Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X: Structure, Cation Position and Adsorption Relationship

Sorption of Methane and Nitrogen on Cesium Exchanged Zeolite-X:Structure, Cation Position and Adsorption RelationshipGovind Sethia,† Rajesh S. Somani, and Hari C. Bajaj*

Discipline of Inorganic Materials and Catalysis, CSIRCentral 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 ofcesium ions was carried out using volumetric gas adsorption method. The dynamic breakthrough measurements were carried outusing a fixed bed breakthrough reactor and binary (methane + nitrogen) gas mixture. The cesium ion exchanged zeolite sampleswere characterized by Brunauer−Emmett−Teller measurements, X-ray diffraction, scanning electron microscopy, and inductivecoupled plasma-optical emission spectrophotometer analysis. Methane and nitrogen adsorption capacities depend on thepercentage and position of cesium ions in the zeolite. The adsorption properties of ion exchanged zeolite were studied incorrelation with cesium ion positions in the zeolite. Above 36% cesium ion exchange in NaX the methane adsorption capacityincreases, while nitrogen adsorption capacity decreases. The cesium exchanged zeolite showed methane adsorption capacity of21.1 molecules/(unit cell) and methane selectivity over nitrogen of 3.84 at 288 K, higher than that of zeolite NaX. The selectivityfor 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 ofthe cesium exchanged zeolites showed nitrogen adsorption capacity less than that of NaX while, Cs(84)NaX and Cs(68)NaXshowed methane adsorption capacity more than NaX. The adsorption isotherms were fitted using the Langmuir model equationand the virial equation. The methane stoichiometric adsorption capacity also increases on cesium ion exchange; NaX andCs(80)NaX showed stoichiometric methane adsorption capacities of 3.5 and 5.7 molecules/(unit cell), respectively. Thestoichiometric 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 awell-established process technology and is used to serve thechemical, petrochemical, environmental, and pharmaceuticalindustries.1−5 During the past three decades there has beenextraordinary growth in the development of adsorption basedtechnologies for the separation and purification of different gasmixtures.6 The separation of CH4 and N2 is one of the greatindustrially significant separation processes.7 Natural gasconsists of mainly CH4 (80−95%) with variable amounts ofimpurities such as N2, CO2, and other minor impurities such ashigher hydrocarbons, O2, and Ar. For pipeline quality naturalgas, 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 N2in variable composition along with other gases. The recovery ofCH4 from off gases is also relevant as this is one of the majorcontributors to global warming with 20 times higher globalwarming potential than that of CO2.

9 Methane recovered withrequired purity from such industrial waste gases can be used asstarting material for fine chemicals synthesis and fuel.Generally, CH4 and N2 mixtures are separated by cryogenic,

membrane, or adsorption separation. Cryogenic separation hasthe drawbacks of high energy requirement and not beingsuitable for low flow rates, while membrane separation does nothave high selectivity and thus is not economical for bulkseparation.10 The adsorption separation is economical inmedium-scale separation only and is not recommended forthe large-scale CH4−N2 separation. The large-scale adsorptive

separation of CH4 from N2 is a big challenge because of the lackof efficient adsorbent having high adsorption capacity andselectivity.11−15

Many materials have been developed for the selectiveseparation of nitrogen from methane since in most cases it isdesirable to remove N2 from a predominately CH4-rich stream.However, the N2 content increases with time after the naturalgas reservoirs are in service for a long time. Due to the high N2

content, the nitrogen−methane separation is no moreeconomic and, therefore, much of the natural gas resourcesare not readily usable.16 The development of methane-selectiveadsorbent 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. Thematerials developed for methane−nitrogen separation can becategorized into nitrogen-selective7,8,13,17−24 or methane-selective16,25−37 adsorbents.The selection of an adsorptive separation process such as P/

V/TSA or their combinations requires accurate data on pureand multicomponent equilibrium and dynamic adsorption, withkinetics and heats of adsorption.6 Maurin et al.38 reportedexperimental and theoretical adsorption of N2 in zeolite-X withvarious alkali and alkaline earth metal ions as extraframeworkcations. Talu et al.39 studied the CH4 adsorption in alkali metal

Received: January 21, 2014Revised: March 30, 2014Accepted: March 31, 2014Published: March 31, 2014

