Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X

Article pubs.acs.org/IECR

Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X: Structure, Cation Position and Adsorption Relationship Govind Sethia,† Rajesh S. Somani, and Hari C. Bajaj* Discipline of Inorganic Materials and Catalysis, CSIRCentral Salt & Marine Chemicals Research Institute, Bhavnagar 364 002, Gujarat, India S Supporting Information *

ABSTRACT: The equilibrium adsorption study of methane and nitrogen on zeolite-X exchanged with different percentages of cesium ions was carried out using volumetric gas adsorption method. The dynamic breakthrough measurements were carried out using a fixed bed breakthrough reactor and binary (methane + nitrogen) gas mixture. The cesium ion exchanged zeolite samples were characterized by Brunauer−Emmett−Teller measurements, X-ray diffraction, scanning electron microscopy, and inductive coupled plasma-optical emission spectrophotometer analysis. Methane and nitrogen adsorption capacities depend on the percentage and position of cesium ions in the zeolite. The adsorption properties of ion exchanged zeolite were studied in correlation with cesium ion positions in the zeolite. Above 36% cesium ion exchange in NaX the methane adsorption capacity increases, while nitrogen adsorption capacity decreases. The cesium exchanged zeolite showed methane adsorption capacity of 21.1 molecules/(unit cell) and methane selectivity over nitrogen of 3.84 at 288 K, higher than that of zeolite NaX. The selectivity for methane over nitrogen was found to be in the order of Cs(84)NaX > Cs(68)NaX > Cs(53)NaX > Cs(36)NaX > NaX. All of the cesium exchanged zeolites showed nitrogen adsorption capacity less than that of NaX while, Cs(84)NaX and Cs(68)NaX showed methane adsorption capacity more than NaX. The adsorption isotherms were fitted using the Langmuir model equation and the virial equation. The methane stoichiometric adsorption capacity also increases on cesium ion exchange; NaX and Cs(80)NaX showed stoichiometric methane adsorption capacities of 3.5 and 5.7 molecules/(unit cell), respectively. The stoichiometric adsorption capacity for methane increases with an increase in the partial pressure of methane in the gas mixture.

1. INTRODUCTION Separation and purification of gas mixtures by adsorption is a well-established process technology and is used to serve the chemical, petrochemical, environmental, and pharmaceutical industries.1−5 During the past three decades there has been extraordinary growth in the development of adsorption based technologies for the separation and purification of different gas mixtures.6 The separation of CH4 and N2 is one of the great industrially significant separation processes.7 Natural gas consists of mainly CH4 (80−95%) with variable amounts of impurities such as N2, CO2, and other minor impurities such as higher hydrocarbons, O2, and Ar. For pipeline quality natural gas, N2 and CO2 content should not exceed 4% and 2%, respectively.8 Some of the waste gases from chemical, petrochemical, and fertilizer plants also contain CH4 and N2 in variable composition along with other gases. The recovery of CH4 from off gases is also relevant as this is one of the major contributors to global warming with 20 times higher global warming potential than that of CO2.9 Methane recovered with required purity from such industrial waste gases can be used as starting material for fine chemicals synthesis and fuel. Generally, CH4 and N2 mixtures are separated by cryogenic, membrane, or adsorption separation. Cryogenic separation has the drawbacks of high energy requirement and not being suitable for low flow rates, while membrane separation does not have high selectivity and thus is not economical for bulk separation.10 The adsorption separation is economical in medium-scale separation only and is not recommended for the large-scale CH4−N2 separation. The large-scale adsorptive © 2014 American Chemical Society

separation of CH4 from N2 is a big challenge because of the lack of efficient adsorbent having high adsorption capacity and selectivity.11−15 Many materials have been developed for the selective separation of nitrogen from methane since in most cases it is desirable to remove N2 from a predominately CH4-rich stream. However, the N2 content increases with time after the natural gas reservoirs are in service for a long time. Due to the high N2 content, the nitrogen−methane separation is no more economic and, therefore, much of the natural gas resources are not readily usable.16 The development of methane-selective adsorbent and processes may find application at this stage. Yet, less attention has been devoted to the separation of nitrogen− methane mixtures using methane-selective adsorbents. The materials developed for methane−nitrogen separation can be categorized into nitrogen-selective7,8,13,17−24 or methaneselective16,25−37 adsorbents. The selection of an adsorptive separation process such as P/ V/TSA or their combinations requires accurate data on pure and multicomponent equilibrium and dynamic adsorption, with kinetics and heats of adsorption.6 Maurin et al.38 reported experimental and theoretical adsorption of N2 in zeolite-X with various alkali and alkaline earth metal ions as extraframework cations. Talu et al.39 studied the CH4 adsorption in alkali metal Received: Revised: Accepted: Published: 6807

