Characterization of zeolite membranes by temperature programmed permeation and step desorption

Journal of Membrane Science 195 (2002) 125–138

Characterization of zeolite membranes by temperatureprogrammed permeation and step desorption

M.P. Bernal, J. Coronas, M. Menéndez, J. Santamarıa∗Department of Chemical and Environmental Engineering, Faculty of Science, University of Zaragoza, 50009 Zaragoza, Spain

Received 15 February 2001; received in revised form 13 June 2001; accepted 28 June 2001

Abstract

A temperature programmed permeation (TPP) system was employed to measure the single gas permeances of H2, N2, CO2,methane, ethane, propane andn-butane through MFI membranes prepared on tubular alumina and stainless steel supports.The same system was used to measure the CO2/N2 separation selectivity using mixed feeds. The results obtained by TPPwere in good agreement with permeation data gathered by on-line gas chromatography at fixed temperatures. However, theamount of information that could be obtained from TPP experiments in a comparable experimental time was much larger. Stepdesorption (SD) experiments were also carried out in which the response of the membrane to a step change in the feed sideconcentration was continuously analyzed. The permeation-temperature and permeation-time data gathered in the TPP and SDexperiments with model compounds and mixtures provided valuable information on the permeation regimes, the adsorptionprocesses taking place and the overall quality of the membranes. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Zeolite membrane; Silicalite; ZSM5; Temperature programmed permeation; Transient desorption

1. Introduction

In recent years, the availability of microporouszeolite membranes has opened up new fields of mem-brane application. Thus, zeolite membranes allow tocarry out difficult separations (e.g. of mixtures ofcompounds with close boiling-points, similar molec-ular weight or even of species that form azeotropes).On the other hand, zeolite membranes permit the inte-gration of reaction and effective separation in a singlestage. This is possible by the stability of zeolites ina wide range of operating conditions. In this respect,the use of zeolite membranes in reactors has beenadvocated in different functions: to separate the prod-

∗ Corresponding author. Tel.:+34-976-76-11-53;fax: +34-976-76-21-42.E-mail address: [email protected] (J. Santamarıa).

ucts formed, to remove inhibitors or to add reactantsin a variety of reaction scenarios. In addition, thezeolite materials that constitute the membranes oftenhave intrinsic catalytic properties, and this makes itpossible to conceive reactors where the membraneitself performs the reaction and separation functions.

Concerning the first group of applications (dif-ficult separations), a wide variety of examples canbe found in the literature [1], although until now nolarge-scale applications have been developed, and theonly commercial application is solvent de-wateringat a relatively small throughput [2]. The combinationof reaction and separation is still at a germinal stage,and examples are scarce. Among the few illustra-tions are some pioneering patents in the field [3–5],and published research works on the use of zeolitemembranes for the dehydrogenation of isobutane toisobutene using a silicalite tubular membrane [6];

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(01)00557-9

126 M.P. Bernal et al. / Journal of Membrane Science 195 (2002) 125–138

methatesis of propene to ethylene and 2-butene tocis-2-butene andtrans-2-butene [7]; MTBE synthesisfrom methanol andtert-butanol [8], dimerization ofisobutene [9], olefin polymerization [10] and esterifi-cation of ethanol [11,12]. These reactions take placeat temperatures at which adsorption still exerts a verysignificant influence on membrane performance. Thesame can be said of most of the high-selectivity gasseparations reported to date in the literature [1,13],which are adsorption-controlled, while there are fewerexamples of selective separations driven by molecularsieving or by differences in diffusion.

The permeation regime often determines the sepa-ration performance. Burggraaf et al. [14] studied thepermeation of different individual gases on silicalitemembranes and found that CH4 permeated in theHenry sorption regime (flux increases linearly with thepressure of the permeating gas at the feed side), ethane,propane andn-butane permeated in the Langmuir sorp-tion regime (non-linear dependency of pressure, max-ima in the flux versus temperature curves), benzene inthe saturation regime (flux independent of pressure),and molecules with kinetic diameters in excess of thezeolite pore diameter (e.g. 2,2-dimethylbutane) wereexcluded from the zeolite channels (size exclusionregime), and permeated only through defects.

