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 in AgxLi96-x-LSX on dehydration 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 AIP ADVANCES 6, 125115 (2016) Influence of alternative cations distribution in Agx Li96-x -LSX on dehydration kinetics and its selective adsorption performance for N2 and O2 Hamida Panezai, Jihong Sun,a and 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 N2 and O2 on various silver exchanged low silica X-type (Agx Li96-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 Agx Li96-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 Ag87.08 Li7.94 Na0.98 -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 © 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.1 Therefore 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.2,3 In these processes, the selection and combination 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,4 The 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 /Al2 O3 (1) ratio is one of the widely used adsorbents for selective adsorption of nitrogen/oxygen in PSA process.4–6 Therefore, 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, Habgood7 found 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 125115-2 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) Seff 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 mechanism 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 /O2 selectivity and higher N2 adsorption capacity of bi-metallic Lix Aly -X zeolite have been reported, the Lix Na96-x -LSX and Agx Li96-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 Agx Li96-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 Agx Li96-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 N2 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 N2 adsorption 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 second 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 bimetallic Agx Li96-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 Li95.95 Na0.05 -LSX and obtained bi-metallic Agx Li96-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 Li95.95 Na0.05 -LSX and Agx Li96-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 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 % Lix Na96-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 Lix Na96-x -LSX samples were dried at 363 K for overnight A total of 4.00 g of ion exchanged form of Lix Na96-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) AgNO3 at 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 125115-3 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) 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 analysis 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 Table I 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 (Ag3.87 Li88.82 Na3.31 -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 parameters of the thermal dehydration and decomposition of parent Lix Na96-x -LSX and Agx Li96-x –LSX zeolites.18,19 Kinetics 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 experimental data.19 Consequently, 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 Model-free method a Kissinger method.21 The equation used for the calculation of E value is: E AR β =− + ln (1) RTP E TP 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 ln 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 E − 5.331 − 1.052 Rg (a) RTP (2) 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 (m2 /g), and pore volume of Li95.95 Na0.05 -LSX and Agx Li96-x -LSX zeolites Weight Loss (wt %) Samples Li95.95 , Na0.05 -LSX Ag3.87 , Li88.82 , Na3.31 -LSX Ag18.67 , Li72.84 , Na4.48 -LSX Ag32.54 , Li71.6 , Na2.53 -LSX Ag85.62 , Li8.77 , Na1.61 -LSX Ag87.08 , Li7.94 , Na0.98 -LSX a BET surface area volume c deriving pore size distribution b pore Total S a BET V b pore 300-460 460-620 620-780 (wt %) (m2 /g) (cm3 /g) Mean pore sizec (nm) 19.31 18.66 18.94 17.93 12.76 12.57 5.37 5.40 4.98 4.61 2.51 2.39 2.21 1.90 1.88 1.67 1.07 1.01 26.90 25.96 25.80 24.21 16.34 15.97 938.8 800.9 785.8 670.2 499.5 493.2 0.59 1.33 1.15 1.14 0.95 0.34 5.35 6.80 5.65 4.70 5.35 5.35 125115-4 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) slope = d(ln β) d( T1P ) = −1.052 E R (3) Model-fitting method a Coats and Redfern method.23 The following equation is used for the