Indian Journal of Chemistry Vol. 51A, Sept-Oct 2012, pp. 1238-1251

Adsorption selectivity of CO2 over N2 by cation exchanged zeolite L: Experimental and simulation studies

Ganga P Dangi, Munusamy K, Rajesh S Somani* & Hari C Bajaj* Discipline of Inorganic Materials and Catalysis,

CSIR-Central Salt & Marine Chemicals Research Institute, GB Marg, Bhavnagar 364 021, Gujarat, India Email: rssomani@csmcri.org (RSS)/ hcbajaj@csmcri.org (HCB)

Received 25 May 2012; revised and accepted 18 June 2012

CO2 and N2 adsorptions of alkali and alkaline earth metal cation exchanged zeolite L have been investigated by volumetric measurements and Grand Canonical Monte Carlo simulation. The zeolite KL shows lower degree of exchange because of its unique open-channel framework, linked cancrinite and intercage sites. Structural characteristics have been evaluated using X-ray diffraction analysis and surface area measurements. CO2 and N2 adsorption isotherms have been obtained for zeolite KL and its cation exchanged form up to 101.3 kPa at 293 and 303 K and the corresponding heats of adsorption estimated by the Clausius-Clapeyron equation. The experimental results are compared with those obtained from GCMC simulation. The texture of the materials and their selectivity for CO2 over N2 adsorption varies with the nature of the exchanged cations. Zeolite-CaL shows remarkably high CO2 selectivity (31 times) over N2 amongst the studied alkali and alkaline cation exchanged zeolite L. It has been shown that zeolite L can be used as a potential adsorbent for the removal of CO2 from industrial effluent gases.

Keywords: Zeolites, Alkali metals, Alkaline earth metals, Ion exchange, Gas adsorption, Carbon dioxide adsorption, Nitrogen adsorption, Grand Canonical Monte Carlo simulation

The structure of zeolites consists of SiO4 and AlO4 tetrahedra arranged in such a way that each oxygen atom is shared between two tetrahedra. As aluminium has one less positive charge than silicon, the framework has a net negative charge of one at the site of each aluminium atom balanced by the exchangeable cation. The presence of cations induces high electric field gradients within the cavities, and the framework itself can possess acidic or basic character. Thus, in addition to steric, kinetic and equilibrium effects, the selectivity of zeolites for a particular adsorbate will also depend on the magnetic susceptibility, polarizability, permanent dipole moment and quadrupole moment of the adsorbate molecules.1

The extent and strength of adsorption of molecules in zeolitic pores can be dominated by interactions of the adsorbate with the electric-field induced by the cations, but the acid–base properties of the zeolite framework can also play an important role in determining adsorption properties in certain pressure regions. The exchangeable cation is an acid site and the framework oxygen nearest to the cation provides a basic site. The basicity increases with an increase in framework aluminum content and also increases as

the cation electronegativity decreases.2 The strength of these zeolite acid–base pairs can be easily changed by exchanging the cations3 or adjusting the aluminum content in the framework.4 Cations are the most electropositive atoms in the zeolite and form acid–base pair sites with the rings they are associated with. These cations are not typically located at the rings in a perfectly symmetrical fashion, and thus, dipoles can be created in the cavity of walls that are, in addition to the electric field, created solely by the cations and the field between charges across the pores. Clearly, there are complex factors at play between cation type, acidity/basicity and electrostatic interactions within zeolites that can differ for various types of adsorbate.

Carbon dioxide, a significant greenhouse gas, has implication in global warming. Consequently, the development of new methods for capturing CO2 is an increasingly important research area since the International Energy Agency claimed the need for more energy efficient and less costly CO2 capture technologies. At present, the most widely used technology for capturing CO2 from flue gas is absorption using amine solvents.5 The major drawback of the amine based processes is its high energy demands to recover the amines for recycling,

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together with the adverse health and environmental effects caused by the loss of volatile amines. Developing new materials for CO2 capture and separation is critically important. Zeolites are amongst the most widely reported adsorbents for CO2 capture in the published literature.6-15 Zeolites constitute the primary adsorption material for commercial hydrogen production (involving H2/CO2 separation) using pressure swing adsorption, with the most popular of these based on Zeolite 13X.16 Some other materials like aluminophosphates,17,18 carbon based materials,19,20 mesoporous silica based materials,21 amine functionalized materials and metal organic frameworks (MOFs) are also studied for capture of carbon dioxide.6,22,23

Recently, attention has turned to experimental and computational screening studies to assess CO2

removal from low pressure flue gas using zeolites, such as X, Y, 4A, 5A and all-silica zeolites like MFI, MOR, ISV, ITE, CHA, DDR.24-27 Currently, molecular simulations are playing an important role in developing the understanding of the relationship between microscopic and macroscopic properties of confined molecular fluids in zeolites. Garcia-Sanchez et al.

25 developed a new force field which is applicable to reproduce CO2 adsorption in zeolites and transferable to all zeolite structures with different Si/Al ratio. Akten et al.