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ion exchanged zeolite-X. Sethia et al.40,41 reported nitrogenadsorption on zeolite ZSM-5 with different silica−aluminaratios.Maple and Williams17 studied nitrogen-selective SAPO-18,

SAPO-34, and ETS-4 for nitrogen−methane separation. Theseparation in SAPOs was achieved by thermodynamicselectivity of the gas molecules in the pores, while in ETS-4and clinoptilolite the separation is achieved by kineticseparation. Bhadra and Farooq18 and Butwell et al.19 alsostudied nitrogen-selective ETS-4 based adsorbents for theseparation of nitrogen−methane mixtures. Jayaraman et al.,7,8

Kouvelos et al.,20 and Guest and Williams21 have also studiednitrogen-selective clinoptilolite in detail for nitrogen−methaneseparation. Cavenati et al.13,22,23 and Fatehi et al.24 studiedcarbon molecular sieves for nitrogen−methane separation. ASr-ETS-4/Na-ETS-4 based pressure swing adsorption (PSA)process is the most promising available nitrogen-selectiveprocess for nitrogen−methane separation but has the drawbackof low adsorption capacity.19

Knaebel,25 Reinhold et al.,26,27 and Dolan and Butwell28

reported methane−nitrogen separation using methane-selectiveadsorbents. Lopes et al.29 studied ion exchanged zeolite 13X forthe separation of steam methane re-forming off gas mixture.Delgado et al.30,31 studied the PSA cycle for methane−nitrogenseparation 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 carbonbased materials for methane−nitrogen separation. Dong et al.37

studied a two-bed PSA process for the selective separation ofcarbon dioxide, methane, and nitrogen. Despite severaladvantages of adsorptive separation processes and developmentof many adsorbents, methane−nitrogen separation has beenfound particularly difficult because of the lack of satisfactorysorbent.The molecular properties of gas molecules determine their

adsorption capacity, selectivity, and heat of adsorption toward aparticular adsorbent. CH4 and N2 have different molecularproperties; this dissimilarity can be utilized for theirseparation.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 andvolumetric measurements, we reported that cesium exchangedzeolite shows the highest equilibrium methane adsorptioncapacity and selectivity over nitrogen among all alkaliexchanged zeolite-X adsorbents.9 Herein, we studied thecesium exchanged zeolite-X in detail as a potential adsorbentfor methane−nitrogen separation. The adsorption capacity,equilibrium, and dynamic selectivity were studied in relationwith different degree of cesium ion exchange and their positionsin zeolite cavity. The study of the equilibrium and dynamicadsorption of CH4 and N2 in cesium exchanged zeolite-X withcation position and their adsorption properties correlation willbe of great scientific and industrial interest. The purecomponent equilibrium and dynamic adsorption of mixtureare used to understand the separation mechanism of methane−nitrogen gases at the molecular level.

2. EXPERIMENTAL SECTION2.1. 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 forcation exchange was purchased from S. D. Fine Chemicals,Mumbai, India. N2 (99.999%), CH4 (99.9%) and helium

(99.999%) from Inox Air Products, Mumbai, India, were usedfor the adsorption isotherm measurements.

2.2. Cesium Ion Exchange. Typically, the zeolite-Xsamples with different percentages of cesium ions wereprepared by repeatedly treating zeolite-X with 0.05 M aqueoussolution of cesium chloride in a batch process with a solid toliquid ratio of 1:80 at 353 K for 4 h. Then the solids werefiltered, washed with hot distilled water until the washings werefree from chloride ions as tested with AgNO3 solution, anddried in an air oven at 353 K for 24 h. The extent of the cesiumexchange was determined by inductive coupled plasma-opticalemission spectrophotometer (ICP-OES) analysis of zeolite-Xand their cesium exchanged forms. The following terminologyis used to describe the ion exchanged samples: the first lettershows the exchanged cation and the number in brackets showsthe percentage of sodium cations exchanged with cesiumcations: e.g., Cs(36)NaX indicates that 36% of the total sodiumcations present in the zeolite-X are exchanged with the cesiumcations.

