the strong effects of silver exchange on FAU framework.26

SEM image of Ag3.87Li88.82Na3.31-LSX is shown in Fig 1B (inset), while rest of SEM images of parent Li95.95Na0.05-LSX and four bi-metallic zeolites (Ag18.67Li72.84Na4.48-LSX,

Ag32.54Li71.6Na2.53-LSX, Ag85.62Li8.77Na1.61-LSX, and Ag87.08Li7.94Na0.98-LSX) are not shown due

to similarity with Ag3.87Li88.82Na3.31-LSX image The particle sizes of Li95.95Na0.05-LSX varied between 5.1–5.35 µm and morphologies were fairly uniform octahedral, and found in good agree-ment with the previously reported results.16While the SEM results of AgxLi96-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 In addition, with the increasing Ag+ content up to 91.00 %, the cell parameters from parents LixNa96-x-LSX to bi-metallic AgxLi96-x-LSX, were increased, whereas relative crystallinity was decreased as shown in Fig.1B

B N 2 adsorption/desorption isotherms

Fig.2 A (a–f)presents N2 adsorption/desorption isotherms of parent LixNa96-x-LSX and bi-metallic (AgxLi96-x-LSX) zeolites prepared by exchanging with different percentages of silver ions measured at 77 K Their surface area, micropore volume and mean pore size are listed in TableI As can be seen, the isotherms of all samples obtained are classified according to the IUPAC as type IV plus type I A steep increase occurred in the isotherm of LixNa96-x-LSX at relative pressures of 10-3

∼0.01, corresponding to the filling of micropores and another step in the isotherm is the increase in

volume adsorbed at higher P/P 0 of 0.49 ∼ 0.1, corresponds to adsorption in mesopores as well as hysteresis loop of type H4 An important feature here is the distinct increase in adsorbate volume in

the low P/P 0region in type IV isotherms that indicates the presence of micropores associated with mesopores In the parent Li Na -LSX samples the closed hysteresis loop with less steepness is

FIG 2 (A) N 2 -adsorption/desorption isotherms of Li 95.95 Na 0.05 -LSX (a), Ag 3.87 Li 88.82 Na 3.31 -LSX (b),

Ag 18.67 Li 72.84 Na 4.48 -LSX (c), Ag 32.54 Li 71.6 Na 2.53 -LSX (d), Ag 85.62 Li 8.77 Na 1.61 -LSX (e), and Ag 87.08 Li 7.94 Na 0.98 -LSX (f), (B) corresponding their pore size distribution.

observed, which is attributed to more uniform pore system, having higher surface area (938.80m2/g) and pore volume (0.59 cm3/g) containing capillaries with wider profile bodies and narrow short necks However, modification made by cation exchange (Ag+) could cause structural variations, which results in strong effects on BET surface area and micro-meso porosity in both from LixNa96-x-LSX

to bi-metallic AgxLi96-x-LSX as well as within the AgxLi96-x-LSX zeolites with increasing contents

of Ag+, but it does not highly affect the isotherm shape (Fig.2A(b–f)) As compared to parent

Li95.95Na0.05-LSX, the bi-metallic AgxLi96-x-LSX samples exhibit markedly decreased surface area and micropore volume as listed in TableI 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 N2 molecules into the cavities of zeolite.10,15In addition, all the isotherms of AgxLi96-x-LSX samples displayed hysteresis loop of type H4 similar to parent LixNa96-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 0of 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.1A (b–f)) after silver ion exchange, leads

to a decrease in the surface area of all the AgxLi96-x-LSX samples This significant reduction in surface area and micro-meso porosity, after the exchange of Ag+ions, is not only good evidence of successful exchange in LSX zeolite, but also verify the distribution of silver clusters in these pores and found in close agreement with the literature.27Moreover, as can be seen in Fig.2B (b–f), the mesopore size distribution showed narrow slit like and uniform pores in the silver exchanged samples that can possibly be attributed to the aggregated crystallites, which in turn results in the mesoporous structure formed by the intra-crystalline voids in the spherical particles and found in similarity with the literature.28

C Influence of presorbed water on N 2 /O 2 selectivity of Li 95.95 Na 0.05 -LSX and Ag x Li 96-x -LSX

Fig 3 presents the amounts of water retained by Ag3.87Li88.82Na3.31-LSX (a), and

Ag87.08Li7.94Na0.98-LSX (b) determined from weight loss measurements carried out in TG analy-sis at 300-800 K The amount of presorbed water obtained for LixNa96-x-LSX is around 26.90 wt%

as shown in Fig SI(a) of thesupplementary material

As can be seen in Fig.3(a–b)and Fig SI of thesupplementary materialthe presorbed water in

AgxLi96-x-LSX zeolite decreases from 25.96-15.97 wt% with the increasing silver content However,

this effect is more pronounced beyond 87.90 % of silver exchange It is therefore interesting to note that the highly Ag+exchanged zeolites show drastic suppression of N2/O2adsorption selectivity by

