alkali-free zsm-5 membranes: preparation conditions and separation performance

Alkali-Free ZSM-5 Membranes: Preparation Conditions andSeparation Performance

Vu Anh Tuan, John L. Falconer,* and Richard D. Noble

Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424

Alkali-free, H-ZSM-5 membranes were synthesized by in-situ crystallization on porousR-alumina, γ-alumina, and stainless steel tubular supports. Membranes prepared from differentSi sources were characterized by X-ray diffraction, scanning electron microscopy, and electronprobe microanalysis. Membranes prepared under different conditions were also characterizedby single-gas permeances of H2, N2, n-C4H10, i-C4H10, and SF6 and by separation selectivities ofn-C4H10/i-C4H10 mixtures from 300 to 473 K. The effects of preparation procedure, crystallizationtime and temperature, number of synthesis layers, gel dilution, Si source, and type of supportwere studied. The permeation and separation properties of the membranes depend strongly onthe preparation procedure. Permeating synthesis solution into the pores of the support beforehydrothermal treatment allows zeolites crystal growth within those pores. Increasing thecrystallization time and temperature increases the n-C4H10/i-C4H10 separation selectivities,whereas increasing the number of synthesis layers and gel dilution decreases the selectivities.Membranes were prepared with high ideal and separation selectivities for n-C4H10/i-C4H10. Thehighest separation selectivity for n-C4H10/i-C4H10 is 111, which was obtained at 429 K, and thehighest separation selectivity at 473 K is 36. Ideal selectivities do not correlate with mixtureselectivities for n-butane/i-butanes at low temperature, but they correlate at 473 K.

Introduction

Inorganic membranes have potential for gas separa-tions owing to their superior thermal, mechanical,chemical, and structural properties compared to poly-meric membranes. Inorganic membranes with poresgreater than 2 nm in diameter are not expected toexhibit high selectivities for gas separations. When thepore size is comparable to the molecular dimensions,however, separation is possible by molecular sieving,preferential adsorption, and differences in diffusionrates. Zeolites, a class of microporous, crystalline alu-minosilicate materials, have extremely narrow pore sizedistributions, and they have the following advantageswhen used as membranes:

(1) The pore size can be tuned by choosing theappropriate zeolite (0.2-0.8-nm pore diameter) and byexchanging cations of different diameters.

(2) The hydrophilic/hydrophobic nature can be modi-fied by changing the Si/Al ratio in the framework. Thebasic/acidic nature can be modified by exchangingalkaline cations with H+.

(3) Their catalytic properties can be used for catalyticmembrane reactors.

In 1987, Suzuki et al.1 patented preparation of zeolitemembranes and published studies appeared in 1991.2,3

Small-pore (A-type),4-6 medium-pore (MFI- and FER-type),7-13 and large-pore (MOR- and Y-type)14,15 zeolitemembranes have been made. Membrane preparationwas extended to related materials such as crystallinesilicoaluminophosphates (SAPO) for large-pore SAPO-5membranes16 and small-pore SAPO-34 membranes.17,18

Most gas separations studies with zeolite membranes

have used MFI-type zeolites such as silicalite-1 (puresilica) and ZSM-5 zeolites (containing Al), which havepore dimensions of 0.53 × 0.56 nm2.19 These zeolites canadsorb molecules whose kinetic diameters are largerthan these XRD dimensions, however. High-separationselectivities in these membranes have mainly beenreported for mixtures of organic molecules, and theselectivities in many studies have been due to differ-ences in adsorption properties rather than molecularsieving.20-22

The membrane pore structure can be modified bychemical vapor deposition of silica via reaction with asilicon alkoxide or other silylation agents. In one study,the separation selectivity for a n/i-C4H10 mixture in-creased from 9.1 to 87.8.23 Coke deposition within themembrane has also been reported to improve perfor-mance.24 However, these treatments may block accessto pores or reduce pore entrance diameters and conse-quently decrease fluxes. Therefore, optimizing param-eters that affect zeolite membrane performance withoutpost-treatments is also desirable.

Yan et al.25 reported that the choice of synthesiscomposition is critical for preparing good quality mem-branes. After searching a wide composition range ofprecursor gel components (Si, Al, NaOH, and H2O), theyarrived at an optimal composition producing a mem-brane with a n/i-C4H10 ideal selectivity of 31 at 458 K.The choice of synthesis conditions is also critical forobtaining high-quality membranes. Vroon et al.26,27

reported preparation of MFI membranes with separa-tion selectivities of 50 and 11 at 298 and 473 K,respectively, for a 50/50 mixture of n/i-C4H10. Thesemembranes were synthesized at 373-393 K, which ismuch lower than the typical temperature used for MFImembranes (443-453 K). Geus et al.28 and Bakker etal.29 studied the growth of ZSM-5 membranes on porous

* To whom correspondence should be addressed. E-mail:[email protected]. Fax: (303) 492-4341.