2.3. Characterization. X-ray powder diffraction patterns ofadsorbents were obtained using a Philips X’pert MPD system inthe 2θ range of 2−60° using Cu Kα1 (λ = 1.54056 Å). Thediffraction pattern of the materials indicated that they are highlycrystalline, showing reflections in the range of 5−35° which aretypical of zeolites. The percent crystallinity of the cesium cationexchanged zeolites was determined from the X-ray diffractionpattern by considering the intensity of 10 major peaks. TheNaX was considered as an arbitrary standard (i.e., 100%crystallinity) for comparison. The surface areas of the cesiumion exchanged zeolite-X samples were determined from N2adsorption data at 77 K using a surface area and pore sizeanalyzer, Model ASAP 2020, (Micromeritics Inc., Norcross,GA, USA). Before N2 adsorption the samples were activated at623 K under vacuum. Surface areas of various samples weredetermined from Brunauer−Emmett−Teller (BET) method.Microscopic images of cesium exchanged zeolite-X sampleswere collected using a LEO 1430 VP variable pressure scanningelectron microscope. An inductive coupled plasma-opticalemission spectrophotometer (Optima 2000 DV, PerkinElmer)was used to determine the percentage of the different elementsin the ion exchanged zeolites. For ICP-OES analysis thesamples were dissolved in a minimum quantity of HF anddiluted for less than 10 ppm concentration of ions in solution.

2.4. Adsorption Isotherm Measurements. The presenceof water in the zeolite significantly affects the adsorptionisotherm; therefore, the samples were dried at 353 K for 24 h inthe oven. Prior to adsorption measurements, the samples wereactivated in situ by heating up to 623 K, at a heating rate of 1 Kmin−1 under vacuum (5 × 10−3 mmHg) for 12 h using adegassing system. N2 and CH4 adsorption isotherms weremeasured at 288 and 303 K using a static volumetric system(Micromeritics ASAP 2020). Adsorption temperature wasmaintained (±0.1 K) by circulating water from a constant-temperature water bath (Julabo F25, Seelbach, Germany).Adsorption capacity, as the volume of gas adsorbed per gram ofadsorbent, and selectivity of adsorption were determined fromthe adsorption isotherms measured at 288 and 303 K.The pure component selectivity of gases A and B was

calculated by using

α = V V[ / ]P TA/B A B , (1)

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

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The number of gas molecules adsorbed per unit cell ofadsorbent was calculated by multiplying the volume of gasadsorbed with a conversion factor which was determined bydividing the number of gas molecules adsorbed with the totalnumber of unit cells in unit gram of that adsorbent. Thenumber of gas molecules adsorbed was calculated from theideal gas equation (PV = nRT), and the number of unit cells pergram of adsorbent was calculated from the molecular weight ofadsorbent.2.5. Langmuir Model Fitting and Henry’s Constant.

The adsorption data obtained is fitted in the Langmuir equationand the virial equation. The values for Langmuir constant andHenry’s constant were determined from these data.43,44

Langmuir equation:

= +P qP bq P q P/ (1/ ) ( / )0m m

0(2)

virial equation:

= + + +P q A Bq Cqln( / ) ...2(3)

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

= −K Aexp( ) (4)

where q is the amount of gas adsorbed per unit weight of theadsorbent, qm is the monolayer capacity of the adsorbent, b isthe Langmuir constant, P is the equilibrium pressure, P0 is thesaturation 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 ionexchanged 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 ϕ ϕ= +− −adsorbate adsorbate adsorbate adsorbent (5)

ϕ ϕ ϕ ϕ ϕ ϕ= + + + +μF FQD R ind (6)

where ϕ = adsorbate−adsorbent interaction potential, ϕD =dispersion energy, ϕR = close-range repulsion energy, ϕind =induction energy (interaction between an electric field and anelectric dipole), ϕFμ = interaction between an electric field (F)and a permanent dipole (μ), and ϕFQ = interaction between afield 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 ϕFQare specific interactions and arise between ionic framework,cations, and gas molecules. Since zeolite is an ionic solid, duringadsorption electrostatic interactions (ϕind, ϕFμ, and ϕFQ)dominate.N2 has zero dipole moment and so has zero field dipole

interaction (ϕFμ).