January 21, 2014 March 30, 2014 March 31, 2014 March 31, 2014 dx.doi.org/10.1021/ie5002839 | Ind. Eng. Chem. Res. 2014, 53, 6807−6814

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ion exchanged zeolite-X. Sethia et al.40,41 reported nitrogen adsorption on zeolite ZSM-5 with different silica−alumina ratios. Maple and Williams17 studied nitrogen-selective SAPO-18, SAPO-34, and ETS-4 for nitrogen−methane separation. The separation in SAPOs was achieved by thermodynamic selectivity of the gas molecules in the pores, while in ETS-4 and clinoptilolite the separation is achieved by kinetic separation. Bhadra and Farooq18 and Butwell et al.19 also studied nitrogen-selective ETS-4 based adsorbents for the separation of nitrogen−methane mixtures. Jayaraman et al.,7,8 Kouvelos et al.,20 and Guest and Williams21 have also studied nitrogen-selective clinoptilolite in detail for nitrogen−methane separation. Cavenati et al.13,22,23 and Fatehi et al.24 studied carbon molecular sieves for nitrogen−methane separation. A Sr-ETS-4/Na-ETS-4 based pressure swing adsorption (PSA) process is the most promising available nitrogen-selective process for nitrogen−methane separation but has the drawback of low adsorption capacity.19 Knaebel,25 Reinhold et al.,26,27 and Dolan and Butwell28 reported methane−nitrogen separation using methane-selective adsorbents. Lopes et al.29 studied ion exchanged zeolite 13X for the separation of steam methane re-forming off gas mixture. Delgado et al.30,31 studied the PSA cycle for methane−nitrogen separation using methane-selective silicalite and mordenite. Rufford et al.,32 Olajossy et al.,33,34 Liu et al.,35 Baksh et al.,16 and Warmuzinski et al.36 studied methane-selective carbon based materials for methane−nitrogen separation. Dong et al.37 studied a two-bed PSA process for the selective separation of carbon dioxide, methane, and nitrogen. Despite several advantages of adsorptive separation processes and development of many adsorbents, methane−nitrogen separation has been found particularly difficult because of the lack of satisfactory sorbent. The molecular properties of gas molecules determine their adsorption capacity, selectivity, and heat of adsorption toward a particular adsorbent. CH4 and N2 have different molecular properties; this dissimilarity can be utilized for their separation.42 In our earlier work, sorption of carbon monoxide, methane, and nitrogen in alkali metal ion exchanged zeolite-X, with use of grand canonical Monte Carlo simulation and volumetric measurements, we reported that cesium exchanged zeolite shows the highest equilibrium methane adsorption capacity and selectivity over nitrogen among all alkali exchanged zeolite-X adsorbents.9 Herein, we studied the cesium exchanged zeolite-X in detail as a potential adsorbent for methane−nitrogen separation. The adsorption capacity, equilibrium, and dynamic selectivity were studied in relation with different degree of cesium ion exchange and their positions in zeolite cavity. The study of the equilibrium and dynamic adsorption of CH4 and N2 in cesium exchanged zeolite-X with cation position and their adsorption properties correlation will be of great scientific and industrial interest. The pure component equilibrium and dynamic adsorption of mixture are used to understand the separation mechanism of methane− nitrogen gases at the molecular level.