26 reported the single and binary component adsorption of CO2, N2, and H2 in zeolite Na, usually known as molecular sieve 4A by experiment and GCMC simulation. Garcia-Perez et al.

27 studied the adsorption properties of CO2, N2, and CH4 molecules in MFI and other all-silica and zeolites

using molecular simulation. Plant et al.28 studied

interaction between CO2 and the Li+, Na+, K+ and Cs+

cations in NaX and NaY zeolites by DFT method and GCMC simulation. Sorption behavior of N2, O2, and Ar with Ca2+ locations in zeolite-A was investigated by Pillai et al.

29 However, selective CO2 capture particularly at ambient temperature and atmospheric pressure still remains a major challenge as some materials show high selectivity but low adsorption capacity while others show high adsorption capacity but very low selectivity.

Cation effects on CO2 adsorption in zeolite X, Y, beta and Chabazite have been widely studied.30-33 Effects of various alkali metal cations on CO2 adsorption in X, Y and Chabazite zeolites were studied and it was concluded that Li+ cation exchanged zeolites have the largest CO2 capacities as

a result of greatest ion–quadrupole interactions with CO2, although no CO2/N2 selectivity was reported.31 The effects of different alkaline earth metal cations on CO2 adsorption in zeolite beta and Chabazite were also studied and it was reported that Mg2+ cation exchanged zeolite beta shows the highest CO2

adsorption capacity, while in the case of Chabazite, the Ca2+ exchanged form shows the maximum CO2

adsorption capacity.32,33 The present study reports the behavior of alkali and

alkaline earth cation exchanged form of zeolite L for separation of CO2 and N2 gases at 293 and 303 K. An attempt has also been made to correlate the experimental data with the simulated adsorption data, using Grand Canonical Monte Carlo (GCMC) simulation technique. Materials and Methods

Zeolite KL powder (Zeochem) with chemical composition K9Al9Si27O72·20H2O and analytical grade chloride salts of alkali and alkaline earth cations (Central Drug House (P) Ltd, New Delhi, India) were used as starting materials for preparation of the adsorbents. Nitrogen (99.999 %), Carbon dioxide (99.99 %) and helium (99.999 %) were used for the adsorption isotherm measurements. Cation exchange

The ion exchange was carried out by four successive contacts of as-obtained zeolite KL with the monovalent lithium, sodium and cesium cations and divalent magnesium, calcium, strontium and barium cations as described by Walton et al.31 In each cycle, fresh salt solution (1 M) was used keeping solid to liquid ratio of 0.1 at 343 K for 4 h. The cation-exchanged zeolite was filtered and washed with distilled water, until the washings were free from ions and dried overnight in an air-oven at 393 K. The degree of cation exchange in the ion-exchanged samples was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis (Perkin-Elmer Instruments, Optima 2000DV). The various cation exchanged forms of zeolite L are denoted as LiL, NaL, KL, CsL, MgL, CaL, SrL, and BaL, for the alkali and alkaline earth exchanged cations. X-ray powder diffraction

The X-ray powder diffraction (XRD) patterns of the zeolite L based adsorbents were recorded at ambient temperature using a Philips X’pert MPD

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system in the 2θ range of 5-50° using Cu-Kα1 radiation (λ = 1.54056 Å). Adsorption-desorption measurement

CO2 and N2 adsorption isotherms were measured using ASAP-2020 (Micromeritics Inc., USA) at 303 and 293 K. Before measuring each isotherm, the sample was evacuated (1 × 10-3 mm Hg) at 623 K for 10 h to remove any adsorbed gas molecules. The temperature was maintained during sorption measurements by circulating water from a constant temperature bath (Julabo F25, Germany). Requisite amount of the adsorbate gas was injected into the volumetric setup at volumes required to achieve a targeted set of pressures ranging from 0.1 − 850 mm Hg. A minimum equilibrium interval of 5 s, with a relative target tolerance of 5 % of the targeted pressure and an absolute target tolerance of 5 mm Hg were used to determine equilibrium sorption for each measurement point.

The pure-component capacity selectivity of gas X over gas Y can be calculated using Eq. (1), SX/Y = (VX / VY)P, T … (1) where VX and VY are the volume of gases X and Y, respectively, adsorbed at pressure P and temperature T.

Isosteric heats of adsorption were calculated from the adsorption data collected at 293 and 303 K using the Clausius-Clapeyron equation (Eq. 2),

ads

θ

lnR

(1/ )

PH

T

∂−∆ ° =

∂ … (2)

where R is the universal gas constant and P is the equilibrium pressure at the particular coverage θ and temperature T. Surface area measurements

The BET surface area measurements of the cation-exchanged zeolite L was carried out using ASAP 2020 (Micromeritics Inc., USA), a volumetric static adsorption instrument, by nitrogen adsorption at 77 K. Prior to the nitrogen adsorption, the samples were activated for 10 h at 623 K under vacuum (1 × 10-3 mm Hg). Computational methodology

The Cerius2 suite of software (Accelrys Software Inc., USA), utilizing grand canonical ensemble (fixed potential) Monte Carlo simulation method, was used

for computation of adsorption data. The “crystal builder” module was used to construct the crystal structure of the zeolites and the “sorption” module was used for the gas adsorption simulation studies.34 The simulations were performed on a Silicon Graphics Fuel workstation running on an IRIX v.6.5 platform.