3635Ind. Eng. Chem. Res. 1999, 38, 3635-3646

10.1021/ie980808g CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 07/30/1999

alumina, zirconia, clay, and sintered stainless steelsupports. The support surface played an important rolein determining the type of zeolite membranes. Forexample, for the same synthesis composition and tem-perature, analcime grew on R-alumina and MFI zeolitesgrew on clay, whereas a mixture of analcime and MFIgrew on a ZrO2/R-Al2O3 composite. Continuous layersof MFI crystals formed on stainless steels disks.

The method of contacting the synthesis solution withthe support is also important. Recently, Oh et al.30

developed a new method for synthesis of MFI mem-branes by applying pressurized sol-gel coatings. Silicasol was forced into the pores of an R-alumina supportby pressure, and the outer wall of the tube wascontactedwithtetrapropylammoniumhydroxide(TPAOH)solution. Continuous layers of MFI crystals formed onthe inner wall.

The current study reports the preparation and char-acterization of ZSM-5 membranes on the inner wall andwithin the pores of support tubes. In-situ crystallizationis used, but in contrast to previous preparations ofZSM-5 membranes, the synthesis gel does not containNaOH. This synthesis approach directly yields acidicH-ZSM-5 membranes. The main factors affecting gasseparation behavior were systematically investigated:preparation procedure, crystallization time and tem-perature, number of synthesis layers, nature of Sisource, supports, and gel dilution. Because of the largernumber of parameters that affect the membrane prop-erties, not every combination of conditions was tried.Thus, the conditions that were used to obtain high-quality membranes are not optimal.

One application of zeolite membranes may be incatalytic membrane reactors. Since only a small amountof zeolite deposits on a porous support, high exchangeefficiency of Na+ to H+ in a Na-ZSM-5 zeolite is neededfor catalytic applications, but exchanging 90% or moremay require multiple exchanges. Membranes preparedwithout NaOH in the synthesis gel are already in theacidic form.

Experimental Methods

Zeolite ZSM-5 membranes were prepared by in-situcrystallization from gels onto three types of multilayer,porous support tubes (o.d. ) 1 cm):

(1) R-Alumina with an inner layer of γ-alumina with5-nm diameter pores (0.70-cm i.d., US Filter).

(2) R-Alumina with an inner layer of R-alumina with200-nm diameter pores (0.70-cm i.d., US Filter).

(3) Porous stainless with an inner layer of stainlesssteel with 500-nm diameter pores (0.65-cm i.d., MottMetallurgical Co.).

To avoid bypass during permeance measurements,the ends of 4.7-cm long alumina supports were sealedwith a glazing compound (GL 611A, Duncan), which wascalcined at increasing temperature with a final hold at1173 K for 0.5 h. Nonporous, stainless steel tubes werewelded to each end of the porous stainless tubes.

Two types of gels were used for synthesis. Clear gelswere prepared by using tetraethyl orthosilicate (TEOS)or silica sol (Ludox AS40) as the Si source and alumi-num alkoxide (Al(i-C3H7O)3) as the Al source. Thesecond type of gel was not clear and was prepared usingfumed silica (Aerosil-200) and aluminum alkoxide. Bothgels had Si/Al atomic ratios of 600. The molar composi-tion was 1.0 TPAOH/0.0162 Al2O3/19.46 SiO2/438 H2O,where TPAOH is tetrapropylammonium hydroxide. ThepH of the gel was approximately 11. Note that NaOHwas not used in the preparation.

Three procedures were used to synthesize ZSM-5membranes:

Procedure I. One end of the support tube waswrapped with Teflon tape and plugged with a Tefloncap, and about 2 mL of the synthesis gel was used tofill the inside of the tubular support. In the procedureused by Jia et al.,20 the other end was then wrappedwith tape, plugged with a Teflon cap, and placedvertically in the Teflon-lined autoclave for synthesis.However, zeolite crystals preferentially deposit nearerthe bottom of the vertical tube. To obtain more uniformdeposition, the filled tube was left overnight at roomtemperature. During this time the porous supportsoaked up almost all the synthesis gel. The tube wasagain filled with synthesis gel, plugged with a Tefloncap, and put into an autoclave to crystallize.

Procedure II. The outer wall of the tube waswrapped with four layers of Teflon tape, and the endsof the tube were not plugged. The tube was placedvertically in the Teflon-lined autoclave (100-mL vol-ume), and 70-80 mL of synthesis gel were poured overthe tube. The top of the gel was approximately 8 cmabove the top of the tube.

Procedure III. This procedure is the same as pro-cedure II except that the outer wall of the tube was notwrapped with Teflon tape. By using this procedure,zeolite crystals can deposit on the outer and inner wallof the tube.