(99.999%) from Inox Air Products, Mumbai, India, were used for the adsorption isotherm measurements. 2.2. Cesium Ion Exchange. Typically, the zeolite-X samples with different percentages of cesium ions were prepared by repeatedly treating zeolite-X with 0.05 M aqueous solution of cesium chloride in a batch process with a solid to liquid ratio of 1:80 at 353 K for 4 h. Then the solids were filtered, washed with hot distilled water until the washings were free from chloride ions as tested with AgNO3 solution, and dried in an air oven at 353 K for 24 h. The extent of the cesium exchange was determined by inductive coupled plasma-optical emission spectrophotometer (ICP-OES) analysis of zeolite-X and their cesium exchanged forms. The following terminology is used to describe the ion exchanged samples: the first letter shows the exchanged cation and the number in brackets shows the percentage of sodium cations exchanged with cesium cations: e.g., Cs(36)NaX indicates that 36% of the total sodium cations present in the zeolite-X are exchanged with the cesium cations. 2.3. Characterization. X-ray powder diffraction patterns of adsorbents were obtained using a Philips X’pert MPD system in the 2θ range of 2−60° using Cu Kα1 (λ = 1.54056 Å). The diffraction pattern of the materials indicated that they are highly crystalline, showing reflections in the range of 5−35° which are typical of zeolites. The percent crystallinity of the cesium cation exchanged zeolites was determined from the X-ray diffraction pattern by considering the intensity of 10 major peaks. The NaX was considered as an arbitrary standard (i.e., 100% crystallinity) for comparison. The surface areas of the cesium ion exchanged zeolite-X samples were determined from N2 adsorption data at 77 K using a surface area and pore size analyzer, Model ASAP 2020, (Micromeritics Inc., Norcross, GA, USA). Before N2 adsorption the samples were activated at 623 K under vacuum. Surface areas of various samples were determined from Brunauer−Emmett−Teller (BET) method. Microscopic images of cesium exchanged zeolite-X samples were collected using a LEO 1430 VP variable pressure scanning electron microscope. An inductive coupled plasma-optical emission spectrophotometer (Optima 2000 DV, PerkinElmer) was used to determine the percentage of the different elements in the ion exchanged zeolites. For ICP-OES analysis the samples were dissolved in a minimum quantity of HF and diluted for less than 10 ppm concentration of ions in solution. 2.4. Adsorption Isotherm Measurements. The presence of water in the zeolite significantly affects the adsorption isotherm; therefore, the samples were dried at 353 K for 24 h in the oven. Prior to adsorption measurements, the samples were activated in situ by heating up to 623 K, at a heating rate of 1 K min−1 under vacuum (5 × 10−3 mmHg) for 12 h using a degassing system. N2 and CH4 adsorption isotherms were measured at 288 and 303 K using a static volumetric system (Micromeritics ASAP 2020). Adsorption temperature was maintained (±0.1 K) by circulating water from a constanttemperature water bath (Julabo F25, Seelbach, Germany). Adsorption capacity, as the volume of gas adsorbed per gram of adsorbent, and selectivity of adsorption were determined from the adsorption isotherms measured at 288 and 303 K. The pure component selectivity of gases A and B was calculated by using

2. EXPERIMENTAL SECTION 2.1. Materials. Zeolite-X (NaX) in powder as well as granular form was procured from Zeochem LLC, Uetikon, Switzerland, and used as received. Cesium chloride used for cation exchange was purchased from S. D. Fine Chemicals, Mumbai, India. N2 (99.999%), CH4 (99.9%) and helium

αA/B = [VA /VB]P , T

(1)

where VA and VB are the volumes of gases A and B, respectively, adsorbed at any given pressure P and temperature T. 6808

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For CH4 adsorption

The number of gas molecules adsorbed per unit cell of adsorbent was calculated by multiplying the volume of gas adsorbed with a conversion factor which was determined by dividing the number of gas molecules adsorbed with the total number of unit cells in unit gram of that adsorbent. The number of gas molecules adsorbed was calculated from the ideal gas equation (PV = nRT), and the number of unit cells per gram of adsorbent was calculated from the molecular weight of adsorbent. 2.5. Langmuir Model Fitting and Henry’s Constant. The adsorption data obtained is fitted in the Langmuir equation and the virial equation. The values for Langmuir constant and Henry’s constant were determined from these data.43,44

ϕmethane = ϕD + ϕR + ϕind

The electrostatic interactions are governed by following relationships field induced dipole:

(ϕind) ∝ q2α /r 4

0

P /qP = (1/bqm) + (P /qmP )

(ϕFμ) ∝ qμ/r 2 (ϕFQ ) ∝ qQ /r 3

(2)

