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Available online at www.sciencedirect.com

Journal of Membrane Science 307 (2008) 277–283

Hydrogen purification using a SAPO-34 membrane

Mei Hong, Shiguang Li, John L. Falconer ∗, Richard D. NobleDepartment of Chemical and Biological Engineering, University of Colorado,

Boulder, CO 80309-0424, United States

Received 23 June 2007; received in revised form 17 September 2007; accepted 18 September 2007Available online 25 September 2007

bstract

A SAPO-34 membrane separated CO2/H2 and H2/CH4 mixtures at feed pressures up to 1.7 MPa. Strong CO2 adsorption inhibited H2 adsorptionnd decreased H2 permeances significantly, especially at low temperatures, so that CO2 preferentially permeated and CO2/H2 selectivities wereigher at low temperatures. At 253 K, CO /H separation selectivities were greater than 100 with CO permeances of 3 × 10−8 mol m−2 s−1 Pa−1.

2 2 2

he CO2/H2 separation exceeded the upper bounds (selectivity–permeability plot) for polymer membranes. The SAPO-34 membrane separated2 from CH4 because CH4 is close to the SAPO-34 pore size and has a lower diffusivity than H2. The H2/CH4 separation selectivity had a smallaximum with temperature, and decreased slightly with feed pressure and CH4 feed concentration.2007 Elsevier B.V. All rights reserved.

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eywords: Gas separation; Zeolite membrane; SAPO-34; Hydrogen purificatio

. Introduction

Hydrogen is a clean energy carrier and is in increasingemand in petroleum refining and petrochemical production1–3]. About 80% of hydrogen is produced from steam reform-ng of natural gas [4], and it has to be separated from CO2, CH4,

2O, CO, and other impurities before it can be used in a fuel cell5]. Most H2 is separated by pressure-swing amine adsorption6,7]. Membrane separation is an attractive alternative becausef its ease of operation, low energy consumption, and cost effec-iveness even at low gas volumes [8]. Most membranes for H2eparation preferentially permeate H2, which is obtained fromhe permeate side at low pressures, and it must then be recom-ressed at significant energy cost. Reverse-selective membranes,.e., membranes that preferentially remove larger molecules, can

inimize H2 recompression [9]. Ritter and Ebner overviewedhe issues in H2 production from natural gas steam reforming10]. The objective of the current study was to determine if aeolite membrane has high selectivities for separating hydrogen

rom binary mixtures.

Hydrogen has a higher diffusivity than most molecules ineolite materials, but it adsorbs weakly [11]. Thus diffusion

∗ Corresponding author. Tel.: +1 303 492 8005; fax: +1 303 492 4341.E-mail address: john.falconer@colorado.edu (J.L. Falconer).

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376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.09.031

rbon dioxide; Methane

avors H2 permeation but competitive adsorption favors per-eation of other molecules. Due to the difficulty in preparing

mall-pore zeolite membranes [12], only a few studies haveeported H2 selectivities, mostly ideal selectivities, through zeo-ite membranes [13–23]. The studies that separated H2 mixtures16,19,23] used Wicke-Kallenbach (WK) systems with a He orr sweep gas on the permeate side and no pressure drop across

he membrane. This arrangement does not provide an indicationf the practical application of the membrane for H2 separation.

In this study, we used the pressure-drop method to inves-igate H2 purification with a SAPO-34 membrane. Molecularieve SAPO-34 has a composition of (SixAlyPz)O2, where= 0.01–0.98, y = 0.01–0.60, z = 0.01–0.52, and x + z = y [28].he SAPO-34 structure is analogous to natural zeolite chabazite,hich has a XRD pore diameter of 0.38 nm. The internal cages

re ∼1.4 nm in diameter, and each cage has six pores. Carbonioxide adsorbs more strongly than CH4 and H2 on SAPO-34rystals [25]. Mixtures of CO2/H2 and H2/CH4 were separatedt various temperatures, feed compositions, and feed pressuresp to 1.7 MPa with a SAPO-34 membrane. Due to strong CO2dsorption, CO2/H2 separation selectivities were greater than00 with high CO2 permeances. The membrane separated H2

rom CH4 because CH4 diffused slower than H2. Similar mem-ranes have been used previously to separate CO2/CH4 mixturesith selectivities higher than 150 [24–27] because of strong CO2

dsorption and slow CH4 diffusion.