Construction of zeolite models

The crystal structure of the zeolite L was built according to the crystallographic data of Barrer and Villager.35 The zeolite L structure is hexagonal with unit-cell dimensions: a = 18.4 Å and c = 7.5 Å with the space group P6/mmm. The framework was built with a strictly ordered alternation of aluminum and silicon atoms in accordance with the Lowenstein’s Al-O-Al avoidance rule. The chemical composition of zeolite L unit cell is M(9/n)Al9Si27O72, where n is the valency of the cation and M = K+, Li+, Na+, Cs+, Mg2+, Ca2+, Sr2+ and Ba2+. The second step consisted of modeling the distribution of the extra framework cations among the different crystallographic sites.

Each zeolite of the aluminosilicate family has unique pore structure and offers a wide variety of substrate for cation exchange. Most of these exchangers have structures made up from two- or three-dimensional assemblages of interlinked channels and cavities (e.g. mordenite and the synthetic faujasite). The aluminosilicate framework of Zeolite L (Fig. 1) consists of polyhedral cages composed of 5 six-membered and 6 four-membered oxygen rings linked together. The cages are joined through the planes of their upper and lower

Fig. 1The framework structure of zeolite L. A, B, C, D′ and D′′

are extra-framework cation sites in zeolite L. Note that site D′ is surrounded by three Al atoms in the framework, while site D′′ has only one neighbor Al atom. Both D′ and D′′ are supposed to take part in the ion exchange process.

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6-membered rings to produce columns, which, in turn, are linked by single oxygen bridges to three similar columns. The bridging creates a series of 12-membered oxygen rings delineating a series of "one-dimensional" channels aligned parallel to each other through the zeolite framework.36

Allocation of cation sites in the original structural analysis was done as suggested by Barrer and co-workers35,37 having five possible environments for cations and/or water molecules. Site A is located in the centre of the hexagonal prism with six-fold coordination to framework oxygen atoms at 3 Å; there are 12 more oxygen atoms at 3.65 Å. Site B is in the center of the cancrinite cage with a coordination shell of 12 oxygen atoms. Site C is located midway between two adjacent cancrinite cages and also have twelve fold coordination to framework oxygen atoms. Site D is near the wall of the main channel and cations in this site are coordinated to two water molecules and six framework oxygen atoms. On dehydration, small ions located on site D may withdraw to a new site D” located between adjacent hexagonal prisms.38 Among these sites, sites A and B are blocked and only sites C and D are cation exchangeable sites.35 In the case of Cs+ exchanged zeolite L where high percentage of Cs+ exchange was observed, there is a possibility of one K+ exchange at site A also. The percentage cation exchange was obtained from the ICP analysis and the required number of cations per unit cell was placed at different crystallographic sites (site A-D) according to the cationic positions given in the literature.35, 38-40 The positions of cations at different sites in zeolite L are given in the Table 1.

Simulation studies

The total energy of the zeolite framework and adsorbed molecules (U) is expressed as the sum of the interactions energy between the adsorbate and zeolite (UAZ) and that between the adsorbate (UAA) molecules.

Both UAZ and UAA are written as the sums of pair wise additive potentials, Uij in the form Eqs (3) and (4),

AAU

AZUU +=

… (3)

+

−

=

ijr

jq

iq

ijr

ijσ

ijr

ijσ

ijε

ijU

612

4

… (4)

where the first term is the repulsion-dispersion Lennard-Jones (LJ) potential with εij, σij corresponding to the parameter sets for species “i” and “j” and the second term represents the columbic interaction potential between point charges qi and qj of species “i” and “j” separated by a distance rij. Cross-terms for the Lennard-Jones potentials were calculated using Lorentz- Berthelot mixing rules.

For N2, we used the three-point charge model,41 where the two outer sites are separated by a distance of 1.098 Å have a charge of q = −0.4048e, and the third midpoint has a point charge of q = +0.8096e. The σN and εN LJ parameters were given as 3.318 Å and 0.0724 kcal mol-1 respectively. For CO2, we used an atomic point charge model with C-O bond length as 1.149 Å, in which carbon has a charge of q = +0.72e and oxygen a charge of q = −0.36e. Charges and LJ potential parameters of CO2 were taken from Ref. 42 and are given in Table 2. Another significant factor in

Table 1Cation locations in cation exchanged zeolite La

Sample

Site A Site B Site C Site D

K+ M K+ M K+ M K+ M

KL 1 - 2 - 3 - 3 -

LiL 1 - 2 - 3 - - 3

NaL 1 - 2 - 3 - - 3

CsL 1 1 2 - - 1 1 3

MgL 2 - 2 - 1 - 1 1

CaL 2 - 2 - 1 - 2 1

SrL 1 - 2 - 2 - - 2

BaL 1 - 2 - 2 - - 2 aData taken from Refs 35, 38-40.