For the size exclusion regime to be observed,the membrane must be of a high quality (low con-centration of inter-crystalline defects), such as thesilicalite membrane used by Burggraaf et al. [14];otherwise, permeation through defects such as meso-pores destroys the intrinsic membrane selectivity.In addition, adsorption effects may mask both thesize exclusion regime and diffusion-controlled per-meation. Perhaps, the best known case is that of theseparation ofn-butane and H2 through MFI (sili-calite and ZSM5)membranes. In this case, the largestmolecule (n-butane) has the lowest diffusion rate, butits stronger adsorption blocks hydrogen diffusion atlow temperatures [1]. In the mixture,n-butane perme-ates mainly by surface diffusion, at a rate similar tothat observed for butane as a single gas. As a conse-quence, a representative measurement of the relativediffusion rates ofn-butane and H2 in the zeolite mi-cropores (activated gaseous diffusion) is only obtainedfrom single gas permeation experiments, of from ex-periments carried out with mixtures under conditionswhere adsorption effects become negligible.

Fig. 1. Qualitative variation of the permeance of a single gas withtemperature.

One way to decrease adsorption effects is to in-crease the operating temperature. This can be ascer-tained qualitatively from the permeation-temperaturediagram for an adsorbable gas shown in Fig. 1 [15,16].At low temperature, the occupancy is high due toadsorption, and initially the permeance increasesbecause the increase in temperature enhances the mo-bility of adsorbed species, even though the amount ofphysically adsorbed material starts to decrease. Even-tually, point B is reached, and from this temperaturethe decline in occupancy prevails, which gives riseto a decrease in permeance. At a certain temperature(C) adsorption effects become negligible, and thepermeance increases monotonically with temperature,driven by the Arrhenius-like dependency of activatedtransport through micropores.

In this work, we advocate the use of temperatureprogrammed permeation (TPP) of single compoundsand mixtures as a fast and reliable tool to investigatethe characteristics of zeolite membranes. Thus, inthe high temperature region, the calculated values ofapparent activation energy and the goodness-of-fit ofArrhenius plots could in principle be used as indica-tors of the membrane quality. The rationale behindthis method is that the presence of Knudsen-typedefects would give rise to a lower activation energyand to a poor fit of the data to the Arrhenius expres-sion. Similarly, the presence of significant amountsof adsorbed material would also interfere withpermeation-temperature data in the activated diffusionregion, and could be detected either from the lack oflinearity of the Arrhenius plot or from the hysteresis ofthe permeation-temperature curves. Finally, since themaximum and minimum in Fig. 1 are related to the ad-sorptive interaction between the permeating moleculesand the membrane, the relative position of these points(B and/or C) in a series of permeation-temperature

M.P. Bernal et al. / Journal of Membrane Science 195 (2002) 125–138 127

curves for different compounds can give an indicationof their relative strengths of adsorption. Complemen-tary information can be gathered from step desorption(SD) experiments, carried out on the same experi-mental set up used for the TPP measurements. In thiscase, a step change in the feed side concentration isinduced, and its effects on the concentration of theexit gases are continuously monitored. The analysisof the concentration-time curves provides valuableinsight on the adsorption phenomena taking place ata given temperature on the membrane.