ϕ ϕ ϕ ϕ ϕ= + + + FQnitrogen D R ind (7)

N2 has a high quadrupole moment which causes high fieldquadrupole interaction (ϕFQ) and thus contributes moretoward the total interaction energy.CH4 has zero quadrupole moment and thus has zero field

quadrupole interaction (ϕFQ), but methane has polarizabilityhigher than N2 and so field induced dipole interaction (ϕind)has a major contribution in the total interaction energy.

For CH4 adsorption

ϕ ϕ ϕ ϕ= + +methane D R ind (8)

The electrostatic interactions are governed by followingrelationships

field induced dipole:

ϕ α∝ q r( ) /ind2 4

(9)

field dipole:

ϕ μ∝μ q r( ) /F2

(10)

field gradient−quadrupole:

ϕ ∝ qQ r( ) /FQ3

(11)

where r (equilibrium distance) = r1 (ionic radius) + r2 (radiusof 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. Theexperimental setup used for methane nitrogen breakthroughmeasurement is shown in Supporting Information Figure S1. Astainless-steel column (20 cm length × 3 cm inner diameter)was packed with Cesium exchanged zeolite-X granules (0.2−0.3cm diameter); the top and bottom of the adsorption columnwere plugged with glass wool. The adsorbent was first activatedovernight at 623 K under N2 flow (200 mL/min) to removemoisture and other adsorbed species, and then the temperaturewas lowered to 303 K. The gas stream was then switched to amethane−nitrogen gas mixture. The flow rate was maintainedat 100 mL/min via mass flow controllers. Different feedcompositions of CH4 and N2 were ascertained by analyzing thefeed 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 N2flow (100 mL/min.) at the same temperature and pressure. Theraffinate was analyzed by gas chromatograph, having amolecular sieve column and thermal conductivity detector(TCD). Helium was used as a carrier gas for GC analysis. Theflow rate of helium was maintained at 40 mL/min.

3. RESULTS AND DISCUSSION3.1. X-ray Powder Diffraction. X-ray powder diffraction

patterns of the zeolite-X showed that they are a highlycrystalline material giving the reflections in the range of 5−35°(Supporting Information Figure S2) typically of zeolites. Thestructure of the zeolite-X is retained even after cesium ionexchange. However, the crystallinity of the zeolite-X decreaseswith an increasing degree of cesium ion exchange. This was dueto the effect of the cesium cations on the framework rather thanthe collapse of the crystalline structure. Exchangeable cesiumcations having a size greater than the sodium cations adverselyaffects the framework of zeolite.

3.2. Scanning Electron Microscopy and InductiveCoupled Plasma-Optical Emission SpectrophotometerAnalysis. The morphology of cesium exchanged zeolite-X(Supporting Information Figure S3) shows that the zeolitecrystals are octahedral in shape and the morphology of thezeolite-X has not changed even after the cesium ion exchange.The ICP-OES analysis was carried out to determine thepercentage of the different elements in the cesium exchangedzeolite samples. The ICP-OES analysis showed that during ion



exchange the extraframework sodium ions in the zeolite arereplaced with cesium ions from the solution. The percentage ofcesium exchange increases with an increasing number ofexchange 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 exchangedwith different percentages of cesium ions were determined fromthe N2 adsorption/desorption isotherm, and the results areshown in Table 1. The surface area and micropore volume ofzeolite-X decreases on cesium ion exchange because the sizeand atomic mass of cesium ion is more than exchangeablesodium cation. Generally the surface area and microporevolume decrease upon exchange of sodium ions with ions ofhigher molecular weight and bigger size. The decrease incrystallinity on cesium ion exchange also leads to a decrease inthe surface area.3.4. Adsorption Isotherm and Selectivity. CH4 and N2

molecules can interact with the zeolite surface through latticeoxygen atoms, accessible extraframework cations, and Si and Alatoms. The Si and Al atoms present at the center of tetrahedraare 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 thezeolite surface are through lattice oxygen atoms and extraframe-work cations. The variation in the electrostatic interactionsbetween CH4 and N2 and the extraframework cesium cations ofthe zeolite depend on the difference in their physical properties(Supporting Information Table S1). The pure component CH4and N2 adsorption equilibrium isotherms for cesium zeolite-Xhaving different percentages of cesium ions were generated andare found to be of type I (Figure 1) as per the IUPACclassification. The pure component equilibrium adsorptioncapacities and CH4/N2 selectivity at 760 mmHg and at 288 and303 K (Supporting Information Table S2) were determinedfrom the adsorption isotherms. The CH4/N2 selectivityincreases from 1.8 for NaX to 3.9 for Cs(84)NaX. Above36% cesium ion exchange the selectivity increases with anincreasing degree of cesium ion exchange, due to an increase inCH4 adsorption capacity and a decrease in N2 adsorptioncapacity.On maximum cesium ion exchange, the CH4 adsorption

