ion-exchanged sapo-34 membranes for light gas separations

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Microporous and Mesoporous Materials 106 (2007) 140–146

Ion-exchanged SAPO-34 membranes for light gas separations

Mei Hong, Shiguang Li, Hans F. Funke, John L. Falconer *, Richard D. Noble

Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0424, United States

Received 9 October 2006; received in revised form 16 February 2007; accepted 20 February 2007Available online 25 February 2007

Abstract

Ion exchange of H–SAPO-34 zeolite membranes with Li+, Na+, K+, NHþ4 , and Cu2+ cations in non-aqueous solutions increasedCO2/CH4 ideal and separation selectivities up to 60%, but increased H2/CH4 ideal and separation selectivities less than 18%. Ionexchange decreased permeances, and the decrease was larger for large cations, apparently due to steric hindrance. Exchange did notdegrade SAPO-34 crystals or membranes, but changed adsorption properties for some ions.� 2007 Elsevier Inc. All rights reserved.

Keywords: Ion exchange; Gas separation; Zeolite membrane; SAPO-34; Gas adsorption

1. Introduction

Zeolites are microporous crystalline materials with uni-form-shaped pores of molecular dimensions. When zeolitecrystals intergrow to form a continuous layer with fewdefects, the resulting membranes can effectively separateliquid and gas mixtures due to preferential adsorption, dif-ferences in diffusion rates, and/or molecular sieving.Because ion exchange can change both adsorption and dif-fusion properties of zeolites [1], it was used in this study inan attempt to increase H2/CH4 and CO2/CH4 separationselectivities in SAPO-34 membranes. The SAPO-34 struc-ture is three-dimensional, with a pore diameter of0.38 nm, as determined by X-ray diffraction (XRD). Silico-aluminophosphates (SAPO) have cations for charge com-pensation because Si isomorphously substitutes for P inAlPO4, and H–SAPO-34 has mild to moderate acidity [2].

Ion exchange has been reported previously to changezeolite membrane permeance and selectivity. Permeanceswere lower for NaY zeolite membranes exchanged withalkaline earth cations than with alkali cations [3]. TheCO2/N2 separation selectivity of a NaY membrane

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doi:10.1016/j.micromeso.2007.02.037

* Corresponding author. Tel.: +1 303 492 8005; fax: +1 303 492 4341.E-mail address: [email protected] (J.L. Falconer).

increased from 19 to 34–40 following exchange with K+,Rb+, and Cs+ [4] and decreased from 30 to 9.9 followingexchange with Li+ [5]. Some ion exchange salts remainedif the membranes were not completely washed, and theypartially blocked pores. Thus, when washing was incom-plete for Rb and Cs salts in NaY membranes, the CO2/N2 selectivity increased to 59–149 and the CO2 permeancedecreased significantly [6].

Both the Si/Al ratio and the ion size affected permeancesof ion-exchanged ZSM-5 membranes [7]. Single gas perme-ances increased in the order: K+ < Ba2+ � Ca2+ < Cs+ <Na+ � H+; except for Cs+, this trend follows decreasingion size. Guan et al. [8] found that as the pore sizedecreased for ion-exchanged LTA membranes, the perme-ance decreased. The CO2/N2 separation properties alsodepended on adsorption properties of LTA membranes[8]. For ETS-4 membranes, when Na+ was exchanged withLi+ or Sr2+, N2/O2 and N2/CH separation selectivitiesincreased, but permeances decreased [9].

The SAPO-34 membranes in the current study were syn-thesized on the inside of tubular stainless steel supports[10–12]. Previous SAPO-34 membranes had H2/CH4 mix-ture selectivities of 8 up to 480 K [11], and the highestCO2/CH4 separation selectivity was 170 at 297 K [13],but the starting membranes used in this study had lowerCO2/CH4 selectivities.