2. Experimental

The zeolite materials membranes used were sili-calite (termed SIL in the remainder of this work), andZSM5. They were prepared on alumina (inocermicor SCT) and stainless steel (Mott) commercial sup-ports, with 5, 200 and 500 nm pore diameters, respec-tively. ZSM5 membranes were prepared using the geldescribed in [17], whose molar composition was: 21SiO2:987 H2O:3 NaOH:1 TPAOH:0.105 Na2Al2O4.For the silicalite membranes the gel was similar, but itdid not contain aluminum [18]. Two different proce-dures were employed to prepare membranes. In proce-dure (a) the tubular support was wrapped with Teflontape on the outside and introduced into the autoclave,which was then filled with the respective gel. In pro-cedure (b) before the introduction of the support in theautoclave, one end of the wet tube was wrapped withTeflon tape, plugged with a Teflon cap and filled withthe synthesis gel, then the other end was wrapped withtape and plugged with another Teflon cap, and the sup-port was introduced in an autoclave containing 3 ml ofwater. The autoclave was placed in a convection ovenand (in the case of procedure (a)) the first synthesiswas carried out with continuous horizontal rotationof the autoclave) at 443 K for periods between 8 and23 h. The synthesis cycle was repeated until an imper-meable membrane (before template removal) was ob-tained. Procedure (a) tends to concentrate most of thezeolite material inside the support pores (giving riseto, what is later called a type A membrane), while withprocedure (b) most of the zeolite material resides inthe thin zeolite layer on top of the support. For someof the membranes prepared on stainless steel tubes amixed preparation procedure was used [19] where an

(a) synthesis was first carried out (to deposit zeolitecrystals in the support) and then several type (b) verti-cal syntheses were repeated until impermeability wasobtained. In all cases, the template was removed byheating up to 713–753 K at 1 K/min and then main-taining the final temperature for 8 h.

Permeation experiments were carried out on twodifferent systems. A TPP device (see Fig. 2) with aTCD cell as was employed to obtain single gas perme-ances of H2, N2, CO2, methane, ethane, propane andn-butane as a function of the temperature. In this sys-tem, a mass-flow controlled stream of the gas whosepermeance is to be tested is fed into the tube side (re-tentate) of the membrane. A He or N2 stream is sent tothe reference channel of the TCD cell and then mixedwith the permeate from the membrane. The resultingstream is then directed to the second channel of theTCD cell, giving a concentration-dependent signal thatis acquired by the controlling computer every 0.5 s.The computer also controls the temperature of themembrane, which is either constant or varied under afixed heating ramp. The pressure difference across themembrane is controlled by means of a back pressureregulator, and is, together with the temperature, contin-uously registered. The permeances are (mol/m2 s Pa),referred to the permeable area of the membrane.

The TPP system can also measure the permeanceof each of the components in binary mixtures by com-bining the results of two separate experiments. In thiscase, a mixture containing components A and B is fedin the retentate side of the membrane. At the sametime, a stream containing only component A is passedthrough the reference channel in the TCD cell andthen mixed with the permeate stream from the mem-brane. The resulting stream enters the second (mea-suring) channel of the TCD cell, giving a signal thatallows the concentration of B in permeate stream tobe calculated. The experiment is rerun under the sameconditions, but in this case the reference in the TCDcell is made by component B, and the signal is used tocalculate the concentration of A in permeate stream.A mass balance around the membrane allows to cal-culate the flow-rate and composition of the perme-ate and retentate streams, and the permeances of bothcomponents. In order to check the accuracy of theconcentrations determined by this system, the con-centrations could also be determined by on-line gaschromatography.


Fig. 2. Experimental setup: 1, mass-flow meter; 2, control unit; 3, and 4, back pressure regulator; 5, data logger; 6, TCD; 7, furnace; 8,temperature controller; and 9, computer.

In the SD experiments, the feed to the retentate sideof the membrane contained a binary mixture (A/B),while the permeate side was swept with the pure com-ponent B, which was also fed to the reference sideof the TCD cell. The exit stream from the permeateside was directed to the second channel of the TCDcell, whose signal was used to calculate the concen-tration of A in the permeate side. After steady statewas reached, as indicated by the constancy of the TCDsignal, the feed of A to the membrane was switchedoff and, at the same time, either the exit stream fromthe permeate side or the mixed permeate and reten-tate streams were sent to the measuring channel of theTCD cell. In this way the evolution of A desorbingfrom the membrane could be followed as a function oftime. In some experiments, the temperature was raised(temperature programmed desorption) and the amountof A desorbed as a function of temperature was ob-tained. By repeating the experiment with the roles of A

and B switched the desorption of component B couldbe followed. In this way the desorption-temperaturecurves of different experiments could be used to as-sess the relative strengths of adsorption of differentcomponents.