capacity increases from 10.1 to 14.9 while the N2 adsorptioncapacity decreases from 5.5 to 3.8 molecules/(unit cell) at 303K and 760 mmHg. The major interactions of N2 with zeoliteNaX and Cs(84)NaX are field gradient quadrupole and fieldinduced dipole interaction which are inversely proportional toionic radii of cations. Since cesium has ionic radii (167 pm)higher than that of sodium ion (97 pm), cesium zeolite-Xshowed a lower electrostatic interaction potential whichresulted in less N2 adsorption capacity. The ionic radius ofcation was important for both nonspecific and specific(electrostatic) interactions while cationic charge is importantonly for electrostatic interactions.

CH4 has neither a dipole nor a quadrupole moment but hashigh polarizability (26 × 10−25 cm3); therefore, field induceddipole interactions dominates. The dispersion interactionpotential for CH4 increases with polarizability of the surfaceions. The polarizability of cesium ions (1.73 × 10−24 cm3) ishigher than that of sodium ions (0.18 × 10−24 cm3); thus,Cs(84)NaX showed CH4 adsorption capacity higher than thatof NaX. The adsorption capacity also depends on the polarizingpower of extraframework cations. The polarization power ofcesium ions (9.58 × 10−10 m−1) is less than that of sodium ions(16.5 × 10−10 m−1) though cesium exchanged zeolite showedincreased methane adsorption capacity, this is, due to shieldingof smaller size sodium cation by the first few methanemolecules of a cavity (first shell of adsorbate), which wouldlimit sodium ions interaction with other methane molecules.The larger cesium cation would experience less shielding andhence would be able to affect a larger number of methanemolecules in the outer shell, even though the imposed potentialis less than that imposed by a small sodium cation. Moreover,CH4 is nonpolar, but due to asymmetric vibrations it acquiredsome polar character. When CH4 molecules are in closeproximity to the cation within the structure and frameworkoxygen atoms, they cause an instantaneous shift in the timeaveraged neutral electrostatic field of CH4 and this inducedpolarity also results into the high adsorption capacity of CH4.

3.5. Structure, Cation Locations and AdsorptionCapacity in Zeolite-X. Zeolite-X is a synthetic aluminum-rich analogue of the naturally occurring mineral faujasite, havinga structure as shown in Figure 2. The 14-hedrons with 24vertices known as the sodalite cavity or β-cage may beconsidered as its principal building block. These β-cages areconnected tetrahedrally through six-member rings by bridgingoxygen to give double six-member rings (D6R, hexagonalprisms) and concomitantly, an interconnected set of even largercavities (supercage), accessible in three dimensions through 12-ring windows. The Si and Al atoms occupy the vertices of thesepolyhedra while oxygen atoms lie approximately midwaybetween each pair of Si and Al atoms but are displaced fromthose points to give near-tetrahedral angles about Si and Al.Silicon and aluminum atoms alternate at the tetrahedralintersections, except that Si substitutes for Al at about 8% ofthe Al positions. Single six-member rings (S6R) are shared bysodalite and supercage and may be viewed as the entrances tothe sodalite units. Each unit cell has 8 sodalite units, 8supercages, 16 D6R, 16 12-rings, and 32 S6R. The extraframe-work sodium and cesium cations which balance the negativecharge of the aluminosilicate framework are found at differentsites (Figure 2) within the zeolite cavities. Site I is located at thecenter of the D6R, site I′ is in the sodalite cavity on theopposite side of one of the D6R, sites I and II′ are inside thesodalite cavity near a S6R, site II is at the center of the S6R ordisplaced from this point into a supercage, site III is in thesupercage on a 2-fold axis opposite a 4-ring between two 12-

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

samples unit cell formula dry (wt %) micropore vol (cm3/g) BET surface area (m2/g) micropore surface area (m2/g)