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M. Hong et al. / Microporous and Mesoporous Materials 106 (2007) 140–146 141

2. Experimental methods

2.1. Preparation and ion exchange of SAPO-34 membranes

The SAPO-34 membranes were prepared by in situ crys-tallization onto tubular, stainless steel supports (0.8-lmpores, Pall Corp.). Non-porous, stainless steel tubes werewelded onto each end of the supports. The permeate areawas approximately 7.8 cm2. Before synthesis, the supportswere boiled in distilled water for 3 h and dried at 373 Kunder vacuum for 30 min.

For membranes 1–8, the synthesis gel had a molar com-position of Al2O3:P2O5:0.6SiO2:1.07TEAOH:56H2O [11],which corresponds to a Si/Al ratio of 0.3. For membrane9, the gel composition was Al2O3:P2O5:0.3SiO2:1.2-TEAOH:55H2O, which has a Si/Al ratio of 0.15. The gelwas prepared by stirring H3PO4(85 wt% aqueous solution),Al(i-C3H7O)3 (>99.99%, Aldrich), and H2O at room tem-perature for 12 h. The template, tetra-ethyl ammoniumhydroxide (TEAOH, 35 wt% aqueous solution, Aldrich),was then added, and the mixture was stirred for 30 minbefore the colloidal silica sol (Ludox AS40, 40% aqueoussolution, Aldrich) was added. The pH of the synthesis gelwas about 3.5. The outside of the support tube waswrapped in Teflon tape and placed in an autoclave withsynthesis gel both inside and outside of the tube. Hydro-thermal synthesis was carried out at 468 K for 20 h, themembrane was then washed with distilled water and driedat 373 K in a vacuum oven for 2 h. Three additional layerswere applied using the same procedure. The membraneswere calcined in static air at 663 K for 20 h, with heatingand cooling rates of 0.6 and 0.9 K/min, respectively, andthey were stored in a 473 K oven.

The acetate salts (Aldrich) of Li+ (99.99%), Na+

(99+%), K+(99+%), NHþ4 ð99:999%Þ, and Cu2+ (monohy-drate, 98+%) were dissolved in ethanol for ion exchange.Each membrane was tied to a magnetic stirrer with Teflontape and placed in exchange solution in a flask that had areflux system. The membranes were stirred at �100 rpm in150 mL of the exchange solution at 348 K for 4 h. Mem-brane 7 was exchanged at 328 K in methanol solvent. Afterexchange, the membranes were washed in �100 mL etha-nol three times, for 30 min with each time, while stirring

Table 1Ion exchange conditions for SAPO-34 membranes

Membrane Precursor ions

Ion Ion radius (nm)

1 Li+ 0.0682 Na+ 0.0983 Na+ 0.0984 Li+ 0.0685 Cu2+ 0.0726 NHþ4 0.1437 Cu2+ 0.0728 K+ 0.1339 Li+ 0.068

at �100 rpm. They were dried at 340 K under vacuumfor 2 h and then at 473 K overnight. The exchange processwas repeated for most membranes (Table 1).

2.2. Membrane permeation and separation

Single-gas permeances were measured in a dead-end,stainless steel module that had one end of the membranetube blocked off. Fifty/fifty mixtures of H2/CH4 andCO2/CH4 were separated in a continuous flow system with-out a sweep gas. The pressure drop across the membranewas 138 kPa, and the permeate side pressure was 84 kPaif not otherwise noted. Before permeances were measuredwith a soap film flowmeter, the membrane was heated to473 K and both sides were swept with the gas to be studied.The lowest measurable flow rate was estimated to be1 · 10�11 mol m�2 s�1 Pa�1. Compositions were analyzedon-line by a HP 5890 or a SRI 8610C GC with a thermalconductivity detector. Log-mean partial pressure differ-ences were used to calculate permeances. To minimizewater adsorption, the membranes were removed from theoven, which was at 473 K, and quickly placed in the mod-ule under flowing gas. This caused the membrane tempera-ture to drop as much as 30 K/min, but permeationmeasurements were reproducible.