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rilar to SAPO-34 crystals synthesized from a gel with a Si/Alratio of 0.3 reported previously [25], these crystals adsorbedCO2 more strongly than CH4, and they adsorbed CH4 morestrongly than H2, as shown in Fig. 1. At 295 K and 100 kPa, the

78 M. Hong et al. / Journal of Mem

. Experimental methods

.1. Membrane synthesis and characterization

The SAPO-34 membrane was prepared by in situ crys-allization and is the same membrane (M3) used previouslyor CO2/CH4 separation [26]. The membrane was grownn a tubular stainless steel support (0.8-�m pores, Pallorp.). Non-porous, stainless steel tubes were welded ontoach end of the support to provide non-porous surfaces forealing the membranes in the module. The permeate areaas 7.8 cm2. The synthesis gel had a molar composition ofl2O3:P2O5:0.3SiO2:1.2TEAOH:55H2O, with a Si/Al ratio of.15. The gel was prepared by stirring H3PO4 (85 wt% aque-us solution), Al(i-C3H7O)3 (>99.99%, Aldrich), and H2O atoom temperature for 12 h. The template, tetra-ethyl ammoniumydroxide (TEAOH, 35 wt% aqueous solution, Aldrich), washen added, and the mixture was stirred for 30 min before col-oidal silica sol (Ludox AS40, 40% aqueous solution, Aldrich)as added.The porous support, with its outside wrapped in Teflon tape,

as placed in an autoclave with synthesis gel both inside and out-ide the tube. Hydrothermal synthesis was carried out at 473 Kor 24 h. After synthesis, the membrane was washed with dis-illed water at room temperature and dried at 373 K in a vacuumven for 2 h. Additional layers were applied using the samerocedure, and membrane was prepared with four layers. TheAPO-34 membrane was calcined in air at 663 K for 10 h, with aeating and cooling rate of 0.6 and 0.9 K/min, respectively, andt was stored in a ∼500 K oven. The membrane was about 5 �mhick, as estimated from an SEM image of another membraneynthesized under the same conditions [26].

Some SAPO-34 crystals formed at the bottom of the auto-lave during membrane synthesis. They were cleaned byepeated centrifugation and decanting, and then calcined at73 K for 10 h. Adsorption isotherms on the crystals were mea-ured in an Autosorb-1 system (Quantachrome Corp.). Prioro each adsorption experiment, the sample was outgassed inacuum at 493 K for about 12 h. The standard deviation fordsorption loading was ±5%.

.2. Gas permeation and separation

Fluxes of single gases and mixtures of CO2/H2 and H2/CH4ere measured in a continuous flow system [29]. The mem-ranes were mounted in a stainless steel module and sealed atach end with silicone O-rings. The permeate pressure was about4 kPa and the feed pressure was controlled up to 1.7 MPa withbackpressure regulator. A solid tube was used to verify that the-rings sealed at these pressures. Feed, permeate, and retentateuxes were measured using soap film flowmeters. Their compo-itions were analyzed on-line by a HP 5890 GC with a thermalonductivity detector and a HAYSEP-DB column (Alltech). The

C oven, injector, and detector temperatures were kept at 353,23, and 523 K, respectively. For measurements above roomemperature, the membrane module and some system lines wereeated with a heating tape. For temperatures below room temper-

e Science 307 (2008) 277–283

ture, the module and some lines were immersed in a refrigeratedthylene glycol/water bath controlled by a chiller. The highestemperature investigated was 523 K and the lowest was 253 K.he membrane was calcined at 633 K with a heating and cool-

ng rate of 0.6 and 0.9 K/min, respectively, before the start of theeasurements.For a mixture, the permeance of component i is

i = Ji

�pln,i

(1)

here Ji (mol/(m2 s)) is the steady-state flux of component i.ince the module has a cross-flow design, a log-mean pressurerop (�pln,i, Pa) was used to calculate the driving force fromhe partial pressures

pln,i = (pf,i − pp,i) − (pr,i − pp,i)

ln[(pf,i − pp,i)/(pr,i − pp,i)](2)

here pf,i (Pa), pr,i (Pa), and pp,i (Pa) are partial pressures foromponent i in feed, retentate, and permeate sides, respectively.he ideal selectivity, αideal

i/j , is the ratio of the single-gas per-

eances, and the separation selectivity, αsepi/j , is the ratio of

he permeances for mixtures. In comparison, the compositioneparation selectivity was calculated as

comi/j =

(yi

yj

)/

(xi

xj

)(3)

here x and y are the molar fractions in the feed and permeate,espectively.