NaX Na(88)Al(88)Si(104)O(384) 0.30 692 647Cs(36)NaX Cs(32)Na(56)Al(88)Si(104)O(384) 0.26 594 544Cs(53)NaX Cs(47)Na(41)Al(88)Si(104)O(384) 0.22 530 487Cs(68)NaX Cs(60)Na(38)Al(88)Si(104)O(384) 0.19 493 453Cs(84)NaX Cs(74)Na(14)Al(88)Si(104)O(384) 0.17 423 392



rings, and site III′ is somewhat or substantially off site III (offthe 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 thecrystal structure of NaX, 30 Na+ ions are located at site I′ and32 Na+ ions at site II, and the remaining 26 Na+ ions arelocated at site III′. Cations at sites I, I′, and II′ are the mostdifficult to exchange. Cesium ions (diameter, 3.38 Å) are too

large under normal conditions to pass through the 2.2 Å six-ring window openings, but they are observed at the sites in theD6R and β-cages.45 The maximum cesium ion exchange inzeolite-X has been reported to be around 82%.46 The ionexchange selectivity varies with the degree of cation exchange.Below a level of 40% exchange the selectivity series for alkalimetal ions was observed in terms of decreasing selectivity, to beCs > Rb > K > Na > Li. This series corresponds to the

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.



occupancy of the most accessible cation sites, i.e., the supercagein zeolite-X. At 50% exchange, which includes site II′ in the six-ring adjacent to the supercage, the selectivity series was foundto be Na > K > Rb > Cs > Li.46

In a single unit cell of zeolite-X there are 192 possible cationsites for 88 cations. On activation of zeolite at 350 °C undervacuum the cesium and sodium ions migrate to the sites oflower energy with maximum coordination. Migration of cationis a process which depends on the temperature, time ofactivation, and size of the cation. The sites with the leastenergies are hidden and are not exposed to the supercagecavity; thus, only 40−50% of cations are located at exposedsites.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 ispartially shielded by the surrounding oxygen atoms. Because ofthis shielding the electrostatic and induction interactions areexpected to be lower than those of an isolated ion. Further, thedispersion forces acting on the molecule will be higher becausethe 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 high-temperature activation the ions migrated inside the β-cages andsix-member ring from the supercage due to cation crowding bylarge size cesium ions in the super cage and thus not availablefor adsorption of gas molecules. Cs(53)NaX shows adsorptioncapacity more than that of Cs(36)NaX because due to thelarger size of the cesium ions the cation crowding inside the β-cages increases; that shifts the site II ions inside the supercagesand thus increases their interactions with CH4 and N2molecules.45 The CH4 and N2 adsorption capacity in cesiumexchange zeolite follows the order Cs(36)NaX, Cs(53)NaX,Cs(68)NaX, and Cs(84)NaX. Above 36% Cs ion exchange, theadsorption capacity increases with increasing cation crowding

and cation shift toward the supercage. Moreover, at a highdegree of cesium exchange the number of cesium ions in the β-cages also increases. Moreover, due to the large size of thecesium cations they do not sit crystallographically very low inthe face of the single six rings (SR6, the SII position), allowingthe electric field to be poorly shielded by the surroundingframework oxygen and thus interact strongly with CH4 and N2molecules.

3.6. Langmuir Model Fitting, and Henry’s Constant.The adsorption data obtained at 288 and 303 K were fitted intothe Langmuir model; and the values of the slope and Langmuirconstant (b) for the adsorption of CH4 and N2 on zeoliteCs(84)NaX and NaX are given in Supporting InformationTable S3. The Langmuir model fitted well for methane andnitrogen adsorption at both 288 and 303 K. On cesium ionexchange the values of slope and b decrease for N2 adsorption,due to a decrease in the strength of the quadrupolar interactionpotentials. However, the slope and b increase for CH4adsorption. The high values of slope and b for methaneadsorption are due to an increase in the dispersion interactions.The opposite trend in the values of slope and b for N2 and CH4adsorption is in agreement with their heats of adsorption,Henry’s constants, adsorption capacities, and cesium cationpositions 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 thevirial coefficients and Henry’s constants for the adsorption ofCH4 and N2 on zeolite Cs(84)NaX and NaX are given inSupporting Information Table S4. Henry’s constant is themeasurement for the strength of adsorption interactions andheat of adsorption. The higher Henry’s constant is the higherwill be the heat of adsorption. On cesium ion exchange thevalue of Henry’s constant K for the N2 adsorption decreaseswhile that for CH4 adsorption increases. The magnitude ofHenry’s constant is higher for CH4 as compared to N2. Thevalue of Henry’s constant confirms the strong dispersioninteractions of the CH4 molecule with the cesium cations in thezeolite-X.