2.3. Zeolite crystal characterization

SAPO-34 crystals (Si/Al ratio of 0.3 in the gel) were col-lected from the bottom of the autoclave after membranesynthesis and washed with deionized water by repeatedcentrifugation, decanting, and redispersion until the pHof the upper solution was less than 8. The crystals were cal-cined at 823 K for 8 h in static air, and then ion exchangedat 348 K for 4 h in 0.01 M copper(II) acetate or 0.1 M lith-ium acetate solutions in ethanol. The crystals were cleanedwith ethanol by repeated centrifugation, decanting, andredispersion to wash off excess acetate salts. The crystalswere then dried in a vacuum oven at 340 K for 2 h and thenovernight at 473 K. The crystal structure was determinedby XRD (Scintag PAD-V diffractometer, Cu Ka radia-tion), and the chemical composition was measured byICP (Varian UltraMass 700 inductively coupled plasma,

Ion concentration (M)

First time Second time Third time

0.1 0.1 –0.01 0.01 –0.01 0.01 –0.01 0.1 0.10.01 0.1 –0.01 0.1 –0.01 0.01 0.010.01 0.01 –0.1 – –

Fig. 2. Adsorption isotherms for N2 on SAPO-34 crystals at 77 K.

Table 2

142 M. Hong et al. / Microporous and Mesoporous Materials 106 (2007) 140–146

standard deviation ±5%). Adsorption isotherms were mea-sured in an Autosorb-1 system (Quantachrome Corp.).Samples were outgassed in vacuum at 493 K for about12 h before adsorption. The standard deviation for adsorp-tion loading was ±5%. The BET area and the microporevolume (Dubinin–Radushkevich (DR) equation) weredetermined with N2 adsorption at 77 K. The standard devi-ation for BET area and DR micropore volume was ±3%.

3. Results and discussion

3.1. Crystal characterization

The unit cell composition, for H–SAPO-34 crystals syn-thesized from a gel with a Si/Al ratio of 0.3, was deter-mined by ICP to be H2:8Si3:9Al17:4P14:7O72. The H and Ostoichiometries were calculated for an ideal SAPO-34 struc-ture. The calculated acid site density is 2.8 H+ ions per unitcell, and each unit cell has 3 CHA cages, so on average,almost one cation resides in each cage. About 22% of theH+ sites exchanged with Cu2+ in 0.01 M CuAc2 solution,and almost 100% exchanged with Li+ in 0.1 M LiAc.

As shown in Fig. 1, when SAPO-34 crystals wereexchanged with Cu2+ and washed once, copper acetatesalts [14] were detected by XRD. These salts were removedafter 3 washings. The XRD powder patterns did notchange after exchange with Cu2+ or Li+ and completewashing, and all the patterns matched the intensities andline positions reported for SAPO-34 [14,15]. Exchangedid not degrade the crystals, whereas Frache et al. observedthat H–SAPO-34 lost much of its crystallinity followingCu2+ exchange in 0.1 M CuAc2 aqueous solution at323 K for 5 h [16]. Prolonged exposure to water vaporhas been reported to permanently degrade H–SAPO-34crystals because Si–OH–Al bonds break upon hydration[17]. The use of ethanol instead of water solvent mayaccount for the SAPO-34 crystal stability in the currentstudy.

Fig. 1. XRD patterns of SAPO-34 crystals; the asterisk denotes peaksfrom the supporting aluminum plate.

3.2. Adsorption

As shown in Fig. 2, the N2 saturation loading was iden-tical for the H and Cu forms of SAPO-34, but it was 17%lower for Li–SAPO-34. Thus, Cu2+ exchange had almostno effect on the BET surface area and micropore volume(Table 2), perhaps because only 22% of the sites exchangedwith Cu2+. In contrast, Li+ exchange decreased BET areaand pore volume by approximately 15%, and, as shownbelow, the CO2 and CH4 adsorption loadings alsodecreased. The monovalent cations should occupy thesame locations in SAPO-34 [18], and the ionic radiusincreases in the order: Hþ < Liþ < Naþ < Kþ < NHþ4 , sothe larger monovalent cations might decrease the pore vol-ume even more for the same degree of exchange.