24.21-15.97 wt% of presorbed water in the last three zeolites as listed in TableII

Moreover as shown in Fig.3and Fig SI of thesupplementary material, among three desorption peaks the first two peaks corresponding to low energy, have been attributed to water (physisorbed) des-orption from the surface and sodalite-cage (SI), whereas the high-energy third peak has been ascribed

FIG 3 TG-DTG profiles of Ag 3.87 Li 88.82 Na 3.31 -LSX (a), and Ag 87.08 Li 7.94 Na 0.98 -LSX (b) zeolites measured under N 2 flow

at three heating rates (5, 10, and 15 K).

to water (chemisorbed and crystalline) desorption from the α-cage (SII and SIII) of the AgxLi96-x -LSX zeolites In bi-metallic AgxLi96-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 LixNa96-x-LSX (620-800 K) zeolite and is found in good agreement with our previously reported results and literature.29Furthermore, 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 adsorp-tion capacity16,17as can be seen in case of Ag3.87Li88.82Na3.31-LSX and Ag18.67Li72.84Na4.48-LSX (1.90 and 1.88 wt %)

However, the near fully exchanged Ag87.08Li7.94Na0.98-LSX sample does not show an increase

in N2capacity after dehydration at 573 K and this decrease in N2adsorption 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 N2and O2adsorption on bi-metallic

AgxLi96-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 The same behavior is reported in case of NaCaA zeolite.30It is quite obvious that the ultimate adsorptive characteristics of the silver-containing zeolites strongly depend on the formation of silver clusters and, in turn on the dehydration conditions, therefore fruitful results for N2adsorption can be achieved when the silver exchange would

be carried out to a lesser percentage in bi-metallic LSX zeolites and proper dehydration temperatures would be followed.12,14

D Influence of silver ion exchange

Figs.4A–D (a)show the N2and O2adsorption isotherms, measured at two temperatures 273 and

298 K and partial pressures 0-1.0 atm, of almost 99 % Li+exchanged Na-LSX zeolite after dehydration

at 573 K, which is commonly used in adsorptive air separation.14However, the difference between TABLE II Comparison made between N 2 /O 2 adsorption data and water content of Li x Na 96-x -LSX and Ag x Li 96-x -LSX.

273 K/cm 3 /g 298 K/cm 3 /g Sample N 2 O 2 Selectivity N 2 O 2 Selectivity H 2 O (wt%)

Ag 3.87 , Li 88.82 , Na 3.31 -LSX 30.02 5.55 5.41 19.95 3.45 5.78 25.96

Ag 18.67 , Li 72.84 , Na 4.48 -LSX 23.54 4.92 4.79 17.12 3.10 5.53 25.80

Ag 32.54 , Li 71.6 , Na 2.53 -LSX 22.88 5.94 3.85 19.82 3.72 5.33 24.21

Ag 85.62 , Li 8.77 , Na 1.61 -LSX 18.16 5.44 3.34 16.01 3.64 4.43 16.34

Ag 87.08 , Li 7.94 , Na 0.98 -LSX 17.60 5.62 3.13 15.28 3.43 4.45 15.97

FIG 4 N 2 adsorption isotherms at 273 (A) and at 298 K (B) and O 2 adsorption isotherms at 273 (C) and at 298 K (D) of Li 95.95 Na 0.05 -LSX (a), Ag 3.87 Li 88.82 Na 3.31 -LSX (b), Ag 18.67 Li 72.84 Na 4.48 -LSX (c), Ag 32.54 Li 71.6 Na 2.53 -LSX (d),

Ag 85.62 Li 8.77 Na 1.61 -LSX (e), and Ag 87.08 Li 7.94 Na 0.98 -LSX (f) measured at partial pressure 0-1.0 atm and 298 K after vacuum dehydration at 573 K for 4∼6hrs prior to adsorption.

nitrogen and oxygen adsorption behavior becomes more prominent when Li+ cations was replaced with Ag+cations

The sorption capacities of different cation exchange levels of AgxLi96-x-LSX samples determined from N2 and O2adsorption isotherms measured at 273 and 298 K and their comparison with water content (wt%) are given in Table II As can be seen in Figs 4A–D (b-f), the nitrogen adsorption

decreases from 30.02-17.60 cm3/g at 273 K and 19.95-15.28 cm3/g at 298 K with the increasing silver exchange degree from 3.87 to 87.08, whereas the adsorption of oxygen shows almost the same trend The adsorption selectivity of N2/O2shows dependence on silver exchange degrees in the LSX zeolite (as listed in TableII) in the form of sharp decrease at higher silver contents During the N2and

O2isotherm measurements at 273 and 298 K, the AgxLi96-x-LSX samples were turned to yellow-deep brown color from gray color after dehydration, indicating the formation of silver clusters, which is found in good agreement with the literature.8