Syntheses using the three procedures were conductedover a range of temperatures (403-468 K) and times(15-72 h). Membranes prepared using procedure I ondifferent supports and at different synthesis conditionsare listed in Table 1. The synthesis was repeated untilan uncalcined membrane, after drying at 373 K, wasimpermeable to N2 for a 138-kPa pressure drop at room

Table 1. Membranes Preparation Conditions (Procedure I)

membrane supportcrystallization

time (h)crystallization

temp. (K) Si source gel dilution no. of layers

M1 R-alumina 72 443 TEOS no 2M2 γ-alumina 72 443 TEOS no 2M5 R-alumina 15 443 TEOS no 2M6 R-alumina 25 443 TEOS no 2M7 R-alumina 48 443 TEOS no 2M8 R-alumina 72 443 TEOS no 3M9 R-alumina 72 443 Aerosol no 2M10 R-alumina 72 403 TEOS no 2M11 R-alumina 72 443 Ludox AS40 no 2M12 R-alumina 48 458 Ludox AS40 no 2M13 R-alumina 48 458 Ludox AS40 yes 2M14 stainless steel 48 458 Ludox AS40 no 2M15 γ-alumina 24 469 Ludox AS40 yes 2

3636 Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999

temperature. Since the TPAOH template filled thezeolite pores during synthesis and thus blocked gaspermeation, a membrane with no defects should beimpermeable. However, the template could also fillpores that are larger than the zeolite pores. After thezeolite synthesis was complete, the membranes werewashed, dried, and calcined in air to remove thetemplate from the pores. The calcination procedure wascarried out in a computer-controlled muffle furnace withheating and cooling rates of 0.6 and 1.1 K/min, respec-tively. The maximum temperature was 753 K, and themembrane was held there for 8 h and then stored atroom temperature under vacuum.

Some membranes were broken and analyzed by SEM(IS1-SX-30) operating at 30 keV and by X-ray diffraction(XRD) (Scintag PAD-V diffractometer with Cu KRradiation). The Si and Al concentrations were measuredby electron probe microanalysis (EPMA) with a JEOLJXA-8600 Superprobe.

Single-gas permeation rates were measured for H2,N2, n-C4H10, i-C4H10, and SF6 over a range of temper-atures for some membranes, but most membranes werecharacterized by n-C4H10 and i-C4H10 single-gas andmixture permeances. The membrane was sealed in astainless steel module by silicone O-rings, and thepressure drop was 138 kPa. The ratio of single-gaspermeances is referred to as the ideal selectivity. Asimilar module was used for separations of n-C4H10/i-C4H10 (50/50) gas mixtures. Gas flow rates were con-trolled by Tylan mass flow controllers (60 cm3/min forthe mixture), and the pressure drop was 138 kPa or less.A sweep gas was not used for most of the measurementsreported here, but a comparison between pressure dropand sweep gas is presented for one membrane. Thepermeate and the retentate were analyzed on-line by aHP5890 gas chromatograph with a TC detector and apacked column (0.1% Alltech AT-1000 on Graph-GC).The n-C4H10/i-C4H10 separation selectivities are theratios of permeances, and the log-mean partial pressuredrops were used in the calculation.

Results and Discussion

Characterization of Membranes. To determine thestructure, morphology, and Al incorporation, threemembranes prepared on R-alumina supports were char-acterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and electron probe microanalysis(EPMA). These membranes were prepared by the sameprocedure (Table 1) but with different Si sources:membrane M9 was prepared with Aerosol, M1 withTEOS, and M11 with Ludox. The XRD patterns showthe presence of both ZSM-5 and alumina phases (Figure1). The XRD pattern for ZSM-5 crystals is in agreementwith the pattern reported for MFI zeolite.31 No ad-ditional diffraction lines are seen, and the baseline isflat, indicating that the membranes do not have sig-nificant amounts of amorphous materials. The SEMmicrographs of the top layers (Figure 2) clearly showthe presence of zeolite crystals on the alumina support.These SEM micrographs of membranes M9, M1, andM11 show intergrown crystals that are approximately5-6, 10-12, and 20-25 µm in diameter, respectively.The crystal shapes are cubic (M9) or prismatic (M1 andM11). The cross-sectional views in Figure 3 indicatemembranes M9, M1, and M11 have continuous layersof ZSM-5 with approximate thicknesses of 15, 40, and70 µm, respectively.

The Si and Al concentrations were measured byEPMA as a function of distance from the membraneinner surface. The Si/Al ratio provides an approximatemeasure of the zeolite layer thickness and of the depthof penetration of the zeolite into the porous support. Formembranes M9 and M11, the Si/Al ratio is about 300at the top of the crystal layer, but it is 1200 for

Figure 1. XRD patterns of ZSM-5 membranes supported onR-alumina. These membranes were prepared under the sameconditions, but with different Si sources: M9 (Aerosol), M1 (TEOS),and M11 (Ludox).