For TPP and SD experiments, the membrane wasplaced in a stainless steel module where it was sealedwith silicone o-rings or graphite gaskets, dependingon the range of temperatures used. Before running anyexperiment the membranes was heated to 753 K at arate of 1 K/min and then calcined at this temperaturefor 4–8 h in order to remove any adsorbed species.The separation selectivities given below for mixtureswere calculated as the ratio of permeances, using thelog-mean partial pressure difference in the calcula-tions. Mass balance closures for the different speciesbased in the composition and flow-rate of the feed andthe two exit streams were in the 100±5% interval forthe experiments reported in this work.


3. Results and discussion

3.1. Temperature programmed permeation(TPP) experiments

The method of preparation and some importantcharacteristics of the membranes used in this workare given in Table 1. The N2/n-butane ideal selectiv-ity (ratio of single gas permeances), is often used asan indicator of the quality of MFI membranes, andusually gives reliable results, provided that they havebeen prepared using the same procedure.

As a preliminary experiment, Fig. 3 shows the per-meance ofn-butane through membrane SIL-4 as a

Table 1Some properties of membranes tested in this work

Membrane Support Preparation procedure N2 permeance(mol/m2 s Pa)

N2/n-butaneideal selectivity

SIL-1 �-Alumina (b) 14× 10−7 68SIL-2 �-Alumina (b) 7.3× 10−7 35SIL-3 �-Alumina –a 1.3 × 10−7 3.1SIL-4 Stainless steel (a)+ (b) 4.7 × 10−7 33.0b

ZSM5-1 �-Alumina (a) 1.5× 10−7 30ZSM5-2 Stainless steel (a) 2.1× 10−7 3.7ZSM5-3 Stainless steel (a)+ (b) 1.7 × 10−7 33.0b

ZSM5-3 �-Alumina (a) 5.6× 10−8 38.6b

a This silicalite membrane was prepared using a different procedure described in [20].b N2/SF6 ideal selectivity.

Fig. 3. Permeance ofn-butane vs. temperature, at three different heating rates. Membrane SIL-4.

function of temperature, in a TPP experiment. Threedifferent heating rates of 0.5, 1 and 1.9 K/min wereused, and it can be seen that the results of the threeexperiments are coincident within experimental error,even thoughn-butane adsorbs strongly in the silicalitepores. In view of this lack of effect of the heating ratein the interval studied, a value of 1 K/min was used inthe rest of experiments reported in this work.

3.1.1. H2 single gas permeancesIn Fig. 4 the H2 permeance is plotted as a func-

tion of temperature for several membranes, includinga �-alumina mesoporous ultrafiltration membranewith 5 nm pores in the separation layer. The


Fig. 4. Permeance of H2 as a function of temperature for a�-alumina support and for membranes SIL-1, SIL-2, SIL-3, SIL-4 and ZSM5-2.Note: the data corresponding to membrane SIL-4 were obtained in different types of experiment: with variable pressure drop (0.35–0.7 bar)across the membrane (open circles), and with a constant (0.2 bar) pressure drop (solid circles).

permeation-temperature behavior is clearly distinctfor the different membranes studied. Thus, the H2 per-meance decreases with temperature for the�-aluminasupport and for membrane SIL-3, shows a very shal-low minimum for membrane ZSM5-2, and a markedone for membranes SIL-1 and SIL-4. It may be no-ticed that two different experiments were carriedout with membrane SIL-4: with a constant pressuretransmembrane pressure gradient of 0.2 bar (solidcircles) and with a variable pressure gradient in the0.35–0.7 bar range (open circles). The results of theTPP system were not affected by this difference inexperimental conditions.