Table 2Lennard-Jones potential parameters used for adsorbate-adsorbate and adsorbate-zeolite interactionsa

Interacting pairs

σ (Å) ε (kcal mol-1) Interacting pairs

σ (Å) ε (kcal mol-1)

N-N 3.318 0.0724 K-Oc 2.824 0.1166 Oz-N 3.179 0.1555 Li-Cc 2.464 0.2583 K-N 2.803 0.0805 Li-Oc 2.229 0.3307 Li-N 2.458 0.2283 Na-Cc 2.788 0.0967 Na-N 2.532 0.0851 Na-Oc 2.553 0.1233 Cs-N 3.021 0.0319 Cs-Cc 3.277 0.0360 Mg-N 3.170 0.0896 Cs-Oc 3.042 0.0462 Ca-N 3.359 0.1312 Mg-Cc 3.426 0.1014 Sr-N 3.480 0.1304 Mg-Oc 3.190 0.1299 Ba-N 3.511 0.1623 Ca-Cc 3.615 0.1485 Cc-Cc 3.830 0.0927 Ca-Oc 3.380 0.1902 Oc-Oc 3.360 0.1519 Sr-Cc 3.736 0.1476 Cc-Oc 3.310 0.0364 Sr-Oc 3.500 0.1889 Oz-Cc 3.90 0.0837 Ba-Cc 3.767 0.1837 Oz-Oc 3.480 0.1386 Ba-Oc 3.532 0.2352 K-Cc 3.059 0.0910

aData taken from Refs 41-44.

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adsorption simulation is the assignment of charges in zeolite. The zeolite L system is assumed to be semi-ionic with atoms carrying the following partial charges (in electron units): Si (+2.4), Al (+1.7), Oz (−1.2), K+, Li+, Na+, Cs+ (+0.7) and Mg2+, Ca2+, Sr2+, Ba2+ (+1.4). The polarizability of silicon and aluminum atoms which are much lower than those of the oxygen atoms suggests that the repulsive dispersion contribution of the zeolite can be assigned only to the oxygens of the framework (Oz) and extra-framework cations. For K+, Li+ and Na+, the LJ interaction potentials were taken from Ref. 43 and for Cs+ we calculated the σ and ε from the equations given in Ref. 43, using the ionic radii of Cs+ as 1.81 Å and polarizability as 2.56 Å. For alkaline earth metal cations, the LJ parameters44 were taken from the universal force-field of Cerius2. The force field LJ parameters used for the simulation of adsorption of N2 and CO2 in zeolite L are summarized in Table 2.

Absolute adsorption isotherms were then computed using GCMC calculation algorithm, as implemented in the “sorption” module of Cerius2 software suite, where the chemical potential, the temperature and the volume were kept fixed. The simulations were performed at 293 and 303 K using 16 unit cells (2×2×4) of each zeolite for 1×107 Monte Carlo steps. The algorithm involved four types of trial moves: attempts to translate a molecule, rotate a molecule, create a new molecule, and, delete an existing molecule. The zeolite framework was assumed to be rigid during the sorption process and the extra framework cations were maintained fixed in their original positions. The Lennard-Jones potential for the adsorbate-zeolite interactions and both the Lennard-Jones and columbic terms of the adsorbate-adsorbate interactions were calculated using the minimum image convention with a real space potential cut-off distance of 12.0 Å. The columbic term for the

adsorbate-zeolite interactions was evaluated using the Ewald summation method.

Results and Discussion

The percentage crystallinity of the cation exchanged zeolite L samples is reported in Table 3, along with the unit cell composition and the surface area data. The framework structure of zeolite L was retained after repeated alkali and alkaline earth metal cation exchange processes as seen from the retention of all major characteristic peaks of zeolite L in the XRD patterns (Fig. 2). The percentage crystallinity of the cation-exchanged zeolite L samples was determined from the XRD pattern by a summation of the intensities of seven major characteristic peaks at 2θ values of 5.5°, 19.2°, 22.5°, 25.5°, 27.9°, 29.0° and 30.6° (Fig. 2). The potassium form of the zeolite L was considered to be an arbitrary standard for comparison. In each case, retention of zeolite L framework crystallinity was observed. Upon ion exchange by various alkali and alkaline earth metal cations, the intensities of the characteristic peaks for Cs+, Mg2+ and Ba2+ exchanged zeolite L were significantly reduced as compared with the KL before ion-exchange. This was ascribed to the partial blocking of pores and

Table 3Chemical compositions, percentage crystallinity and surface area of alkali and alkaline earth cation exchanged zeolite L