Figure 2. Top-view SEM micrographs of membranes supportedon R-alumina. These membranes were prepared under the sameconditions, but with different Si sources: M9 (Aerosol), M1 TEOS),and M11 (Ludox).

Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3637

membrane M1. The Si/Al ratio is almost unchangedwithin 15, 30, and 50 µm from the top for M9, M11, andM1, respectively. The high Si/Al ratio at the top indi-cates this part of the zeolite layer is not in the aluminapores, and the Al content is due to zeolitic Al. Theregions with constant Si/Al ratios correspond to thezeolite layer thickness determined by SEM.

These three membranes were prepared from gel witha Si/Al ratio of 600; the different Si/Al ratios in thezeolite layer correspond to different incorporations ofAl into the zeolite framework. For ZSM-5 crystals,30 theSi/Al ratio in the zeolite framework usually correspondsto that in the gel at Si/Al ratios greater than 100. Atlower gel ratios, the Si/Al ratio in the framework ishigher than that in the gel. The Si/Al ratio of 300 formembranes M9 and M11 is lower than the gel ratio of600, as was observed by Yan et al.25 Since alumina fromthe support can dissolve in the high-pH gel, the gel ratioduring synthesis may be lower than 600. For membraneM1, the Si/Al ratio in the zeolite framework is twice thatin the gel. Membrane M1 was prepared using mono-meric Si (TEOS), which is easily converted to Si(OH)4in the presence of water. These Si(OH)4 species rapidlycondense, forming Si-O-Si bonds. Thus, Al incorpora-tion appears to be slower in this case, and the Si/Al ratioin the zeolite framework is higher than that in the gel.

For membrane M1, the Si/Al ratio drops rapidly at adepth around 50 µm from 1200 to about 50, and then itdecreases to 10 over the next 20 µm. It then drops againand fluctuates between 0.2 and 0.02 for the next 20-30 µm. For membrane M9, the Si/Al ratio drops from300 to 120 around 15 µm, and between 15 and 25 µm itdecreases from 120 to 5. The ratio drops again 25 µmfrom the top and fluctuates between 0.1 and 0.01 forthe next 40 µm. For membrane M11, the Si/Al ratiodrops from 300 to 170, and between 30 and 150 µm fromthe top, it decreases from 170 to 20. It then drops at150 µm and fluctuates between 0.08 and 0.003 for thenext 20 µm. The low values of the Si/Al ratio, which arehigher however than that of the alumina support,indicate that some zeolite lies within the support pores.The lowest Si/Al ratio reported for ZSM-5 crystals is12.33 What is significant is that, even 150 µm from the

surface, the Si/Al ratio is as high as 20 on membraneM11, indicating that zeolite crystals are present deepinto the R-alumina support. This is confirmed by EPMAmicrographs of cross sections shown in Figure 4. Thezeolite layers (lighter color indicates a higher concentra-tion of Si) are approximately 15- and 50-µm thick formembranes M9 and M1, respectively. Similar thick-nesses were measured by SEM. However, zeolite wasdispersed 15-20-µm deep into the support for mem-branes M1 and M9. This could not be seen by SEM. Formembrane M11, zeolite crystals are approximately 100µm into the support, in addition to the 70-µm layer onthe inner surface. This is consistent with XRD data thatthe peaks intensities (zeolite content) of membrane M11are 2-3 times higher than those of membranes M1 andM9.

Effect of Preparation Procedure. Membranes ontubular supports require different synthesis proceduresfrom those on flat disks, and the method of contactingthe gel with the support can change the membraneproperties. For synthesis on the inside of a porous tube,gel composition can change as crystals form and liquidpermeates through the support. In procedures I and II,the synthesis solution only contacts the inner surfaceof the tube, whereas the solution contacts both the innerand outer surfaces in procedure III. The preparationconditions for membranes M1 and M2, which wereprepared by procedure I, are listed in Table 1. They wereprepared with TEOS for 72 h at 443 K on R- andγ-alumina, respectively. Membranes M3 and M4 wereprepared on R-alumina by procedures II and III, respec-tively, for 72 h at 443 K, but required three layers.