In the literature, a variety of permeation-temperaturebehaviors has been described, depending on the typeof membrane and the operating conditions, that maycorrespond to different stages depicted in Fig. 1. Thus,different authors have found that the H2 permeancedecreases with temperature (e.g. [20]), goes througha minimum (e.g. [17,21]), or increases with tempera-ture (e.g. [17,22]). Regarding the results in Fig. 4, themesoporous membrane shows the expected behavior— a decrease of permeance with temperature owingto Knudsen permeation through mesopores, with per-haps some contribution from larger defects. Regardingthe microporous membranes, from the TPP results it

seems evident that the quality of all these membranesin terms of defect density is not the same. Because atambient temperature and atmospheric pressure H2 isonly weakly adsorbed on MFI zeolites, in agreementwith the previous discussion one could expect thepermeance of H2 to be governed by activated diffu-sion and display a clear increase with temperature,as shown in Fig. 4 for membranes SIL-1 and 4 afterthe minimum. However, H2 is a small molecule capa-ble of high permeation rates through inter-crystallinedefects. Thus, in membranes with a greater defectdensity the effect of permeation through these defectscould overcome activated permeation in the zeolitepores. Therefore, the shallow minimum presented bymembrane ZSM5-2 and the descending pattern ofSIL-3 can be considered as reliable indicators of thepresence of defects. This is in good agreement withthe ideal selectivities in Table 1, which are 10 timeslower for SIL-3 and ZSM5-2 than for the SIL-1 andSIL-4 membranes.

3.1.2. Type A and type B MFI membranesRegarding the distribution of the zeolitic material

on the support, two types of zeolite MFI membraneshave sometimes been distinguished, taking into ac-count the distribution of the zeolite in the support


[1,17,23,24]. In type A membranes [1,17,23] (SIL-3,SIL-4, ZSM5-1, ZSM5-2 and ZSM5-3) the zeolite ispreferentially deposited inside the porous structure ofthe support; while in type B membranes (SIL-1 andSIL-2) most of the zeolite material exists as a thin layeron top of the porous support. The pattern of zeolitedistribution achieved at the end of the hydrothermalsynthesis depends on the type of support and synthesisprecursor gel, and on the procedure used to put them incontact. Fig. 5a and b show SEM photographs of typeA and B membranes. In both types of membranes thedependence of permeance with temperature and eventhe separation performance are often different. Thus, ithas been shown [17,23] that type A membranes main-tain their separation properties at higher temperatures,a behavior that could be explained as a consequence ofstronger adsorption effects on these membranes. Thishypothesis can be tested in TPP experiments. Fig. 6shows the propane single gas permeances for mem-branes SIL-2 (type B) and ZSM5-1 (type A). It is in-teresting to compare them while keeping in mind thesingle gas permeation behavior shown qualitatively inFig. 1. The permeance of propane in the type B mem-brane presents a minimum near 400 K, which corre-sponds to point C of Fig. 1. In contrast, the ZSM5-1membrane (type A) presents a maximum, rather than aminimum in the temperature range studied, indicatingthat even higher temperatures are needed to reach pointC of Fig. 1, i.e. a significant influence of adsorptionstill exists at these higher temperatures. These resultsare consistent with a stronger adsorption of propanein type A MFI membranes, where the distribution ofzeolite material seems to enhance adsorption effects.