Adsorbent Unit cell formula (dry basis) Crystallinity (%) Hydrated ionic radii (Ǻ)

Surface area (m²/g) BET Langmuir External

KL K9Al9Si27O72 100 3.3 336 443 26 LiL Li2.97K6.03Al9Si27O72 100 3.8 384 485 44 NaL Na2.79K6.21Al9Si27O72 100 3.6 314 398 33 CsL Cs4.91K4.09Al9Si27O72 40 3.3 227 287 23 MgL Mg1.24K6.52Al9Si27O72 48 4.3 261 330 33 CaL Ca1.22K6.56Al9Si27O72 93 4.1 366 462 40 SrL Sr1.57K5.86Al9Si27O72 79 4.2 239 306 29 BaL Ba1.66K5.68Al9Si27O72 51 4.1 298 377 30

Fig. 2X-ray diffraction patterns of alkali and alkaline earth cation exchanged zeolite L.

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decreased crystallinity of the structure by the incorporation of cations.45 Surprisingly, lower degrees of ion exchange were obtained for all the cations under study for zeolite L. In the case of zeolite L, chains contain 4-, 6-, and 8-membered rings. Within the chains one can distinguish additional cages (similar but not identical to cancrinite cages) which contain occluded K+ ions within the cages. These potassium cations are trapped and cannot be removed or exchanged for other cations. This is probably the reason for relatively lower ion exchange capacities of zeolite L.

The K+ ions located on site D are the ones most likely to be exchanged. There are six such cations present at this site; this may be the reason for lower degree of exchange (less than 60 %). Relatively higher degree of

exchange for cesium can be attributed to the lowest hydrated ionic radii amongst all the alkali and alkaline cations (Table 3) studied. The crystallinity values for Cs+, Mg2+, and Ba2+ exchanged zeolite L were considerably smaller than those for other zeolite samples, probably due to higher degree of metal ion exchange which results in partial disruption of the zeolite framework. All over lower values of crystallinity were observed for alkaline earth metal zeolite L. BET surface area decreased with increase in cation size because of partial blocking of pores by larger cations making the pores unavailable for N2 adsorption.

Simulation of the adsorption of CO2 and N2 in alkali and alkaline earth metal ion exchanged zeolite-L were carried out at 293 and 303 K. Figures 3 and 4 show

Fig. 3Experimental (E), and, simulated (S) adsorption isotherms of N2 and CO2 at 293 K and 303 K. [(a) LiL; (b) NaL; (c) KL; (d) CsL].

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both experimental and simulated adsorption isotherms of CO2 and N2 in zeolite-L having alkali and alkaline earth metal extra framework cations obtained at 293 and 303 K, respectively. Presence of cation alters the electrostatic field within the zeolite cavities and hence, influences the amount of N2 and CO2 adsorbed on zeolite surface. Higher adsorption capacities for CO2 in ion-exchanged zeolite L compared to N2 are due to its high quadrupole moment23 (CO2 = −13.71 × 1040 cm2, N2 = −4.91 × 1040 cm2) and high polarizability (CO2 = 29.1 × 10-25 cm3, N2 = 17.4 × 10-25 cm3). N2 and

CO2 adsorptions show different patterns towards different groups of cations. The capacity for CO2 and N2 adsorption near 101.3 kPa on cation-exchanged zeolite L increases with decreasing cation size (Figs 3 and 4). Larger cations induce higher basicity in the zeolite framework. Thus, it was expected that CO2 would interact more strongly with Cs+ and Ba2+ exchanged zeolites rather than with the other forms because of its weak acidity. Li+ cations are the smallest of the alkali metal series and hence, while it has least basicity for all the ion-exchanged forms, the

Fig. 4Experimental (E), and, simulated (S) adsorption isotherms of N2 and CO2 at 293 K and 303 K. [(a) MgL; (b) CaL;(c)SrL; (d) BaL].

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ion–quadrupole interaction will be much stronger than the other cations due to its larger charge density.31

Interestingly, the trend for CO2 uptake did not match with the cationic radii in the case of alkaline earth cation exchanged zeolite L. Here, basicity of the incorporated cations also plays an important role. The balance between the two main factors, i.e., ionic radii which induces steric effects in the zeolite micro pores, and, the electropositive character of the metal ions that induces basicity for adsorbing acidic gases, decides the CO2 uptake capacity. Huang46 and Xie et al.47 have also shown that CO2 adsorption is greatly affected when the basicity of X and Y zeolites is weakened by cation exchange.

Table 4 shows the equilibrium adsorption capacities of CO2 and N2 at 101.3 kPa determined from adsorption isotherms, and Table 5 depicts the equilibrium adsorption selectivity for CO2 over N2 at pressure of 3.33 kPa and 101.3 kPa at 293 and 303 K. The CO2 equilibrium adsorption capacities showed the decreasing order as LiL > NaL > KL > CsL for alkali metal type zeolite L and CaL > MgL > SrL > BaL for alkaline earth cation exchanged zeolite L. However, the CO2 and N2 adsorption capacities of cation exchanged zeolite L at 293 K are higher than those at 303 K.