. Results and discussion

.1. Adsorption

SAPO-34 crystals were synthesized from the gel with a Si/Alatio of 0.15 and were used for adsorption measurements. Sim-

Fig. 1. Adsorption isotherms at 295 K on SAPO-34 crystals.

M. Hong et al. / Journal of Membrane Science 307 (2008) 277–283 279

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tsH2 single-gas permeance did not depend on pressure, and thusthe H2 single-gas permeance (Fig. 2) was measured at a higherpressure than its partial pressure in the mixture (Fig. 3).

ig. 2. CO2 and H2 single-gas permeances as a function of temperature for aeed pressure of 0.6 MPa for CO2 and 1.4 MPa for H2.

O2 loading was 3.5 mmol/g, whereas it was only 2.8 mmol/gor the crystals with a Si/Al ratio of 0.3. The sorption selectiv-ties were almost the same for the two SAPO-34 crystals. At95 K and 100 kPa, the sorption selectivity was 5 for CO2/CH4nd 100 for CO2/H2. For the previous SAPO-34 crystals, theeat of adsorption was 24 kJ/mol for CO2 and 16 kJ/mol forH4 [25]. Hydrogen adsorption isotherms were measured at 77,43, and 295 K, and its heat of adsorption was calculated to be.7 kJ/mol.

.2. Carbon dioxide/hydrogen separation

Although CO2 adsorbs more strongly than H2, its kineticiameter (0.33 nm) is larger than that of H2 (0.289 nm), and thust diffuses more slowly. Increasing the membrane temperatureecreases the surface loadings but increases the diffusivities.he CO2/H2 ideal selectivity was around 2, and it decreased at

ower temperatures, as shown in Fig. 2. The CO2 and H2 single-as permeances were not strong functions of temperature from53 to 308 K, and the reproducibility for permeances was about7%.The separation selectivities for CO2/H2 mixtures (Fig. 3a)

ere much higher than the ideal selectivities, and at 253 K,he CO2/H2 separation selectivity was over 100. The separa-ion measurements were repeated after calcining the membranet 523 K, and the standard deviations for two sets of data arehown in Fig. 3. Carbon dioxide adsorption significantly inhib-ted H2 adsorption; at 253 K, the H2 permeance in the mixtureas less than 1% of its single-gas permeance. The CO2 inhibitionas much stronger at low temperatures, where the CO2 loadingsere higher, and thus the CO2/H2 separation selectivities wereigher at lower temperatures. The H2 permeance in the mixturencreased over an order of magnitude from 253 to 308 K, andhe membrane became H2 selective at high temperatures, but

2/CO2 selectivity was only 2 at 473 K.Because H2 adsorbs weakly on SAPO-34, it did not affect

O2 permeances much. The CO2 permeance in the mixture wasnly slightly lower than its single-gas permeance (Fig. 4). Note

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ig. 3. CO2/H2 separation selectivities (a), and CO2 and H2 permeances (b) forCO2/H2 mixture (43/57) as a function of temperature at a feed pressure of

.6 MPa from two repeated measurements.

hat the CO2 single-gas permeance was measured at a feed pres-ure similar to the CO2 feed partial pressure in the mixture. The

ig. 4. CO2 permeances for single gas and for CO2/H2 mixtures at 253 K as aunction of partial pressure drop.

280 M. Hong et al. / Journal of Membrane Science 307 (2008) 277–283

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ig. 5. Permeances and CO2/H2 separation selectivity at 253 K as a function ofO2 feed concentration at a feed pressure of 1.16 MPa.

The CO2 loading approached saturation at high CO2 pres-ures, and thus its driving force did not increase linearly witheed pressure [29]. Therefore, the CO2 permeance decreasedignificantly as the pressure drop increased for both single gasnd mixtures (Fig. 4). The log-mean partial pressure drop wassed for mixtures. Hydrogen permeance did not change withressure because H2 adsorption was in the Henry’s adsorptionegime. The single-gas H2 permeance changed less than 3% ashe feed pressure increased from 0.23 to 1.4 MPa at 253 K.

The CO2/H2 separation selectivity at 253 K had a maximumor a feed concentration around 45% CO2 at a feed pressure of.16 MPa (Fig. 5). When the CO2 feed concentration increased,he CO2 flux increased and the H2 flux decreased. For a pressurerop of 1.1 MPa and 12% CO2 in the feed, the CO2 permeate con-entration was 77%. For 43% CO2 in the feed, the CO2 permeateoncentration was 98.8%. The CO2/H2 separation selectivityad a maximum value of 140, as shown in Fig. 5, and theorresponding composition selectivity was 110.