(3)

(4)

where q is the amount of gas adsorbed per unit weight of the adsorbent, qm is the monolayer capacity of the adsorbent, b is the Langmuir constant, P is the equilibrium pressure, P0 is the saturation vapor pressure, and A, B, and C are the first, second, and third virial coefficients, respectively. Henry’s constant is the measure of strength of adsorption interactions between CH4, N2 gas molecules and cesium ion exchanged zeolite-X. The total energy of CH4 and N2 adsorption on cesium metal ion exchanged zeolite-X is the sum of the total molecular− molecular and molecular−adsorbent interaction potential.4 ϕ = ϕadsorbate−adsorbate + ϕadsorbate−adsorbent

(5)

ϕ = ϕD + ϕR + ϕind + ϕFμ + ϕFQ

(6)

where ϕ = adsorbate−adsorbent interaction potential, ϕD = dispersion energy, ϕR = close-range repulsion energy, ϕind = induction energy (interaction between an electric field and an electric dipole), ϕFμ = interaction between an electric field (F) and a permanent dipole (μ), and ϕFQ = interaction between a field gradient (F) and a quadrupole (Q). ϕD and ϕR are nonspecific interactions, occuring between CH4 or N2 gas molecules and zeolites, while ϕind, ϕFμ and ϕFQ are specific interactions and arise between ionic framework, cations, and gas molecules. Since zeolite is an ionic solid, during adsorption electrostatic interactions (ϕind, ϕFμ, and ϕFQ) dominate. N2 has zero dipole moment and so has zero field dipole interaction (ϕFμ). ϕnitrogen = ϕD + ϕR + ϕind + ϕFQ

(11)

where r (equilibrium distance) = r1 (ionic radius) + r2 (radius of gas molecule), q = electronic charge of ion, α = polarizability, F = electric field, μ = permanent dipole moment, and Q = quadrupole moment. 2.6. Dynamic Breakthrough Measurements. The experimental setup used for methane nitrogen breakthrough measurement is shown in Supporting Information Figure S1. A stainless-steel column (20 cm length × 3 cm inner diameter) was packed with Cesium exchanged zeolite-X granules (0.2−0.3 cm diameter); the top and bottom of the adsorption column were plugged with glass wool. The adsorbent was first activated overnight at 623 K under N2 flow (200 mL/min) to remove moisture and other adsorbed species, and then the temperature was lowered to 303 K. The gas stream was then switched to a methane−nitrogen gas mixture. The flow rate was maintained at 100 mL/min via mass flow controllers. Different feed compositions of CH4 and N2 were ascertained by analyzing the feed gas using gas chromatograph (Chemito Inc., model 7610). The breakthrough studies were carried out at 303 K and 1 atm. pressure. Desorption was carried out countercurrently under N2 flow (100 mL/min.) at the same temperature and pressure. The raffinate was analyzed by gas chromatograph, having a molecular sieve column and thermal conductivity detector (TCD). Helium was used as a carrier gas for GC analysis. The flow rate of helium was maintained at 40 mL/min.

Henry’s constant, K, was determined from the first virial coefficient using the equation

K = exp( −A)

(10)

field gradient−quadrupole:

virial equation: ln(P /q) = A + Bq + Cq2 + ...

(9)

field dipole:

Langmuir equation: 0

(8)

3. RESULTS AND DISCUSSION 3.1. X-ray Powder Diffraction. X-ray powder diffraction patterns of the zeolite-X showed that they are a highly crystalline material giving the reflections in the range of 5−35° (Supporting Information Figure S2) typically of zeolites. The structure of the zeolite-X is retained even after cesium ion exchange. However, the crystallinity of the zeolite-X decreases with an increasing degree of cesium ion exchange. This was due to the effect of the cesium cations on the framework rather than the collapse of the crystalline structure. Exchangeable cesium cations having a size greater than the sodium cations adversely affects the framework of zeolite. 3.2. Scanning Electron Microscopy and Inductive Coupled Plasma-Optical Emission Spectrophotometer Analysis. The morphology of cesium exchanged zeolite-X (Supporting Information Figure S3) shows that the zeolite crystals are octahedral in shape and the morphology of the zeolite-X has not changed even after the cesium ion exchange. The ICP-OES analysis was carried out to determine the percentage of the different elements in the cesium exchanged zeolite samples. The ICP-OES analysis showed that during ion