3.1.3. Single gas permeances of differenthydrocarbons

In Fig. 7, the methane, ethane, propane andn-butanesingle gas permeances are plotted as a function of tem-perature for membrane ZSM5-2. Note that, the sin-gle gas permeance of methane has been multiplied bya factor of 0.5. The behavior observed for the singlegas permeances a function of temperature is the samefound by Bakker et al. for silicalite [25]. Initially, thepermeance goes through a maximum with increasingtemperature (which Bakker et al. [15] found at 243,325, 358 and 418 K for methane, ethane, propane andn-butane, respectively; and Burggraaf et al. [14] andBurggraaf [26] at 350, 380 and 370–440 K for ethane,

Fig. 5. SEM photographs: (a) type A ZSM5 membrane on�-Al2O3

support; and (b) type B ZSM5 membrane on stainless steel support.

propane andn-butane, respectively), and if the tem-perature is further increased, a minimum is observedin the permeance. The sequence found in this workfor the permeance maxima is the same as in the workscited: methane (the temperature used was too highto find a maximum) < ethane(373 K) < propane(418 K) < n-butane (454 K), although the individ-


Fig. 6. Permeances of H2 and propane for membranes SIL-1 (type A); and ZSM5-1 (type B) as a function of temperature.

ual temperatures are higher. In this respect, it mustbe taken into account that our single gas permeationexperiments were carried out in the pressure gradi-ent mode (without using any sweep gas), which couldexplain the displacement of the maximum in the per-meance in comparison with the other reported data,as demonstrated by Kapteijn et al. [27], who found

Fig. 7. Permeances of methane, ethane, propane andn-butane for membrane ZSM5-2 as a function of temperature.

displacements in the temperature of maxima depend-ing on the conditions on the permeate side.

At temperatures beyond point C the maxima andminima disappear, and a monotonic increase withtemperature is observed in TPP experiments, as shownin Fig. 8 for the permeances of He, methane, ethaneand n-butane in the 500–773 K temperature range.


Fig. 8. Permeances of He, methane, ethane andn-butane for membrane ZSM5-2 as a function of temperature.

At these temperatures, for any of these compounds,the occupancy in the pores of the membrane is lowand activated diffusion is the prevailing mechanism.Again, valuable information can be gathered from theexamination of the TPP results, which are presentedin this case as Arrhenius plots (Fig. 9). It can be seenthat a nearly perfect linear fit was obtained for He(R = 0.99997), and CH4 (R = 0.99959), although inthe latter case some deviation can be observed at the

Fig. 9. Arrhenius plot for the permeances of Fig. 8.

lower temperatures. The deviation becomes larger forethane and butane, where larger adsorption effects canbe expected. If we take the linear part of each plot andextrapolate it to lower temperatures, we find that forthe larger hydrocarbons the predicted permeances arelower than the experimental ones, which can be at-tributed to the contribution of surface diffusion whenthe temperature becomes low enough for adsorption toplay a role. Thus, while at 600 K the deviation for He


Table 2Arrhenius fit of the data of activated permeation of different gasesthrough membrane ZSM5-2 (see Fig. 9)

Gas Ra Tlowb Eact

c

He 0.99997 423 16.91CH4 0.99959 512 15.29C2H6 0.99791 656 17.65C4H10 0.99647 667 14.49

a Regression coefficient of the Arrhenius plot calculated overthe whole temperature interval of Fig. 9 (600–770 K).

b Lower temperature limit (K) for a good-fit (R ≥ 0.997).c Activation energy (kJ/mol), calculated for the temperature

interval betweenTlow and 770 K.

is still negligible (which means that all of the observedpermeance can still be explained as a result of acti-vated diffusion), for butane the measured permeanceis significantly (14%) higher than the predicted fromlinear extrapolation as shown in Fig. 9. Table 2 liststhe regression coefficients calculated from the fit of thepermeances by the Arrhenius law, which as expecteddecrease as we move from He (nearly perfect fit) ton-butane, i.e. as the adsorption strength increases. It isinteresting to calculate the limit of lower temperaturethat can be used in order to obtain a good linear fit(which we have defined atR ≥ 0.9970) from the Ar-rhenius expression. This temperature is also shown inTable 2 asTlow and represents the temperature at whichadsorption effects are significant enough to cause asignificant deviation in the Arrhenius plot. For Hethis temperature is not reached in the interval tested,while for the other gases it becomes higher (i.e. thegood-fit interval narrows) as the molecular weight in-creases. Finally, Table 2 also gives the activation ener-gies, which were calculated from the linear regions ofFig. 9, i.e. fromTlow to the highest temperature tested.