Selectivity of the adsorbent for CO2 and N2 were calculated by taking the ratio of equilibrium adsorption for CO2 and N2 at 3.33 kPa and 101.3 kPa. At lower pressure region (3.33 kPa), which is comparable with flue gas composition, the selectivity is quite high, up to 247. In the case of alkaline earth cation exchanged zeolite L, zeolite CaL showed highest selectivity. CaL showed high CO2 uptake (62.5 cm3/g) and minimum N2 uptake (2.0 cm3/g) at 303 K which may be due to its high surface area (462 m2/g). The equilibrium selectivity for CO2 over

N2 is 247 at 3.33 kPa and 31 at 101.3 kPa, thereby providing the highest working capacity for CO2 and N2 separation at 303 K. Thus, zeolite CaL can be used as a potential adsorbent for the separation of CO2 and N2 mixtures. However, in the case of equilibrium adsorption selectivity for CO2 over N2 at 293 K, the observed order was: CsL > MgL > CaL > KL > SrL > BaL > NaL > LiL. Overall, the selectivity of all samples observed at 101.3 kPa and 303 K were higher than those at 293 K.

It is clear from the Figs 3(a-d) and 4(e-f) that the simulated CO2 and N2 adsorption isotherms match well with the experimental data. The simulated adsorption isotherms of CO2 and N2 in KL, NaL and LiL are in good agreement with the experimental isotherms, but in the case of CsL and alkaline earth cation ion-exchanged samples, simulated isotherms show higher values than the experimental. The slightly higher values for the simulated adsorption isotherm could be due to the loss of crystallinity during cation exchange (Table 3) or/and due to unestimated parameter selection for simulation. Another factor could be the cation locations used for the simulations that may not exactly match the cation locations inside the adsorbents used for the adsorption experiment. Furthermore, the adsorbent samples used for the experiments would have minor quantities of other phases formed during the synthesis of zeolite powders, whereas we have used unit cell of the zeolite for the simulation study. Moreover, there may be strong interaction of hydrated cations with the zeolite structure that can result in some structural defects or formation of amorphous phase, due to dealumination during the cation exchange or vacuum dehydration process. This is also evidenced by the decrease in the crystallinity of samples with ion exchanged zeolite as shown in Table 3.

Table 4Equilibrium adsorption capacities of CO2 and N2 in alkali and alkaline earth cation exchanged zeolite L at 101.3 kPa

Adsorbent Equilibrium adsorption capacity (cm³/g) 293 K 303 K

CO2 N2 CO2 N2 LiL 75.3 7.6 70.7 5.0 NaL 68.4 5.9 65.1 4.5 KL 65.8 4.2 59.3 3.2 CsL 47.3 2.5 44.5 2.1 MgL 64.4 3.5 60.2 2.9 CaL 65.7 3.9 62.5 2.0 SrL 64.9 4.5 58.4 2.7 BaL 59.7 4.1 54.1 2.8

Table 5Equilibrium adsorption selectivity of alkali and alkaline earth cation exchanged zeolite L for CO2 over N2 at 293 K and 303 K

Adsorbent Equilibrium adsorption selectivity 293 K 303 K

3.33 kPa 101.3 kPa 3.33 kPa 101.3 kPa LiL 96.8 9.9 122.5 14.1 NaL 135.7 11.5 129.4 14.4 KL 146.3 15.5 140.4 18.4 CsL 247.1 18.7 240.4 21.5 MgL 115.2 18.4 178.4 20.8 CaL 144.3 16.9 247.6 31.0 SrL 149.8 14.5 173.5 21.8 BaL 119.2 14.5 129.2 19.4

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The Langmuir equation (also known as the Langmuir isotherm) relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature (Eq. 5),

θ = αP/(1 + αP) ... (5)

Where θ is the fractional coverage of the surface, P is the gas pressure or concentration, α is a constant. The constant α is the Langmuir adsorption constant and increases with an increase in the binding energy of adsorption and with a decrease in temperature.

The adsorption data obtained at 293 and 303 K were fitted in the Langmuir equation, and from that the slope, intercept, and Langmuir constant were determined. The Langmuir fitting data of zeolite L exchanged with alkali and alkaline cations at 293 and 303 K are given in Table 6. The lower values of Langmuir constants indicate micro-porosity of the zeolite L structure. Maximum value of Langmuir constant (α) for CO2 adsorption was observed for Cs+

exchanged zeolite L at both the temperatures studied. The isosteric heats of adsorption values of CO2 and

N2 in cation exchanged zeolite L were calculated from both experimental and simulation data at a coverage of 1 cm3/g for N2 and 10 cm3/g for CO2 (Table 7). The values obtained experimentally match very well with the simulated results. The isosteric heats of adsorption of CO2 are greater than those of N2 in all the alkali and alkaline earth metal zeolites which show that the interaction of CO2 with the cations and zeolite framework is stronger than that of N2. The maximum heat of adsorption was observed for LiL (52. 5 kJ mol-1) for CO2 adsorption followed by zeolite CaL (47.0 kJ mol-1),

which shows the stronger interaction of CO2

molecules with Li+ and Ca2+ ions. The minimum heat of adsorption was observed for zeolite CsL.