(7)

N2 has a high quadrupole moment which causes high field quadrupole interaction (ϕFQ) and thus contributes more toward the total interaction energy. CH4 has zero quadrupole moment and thus has zero field quadrupole interaction (ϕFQ), but methane has polarizability higher than N2 and so field induced dipole interaction (ϕind) has a major contribution in the total interaction energy. 6809



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Table 1. Unit Cell Compositions, Surface Areas, and Pore Volumes of Sodium and Cesium Metal Ion Exchanged Zeolite-X Samples samples

unit cell formula dry (wt %)

micropore vol (cm3/g)

BET surface area (m2/g)

micropore surface area (m2/g)

NaX Cs(36)NaX Cs(53)NaX Cs(68)NaX Cs(84)NaX

Na(88)Al(88)Si(104)O(384) Cs(32)Na(56)Al(88)Si(104)O(384) Cs(47)Na(41)Al(88)Si(104)O(384) Cs(60)Na(38)Al(88)Si(104)O(384) Cs(74)Na(14)Al(88)Si(104)O(384)

0.30 0.26 0.22 0.19 0.17

692 594 530 493 423

647 544 487 453 392

CH4 has neither a dipole nor a quadrupole moment but has high polarizability (26 × 10−25 cm3); therefore, field induced dipole interactions dominates. The dispersion interaction potential for CH4 increases with polarizability of the surface ions. The polarizability of cesium ions (1.73 × 10−24 cm3) is higher than that of sodium ions (0.18 × 10−24 cm3); thus, Cs(84)NaX showed CH4 adsorption capacity higher than that of NaX. The adsorption capacity also depends on the polarizing power of extraframework cations. The polarization power of cesium ions (9.58 × 10−10 m−1) is less than that of sodium ions (16.5 × 10−10 m−1) though cesium exchanged zeolite showed increased methane adsorption capacity, this is, due to shielding of smaller size sodium cation by the first few methane molecules of a cavity (first shell of adsorbate), which would limit sodium ions interaction with other methane molecules. The larger cesium cation would experience less shielding and hence would be able to affect a larger number of methane molecules in the outer shell, even though the imposed potential is less than that imposed by a small sodium cation. Moreover, CH4 is nonpolar, but due to asymmetric vibrations it acquired some polar character. When CH4 molecules are in close proximity to the cation within the structure and framework oxygen atoms, they cause an instantaneous shift in the time averaged neutral electrostatic field of CH4 and this induced polarity also results into the high adsorption capacity of CH4. 3.5. Structure, Cation Locations and Adsorption Capacity in Zeolite-X. Zeolite-X is a synthetic aluminumrich analogue of the naturally occurring mineral faujasite, having a structure as shown in Figure 2. The 14-hedrons with 24 vertices known as the sodalite cavity or β-cage may be considered as its principal building block. These β-cages are connected tetrahedrally through six-member rings by bridging oxygen to give double six-member rings (D6R, hexagonal prisms) and concomitantly, an interconnected set of even larger cavities (supercage), accessible in three dimensions through 12ring windows. The Si and Al atoms occupy the vertices of these polyhedra while oxygen atoms lie approximately midway between each pair of Si and Al atoms but are displaced from those points to give near-tetrahedral angles about Si and Al. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that Si substitutes for Al at about 8% of the Al positions. Single six-member rings (S6R) are shared by sodalite and supercage and may be viewed as the entrances to the sodalite units. Each unit cell has 8 sodalite units, 8 supercages, 16 D6R, 16 12-rings, and 32 S6R. The extraframework sodium and cesium cations which balance the negative charge of the aluminosilicate framework are found at different sites (Figure 2) within the zeolite cavities. Site I is located at the center of the D6R, site I′ is in the sodalite cavity on the opposite side of one of the D6R, sites I and II′ are inside the sodalite cavity near a S6R, site II is at the center of the S6R or displaced from this point into a supercage, site III is in the supercage on a 2-fold axis opposite a 4-ring between two 12-