The permeation through zeolite membranes at hightemperatures has been rarely studied, which is partlydue to the fact that the selectivity usually decreasesas the temperature increases beyond a certain value.Several authors [28,29] pointed out that activatedgaseous diffusion through micropores should ap-proach the Knudsen value at high temperatures. Also,van de Graaf et al. [30], in the case of MFI typemembranes, reported very similar apparent activationenergies for activated gas diffusion of methane, ethaneand n-butane (20.4, 22.8 and 22.4 kJ/mol, respec-tively). The activation energies reported in Table 2 are

also close to each other (15.3, 17.7 and 14.5 kJ/mol,respectively) in spite of the important differences inthe molecular properties of the individual compounds.The average value of the apparent activation energiesfor these compounds is 15.8 kJ/mol, not too far fromthe average value in the work of van de Graaf et al.[30] (21.8 kJ/mol), in spite of the fact that they wereobtained with a different experimental procedure andon different membranes. Finally, at 770 K (the highesttemperature used in this work), the ideal selectivity ofHe with respect to methane, ethane andn-butane was2.1, 3.0 and 6.3, respectively, still above but rapidlyapproaching the Knudsen selectivity values (2.0, 2.7and 3.8, respectively).

3.1.4. Separation of CO2/N2 mixturesFig. 10a and b shows the results of TPP experi-

ments carried out on membrane ZSM5-3 with feedsconsisting of pure N2 and CO2 streams and also witha equimolar CO2/N2 mixture. No sweep gas was usedfor these experiments, and in the mixed feed exper-iment both the CO2 and N2 partial pressures were99 kPa. For the mixed-feed experiment the N2 andCO2 permeances were also calculated by analyzing theretentate and permeate streams in isothermal experi-ments using on-line gas chromatography, and it can beseen that the agreement with the TPP data is excellent.The single gas permeance of CO2 shows a maximumat 322 K, while that of N2 passes through a weak mini-mum. A minimum is also hinted in the single gas CO2curve, although at a considerably higher temperature.All of these observations indicate a stronger adsorptionof CO2, as could be expected. The CO2/N2 ideal selec-tivity can be calculated from the single gas permeancecurves: it is highest (1.9) at the lower end of tempera-tures tested, and decreases to 1.1 at the highest temper-ature tested, where CO2 permeance has its maximum.

In the case of CO2, the differences measured be-tween its single gas permeance and its permeance inthe mixture with N2 were small. On the contrary, theN2 permeance in the mixture was strongly decreasedby the presence of CO2. The separation selectivity formixed feeds is considerably higher (Fig. 10b) than theideal selectivity just discussed, with a maximum valueof 8.3, over four times larger than the ideal selectiv-ity. Although CO2 (kinetic diameter,dk = 0.33 nm)is a smaller molecule than N2 (dk = 0.37 nm)this difference does not seem enough to justify the


Fig. 10. (a) TPP experiments: N2 and CO2 single gas permeances and permeances in the equimolar binary CO2/N2 mixture for membraneZSM5-3. Individual open symbols correspond to permeation data obtained from GC analysis in isothermal experiments; and (b) CO2/N2

separation selectivities for the results of (a).

separation of the mixture by the MFI pores (averagepore size= 0.55 nm). Instead what happens is thatthe CO2 molecules are preferentially adsorbed in thepores of the membrane, hindering the access of N2and its permeation in these pores. As expected ina separation controlled by adsorption effects, as thetemperature was increased the CO2/N2 selectivitydecreased (from 8.3 to 1.6 in the interval tested).