As can be seen in Figs 4A(b) and 4B(b), after 3.87 wt % of Ag+ was exchanged in

Ag3.87Li88.82Na3.31-LSX, the N2adsorption as well as N2/O2separation factor (5.41 and 5.78) were increased as compared to the parent Li95.95Na0.05-LSX (Figs.4A(a)and4B(a)) (5.08 and 5.39) at 273 and 298K, which reveals that small amount of Ag+present at SIII site is responsible for N2adsorption This means that Ag+is the main cause of enhancement of N2adsorption However, as the percentage

of Ag+was increased, the water content, N2adsorption and N2/O2separation factor were decreased, which confirms that Ag+neither has strong affinity for water adsorption, nor is accessible to the N2at

SI, SI’ and SII sites after replacement by Li+ions and found in close agreement with the literature.14 Here a question arises that why N2adsorption capacity decreased with the increasing Ag+content,

if Ag+would be the reason of increase, therefore it is suggested that it might be due to some factors other than water content such as charge density, size, and location of the cations, which influence

adsorption capacity 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 N2/O2 selectivity.12,16 That is why; strengths of adsorption for both N2 and O2 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 AgxLi96-x-LSX decreases sharply with an increase

in the size of cations that agrees well with the literature.31In the case of lithium cations, the curve

shows highest nitrogen adsorption capacity with small cation size Yang et al reported,10the ionic radius of Ag+is (1.26 Å) considerably larger than that of Li+(0.68 Å) Therefore, N2adsorbs more strongly on LixNa96-x-LSX, as the distance between the nucleus of Li+and the center of N2molecule

is shortest as compared to Ag+cations However, the strong bonds between N2and Ag+cannot only

be attributed to the ion-quadrupole interaction, but also relatively slow desorption and high heats

of adsorption involves weak π-complexation bonds that play an important role in the reduction of

N2adsorption and confirmed in the literature.10,31On the basis of these observations, it is therefore inferred here that near fully exchanged AgxLi96-x-LSX zeolites as compared to their LixNa96-x-LSX zeolite analogs are not favorable for use in adsorption-based separations, not only because of the low

N2adsorption capacity and N2/O2selectivity, but also have strong affinity for N2at low pressures that can easily create a low-pressure high “knee” in the adsorption isotherm, which results in difficulty

of N2 desorption for the purpose of adsorbent regeneration as shown in Figs.4A–B (b-f)and found

in good agreement with the literature.14 , 26In addition, the dehydrated (at 573 K) AgxLi96-x-LSX has also low adsorption capacity for O2 as shown in Figs.4C–D (b-f), which is probably due to the low quadrupole moment of O2 molecule as compared to N2 and is found in agreement with the literature.10,12,14 Moreover, as observed in Figs.4A–D (a-f), N2 and O2 adsorption amounts were higher at low temperature (273 K) than that at higher temperature (298 K), which in turn confirmed temperature dependence of saturation loading.32

E Location of cation site

Besides these observations another most important factor, which influences the adsorption capac-ity is the location of cations in the skeleton of LSX zeolite and as reported in the literature,14 , 32Li+ cations in fully Li-exchanged LSX are present in the six-ring (SI, SI0), sodalite cages (SII), and in the supercages at SIII site Among which, cations at SI, SI0and SII sites do not interact with atmospheric gases because of the short distance from the closest framework oxygen Similarly, silver cations in the SI, SI0and SII locations have weak effect on the adsorptive properties However, it is suggested that silver ions present at SIII sites in Ag3.87Li88.82Na3.31-LSX are very active and has higher N2 adsorption capacity

It has been reported that pure Li-LSX contains 96 Li+ions and nitrogen is mainly adsorbed on the Li+cations present at SIII and SIII0sites,2 therefore, in mixed LixNa96-x-LSX, only those Li+ ions located at SIII sites were responsible for interaction with N2molecules Number of Li+cations partially exchanged in LSX and AgxLi96-x-LSX at site SIII could be calculated by the following

Eq (6):2,33

Li+ ions present at SIII = (Li exchanged level % /100 * 96) − 64 (6)

As it is clear from the data listed in TableIII, Li+ions strongly prefer SI and SI0sites, whereas from the above discussion it seems that Ag+ions also follow the same behavior, both of these sites are sterically hindered to the N2 and O2 molecules; so, the overall adsorptive characteristics of the bi-metallic AgxLi96-x-LSX may not be influenced by silver clusters present in these sites Therefore,

it is expected that Ag+ clusters, in these bi-metallic zeolites, are instead formed at the N2 and O2 accessible SIII sites due to competition with the Li+ cations and unavailability of SI, SI0and SII sites in the start of Ag+exchange and found in close agreement with the literature.12As can be seen from the data listed in TableIIIthat Li+cations present at SIII site are only 24.82 % out of the total 92.52 % and it is expected that 4.03% Ag+ is exchanged with SIII site cations exposed to the N2 and O2 molecules, therefore the N2 adsorption capacity in Ag3.87Li88.82Na3.31-LSX is the highest one (30.02 and 19.95 cm3/g) at 273 and 298K However, as the Ag+ exchange was increased, Ag+