exchange the extraframework sodium ions in the zeolite are replaced with cesium ions from the solution. The percentage of cesium exchange increases with an increasing number of exchange cycles, with a maximum exchange of 84%. 3.3. Surface Area and Pore Volume. The surface area and micropore volume of zeolite-X in powder form exchanged with different percentages of cesium ions were determined from the N2 adsorption/desorption isotherm, and the results are shown in Table 1. The surface area and micropore volume of zeolite-X decreases on cesium ion exchange because the size and atomic mass of cesium ion is more than exchangeable sodium cation. Generally the surface area and micropore volume decrease upon exchange of sodium ions with ions of higher molecular weight and bigger size. The decrease in crystallinity on cesium ion exchange also leads to a decrease in the surface area. 3.4. Adsorption Isotherm and Selectivity. CH4 and N2 molecules can interact with the zeolite surface through lattice oxygen atoms, accessible extraframework cations, and Si and Al atoms. The Si and Al atoms present at the center of tetrahedra are not directly exposed to the gas molecules. Consequently, their interactions with the CH4 and N2 molecules are negligible. The principal interactions of CH4 and N2 molecules with the zeolite surface are through lattice oxygen atoms and extraframework cations. The variation in the electrostatic interactions between CH4 and N2 and the extraframework cesium cations of the zeolite depend on the difference in their physical properties (Supporting Information Table S1). The pure component CH4 and N2 adsorption equilibrium isotherms for cesium zeolite-X having different percentages of cesium ions were generated and are found to be of type I (Figure 1) as per the IUPAC classification. The pure component equilibrium adsorption capacities and CH4/N2 selectivity at 760 mmHg and at 288 and 303 K (Supporting Information Table S2) were determined from the adsorption isotherms. The CH4/N2 selectivity increases from 1.8 for NaX to 3.9 for Cs(84)NaX. Above 36% cesium ion exchange the selectivity increases with an increasing degree of cesium ion exchange, due to an increase in CH4 adsorption capacity and a decrease in N2 adsorption capacity. On maximum cesium ion exchange, the CH4 adsorption capacity increases from 10.1 to 14.9 while the N2 adsorption capacity decreases from 5.5 to 3.8 molecules/(unit cell) at 303 K and 760 mmHg. The major interactions of N2 with zeolite NaX and Cs(84)NaX are field gradient quadrupole and field induced dipole interaction which are inversely proportional to ionic radii of cations. Since cesium has ionic radii (167 pm) higher than that of sodium ion (97 pm), cesium zeolite-X showed a lower electrostatic interaction potential which resulted in less N2 adsorption capacity. The ionic radius of cation was important for both nonspecific and specific (electrostatic) interactions while cationic charge is important only for electrostatic interactions. 6810



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Figure 1. Adsorption isotherms of CH4 and N2 on sodium and cesium ion exchanged zeolite-X at 303 K: (a) NaX, (b) Cs(36)NaX, (c) Cs(53)NaX, (d) Cs(68)NaX, and (e) Cs(84)NaX.

large under normal conditions to pass through the 2.2 Å sixring window openings, but they are observed at the sites in the D6R and β-cages.45 The maximum cesium ion exchange in zeolite-X has been reported to be around 82%.46 The ion exchange selectivity varies with the degree of cation exchange. Below a level of 40% exchange the selectivity series for alkali metal ions was observed in terms of decreasing selectivity, to be Cs > Rb > K > Na > Li. This series corresponds to the

rings, and site III′ is somewhat or substantially off site III (off the 2-fold axis) on the inner surface of the supercage. Zeolite-X contains about 88 cations/(unit cell) which may occupy six different sites (Figure 2) in hydrated zeolite. In the crystal structure of NaX, 30 Na+ ions are located at site I′ and 32 Na+ ions at site II, and the remaining 26 Na+ ions are located at site III′. Cations at sites I, I′, and II′ are the most difficult to exchange. Cesium ions (diameter, 3.38 Å) are too 6811