3.2. Step desorption (SD) experiments

Supplementary data on the role of adsorption onthe permeation process can be obtained from SDexperiments. Fig. 11 shows the evolution of concen-trations on the permeate side after reaching stablevalues of permeation with an equimolar feed mixtureof N2 and CO2 at 373 K with ZSM5-4. It can be seen


Fig. 11. Evolution of concentrations on the permeate side of the membrane during a SD experiment. Equimolar N2/CO2 feed, feedflow-rate= 100 N ml/min, retentate pressure= 2 bar, permeate presure= 1 bar,T = 373 K.

that the amount of CO2 desorbed is clearly higher,and also its desorption extends for a longer periodof time, corresponding to a stronger adsorption ofCO2. This agrees well with the separation results justdiscussed, in which the stronger adsorption of CO2explained its selective separation.

With strongly adsorbed components it is possible tocarry out SD experiments under a temperature ramp. In

Fig. 12. Evolution of propane and N2 concentrations in a SD experiment under a temperature ramp. Equimolar feed mixture, retentatepressure= 2 bar, permeate presure= 1 bar, heating rate 1 K/min.

this case the desorption temperature provides a directmeasurement of the strength of adsorption and can beused to rank the interaction of a series of compoundswith the membrane. For the case presented in Fig. 12,an equimolar propane/N2 mixture was adsorbed onthe membrane at 378 K, then both sides were purgedand the temperature was raised at a rate of 1 K/min.The desorption results indicate a very weak adsorption


Fig. 13. Evolution of propane, ethane and N2 concentrations in SD experiments with equimolar ethane/N2 and propane/N2 feed mixtures.Feed flow rate= 100 N ml/min, retentate pressure= 2 bar, permeate presure= 1 bar,T = 298 K.

of N2 at this temperature. Contrarily, a considerableamount of propane is desorbed starting from 378 K,and the desorption continues to considerably highertemperatures. It is interesting to note that after the con-centration of propane becomes null at about 140◦C,a second desorption process starts, with a maximumconcentration around 180◦C. This suggests the ex-istence of different types of adsorption sites (and/oradsorption mechanisms) for propane on the ZSM-5membrane.

Finally, the results of SD experiments with binarymixtures can be used to gain insight into competitiveadsorption processes taking place on the membrane.Fig. 13 shows the evolution of the concentrationof each of the components in SD experiments car-ried out with propane/N2 and ethane/N2 mixtures.As expected, propane shows the strongest adsorp-tion, both in terms of the amount adsorbed and onthe length of the desorption period and N2 adsorbsless than either propane and ethane, as shown inFig. 13. It is interesting to compare the N2 desorp-tion curves in the experiments where it was mixedwith ethane and propane: the SD experiments aresensitive enough to show that there is less N2 ad-sorbed, and its desorption is faster, in the propane/N2mixtures, i.e. the more strongly adsorbed propane dis-places N2 from adsorption sites, reducing the amount

adsorbed on the membrane and giving a shorterdesorption.

4. Conclusions

Temperature programmed permeations and stepdesorptions provide valuable insight on the adsorp-tion processes that seem to govern many of the suc-cessful separations reported with zeolite membranes.The varied information gathered by both TPP andSD experiments (position of maximum/minimum insingle gas TPP, goodness of Arrhenius fit, compari-son between single gas and mixture TPP, SD under atemperature ramp, SD of mixtures), allows to inves-tigate the characteristics of zeolite membranes. Thus,TPP and SD experiments constitute fast and reliablemethods to identify defective MFI membranes. Onthe other hand, with good quality membranes the re-sult of these transient experiments allow to rank thestrength of adsorption of different compounds, and toevaluate the temperature range in which adsorptionprocesses affect permeation.

Acknowledgements

Financial support from DGICYT (PPQ2000-1337)and DGA (P94/97) is gratefully acknowledged.


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Characterization of zeolite membranes by temperature programmed permeation and step desorption