Carbon dioxide adsorbs more strongly than CH4 onSAPO-34 [12], and CO2 and CH4 isotherms are shown inFigs. 3 and 4 for a wide temperature range for H–SAPO-34. The isotherms were similar for Cu–SAPO-34, but for

Adsorption properties of SAPO-34 crystals

Sample H–SAPO Cu–SAPO Li–SAPO

BET surface area (m2/g) 490 490 420Micropore volume (cm3/g) 0.26 0.26 0.22qsat (mmol/g)

CO2 195 K 7.1 7.2 6.0CH4 143 K 6.1 6.3 5.3N2 77 K 7.6 7.6 6.4

qsat (molecules/u.c.)CO2 195 K 16 16 13CH4 143 K 13 14 12N2 77 K 17 17 14

Adsorbed phase density (g/cm3)CO2 195 K 1.1 1.2 1.2CH4 143 K 0.37 0.39 0.38�DH (kJ/mol)

CO2 24.8 ± 3.3 29.8 ± 0.7 25.0 ± 1.5CH4 16.8 ± 0.7 16.6 ± 0.7 16.4 ± 0.4�DS (kJ/mol K)

CO2 �124 ± 12 �142 ± 3 �120 ± 6CH4 �116 ± 3 �114 ± 3 �112 ± 2

Fig. 3. Adsorption isotherms for CO2 on H–SAPO-34 crystals.

Fig. 4. Adsorption isotherms for CH4 on H–SAPO-34 crystals.

Fig. 5. Adsorption isotherms for CH4 on SAPO-34 crystals at 143 K.


Li–SAPO-34, the CO2 and CH4 saturation loadings wereabout 14% lower. As an example, CH4 isotherms at143 K are shown Fig. 5 for H–, Cu–, and Li–SAPO-34crystals. The saturation loadings qsat in Table 2, calculatedby fitting the lowest-temperature isotherms to a Langmuirisotherm, are higher than we previously estimated for H–SAPO-34 from isotherms at 253 K [12]; the pressures usedwere not high enough to saturate the pores at 253 K. Theadsorption equilibrium constants, b, were obtained forLangmuir isotherms, using the same qsat values (Table 2)for all temperatures, and these b values were used to esti-mate heats and entropies of adsorption (Table 2). Thesevalues for H–SAPO-34 were almost the same as determinedpreviously [12]. Although Cu2+ only exchanged 22% of theH+ sites, it increased the CO2 heat of adsorption by 5 kJ/mol, whereas 100% Li+ exchange did not significantlychange the heat of adsorption. Neither Cu2+ nor Li+ chan-ged the heat of adsorption for CH4.

The ions in SAPO-34 affect CO2 adsorption (Table 2)because they change both the basicity of the zeolite frame-work and electrostatic interactions between the extraframework cations and adsorbed molecules [19]. The cat-

ion site is acidic, and the framework oxygen nearest it isbasic. The basicity increases as the cation electronegativitydecreases [19,20], and the cation electronegativity decreasesin the order: H+ > Cu2+ > Li+ > Na+ > K+. The higher thebasicity, the more strongly the framework bonds to theweakly acidic CO2. Thus H–SAPO-34, which is the leastbasic structure, should adsorb CO2 less strongly. Electro-static interactions, however, also affect adsorption strength.The charge density of ions affects the electric field inducedby the ion, which can polarize and attract CO2 because ithas a large quadrupole moment. The charge density ofthe cations decreases in the order: Hþ > Cu2þ >Liþ > Naþ > Kþ > NHþ4 . Cations with higher charge den-sity are expected to interact more strongly with CO2. SinceH+ has a higher charge density than the metal cations, H–SAPO-34 should adsorb CO2 more strongly based on elec-trostatic interactions. Thus, ion exchange increased theframework basicity but decreased the strength of ion-quad-rupole interaction. For Cu2+ exchange, the increase inacid–base interaction apparently dominated, and the CO2

heat of adsorption increased by 5 kJ/mol even though only22% of the sites exchanged. For Li+ exchange, however, theincrease in acid–base interaction was apparently offset bythe decrease in ion-quadrupole force, and the CO2 heatof adsorption did not change.