calculation of E value is: ln g(α) AR E = ln − βE RT T2 (4) 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 preexponential factor; and R is gas constant If the correct g(α) mechanism function is employed, a plot of ln[g(α)/T2 ] 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 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-1 K-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 Lix Na96-x -LSX and Agx Li96-x -LSX samples were characterized by SEM analysis (Hitachi fieldemission 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 N2 and O2 adsorption isotherms 273 and 298 K were measured using a JW-BK-300C equipment All the samples were degassed under helium at 473 K for 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 N2 and O2 adsorption 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 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 1A presents the XRD patterns of parent Li95.95 Na0.05 -LSX and Ag+ exchanged bi-metallic Agx Li96-x -LSX zeolites As can be seen, the diffraction patterns of Li95.95 Na0.05 -LSX and bi-metallic 125115-5 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) FIG (A) XRD patterns of Li95.95 Na0.05 -LSX (a), whereas bi-metallic zeolites: Ag3.87 Li88.82 Na3.31 -LSX (b), Ag18.67 Li72.84 Na4.48 -LSX (c), Ag32.54 Li71.6 Na2.53 -LSX (d), Ag85.62 Li8.77 Na1.61 -LSX (e), and Ag87.08 Li7.94 Na0.98 -LSX (f) (B) Relative crystallinity (%) and Lattice parameters (nm) of Agx Li96-x -LSX samples (inset: SEM image of Ag3.87 Li88.82 Na3.31 -LSX) Agx Li96-x -LSX samples present characteristic peaks such as (111), (220), (331), (533), (553), and (715) in the theta range 5-50◦ , showing a typical FAU structure.25 Peaks present in Li95.95 Na0.05 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, Agx Li96-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 theta values to higher angles due to large ionic radius of Ag+ ion14,26 and reduction of intensity in the diffraction patterns as shown in Figs 1A and B 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 Ag3.87 Li88.82 Na3.31 -LSX is shown in Fig 1B (inset), while rest of SEM images of parent Li95.95 Na0.05 -LSX and four bi-metallic zeolites (Ag18.67 Li72.84 Na4.48 -LSX, Ag32.54 Li71.6 Na2.53 -LSX, Ag85.62 Li8.77 Na1.61 -LSX, and Ag87.08 Li7.94 Na0.98 -LSX) are not shown due to similarity with Ag3.87 Li88.82 Na3.31 -LSX image The particle sizes of Li95.95 Na0.05 -LSX varied between 5.1–5.35 µm and morphologies were fairly uniform octahedral, and found in good agreement with the previously reported results.16 While the SEM results of Agx Li96-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 Lix Na96-x -LSX to bi-metallic Agx Li96-x -LSX, were increased, whereas relative crystallinity was decreased as shown in Fig 1B B N2 adsorption/desorption isotherms Fig A (a–f) presents N2 adsorption/desorption isotherms of parent Lix Na96-x -LSX and bimetallic (Agx Li96-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 Table I 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 Lix Na96-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/P0 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/P0 region in type IV isotherms that indicates the presence of micropores associated with mesopores In the parent Lix Na96-x -LSX samples the closed hysteresis loop with less steepness is 125115-6 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) FIG (A) N2 -adsorption/desorption isotherms of Li95.95 Na0.05 -LSX (a), Ag3.87 Li88.82 Na3.31 -LSX (b), Ag18.67 Li72.84 Na4.48 -LSX (c), Ag32.54 Li71.6 Na2.53 -LSX (d), Ag85.62 Li8.77 Na1.61 -LSX (e), and Ag87.08 Li7.94 Na0.98 -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 Lix Na96-x -LSX to bi-metallic Agx Li96-x -LSX as well as within the Agx Li96-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 Li95.95 Na0.05 -LSX, the bi-metallic Agx Li96-x -LSX samples exhibit markedly decreased surface area and micropore volume as listed in Table I 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.10,15 In addition, all the isotherms of Agx Li96-x -LSX samples displayed hysteresis loop of type H4 similar to parent Lix Na96-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/P0 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 1A (b–f)) after silver ion