In a zeolite framework, SiO2 and AlO2 tetrahedra are connected by sharing oxygen atoms. Al and Si atoms are buried in the tetrahedra of oxygen atoms and are not directly exposed to sorbate molecules. Thus, the main interactions of the adsorbate molecules in a zeolite structure are through lattice oxygen atoms and extra framework cations. The cations are the preferred adsorption sites that are especially important for interacting with polar, quadrupolar and easily polarizable molecules. The molecular graphics snapshots of CO2 and N2 adsorbed

in KL, LiL and CaL zeolites at 101.3 kPa and 293 K are given in Figs 5 and 6, respectively. It is observed that CO2 molecules are adsorbed in the main channel of the zeolite. In zeolite L, the cations present at site A (at the center of the hexagonal prism), at site B (at the center of cancrinite cage) and at site C (located at midway between two adjacent cancrinite cages) are

Table 6The Langmuir constant (α) of the adsorption of CO2 and N2 fitted in the Langmuir equation at 293 and 303 K on alkali and alkaline earth cation exchanged zeolite L

Adsorbent 293 K 303 K Slope α Slope α

CO2 N2 CO2 N2 (10-3) CO2 N2 CO2 N2 (10-3)

LiL 0.012 0.027 0.016 0.331 0.013 0.030 0.014 0.234

NaL 0.014 0.023 0.019 0.202 0.014 0.026 0.014 0.173

KL 0.014 0.021 0.017 0.126 0.015 0.021 0.012 0.093

CsL 0.020 0.032 0.024 0.116 0.021 0.026 0.018 0.075

MgL 0.014 0.043 0.012 0.235 0.014 0.010 0.008 0.041

CaL 0.014 0.028 0.012 0.161 0.014 0.025 0.009 0.070

SrL 0.014 0.017 0.014 0.110 0.015 0.022 0.009 0.082

BaL 0.015 0.017 0.011 0.096 0.016 0.032 0.008 0.126

Table 7Experimental and simulated heats of adsorption of CO2

and N2 on alkali and alkaline earth metal cation exchanged zeolite L

Adsorbent Heat of adsorption (kJ mol-1) Experimental Simulation CO2 N2 CO2 N2

LiL 52.5 20.5 54.6 20.2

NaL 44.6 19.6 48.1 19.3

KL 43.4 18.9 41.3 16.6

CsL 42.7 18.4 42.1 16.5

MgL 49.0 19.9 47.5 17.9

CaL 47.0 24.3 46.4 17.6

SrL 45.6 19.2 50.6 19.4 BaL 45.8 19.7 50.5 19.5

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not accessible to the adsorbate molecules (CO2 or N2), and therefore these cations cannot interact directly with the CO2 or N2 molecules because of the larger distance between the cations and adsorbate molecules adsorbed in the main channel. The cations present at site D (near the wall of the main channel) are easily accessible to the adsorbate molecules and these are the cations which interact directly with the adsorbate molecules adsorbed in the main channel. The most alkali and alkaline earth metal cations are exchanged at site D (Table 1). Different cations at site D interact differently with the adsorbate molecules and lead to the difference in the adsorption capacity of CO2 and N2. Figure 7 shows the typical arrangements of CO2

molecules in the main channel interacting with the cations (Li+ and Ca2+) located at site D. It was observed that two to four molecules of CO2 can be adsorbed per cation located at site D.

The radial distribution functions (RDFs) between four different types of K+ ions (at sites A, B, C and D), Li+ and Ca2+ ions at site D, and CO2 and N2

molecules in zeolite L were obtained at 101.3 kPa and 293 K (Figs 8 and 9). Figure 8(a-c) presents the RDFs between K(A), K(B) and K(C) ions and oxygen atoms of CO2 molecules. It is observed that for K(A), K(B) and K(C) ions, the first peaks are located at around 5.85, 6.95 and 6.55 Å, respectively. These distances are large enough for the direct interaction of K+ ions at sites A, B and C and CO2 molecules. Figure 8(d, e, f) shows the RDFs between K+, Li+ and Ca2+ ions at site D and oxygen atoms of adsorbed CO2 molecules. The RDFs have a first major sharp peak around 2.25 Å and second peak at 4.45 Å corresponding to the direct contact between K(D), Li(D) and Ca(D) ions and CO2 molecules. This means that the CO2 molecules adsorbed in the main channel strongly interact with

Fig. 5Molecular graphics snapshots of CO2 adsorption at 101.3 kPa and 293 K shown in 1 0 0 plane for (a) KL, (b) LiL and (c) CaL, and, (d) 0 0 1 plane of KL. [yellow for Si, pink for Al, red for O, gray for C, green for K ions, cyan for Li ions and violet for Ca ions].