Hydrogen and CH4 are neither acidic nor basic, andthus their adsorption strength should be mainly determinedby electrostatic interactions. Because H+ has a highercharge density than the other cations, and H2 has a smallquadrupole moment, ion exchange of H–SAPO-34 isexpected to decrease the cation–quadrupole interaction,and thus the H2 adsorption strength. Methane has noquadrupole moment and only a small octopole moment.Because the ion–octopole interaction is weak, ion exchangedid not alter the CH4 enthalpy (Table 2).

For a micropore volume of H–SAPO-34 of 0.26 cm3/g(Table 2), the adsorbed-phase density for CO2 at saturationat 195 K (qsat = 7.1 mmol/g) was 1.1 g/cm3, which isbetween the densities of CO2 liquid (1.0 g/cm3 at 253 Kand 19.7 bar) and solid (1.6 g/cm3). The larger CH4 didnot pack as efficiently in the SAPO-34 pores, so its


adsorbed-phase density at saturation at 143 K was 0.37 g/cm3, which is 88% of the liquid CH4 density of 0.42 g/cm3 at 111 K. The adsorbed phase densities were similarfor Cu–SAPO-34 and Li–SAPO-34 (Table 2).

3.3. Single gas permeation

As shown in Fig. 6, the H2 and CO2 permeancesdecreased with increasing temperature for H–SAPO-34and Li–SAPO-34 membranes, whereas the CH4 permeancewas relatively independent of temperature. Thus, the H2/CH4 and CO2/CH4 selectivities decreased with increasingtemperature (Fig. 7). The H2, CO2, and CH4 single-gas per-meances decreased after Li ion exchange (membrane 9),and the percent decrease was higher for CH4 than for H2

or CO2 (Fig. 6), so the H2/CH4 and CO2/CH4 ideal selec-tivities increased after Li ion exchange (Fig. 7). Similarbehavior was observed for Na+-, NHþ4 -, and Cu2+-exchanged membranes (Table 3). At 295 K, CO2 (0.33 nmkinetic diameter) permeated faster than H2 (0.29 nm), but

Fig. 6. Temperature dependencies of single gas permeances for membrane9 before (solid line) and after (dashed line) Li+ exchange.

Fig. 7. Temperature dependencies of H2/CH4 and CO2/CH4 idealselectivities for membrane 9 before (solid line) and after (dashed line)Li+ exchange.

CO2 permeance decreased more with temperature, so thatat 473 K, H2 permeated faster (Fig. 6). Methane(0.38 nm) permeated significantly slower than either H2

or CO2, and thus its permeances are multiplied by 10 inFig. 6. As shown in Fig. 8 for the H–SAPO-34 membrane,the H2 and CH4 single-gas permeances decreased slightlywith increased pressure drop, but the CO2 permeancedecreased more because CO2 adsorption was not in theHenry’s regime at 295 K. Ion exchange did not changeO2/N2 ideal selectivities; they were 1.6–1.8 for both H–SAPO-34 and exchanged membranes.