A maximum in CO2/H2 separation selectivity with CO2 feedoncentration was also seen at other feed pressures, as shown inig. 6. Both CO2 and H2 permeances decreased with increasedO2 feed concentration, but the H2 permeance decreased pro-ortionally less at high CO2 concentrations (Fig. 5). The CO2ermeance decreased because the CO2 loading approached satu-ation (Fig. 4), and the H2 permeance decreased because a higherO2 loading decreased the H2 loading. A similar maximum in

electivity was observed when SAPO-34 membranes separatedO2/CH4 mixtures at 253 K [27].

The maximum in CO2/H2 separation selectivity with CO2eed concentration was possibly caused by a maximum in theO2/H2 sorption selectivity. Sweatman et al. [30] found in

imulations that for an equimolar CO2/H2 mixture in a 1.0-m slit pore, the CO2/H2 sorption selectivity had a maximum.

he adsorption selectivity was 300–400 for a CO2 pressuref ∼500 kPa, and it was less than 100 below 50 kPa or above04 kPa. They [30] attributed this to the proximity of the systemo a capillary condensation transition.

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ig. 6. CO2/H2 separation selectivity at 253 K as a function of CO2 feed con-entration at three feed pressures for the SAPO-34 membrane.

Weinberger et al. [31] calculated the adsorption density pro-les of H2 and CO2 in an equimolar mixture in slits of width.0–3.0 nm in porous graphite nanofibers. They found thattrong CO2 adsorption inhibited H2 adsorption; the inhibitionas stronger when CO2 condensed. Whereas the H2 loading

ncreased linearly with pressure in pure H2 gas, in an equimolarixture with CO2, the H2 loading was lower and did not increaseith pressure.Since the CO2/H2 separation is dominated by adsorption, a

aximum in adsorption selectivity would cause a maximum inhe separation selectivity, as seen in Figs. 5 and 6. When the CO2eed pressure was low, increasing the pressure increased the CO2oading. Carbon dioxide inhibited H2 adsorption more stronglyt higher loadings [30,31], and thus decreased H2 permeanceore. However, at CO2 feed partial pressures close to saturation,

ncreasing the CO2 pressure caused a small increase in loading,o the H2 permeance remained constant but the CO2 permeanceontinued to decrease as the CO2 pressure increased (Fig. 5).

.3. Hydrogen/methane separation

The SAPO-34 membrane separated H2 from CH4 with selec-ivities around 20. Methane is similar in size to SAPO-34 pores0.38 nm), so although CH4 adsorbs more strongly, it diffusesore slowly than H2. The H2 permeances in the mixture were

ower (21% or less) than the single-gas H2 permeances, and theH4 permeances in the mixture were higher (less than 15%) than

he single gas CH4, as shown in Fig. 7a. The Maxwell–Stefanodel indicates that the H2 flux through SAPO-34 membrane

t the same H2 pressure should be lower in the mixture becausehe slower-diffusing CH4 slows the faster-diffusing H2, and thetronger adsorbed CH4 inhibits H2 adsorption [33]. The modellso indicates that the CH4 flux should be higher in the mixturehan for the single gas because H2 speeds up CH4 diffusion [33].s a result, the H2/CH4 ideal selectivity was 13–31% higher than

he separation selectivities, as shown in Fig. 7b.The H2 permeance had a maximum and the CH4 permeance

ad a minimum with temperature, both in the single gas andn the mixture. Thus, the H2/CH4 ideal and separation selec-

M. Hong et al. / Journal of Membrane Science 307 (2008) 277–283 281

Fig. 7. Permeances (a) and selectivities (b) for single-gas H2, single-gas CH4,and a H /CH mixture (54/46) as a function of temperature at a feed pres-see

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i[amFbtceabound (Fig. 10). The maximum CO2/H2 selectivity was 150, andthe maximum CO2 permeance was 5.7 × 10−8 mol/(m2 s Pa).The permeabilities for the SAPO-34 membrane were calculatedbased on the SEM membrane thickness of 5 �m [26]. The sep-

2 4

ure of 1.6 MPa. The closed symbols with solid lines are from single gas (S)xperiments, and the open symbols with dashed lines are from mixture (M)xperiments.

ivities had slight maxima with temperature (Fig. 7b), whereashe selectivity decreased with temperature for SAPO-34 mem-ranes synthesized from other gels [32]. The Si/Al ratio was 0.3or the membranes reported previously [32] and it was 0.15 forhe current membrane.