exchange, leads to a decrease in the surface area of all the Agx Li96-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.27 Moreover, 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 N2 /O2 selectivity of Li95.95 Na0.05 -LSX and Agx Li96-x -LSX Fig presents the amounts of water retained by Ag3.87 Li88.82 Na3.31 -LSX (a), and Ag87.08 Li7.94 Na0.98 -LSX (b) determined from weight loss measurements carried out in TG analysis at 300-800 K The amount of presorbed water obtained for Lix Na96-x -LSX is around 26.90 wt% as shown in Fig SI(a) of the supplementary material As can be seen in Fig 3(a–b) and Fig SI of the supplementary material the presorbed water in Agx Li96-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 /O2 adsorption selectivity by 24.21-15.97 wt% of presorbed water in the last three zeolites as listed in Table II Moreover as shown in Fig and Fig SI of the supplementary material, among three desorption peaks the first two peaks corresponding to low energy, have been attributed to water (physisorbed) desorption from the surface and sodalite-cage (SI), whereas the high-energy third peak has been ascribed 125115-7 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) FIG TG-DTG profiles of Ag3.87 Li88.82 Na3.31 -LSX (a), and Ag87.08 Li7.94 Na0.98 -LSX (b) zeolites measured under N2 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 Agx Li96-x LSX zeolites In bi-metallic Agx Li96-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 Lix Na96-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 capacity16,17 as can be seen in case of Ag3.87 Li88.82 Na3.31 -LSX and Ag18.67 Li72.84 Na4.48 -LSX (1.90 and 1.88 wt %) However, the near fully exchanged Ag87.08 Li7.94 Na0.98 -LSX sample does not show an increase in N2 capacity after dehydration at 573 K and this decrease in N2 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 N2 and O2 adsorption on bi-metallic Agx Li96-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.30 It is quite obvious that the ultimate adsorptive characteristics of the silvercontaining zeolites strongly depend on the formation of silver clusters and, in turn on the dehydration conditions, therefore fruitful results for N2 adsorption 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 N2 and O2 adsorption 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.14 However, the difference between TABLE II Comparison made between N2 /O2 adsorption data and water content of Lix Na96-x -LSX and Agx Li96-x -LSX 273 K/cm3 /g Sample Li95.95 , Na0.05 -LSX Ag3.87 , Li88.82 , Na3.31 -LSX Ag18.67 , Li72.84 , Na4.48 -LSX Ag32.54 , Li71.6 , Na2.53 -LSX Ag85.62 , Li8.77 , Na1.61 -LSX Ag87.08 , Li7.94 , Na0.98 -LSX 298 K/cm3 /g N2 O2 Selectivity N2 O2 Selectivity H2 O (wt%) 26.09 30.02 23.54 22.88 18.16 17.60 5.14 5.55 4.92 5.94 5.44 5.62 5.08 5.41 4.79 3.85 3.34 3.13 19.20 19.95 17.12 19.82 16.01 15.28 3.56 3.45 3.10 3.72 3.64 3.43 5.39 5.78 5.53 5.33 4.43 4.45 26.90 25.96 25.80 24.21 16.34 15.97 125115-8 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) FIG N2 adsorption isotherms at 273 (A) and at 298 K (B) and O2 adsorption isotherms at 273 (C) and at 298 K (D) of Li95.95 Na0.05 -LSX (a), Ag3.87 Li88.82 Na3.31 -LSX (b), Ag18.67 Li72.84 Na4.48 -LSX (c), Ag32.54 Li71.6 Na2.53 -LSX (d), Ag85.62 Li8.77 Na1.61 -LSX (e), and Ag87.08 Li7.94 Na0.98 -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 Agx Li96-x -LSX samples determined from N2 and O2 adsorption 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 /O2 shows dependence on silver exchange degrees in the LSX zeolite (as listed in Table II) in the form of sharp decrease at higher silver contents During the N2 and O2 isotherm measurements at 273 and 298 K, the Agx Li96-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 Ag3.87 Li88.82 Na3.31 -LSX, the N2 adsorption as well as N2 /O2 separation factor (5.41 and 5.78) were increased as compared to the parent Li95.95 Na0.05 -LSX (Figs 4A(a) and 4B(a)) (5.08 and 5.39) at 273 and 298K, which reveals that small amount of Ag+ present at SIII site is responsible for N2 adsorption This means that Ag+ is the main cause of enhancement of N2 adsorption However, as the percentage of Ag+ was increased, the water content, N2 adsorption and N2 /O2 separation factor were decreased, which confirms that Ag+ neither has strong affinity for water adsorption, nor is accessible to the N2 at 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 N2 adsorption 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 125115-9 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) 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 Agx Li96-x -LSX decreases sharply with an increase in the size of cations that agrees well with the literature.