3.4. Separations

The CO2/CH4 separation selectivities for most mem-branes were around 60 at 295 K. These selectivities arehigher than the corresponding ideal selectivities [12]because CO2 adsorbs more strongly than CH4 (Table 2).Ion exchange increased CO2/CH4 separation selectivitiesand decreased CO2 permeances, as shown in Table 4.Repeating the exchange (Table 1) had almost no effect,except for K+ and NHþ4 , the largest cations (membranes6 and 8). The CO2 permeance decreased and the CO2/CH4 selectivity increased after the second K+ exchange.For NHþ4 exchange, the permeance decreased and selectiv-ity increased after the first exchange (0.01 M), but the sec-ond exchange (0.1 M) decreased selectivity below that ofthe original membrane and reduced CO2 permeance bymore than 50%. These decreases might be due to the com-petition between an increase in CO2 adsorption, becausethe NH4–SAPO-34 is more basic [21,22], and a significantdecrease in diffusivity because NHþ4 has a radius of0.143 nm, and is the biggest monovalent cation used.

The percent increase in CO2/CH4 separation selectivitydue to Li+ exchange was smaller for membrane 9 thanfor membranes 1 and 4. Membrane 9 was synthesized bya procedure that used more template and less silica; theSi/Al ratio in the gel for membrane 9 was 0.15, whereasit was 0.3 for the other membranes. Therefore, membrane9 probably had fewer acid sites for exchange, and thusexchange should affect separations less.

The CO2 permeances for the CO2/CH4 mixture werehigher than for the single gas, partly because the CO2 par-tial pressure drop across the membrane (�28 kPa) waslower for the mixture. The total pressure drop (138 kPa)was the same, but the feed mixture had only 50% CO2

and the permeate was enriched in CO2. The CH4 perme-ances were almost the same for the two measurementsbecause the CH4 partial pressure drop (�110 kPa) in themixture was close to the CH4 pressure drop (138 kPa) usedfor single-gas measurements, and CH4 adsorption was inthe Henry’s regime at 295 K.

Ion exchange also increased the H2/CH4 separationselectivity, but by less than 10% for most ions (reproduc-ibility was 7%); for Cu2+ exchange the selectivity increasedfrom 16 to 19 at 473 K (18% increase, membrane 7). Thehighest H2/CH4 separation selectivity at 473 K was 32 for

Table 3Single gas CO2 permeances and ideal selectivities for SAPO-34 membranes

No. Iona T (K) CO2 permeance · 108 (mol m�2 s�1 Pa�1) Ideal selectivity

H2/CH4 CO2/CH4

Original Exchanged Original Exchanged Original Exchanged

2 Na+ 295 5.6 5.1 21 27 36 46473 0.97 0.81 16 20 8.2 8.8

3 Na+ 295 6.4 5.7 20 22 32 36473 1.2 1.1 12 13 5.6 6.3

4 Li+ 295 6.1 5.0 23 26 37 45473 1.1 0.85 17 20 7.9 9.6

5 Cu2+ 295 4.6 3.9 25 28 46 54473 0.65 0.38 18 19 6.4 7.9

6 NHþ4 295 7.1 5.8 19 29 31 47473 1.3 1.2 16 18 8.5 11

9 Li+ 295 7.8 5.9 46 61 82 118473 1.5 1.2 32 32 19 21

a Ion exchange was conducted only once.

Fig. 8. Pressure dependencies of single gas permeances for membrane 2 (inH-form) at 295 K and permeate pressure of 84 kPa.


membrane 9 before exchange and 35 after Li+ exchange.The H2/CH4 separation selectivities were lower than theideal selectivities (Table 3).

Ion exchange decreased permeances for H2/CH4 mix-tures, as shown in Fig. 9 for Li+ exchange. The H2 perme-