The single-gas CH4 permeance changed less than 7% as theeed pressure increased from 0.28 to 1.6 MPa, because CH4dsorption was in the Henry’s regime. In a H2/CH4 mixture,he H2 permeance and H2/CH4 separation selectivity decreasedlightly as the feed pressure increased (Fig. 8). The H2 per-eate concentration was relatively constant (94.3–95.8%), and

hus the composition selectivity, αcomi/j = (yi/yj)/(xi/xj), was

lmost constant. The separation selectivity, αsepi/j , is the ratio of

he permeances [αH2/CH4 = (JH2/�pln,H2 )/(JCH4/�pln,CH4 ) =JH2/JCH4 )/(�pln,H2/�pln,CH4 )]. As CH4 pressure increased,he increase in JH2/JCH4 was smaller than the increase in

pln,H2/�pln,CH4 , and thus separation selectivity decreased.When the CH4 feed concentration increased at 293 K and a

eed pressure of 1.6 MPa, its permeance decreased (Fig. 9). Sim-larly, the H2 permeance and the H2/CH4 separation selectivity

Fa

ig. 8. Permeances and H2/CH4 separation selectivities for a H2/CH4 mixture54/46) at 293 K as a function of feed pressure.

ecreased because the CH4 coverage increased and inhibited H2dsorption more.

.4. Comparison to polymer membranes

Robeson et al. [34,35] reported upper bounds for selectiv-ty versus permeability of polymeric membranes for H2/CO234] and H2/CH4 [35] separations, and Freeman [36] proposedmodel for the upper bound. Permeability is calculated as theembrane permeance multiplied by the membrane thickness.or CO2/H2 separation, the slope of the upper bound is positiveecause CO2/H2 separation is dominated by competitive adsorp-ion [9]. Recently, Lin et al. [9] synthesized 70–500 �m thick,ross-linked poly(ethylene glycol) copolymer membranes thatxceeded the upper bound for CO2/H2 separation (Fig. 10). Sep-rations with the SAPO-34 membrane also exceeded the upper

ig. 9. Permeances and H2/CH4 separation selectivities for H2/CH4 mixturest 293 K as a function of CH4 feed concentration at a feed pressure of 1.6 MPa.

282 M. Hong et al. / Journal of Membran

Fig. 10. CO2/H2 separation selectivity vs. CO2 permeability for the SAPO-34membrane (closed symbols) at 308 K (�), 283 K (�), and 253 K (�) and a cross-linked poly(ethylene glycol) copolymer membrane at 308 K (©), 283 K (�), and2wb

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[26] S.G. Li, J.L. Falconer, R.D. Noble, Improved SAPO-34 membranes for

53 K (♦) from Ref. [9]. The data points at 253 K for the SAPO-34 membraneere measured at different feed pressures and compositions. The previous upperound for polymer membranes is shown.

ration layer could be thicker if the SAPO-34 membrane grewnto the support, and thus, these permeabilities are lower limits.ecause the polymer membranes [9] are 1–2 orders of magnitude

hicker than the SAPO-34 membrane, for the same permeabil-ties, the CO2 permeances of the SAPO-34 membrane are 1–2rders of magnitude higher than those of the polymer mem-ranes. The H2/CH4 separations at room temperature are closeo the upper bound reported for polymer membranes.

. Summary

A SAPO-34 membrane removed CO2 from CO2/H2 mix-ures because CO2 adsorbed more strongly than H2 in SAPO-34,o that H2 fluxes at low temperatures were orders of magni-ude lower in the presence of CO2. At low temperatures andigh pressures, the CO2/H2 selectivity was greater than 100,nd thus a SAPO-34 membrane may have potential for applica-ions. The CO2/H2 selectivity had a maximum as a function CO2eed percentage, and the selectivity decreased as the tempera-ure increased. The SAPO-34 membrane separated H2 from CH4ecause H2 diffused faster, and the separation selectivity was aeak function of temperature, pressure, and feed composition.

cknowledgments

We gratefully acknowledge funding by the US Departmentf Energy (Grant No. DE-FG26-02NT41536). We thank Dr.ans H. Funke for assistance with the high-pressure separationeasurements.

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