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, N2 adsorbs more strongly on Lix Na96-x -LSX, as the distance between the nucleus of Li+ and the center of N2 molecule is shortest as compared to Ag+ cations However, the strong bonds between N2 and 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 N2 adsorption and confirmed in the literature.10,31 On the basis of these observations, it is therefore inferred here that near fully exchanged Agx Li96 -x -LSX zeolites as compared to their Lix Na96-x -LSX zeolite analogs are not favorable for use in adsorption-based separations, not only because of the low N2 adsorption capacity and N2 /O2 selectivity, but also have strong affinity for N2 at 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.14,26 In addition, the dehydrated (at 573 K) Agx Li96 -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 capacity is the location of cations in the skeleton of LSX zeolite and as reported in the literature,14,32 Li+ cations in fully Li-exchanged LSX are present in the six-ring (SI, SI ), sodalite cages (SII), and in the supercages at SIII site Among which, cations at SI, SI and SII sites not interact with atmospheric gases because of the short distance from the closest framework oxygen Similarly, silver cations in the SI, SI and SII locations have weak effect on the adsorptive properties However, it is suggested that silver ions present at SIII sites in Ag3.87 Li88.82 Na3.31 -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 SIII sites,2 therefore, in mixed Lix Na96-x -LSX, only those Li+ ions located at SIII sites were responsible for interaction with N2 molecules Number of Li+ cations partially exchanged in LSX and Agx Li96-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 Table III, Li+ ions strongly prefer SI and SI sites, 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 Agx Li96-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, SI and SII sites in the start of Ag+ exchange and found in close agreement with the literature.12 As can be seen from the data listed in Table III that 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 Ag3.87 Li88.82 Na3.31 -LSX is the highest one (30.02 and 19.95 cm3 /g) at 273 and 298K However, as the Ag+ exchange was increased, Ag+ 125115-10 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) TABLE III Amount of Li+ (%) present at SIII site in Li95.95 Na0.05 -LSX and Agx Li96-x -LSX zeolites Sample Li95.95 , Na0.05 -LSX Ag3.87 , Li88.82 , Na3.31 -LSX Ag18.67 , Li72.84 , Na4.48 -LSX Ag32.54 , Li71.6 , Na2.53 -LSX Ag85.62 , Li8.77 , Na1.61 -LSX Ag87.08 , Li7.94 , Na0.98 -LSX Exchange Li+ % Li+ at SIII Exchange Ag+ % 99.95 92.52 75.88 63.45 9.13 8.27 31.95 24.82 8.84 0.00 0.00 0.00 4.03 19.44 33.91 89.19 91.00 ions replaced SI, SI and SII Li+ ions, so larger content of Ag+ was shifted to these inaccessible sites rather than SIII This strongly affected the N2 adsorption capacity, which is quite obvious in the highly exchanged sample (Ag87.08 Li7.94 Na0.98 -LSX) N2 interaction with Ag-clusters at SIII sites in bi-metallic Agx Li96-x -LSX would be most favorable since these sites have been shown to be not veryinteractive when occupied with Li+ ions The same trend of exchange at SIII site is carried out up to the second (Ag18.67 Li72.84 Na4.48 -LSX) sample and from 3rd to 5th samples there are almost no Li+ cations left in the SIII site, therefore the exchange is continued to SI, SI and SII cations sites as shown in Table III Another reason of decrease in adsorption capacity of Ag87.08 Li7.94 Na0.98 -LSX zeolite after complete occupation of SIII site by Ag+ is the low chemisorbed water content of Ag+ as compared to Li+ cation These observations are found in close agreement with the literature.34 In addition, bi-metallic silver ion exchanged LSX with low