3.7. Dynamic Adsorption from the (CH4 + N2) BinaryGas Mixture. The pore opening and cage structure of cesiumion exchanged zeolite-X is large enough to neglect any stericeffects of the gas molecules with the adsorbent structure.However, the cesium ion location, number, and nature of theadsorbent surface are predominantly the cause of the differencein the dynamic adsorption capacity of CH4. The breakthroughmeasurements for methane from (CH4 + N2) binary gasmixtures on Cs(80)NaX were carried out at 303 K and 1 atmpressure with feed flow of 100 mL/min. The breakthroughresults are given in Supporting Information Table S5. Figures 3and 4 show the CH4 adsorption and desorption breakthroughcurves for methane−nitrogen binary gas mixture on zeolite-NaX 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 stoichiometriccapacity increases with an increase in the percentage ofmethane in the feed gas mixture. The increase in stoichiometricadsorption capacity was due to an increase in the partialpressure of CH4. The comparable breakthrough time (5−6min) for mixtures having different compositions indicated theweak adsorbent−adsorbate interactions as the diffusion of gases

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



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

4. CONCLUSIONEquilibrium adsorption measurements for the adsorption ofCH4 and N2 are performed in zeolite-X with differentpercentages of cesium exchange. More than 36% cesium ionexchanged zeolite-X samples shows increased CH4 selectivityover N2 due to increase in methane and decrease in nitrogenadsorption capacity. The adsorption properties of ionexchanged zeolite was studied in correlation with the cesiumion position in the zeolite. The adsorption capacity for CH4 andN2 decreases for the low cesium ion exchange zeoliteCs(36)NaX, due to a decrease in the number of cations inthe supercage. The large size cesium ions causes cationcrowding in the supercage and forced sodium ions to migrateinside less accessible β-cages. However, on further increase incesium exchange, the adsorption capacity significantly increasesdue to an increased number of cesium ions in the super cage forthe adsorption of CH4 and N2. The adsorption capacity of

cesium exchange zeolite-X depends on the degree of ionexchange and cation migration in β-cages. The methane andnitrogen adsorption isotherms show good Langmuir modelfitting. On cesium ion exchange the values of the slope andLangmuir constant, and Henry’s constant, decrease for N2adsorption, while they increase for CH4 adsorption. Dynamicmixed gas breakthrough adsorption studies are carried out fordifferent CH4 and N2 gas mixtures. The dynamic mixture gasstudy shows CH4 selectivity over N2. However, the CH4breakthrough time for different CH4 + N2 gas mixtures isapproximately similar and the CH4 stoichiometric adsorptioncapacity increases with increasing CH4 percentage in the gasmixture due to the linear nature of CH4 and N2 adsorptionisotherms.

■ ASSOCIATED CONTENT*S Supporting InformationTables listing physical properties of CH4 and N2 gas moleculesand CH4 and N2 adsorption capacity, selectivity, Langmuirmodel parameters, Henry’s constant, virial coefficient, anddynamic adsorption capacity for sodium and cesium ionexchanged zeolite-X and figures showing the experimentalsetup for dynamic adsorption studies, X-ray powder diffractionpatterns, and SEM images for sodium and cesium exchangedzeolite-X. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Address†Center for Catalysis Research and Innovation, BiosciencesComplex, University of Ottawa, 30 Marie-Curie, Ottawa,Ontario, Canada K1N 6N5.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSCSMCRI 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 researchfellowship. The authors also thankful to Analytical ScienceDiscipline of CSMCRI for providing analytical facilities.

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

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) methanedesorption breakthrough curve for a 90% methane and 10% nitrogengas mixture on Cs(80)NaX at 303 K and 1 atm pressure.



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