The N2 permeation rates before calcination and then/i-C4H10 and N2/SF6 ideal selectivities of calcinedmembranes M1-M4 are listed in Table 2. MembraneM1, prepared by procedure I, has better selectivitiesthan membranes M3 and M4, which were prepared byprocedures II and III, respectively. Membrane M1 wasimpermeable to N2 after one synthesis layer, whereasmembrane M4 required three layers. Membrane M3 waspermeable even after three layers. Because both endsof the tube were plugged for procedure I, the synthesissolution penetrated into the R-alumina pores at high

Figure 3. Cross-sectional SEM micrographs of membranes supported on R-alumina. These membranes were prepared under the sameconditions, but with different Si sources: M9 (Aerosol), M1 (TEOS), and M11 (Ludox).


temperature and pressure. Thus, zeolite crystals werepartially dispersed in the pores. Membrane M2 was alsoprepared by procedure I, but used a γ-alumina support,and it has lower selectivities. Dispersion of the synthesissolution is slower in the smaller pores (5 nm) of theγ-alumina support. The γ-alumina support of membraneM2 adsorbed less than 10% of the synthesis solutionwhen it was left overnight, whereas all the solution wasabsorbed by the R-alumina support of membrane M1;the ability of the gel to permeate into the supportappears to correlate with membrane quality.

On one hand, Oh et al. 30 reported preparation of MFImembranes using a sol-gel pressurized coating tech-nique, which dispersed the synthesis solution through-out the pores of an R-alumina support. They concludedthat their membranes were defect-free because theirpermeances were constant with increasing pressure. Onthe other hand, Coronas et al.22 prepared ZSM-5 mem-branes by procedure I on γ- and R-alumina, but withoutfirst soaking the supports overnight in the gel at roomtemperature. These membranes exhibited N2/SF6 idealselectivities of 200-300, but the n/i-C4H10 ideal selec-tivities were only 1.1-4.8 at 300 K. Their N2 per-meances were 4-7 times higher than that of membraneM1, whose N2 permeance is 2.0 × 10-7 mol/m2‚s‚Pa.

Effect of Crystallization Time. Thicknesses of MFIzeolite membranes were reported to increase withincreasing crystallization time (contact time of supportwith the synthesis gel).27,34 In contrast, for A-type zeolitemembranes, which are prepared in a high-alkalinitymedium, zeolite crystals dissolve in synthesis solutionand the membrane thickness decreases with increasing

crystallization time.35 To study the effect of crystalliza-tion time, membranes M1, M5, M6, and M7 wereprepared on R-alumina with crystallization times from15 to 72 h, and all other parameters (443 K, TEOS, twolayers) were kept constant. As shown in Figure 5,i-C4H10 and n-C4H10 permeances at 301 K decrease withincreasing crystallization time, and the n/i-C4H10 idealselectivities (Figure 6) do not show a trend with crystal-lization time. The ideal selectivities decrease withpermeation temperature (Figure 6). The highest ideal

Figure 4. Cross-sectional EPMA micrographs of membranes supported on R-alumina. These membranes were prepared under the sameconditions, but with different Si sources: M9 (Aerosol), M1 (TEOS), and M11 (Ludox)). The light area corresponds to higher concentrationsof Si.

Table 2. Nitrogen Permeances (before Calcination) and Selectivities after Calcination for Membranes Prepared byProcedures I, II, and III

N2 permeation before calcination ideal selectivityamembrane(procedure) one layer two layers three layers n-C4H10/i-C4H10 N2/SF6

n-C4H10/i-C4H10separation selectivitya

M1 (I) imperm. imperm. 20 56 62 (373 K)M2 (I) perm. imperm. 2.5 3.6 5.2 (378)M3 (II) perm. perm. perm.M4 (III) perm. perm. imperm. 1.6 2.5 3.5 (378 K)

a Ideal and mixture selectivities measured at room temperature.

Figure 5. Single-gas permeances at 301 K of n-C4H10 and i-C4H10versus crystallization time at 443 K. The membranes (M1, M5,M6, and M7) were prepared on R-alumina with TEOS as the Sisource.


selectivities at 300 K are obtained for the 72-h crystal-lization time.

The permeances of i-C4H10 in a 50/50 mixture alsodecreases with increasing crystallization time. As thecrystallization time increases from 15 to 72 h, thei-C4H10 permeance decreases more than the n-C4H10permeance at lower temperatures; the permeances aresimilar at 473 K for the four membranes. The separationselectivities of n/i-C4H10 mixtures for these four mem-branes are more similar than their ideal selectivities.As shown in Figure 7, each membrane has a maximumselectivity between 300 and 373 K, and the maximumincreases with crystallization time. Because permeanceswere only measured at three temperatures, the maximaare probably higher for each membrane and are notnecessarily at the same temperature for each mem-brane.

For many MFI zeolite membranes, n-C4H10/i-C4H10separation selectivities are higher than ideal selectivi-ties, except at low and high temperatures. This has beenattributed to preferential adsorption,22,36,37 which is lessimportant at high temperatures where coverages arelow. At intermediate temperature, coverages are high,and n-C4H10 strongly inhibits i-C4H10 permeation andalso inhibits permeation of small gases such as H2.22,36

At room temperature, however, i-C4H10 also inhibits

n-C4H10 permeation. Bakker et al.36 and Vroon et al.26

reported that i-C4H10 reduced n-C4H10 flux by 60% sothat their separation selectivity was half the idealselectivity.