silver loading has a high nitrogen adsorption capacity that was enhanced with the presence of coordinatively unsaturated silver cations in SII sites In this location silver is more easily accessible to the sorbate molecules as compared to those in SII sites near the single six-ring whereas, their presence in SII would be affected by the dehydration temperature.26,29 However, bi-metallic silver LSX with high content of silver has a low nitrogen adsorption capacity due to lack of easily accessible supercage cation in SII’ and SIII sites Therefore these samples showed reduction in nitrogen adsorption even after dehydration F Evaluation of kinetic parameters from thermal dehydration It is interesting to elucidate the mechanism of water desorption, kinetic parameters and thermal stability of the LSX zeolites during thermal treatment Three peaks I, II, and -III of thermal dehydration for Li95.95 Na0.05 -LSX and Agx Li96-x -LSX samples are shown in Fig and Fig SI of the supplementary material The kinetic triplet for the thermal dehydration of water were evaluated first by employing Kissinger and Flynn-Wall-Ozawa, and then by Coats and Redfern methods The comparative study of non-isothermal kinetics of dehydration for Agx Li96-x -LSX zeolites was carried out on the basis of related kinetic models and mechanisms, and results obtained are listed in Table IV As can be seen in Table IV, the E value of peak-I at low temperature (300-500 K) calculated by the three methods was found to be lower than that of both peak-II and -III at high temperatures (500-700 K) in Ag3.87 Li88.82 Na3.31 -LSX The same trend is observed in all of the rest of samples This result suggests the diffusion-based control dehydration mechanism at low temperature (300-500 K) and kinetic-based control dehydration mechanism at high temperatures (500-700 K), and found in close agreement with our previous results.17 It is not difficult to find that the calculated E values obtained from these three methods are more reliable, because these values are practically identical Meanwhile, the E values for Li95.95 Na0.05 -LSX calculated by the Kissinger, Flynn-Wall-Ozawa, Coats and Redfern methods are lower than that obtained for Ag3.87 Li88.82 Na3.31 -LSX, suggested the presence of strong interaction between Ag+ cations and water molecules, whereas the reverse trend is observed up to Ag18.67 Li72.84 Na4.48 -LSX as the number of Ag+ cations increases, but then again a regular increase to last sample is observed Though the total content of water was decreased with the increasing number of Ag+ cations, but the energy enhancement confirmed that the last amount of trace water (crystalline water) present at SIII site was difficult to remove These phenomena support the results obtained from decreasing N2 adsorption capacity with increasing content of Ag+ exchange, depending on the charge density, cation size, potential energy and water content 125115-11 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) TABLE IV Kinetic parameters (including E, A, K and R2 ) of Li95.95 , Na0.05 -LSX and Agx Li96-x -LSX zeolites obtained at three different heating rates using three kinetic methods β Kissinger method Ozawa method A K Coats Redfern method 10K/min T p K E KJ/mol A K E KJ/mol E KJ/mol A K Li95.95 , Na0.05 -LSX Peak-I 406.08 74.86 Peak-II 506.8 121.23 Peak-III 616.07 143.50 2.4x10+5 1.2x10+8 3.5x10+7 2.33x10+5 1.16x10+8 3.38x10+7 82.82 129.62 153.75 2.88x10+11 2.81x10+11 2.26x10+14 2.19x10+14 9.77x10+13 9.48x10+13 85.46 126.22 146.70 Ag3.87 , Li88.82 , Na3.31 -LSX Peak-I 406.5 82.06 3.5x10+11 3.41x10+11 Peak-II 502.58 122.02 1.9x10+8 1.86x10+8 Peak-III 620.9 161.82 1.1x10+9 1.02x10+9 83.03 130.33 172.09 3.5x10+11 3.40x10+11 3.5x10+14 3.43x10+14 3.0x10+15 2.87x10+15 86.9 136 157 5.8x10+5 3.5x10+8 1.1x10+7 5.66x10+5 3.35x10+8 1.04x10+7 Ag18.67 , Li72.84 , Na4.48 -LSX Peak-I 404.63 66.30 2.24x10+4 2.19x10+4 Peak-II 498.02 120.02 1.58x10+8 1.53x10+8 Peak-III 616.85 158.14 5.98x10+8 5.79x10+8 72.98 128.25 168.39 2.66x10+10 2.60x10+10 2.86x10+14 2.77x10+14 1.68x10+15 1.63x10+15 85 128 188 4.8x10+5 6.6x10+7 3.7x10+9 4.65x10+5 6.38x10+7 3.55x10+9 Ag32.54 , Li71.6 , Na2.53 -LSX Peak-I 410.03 54.24 4.74x10+2 4.66x10+2 Peak-II 584.23 119.60 1.06x10+6 1.03x10+6 Peak-III 632.97 