Table 4CO2 permeances and CO2/CH4 separation selectivities at 295 K for SAPO-34

Membrane Ion CO2 mixture permeance · 108 (mol m�2 s�1

Original First

1 Li+ 11 8.02 Na+ 8.8 7.43 Na+ 12 7.64 Li+ 10 7.05 Cu2+ 8.4 5.26 NHþ4 13 7.67 Cu2+ 8.9 –8 K+ 10 7.39 Li+ 11 6.3

ances decreased slightly as the temperature increased from297 to 473 K, but the H2/CH4 separation selectivity did notchange significantly. Similar behavior was observed forNa+-, K+-, NHþ4 -, and Cu2+-exchanged membranes. Thelargest decrease in H2 permeance was for the biggest cat-ions, apparently because the steric hindrance was larger.The H2 permeance decreased 61% after NHþ4 exchange(membrane 6), and 37% after K+ exchange (membrane8), whereas other ions decreased the H2 permeance lessthan 20%. The decrease in permeances after exchange indi-cates that exchange in non-aqueous solutions did not destroymembrane integrity. Poshusta et al. found that water chan-ged intercrystalline and intracrystalline pores in SAPO-34membranes so that permeances increased [23].

The changes in H2/CH4 and CO2/CH4 selectivities fol-lowing ion exchange were due to changes in both adsorp-tion and diffusion. Since the kinetic diameter of CH4 isclose to the SAPO-34 pore size, CH4 permeance throughSAPO-34 pores is expected to be decreased more by ionexchange than CO2 or H2. Thus, the H2/CH4 and CO2/CH4 selectivities increased after exchange (Tables 3 and4), except for NHþ4 exchange at high concentration.

Competitive adsorption affected H2/CH4 and CO2/CH4

selectivities more at lower temperatures, where coverages

membranes

Pa�1) CO2/CH4 separation selectivity

Second Original First Second

8.0 66 86 877.4 59 73 757.7 64 66 707.0 60 76 775.2 64 74 813.6 55 72 487.8 49 – 695.1 40 55 64– 136 144 –

Fig. 9. Temperature dependencies of H2 permeance and H2/CH4 separa-tion selectivity for membrane 4 before (solid line) and after (dashed line)Li+ exchange.


are higher. At 297 K and 120 kPa, the CO2 loading was �5times the CH4 loading, and �100 times the H2 loading [12].For H2/CH4 separations, competitive adsorption favorsCH4 permeation, so the H2/CH4 separation selectivitywas lower than the ideal selectivity. Although ion exchangeincreased the difference between H2 and CH4 diffusivities, itmay have also decreased the H2 adsorption strength, sothat the H2/CH4 separation selectivities were almost thesame.

3.5. Reproducibility

Membranes 2 and 3, which were exchanged with Na+

under the same conditions (Table 1), exhibited similartrends, but the CO2 mixture permeances decreased 16%and 35% after exchange, and the CO2/CH4 separationselectivities increased 27% and 9.4% (Table 4). Membranes1 and 4, which were exchanged with Li+ using different Li+

concentrations (Table 1), had almost the same percentincrease in CO2/CH4 and H2/CH4 selectivities and decreasein permeances (Tables 3 and 4). Membranes 5 and 7, whichwere exchanged with Cu2+ at different temperatures andwith different solvents, exhibited similar trends in CO2/CH4 (Table 4) and H2/CH4 separation performance. WhenLi+ exchange on membrane 4 and Cu2+ exchange on mem-brane 7 were repeated a third time, the CO2/CH4 separa-tion selectivity and CO2 permeance did not change.

4. Conclusions

H–SAPO-34 crystals and membranes were ionexchanged with various cations in anhydrous solutionswithout damaging the crystal structure or membrane integ-

rity. For SAPO-34 crystals, ion exchange decreased thepore volume when some ions were exchanged and changedadsorption properties. The H2, CO2, and CH4 permeancesdecreased through SAPO-34 membranes after exchange,and H2/CH4 and CO2/CH4 ideal and separation selectivi-ties increased because of changes in adsorption and diffu-sion properties. The decrease in gas permeances waslarger for exchange with the largest cations, K+ and NHþ4 .

Acknowledgments

We gratefully acknowledge support by the US Depart-ment of Energy (Grant DE-FG26-02NT41536). We thankMiao Yu for valuable discussions.

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ion-exchanged sapo-34 membranes for light gas separations

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