Effect of Number of Synthesis Layers. Mostmembranes prepared by procedure I are impermeableto N2 after one synthesis layer and before calcination.However, after calcination membranes with one layerhave low N2/SF6 and n/i-C4H10 ideal selectivities, indi-cating the presence of nonzeolite pores. Therefore,additional layers were needed. Yan et al.25 also observedthat membranes with one synthesis layer containeddefects and a second layer improved n/i-C4H10 idealselectivities considerably. Vroon27 observed that increas-ing the number of synthesis layers from one to fourincreased the layer thickness from 2.5 to 6 µm. However,n/i-C4H10 separation selectivities had a maximum aftertwo synthesis layers. Although most MFI-zeolite mem-branes have been prepared with one or two layers,thicknesses varied from 0.5 to 100 µm because ofdifferent preparation procedures, gel compositions, andsynthesis conditions.2,7,9,28,34,38 Also, the effective thick-ness may be quite different from the thickness measuredby SEM since the thinner membranes do not haveproportionally higher permeances.

Membranes M1 (two layers) and M8 (three layers)were prepared on R-alumina using the same procedureand synthesis conditions (443 K, 72 h, TEOS). Single-gas permeances of H2, N2, n-C4H10, and i-C4H10 as afunction of temperature are in Table 3. Most single-gaspermeances and ideal selectivities of membrane M8(three layers) are lower than those of membrane M1(two layers). The lower permeances might be expectedbecause the zeolite layer is expected to be thicker forthree layers, but the lower ideal selectivities are notexpected. Similarly for mixtures, permeances of n-C4H10and i-C4H10 through membrane M1 are a factor of 1.6-3higher than those for membrane M8, as shown in Table4. Separation selectivities have a maximum as a func-tion of temperature for both membranes, and mem-brane M1 has the higher selectivity at 373 K. Inagreement with previous studies for membranes pre-pared with NaOH,25,27 two synthesis layers seem to be

Figure 6. n-C4H10/i-C4H10 ideal selectivities versus permeationtemperature for ZSM-5 membranes prepared for different crystal-lization times at 443 K, as indicated. The support is R-aluminaand the Si source for synthesis was TEOS.

Figure 7. n-C4H10/i-C4H10 separation selectivities versus perme-ation temperature for ZSM-5 membranes prepared at differentcrystallization times at 443 K, as indicated. The support isR-alumina and the Si source for synthesis was TEOS.

Table 3. Single-Gas Permeances and Ideal Selectivitiesof Membranes M1 and M8

permeance ×10-8

(mol/m2‚s‚Pa) ideal selectivity

membrane attemperature H2 N2

n-C4H10

i-C4H10

H2/N2

H2/i-C4H10

n/i-C4H10

M1 at 301 K 37 20 3.4 0.17 1.9 220 20M1 at 373 K 46 22 10 1.0 2.1 46 10M1 at 473 K 74 23 17 9.3 3.2 8.0 1.8M1 at 473 K 74 23 17 9.3 3.2 8.0 1.8M8 at 301 K 23 14 2.1 0.18 1.6 130 12M8 at 373 K 25 15 8.2 0.46 1.7 54 18M8 at 473 K 46 17 12 7.3 2.7 6.3 1.6

Table 4. n/i-C4H10 Mixture Permeances and SeparationSelectivities of Membranes M1 and M8

permeance ×10-8

(mol/m2‚ s‚Pa)separationselectivitymembrane

at temp. n-C4H10 i-C4H10 (n-C4H10/i-C4H10)

M1 at 301 K 1.3 0.052 25M1 at 373 K 6.9 0.11 62M1 at 473 K 13 2.7 4.8M8 at 301 K 0.42 0.053 7.9M8 at 373 K 2.8 0.058 48M8 at 473 K 8.9 1.7 5.2


optimal for producing good membranes. We also ob-served this when we used Ludox AS40 as the Si sourceinstead of TEOS.

Effect of Si Source. The nucleation and crystalliza-tion rates and the structure and morphology of zeolitecrystals can change by changing the Si source.32 TheEPMA and SEM results in Figures 2-4 illustrate thechanges in membrane morphology and thickness. Mem-branes M9, M11, and M1 used polymeric (Aerosil-200),di-trimeric (Ludox AS40), and monomeric (TEOS) Sisources, respectively. The crystal sizes in membranesM9, M1, and M11 are approximately 5, 10, and 20 µm,respectively, and the membrane thicknesses are ap-proximately 15, 50, and 70 µm. Membranes with smallercrystals and thinner layers are obtained from polymericSi; that is, the rate of zeolite crystallization appears tobe lower with polymeric Si.39