129.02 1.03x10+6 1.01x10+6 61.02 129.37 140.00 5.8x10+8 5.71x10+8 2.7x10+12 2.62x10+12 3.0x10+12 2.96x10+12 61 119 138 2.1x10+2 1.3x10+4 9.6x10+4 2.04x10+2 1.32x10+4 9.35x10+4 Ag85.62 , Li8.77 , Na1.61 -LSX Peak-I 398.25 52.25 4.1x10+2 Peak-II 561.5 133.61 7.9x10+7 Peak-III 646.89 152.04 4.9x10+7 4.01x10+2 7.66x10+7 4.75x10+7 58.87 142.96 162.70 4.8x10+8 4.70x10+8 1.83x10+14 1.77x10+14 1.49x10+14 1.44x10+14 56 126 159 8.7x10+1 4.3x10+5 1.3x10+7 8.60x10+1 4.18x10+5 1.24x10+7 Ag87.08 , Li7.94 , Na0.98 -LSX Peak-I 383.52 63.77 3.6x10+4 3.51x10+4 Peak-II 535.79 200.52 1.1x10+15 1.02x10+15 Peak-III 640.90 251 49 8.8x10+15 8.38x10+15 71.59 209.44 262.07 6.3x10+10 6.11x10+10 2.3x10+21 2.18x10+21 2.6x10+22 2.50x10+22 69 200 262 2.2x10+2 2.18x10+2 3.9x10+12 3.74x10+12 4.0x10+15 3.80x10+15 2.87x10+5 2.8 x10+5 2.54x10+7 2.54x10+7 1.61x10+6 1.61x10+6 Moreover, the A values obtained for dehydration reactions may vary from 101 -1015 s-1 in all samples with the exception of the only last Ag87.08 , Li7.94 , Na0.98 -LSX zeolite at peak-II and -III are 1021 and 1022 s-1 Cordes reported,35 that the low factors may indicate a “tight”’ complex, whereas the high factors will usually indicate a “loose” complex because the reactions not depend on surface area On the basis of the aforementioned facts, the thermal dehydration reactions in Agx Li96-x -LSX zeolites may be interpreted as loose complexes that are related to the loss of crystalline water In addition, a linear dependence exists between A and E in a reaction known as kinetic compensation effect, which results from occurrences of reactions acting on active centers of different activation energies.36 In the present study, the apparent kinetic parameters depend strongly on the temperature and particularly on the temperature for a given conversion degree Therefore, the results listed in Table IV indicate that the apparent kinetic parameters confirm the compensation effect The kinetic parameters of dehydration process such as models and decomposition mechanisms of Li95.95 , Na0.05 -LSX and Agx Li96-x -LSX zeolites presented in Table SI of the supplementary material, were evaluated according to the model fitting method (Coats and Redfern) and by substituting the algebraic expressions of forty-one mechanism functions g(α) using TG and DTG curves in the decomposition range 0.01 < α < 0.9 The best fitted model and mechanism for each sample is given in Table V As can be seen in Table V for the first peak (physisorbed water), the R2 determined from mechanism function no 17 was better suited to Ag3.87 Li88.82 Na3.31 -LSX and Ag18.67 Li72.84 Na4.48 LSX Thus, it can be stated that the mechanism function belongs to the mechanism of Nucleation and growth (N and G) and is the most probable describing function for the dehydration step Concerning the second peak of decomposition (chemisorbed water), the R2 was obtained again with mechanism 125115-12 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) TABLE V The most probable mechanism function g(α), kinetics models and the related decomposition mechanisms of Li95.95 , Na0.05 -LSX and different samples of Agx Li96-x -LSX Sample Li95.95 , Na0.05 -LSX Ag3.87 , Li88.82 , Na3.31 -LSX Peak No g(α) Peak-I 17 [− ln(1 − Peak-II 17 [− ln(1 − Peak-III 26 α2 Peak-I 17 17 [− ln(1 − 26 α2 Peak-II Peak-III α)] α)] Kinetics model Decomposition mechanism Avrami-Erofeev N and G Avrami-Erofeev N and G Maple Power law N and G [− ln(1 − α)] Avrami-Erofeev N and G α)] Avrami-Erofeev N and G Maple Power law N and G Avrami-Erofeev N and G Avrami-Erofeev Diffusion 1D N and G N and G Mampel (first order) N and G Avrami-Erofeev N and G N and G 3 α)] α)] Peak-I 17 [− ln(1 − Ag18.67 , Li72.84 , Na4.48 -LSX Peak-II Peak-III 17 1/27 [− ln(1 − α2 Peak-I 16 − ln(1 − α) Ag32.54 , Li71.6 , Na2.53 -LSX Peak-II 28 − (1 − α) Peak-III 28 − (1 − α) Avrami-Erofeev Mampel (first order) N and G Maple Power law Mampel (first order) N and G N and G (n=1) Ag85.62 , Li8.77 , Na1.61 -LSX Ag87.08 , Li7.94 , Na0.98 -LSX 1 Peak-I 16 − ln(1 − α) Peak-II Peak-III 26 16 α2 − ln(1 − α) Peak-I (1 + α) − Peak-II Peak-III 17 Diffusion 3D Anti-Jander equation − (1 − α) Diffusion 3D Jander equation α)] Avrami-Erofeev N and G [− ln(1 − One dimensional diffusion= 1D; Three dimensional