Fig. 6Molecular graphics snapshots of N2 adsorption at 101.3 kPa and 293 K shown in 1 0 0 plane for (a) KL, (b) LiL and (c) CaL. [yellow for Si, pink for Al, red for O, gray for C, blue for N, green for K ions, cyan for Li ions and violet for Ca ions].

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K+, Li+ and Ca2+ ion at site D and are located in the vicinity of ions at site D. In RDFs of K(D)-Oco2, Li(D)-Oco2 and Ca(D)-Oco2, the first two peaks correspond to the contacts of site D ions with the two oxygen atoms of the adsorbed CO2 molecules, that is, the CO2 molecule has one of its ends (oxygen atoms) pointing towards the site D ions. This phenomenon can be explicitly observed from the snapshots in Fig. 7. From the RDFs and the snapshots it can be stated that the confined CO2 molecules coordinate with the site D ions in a near linear geometry in zeolite L.

Similarly, Fig. 9 shows the RDFs between K+ ions at different sites and Li+ and Ca2+ ions at site D and adsorbed N2 molecules. The first peak was observed for K(A) at 6.35 Å, for K(B) at 7.5 Å, and for K(C) at

6.8 Å. This indicates that the N2 molecules are also not in direct contact with the ions at sites A, B and C. For the ions at site D, we observed very sharp peaks at lower distance. For K(D), Li(D) and Ca(D), the first sharp peak is located at 2.90, 2.2 and 2.95 Å and the second peak at 3.77, 3.35 and 3.95 Å, respectively. The first and second peaks correspond to the direct contact of D site ions with the two nitrogen atoms of N2 molecules. From the RDFs and snapshots we observed that the CO2 and N2 molecules can directly interact with the D site ions and thus are adsorbed more at site D.

Table 8 shows the comparison of equilibrium selectivity for CO2/N2 of some zeolitic materials and other porous materials at ~101.3 kPa pressure.

Fig. 7Typical arrangement of CO2 molecules in the main channel of zeolite L interacting with the cations located at site D at 293 K and 101.3 kPa. [(a) LiL and (b) CaL in 0 0 1 plane, (c) LiL, and, (d) CaL in 1 0 0 plane].

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Fig. 8The radial distribution functions (RDFs) between different K ions and Li, Ca ions at site D and oxygen atoms of adsorbed CO2

molecules at 101.3 kPa and 293 K.

Fig. 9The radial distribution functions (RDFs) between different K ions and Li, Ca ions at site D and oxygen atoms of adsorbed N2

molecules at 101.3 kPa and 293 K.

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Zeolite L provides higher CO2/N2 selectivity as compared to other materials of the same type. This indicates that zeolite CaL can be a better prospective adsorbent for the separation of CO2 and N2. Conclusions

Equilibrium adsorption isotherms of CO2 and N2 were studied on alkali and alkaline earth cation-exchanged zeolite L at 293 and 303 K. The basicity of zeolite L can be adjusted via cation-exchange and such modifications have been used to systematically tune the CO2 adsorption capacity of this material. For CO2, the isosteric heat of adsorption falls in the range of 42.7−52.5 kJ mol-1 and for N2, it lies in the range of 18.5−24.3 kJ mol-1. The CO2/N2 selectivity obtained at 101.3 kPa and 303 K is in the range of 14.1–31.0. Simulated adsorption isotherms and isosteric heats of adsorption of CO2 and N2 in alkali and alkaline earth cation exchanged zeolite L match reasonably well with the experimental results. Simulation of the CO2 and N2 adsorption in alkali and alkaline earth cation exchanged zeolite L clearly show that the adsorbed CO2 and N2 molecules are located in the main channel in proximity to the extra-framework cations at site D. The observed CO2 capacity is highest for the LiL where the ion-quadruple interaction is dominant but the highest CO2/N2 selectivity was obtained for the CaL, which provides high separation and better capacity for CO2. The higher adsorption capacities for CO2 in ion-exchanged zeolite L are due to its high quadruple moment and high polarizability compared to N2. The selectivity of CO2 over N2 depends on the basicity, ionic radii and electropositive character of the cation incorporated in the zeolite. From these data, we can

conclude that smaller cation size and larger pore volume in zeolite L may be ideal for adsorbing CO2 at near room temperatures and pressures up to 101.3 kPa for real world applications. Acknowledgement

The authors thank the Analytical Division, CSIR-CSMCRI, Bhavnagar, India, for support in characterization of the samples and Mr. Nilesh Narkhede, CSIR-CSMCRI, Bhavnagar, India, for preparing cation-exchanged zeolite L samples. CSIR, New Delhi, India, is acknowledged for funding the project (NWP-0021).

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