The n/i-C4H10 ideal selectivities for membranes M1,M9, and M11 are shown in Figure 8 as a function oftemperature. Membranes M1 and M11, prepared withmonomeric and di-trimeric Si, exhibit much higher idealselectivities than membrane M9, which was preparedwith polymeric Si. Membrane M9 also has 2-4 timeshigher single-gas permeances for H2, N2, n-C4H10, andi-C4H10. The n/i-C4H10 separation selectivities correlatewith the ideal selectivities. As shown by comparingFigures 8 and 9, ideal and separation selectivities arein the same order at 300, 373, and 473 K (M1 > M11 >M9). Note that the mixture selectivities are higher thanthe ideal selectivities except for membrane M11 at 300K. The maximum selectivities for membranes M1 andM11 are about 7 times higher than that for membraneM9. The mixture permeances for membrane M9 areabout 2 times those of membranes M1 and M11. Thetemperature dependencies of the separation selectivitiesare different for membranes M1 and M11. The maxi-mum selectivity for membrane M11 is at a highertemperature than for membrane M1, and thus itsselectivity at 473 K is 2 times that for membrane M1.Because of the large particle size of the polymeric Sisource (membrane M9), dispersion of synthesis solutionmay be inhibited so that zeolite crystals only depositon the inner surface of the support, as observed formembrane M2 (using small-pore γ-alumina). However,good quality MFI membranes26,40 were prepared withpolymeric fumed silica in the presence of NaOH, whichcan depolymerize polymeric Si.

Effect of Crystallization Temperature. The nucle-ation and crystallization rates increase with tempera-ture,32 and most MFI zeolites are crystallized at 443-450 K. Above 470 K, cristobalite cocrystallizes, andbelow 400 K, the rates are low and long times arerequired for complete crystallization. Vroon et al.,26,27

however, obtained good ZSM-5 membranes at 373-393K. One advantage of low temperature is that thenucleation period is long so that the synthesis solutionhas time to disperse into the support before crystalsform. In the current study, zeolite membranes weresynthesized at temperatures from 403 to 469 K. Asshown in Figure 10, membrane M10, which was pre-pared at 403 K, has the highest n/i-C4H10 ideal selectiv-ity at 300 K, but the selectivity is much lower at 373-473 K. In contrast, membranes M1 and M11, preparedat 443 K, and M12, prepared at 458 K, have lower idealselectivities at 300 K, but their selectivities do not dropas rapidly with temperature. The permeation behaviorsof these three membranes are similar. With increasingcrystallization temperature, the n/i-C4H10 ideal selectiv-ity at 473 K increases. High n/i-C4H10 ideal selectivitiesat 300 K may not indicate high-quality membranes. For

Figure 8. n-C4H10/i-C4H10 ideal selectivities versus permeationtemperature for ZSM-5 membranes prepared at 443 K using theindicated Si sources. The support is R-alumina and the crystal-lization time is 72 h.

Figure 9. n-C4H10/i-C4H10 separation selectivities versus perme-ation temperature for ZSM-5 membranes prepared at 443 K usingthe indicated Si source. The support is R-alumina and thecrystallization time is 72 h.

Figure 10. n-C4H10/i-C4H10 ideal selectivities versus permeationtemperature for ZSM-5 membranes, prepared at the indicatedcrystallization temperatures, versus permeation temperature. Thesupport is R-alumina, and the synthesis time and Si sources areM1 (72 h, TEOS), M10 (72 h, TEOS), M11 (72 h, Ludox), and M12(48 h, Ludox). Membrane M13 was synthesized under the sameconditions as membrane M12, but with twice the water contentin the gel.


example, Geus et al. reported a n/i-C4H10 ideal selectiv-ity of 64 at room temperature, but the selectivity wasonly 1 at high temperature.

The n/i-C4H10 separation selectivities as a function ofpermeation temperature for membranes prepared atdifferent temperatures are presented in Figure 11.Increasing crystallization temperature from 403 K(membrane M10) to 443 K (membranes M1 and M11)increased the maximum selectivity from 19 to 62. Afurther increase of crystallization temperature to 458K (membrane M12) increased the maximum selectivityto 111. The separation selectivity of membrane M12 is20 at 473 K, whereas selectivities for membranes M11and M1 are 9.0 and 4.6, respectively. Note that the idealand separation selectivities do not correlate except at473 K. High crystallization temperatures for alkali-freeconditions appear to yield membranes with high sepa-ration selectivities at high temperature. Thus, mem-brane M2, prepared on γ-alumina at 443 K, showed poorseparation selectivity, whereas membrane M15, pre-pared on γ-alumina at 469 K, exhibited a separationselectivity of 19 at 473 K. Also, good MFI membranesprepared at 465 K have been reported by Bakker etal.29,36,41 and Kusakabe et al.40

Effect of Gel Dilution. The water concentration inthe gel plays an important role in zeolite synthesis.Adding more water reduces the pH, which affects thecrystallization rate and crystal morphology.25,32 Geldilution is a critical factor for obtaining high-quality,A-type zeolite membranes,5 and it has been reported toimprove the quality of MFI-zeolite membranes.30 Dilut-ing the synthesis solution reduces the viscosity andfavors dispersion into the support pores. To study theeffect of gel dilution, membrane M13 was synthesizedusing the same conditions as membrane M12 (Table 1),but twice as much water was used in the synthesissolution.