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and cation shift toward the supercage. Moreover, at a high degree of cesium exchange the number of cesium ions in the βcages also increases. Moreover, due to the large size of the cesium cations they do not sit crystallographically very low in the face of the single six rings (SR6, the SII position), allowing the electric field to be poorly shielded by the surrounding framework oxygen and thus interact strongly with CH4 and N2 molecules. 3.6. Langmuir Model Fitting, and Henry’s Constant. The adsorption data obtained at 288 and 303 K were fitted into the Langmuir model; and the values of the slope and Langmuir constant (b) for the adsorption of CH4 and N2 on zeolite Cs(84)NaX and NaX are given in Supporting Information Table S3. The Langmuir model fitted well for methane and nitrogen adsorption at both 288 and 303 K. On cesium ion exchange the values of slope and b decrease for N2 adsorption, due to a decrease in the strength of the quadrupolar interaction potentials. However, the slope and b increase for CH4 adsorption. The high values of slope and b for methane adsorption are due to an increase in the dispersion interactions. The opposite trend in the values of slope and b for N2 and CH4 adsorption is in agreement with their heats of adsorption, Henry’s constants, adsorption capacities, and cesium cation positions in zeolite-X. The CH4 and N2 adsorption data obtained at 288 and 303 K were also fitted to the virial equation, and the values for the virial coefficients and Henry’s constants for the adsorption of CH4 and N2 on zeolite Cs(84)NaX and NaX are given in Supporting Information Table S4. Henry’s constant is the measurement for the strength of adsorption interactions and heat of adsorption. The higher Henry’s constant is the higher will be the heat of adsorption. On cesium ion exchange the value of Henry’s constant K for the N2 adsorption decreases while that for CH4 adsorption increases. The magnitude of Henry’s constant is higher for CH4 as compared to N2. The value of Henry’s constant confirms the strong dispersion interactions of the CH4 molecule with the cesium cations in the zeolite-X. 3.7. Dynamic Adsorption from the (CH4 + N2) Binary Gas Mixture. The pore opening and cage structure of cesium ion exchanged zeolite-X is large enough to neglect any steric effects of the gas molecules with the adsorbent structure. However, the cesium ion location, number, and nature of the adsorbent surface are predominantly the cause of the difference in the dynamic adsorption capacity of CH4. The breakthrough measurements for methane from (CH4 + N2) binary gas mixtures on Cs(80)NaX were carried out at 303 K and 1 atm pressure with feed flow of 100 mL/min. The breakthrough results are given in Supporting Information Table S5. Figures 3 and 4 show the CH4 adsorption and desorption breakthrough curves for methane−nitrogen binary gas mixture on zeoliteNaX and Cs(80)NaX, respectively. The effect of methane−nitrogen mixture composition on the dynamic adsorption of the mixture was also investigated. Methane stoichiometric adsorption capacities for the 10%, 30%, 50%, 68%, and 90%, methane gas mixture were 1.3, 3.1, 5.2, 7.1, and 9.4 cm3/g, respectively, at 303 K. The stoichiometric capacity increases with an increase in the percentage of methane in the feed gas mixture. The increase in stoichiometric adsorption capacity was due to an increase in the partial pressure of CH4. The comparable breakthrough time (5−6 min) for mixtures having different compositions indicated the weak adsorbent−adsorbate interactions as the diffusion of gases

Figure 2. Framework structure of zeolite-X. Near the center of the each line segment is an oxygen atom. Numbers 1−4 indicate the different oxygen atoms. Extraframework cation positions are labeled with Roman numerals. Reproduced with permission from ref 9. Copyright 2010 American Chemical Society.

occupancy of the most accessible cation sites, i.e., the supercage in zeolite-X. At 50% exchange, which includes site II′ in the sixring adjacent to the supercage, the selectivity series was found to be Na > K > Rb > Cs > Li.46 In a single unit cell of zeolite-X there are 192 possible cation sites for 88 cations. On activation of zeolite at 350 °C under vacuum the cesium and sodium ions migrate to the sites of lower energy with maximum coordination. Migration of cation is a process which depends on the temperature, time of activation, and size of the cation. The sites with the least energies are hidden and are not exposed to the supercage cavity; thus, only 40−50% of cations are located at exposed sites. In zeolite-X, the cations in the β-cages and the D6R (i.e., at sites I, I′, and II′) were sterically inaccessible to CH4 and N2, and hence only the supercage cations, i.e., those at sites II, III, and III′, are available for interaction with these gas molecules. However, the electric field around these supercage cations is partially shielded by the surrounding oxygen atoms. Because of this shielding the electrostatic and induction interactions are expected to be lower than those of an isolated ion. Further, the dispersion forces acting on the molecule will be higher because the gas molecule also interacts with oxygen atoms of the zeolite. The Cs(36)NaX, low cesium exchange zeolite showed the least CH4 and N2 adsorption capacity because during hightemperature activation the ions migrated inside the β-cages and six-member ring from the supercage due to cation crowding by large size cesium ions in the super cage and thus not available for adsorption of gas molecules. Cs(53)NaX shows adsorption capacity more than that of Cs(36)NaX because due to the larger size of the cesium ions the cation crowding inside the βcages increases; that shifts the site II ions inside the supercages and thus increases their interactions with CH4 and N2 molecules.45 The CH4 and N2 adsorption capacity in cesium exchange zeolite follows the order Cs(36)NaX, Cs(53)NaX, Cs(68)NaX, and Cs(84)NaX. Above 36% Cs ion exchange, the adsorption capacity increases with increasing cation crowding 6812