diffusion=3D; Nucleation and growth =N and G function no 17, which corresponds to nucleation and growth mechanism (N and G) and it was found that peaks -I and -II belong to Avrami-Erofeev model While peak-III (crystalline water) of Li95.95 , Na0.05 -LSX and Ag3.87 Li88.82 Na3.31 -LSX correspond to the Mample power law model that was related to nucleation and growth mechanism and that of Ag18.67 Li72.8 Na4.48 -LSX correspond to one dimensional diffusion model related to nucleation and growth mechanism In case of Ag32.54 Li71.6 Na2.53 -LSX and Ag85.62 Li8.77 Na1.61 -LSX, the first peak of thermal decomposition (physisorbed water) is characterized by mechanism function no 16, which was assumed to have integral form g(α)= − ln(1 − α) belongs to the Mampel (first order) model related to nucleation and growth mechanism, whereas the second and third peaks of Ag32.54 Li71.6 Na2.53 -LSX are characterized by function no 28 correspond to Avrami-Erofeev model, which is related to nucleation and growth mechanism The second and third peaks of Ag85.62 Li8.77 Na1.61 -LSX are characterized by function no 26 and 16 belong to Mampel Power law and Mampel (first order) model Both the models are related to the same nucleation and growth mechanism The first and second peaks of Ag87.08 Li7.94 Na0.98 -LSX belong to three dimensional diffusion model related to Anti-Jander equation and Jander equation mechanisms characterized by function no and 6, while third peak corresponds to Avrami-Erofeev model that belongs to nucleation and growth mechanism characterized by function no 17 This was considered to be the most probable reaction mechanism for the description of Agx Li96-x -LSX based on better R2 Therefore integral method of Coats and Redfern has been widely and successfully used for studying the kinetics of dehydration and decomposition of solid substances.24 IV CONCLUSIONS The following conclusions are made from the above observations and results: 125115-13 Panezai, Sun, and Jin AIP Advances 6, 125115 (2016) (i) Impact of increasing silver exchange on nitrogen and oxygen adsorption of bi-metallic Agx Li96-x LSX zeolites is very pronounced and their nitrogen adsorption selectivity decreases with increasing silver content due to low interaction of nitrogen with Ag+ ions located at SI, SI and SII sites This decrease is exponential when the Ag+ exchange degree gets up to 91 % However oxygen adsorption is marginally affected (ii) The increasing amount of silver clusters causes a decrease in BET surface area, pore volume, and crystallinty, whereas lattice parameters were increased The pore size distribution of intercrystallites obtained was in the range 6.8-5.35 nm (iii) Heating of Agx Li96-x -LSX in the presence of water renders the adsorption sites more favorable toward nitrogen adsorption as compared to oxygen adsorption due to its large quadrupole moment (iv) The kinetics of thermal dehydration of Li95.95 Na0.05 -LSX and Agx Li96-x -LSX showed the strong dependence on the choice of model and mechanism function of the process Increasing E value with increased Ag+ content confirmed that Agx Li96-x -LSX zeolites are thermally and comparatively more stable than their parent Li95.95 Na0.05 -LSX (v) Presorbed water, activation procedure as well as percent of silver exchange revealed substantial effect on N2 and O2 adsorption However, lower Ag+ content has the advantages of high N2 adsorption capacity and selectivity in air separation, as compared to higher Ag+ content in Agx Li96-x -LSX zeolites SUPPLEMENTARY MATERIAL See supplementary material for FIG S1 TG-DTG profiles of Li95.95 Na0.05 -LSX (a), Ag18.67 Li72.84 Na4.48 -LSX (b), Ag32.54 Li71.6 Na2.53 -LSX (c), and Ag85.62 Li8.77 Na1.61 -LSX (d) zeolites measured under N2 flow at three heating rates (5, 10, and 15 K) Table SI Summaries of activation energies (E) and related mechanisms of parent Li95.95 ,Na0.05 -LSX and Agx Li96-x -LSX samples obtained by using Coats-Redfern method at a heating rate (10 K/min) ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (21276005 and 21576005), and the Beijing Municipal Natural Science Foundation (2152005) H D Jeong, D S Kim, K I Kim, and I K Song, Solid State Phenomena 119, 143 (2007) E Keller and R L 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