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cesium exchange zeolite-X depends on the degree of ion exchange and cation migration in β-cages. The methane and nitrogen adsorption isotherms show good Langmuir model fitting. On cesium ion exchange the values of the slope and Langmuir constant, and Henry’s constant, decrease for N2 adsorption, while they increase for CH4 adsorption. Dynamic mixed gas breakthrough adsorption studies are carried out for different CH4 and N2 gas mixtures. The dynamic mixture gas study shows CH4 selectivity over N2. However, the CH4 breakthrough time for different CH4 + N2 gas mixtures is approximately similar and the CH4 stoichiometric adsorption capacity increases with increasing CH4 percentage in the gas mixture due to the linear nature of CH4 and N2 adsorption isotherms.

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Figure 3. Dynamic adsorption/desorption breakthrough curves of methane for zeolite-X from 68% methane and 32% nitrogen gas mixture at 303 K and 1 atm pressure.

ASSOCIATED CONTENT

S Supporting Information *

Tables listing physical properties of CH4 and N2 gas molecules and CH4 and N2 adsorption capacity, selectivity, Langmuir model parameters, Henry’s constant, virial coefficient, and dynamic adsorption capacity for sodium and cesium ion exchanged zeolite-X and figures showing the experimental setup for dynamic adsorption studies, X-ray powder diffraction patterns, and SEM images for sodium and cesium exchanged zeolite-X. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Center for Catalysis Research and Innovation, Biosciences Complex, University of Ottawa, 30 Marie-Curie, Ottawa, Ontario, Canada K1N 6N5. Notes

Figure 4. Dynamic adsorption breakthrough curves of methane for (a) 90% methane and 10% nitrogen, (b) 68% methane and 32% nitrogen, (c) 50% methane and 50% nitrogen, (d) 30% methane and 70% nitrogen, and (e) 10% methane and 90% nitrogen, and (f) methane desorption breakthrough curve for a 90% methane and 10% nitrogen gas mixture on Cs(80)NaX at 303 K and 1 atm pressure.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CSMCRI communication No. IMC- 024-14. G.S. thanks Dr. Sunil A. P. for fruitful discussion and CSIR, New Delhi, India, for financial assistance in the form of a senior research fellowship. The authors also thankful to Analytical Science Discipline of CSMCRI for providing analytical facilities.

having linear isotherm is independent of their percentage in the mixture. The adsorbed CH4 could be easily desorbed by countercurrent purging of N2 at 100 mL/min.

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4. CONCLUSION Equilibrium adsorption measurements for the adsorption of CH4 and N2 are performed in zeolite-X with different percentages of cesium exchange. More than 36% cesium ion exchanged zeolite-X samples shows increased CH4 selectivity over N2 due to increase in methane and decrease in nitrogen adsorption capacity. The adsorption properties of ion exchanged zeolite was studied in correlation with the cesium ion position in the zeolite. The adsorption capacity for CH4 and N2 decreases for the low cesium ion exchange zeolite Cs(36)NaX, due to a decrease in the number of cations in the supercage. The large size cesium ions causes cation crowding in the supercage and forced sodium ions to migrate inside less accessible β-cages. However, on further increase in cesium exchange, the adsorption capacity significantly increases due to an increased number of cesium ions in the super cage for the adsorption of CH4 and N2. The adsorption capacity of

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Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X

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