The n/i-C4H10 ideal selectivities for membranes M12and M13 decrease with increasing permeation temper-ature, as shown in Figure 10. The ideal selectivities formembrane M13 are 2 times those for M12 at 300-373K, but the selectivity is lower at 473 K. Single-gas

permeances for membrane M12 were 2-3 times higherthan those for membrane M13. In contrast to the idealselectivities, separation selectivities for membrane M12are 2-4 times those of membrane M13, as shown inFigure 11. Thus, gel dilution causes lower permeancesand separation selectivities, even though higher idealselectivities are obtained. Again, the ideal and mixtureselectivities do not correlate, except at 473 K. Both geldilution and more synthesis layers decrease gas per-meances, presumably because of thicker zeolite layersand better dispersion within the porous support.

Effect of Supports. MFI zeolite membranes havebeen prepared on alumina, clay, cellulose, Teflon, fusedglass, stainless steel, and gold supports.11,28,29,34,38,42 Thezeolite type, orientation, and location of zeolite crys-tals28,29,42 changed with the support. Only silicalitemembranes have previously been prepared on stainlesssteel disks. Recently, ZSM-5 membranes were preparedon tubular stainless steel,37 but they had low ideal andseparation selectivities for n-C4H10/i-C4H10. Figure 12compares ideal selectivities for alkali-free ZSM-5 mem-branes prepared on R-alumina, γ-alumina, and stainlesssteel. Note that the ideal selectivities at 300 K aresimilar for the three membranes, but they are quitedifferent at higher temperatures. The n-C4H10/i-C4H10ideal selectivities of the R-alumina-supported membranedecrease with increasing temperature, whereas selec-tivities have maxima for membranes on γ-alumina andstainless steel. The stainless-steel-supported membranehas the highest ideal selectivity, which is almost 60 at373 K, and at 473 K is almost 6 times that for thealumina-supported membranes. In contrast, for mix-tures, the R-alumina-supported membrane has a maxi-mum selectivity that is 2 times that of the stainless-steel-supported membrane (Figure 13), and the highestselectivity is greater than 100 for the R-alumina-supported membrane. At 473 K, the stainless-steel-supported membrane has the highest separation selec-tivity, as was seen for the ideal selectivities. Again, theideal and mixture selectivities correlate at 473 K butnot at the lower temperatures.

Comparison to Membranes in the Literature.Table 5 compares n/i-C4H10 ideal and separation selec-tivities for MFI membranes in the literature. The idealselectivities for different membranes show quite dif-ferent temperature dependencies. For some mem-branes,21,26,36,43 ideal selectivities decrease with increas-ing temperature, whereas for others,25,40,44 the idealselectivities increase. Similarly, ideal selectivities for

Figure 11. n-C4H10/i-C4H10 separation selectivities versus per-meation temperature for ZSM-5 membranes, prepared at differentcrystallization temperatures (M10, 403 K; M1 and M11, -443 K;M12, 458 K), versus permeation temperature. The support isR-alumina, and the synthesis time and Si sources are: M1 (72 h,TEOS), M10 (72 h, TEOS), M11 (72 h, Ludox), and M12 (48 h,Ludox). Membrane M13 was synthesized under the same condi-tions as membrane M12, but with twice the water content in thegel.

Figure 12. n-C4H10/i-C4H10 ideal selectivities for ZSM-5 mem-branes prepared under similar conditions with Ludox as the Sisource, but on different porous supports, as indicated.


some of our membranes decrease with temperature,whereas other have maxima as a function of tempera-ture. Tsapatsis46 reported that changing the crystalorientation on the supports changed ideal selectivitiesfrom 20 to 0.02. That is, either n-butane or i-butanepreferentially permeated.

Vroon et al.26 and Moueddeb et al.45 found that n/i-C4H10 separation selectivities decreased with increasingtemperature, whereas separation selectivities for all ourmembranes have maxima. Some of our membranes canseparate n/i-C4H10 mixtures with high selectivities athigh temperature. This is an important feature forcatalytic membrane reactors. A comparison of ideal andmixture selectivities shows that organic separations inmembranes prepared previously in our laboratory is

mostly due to preferential adsorption and not molecularsieving.22,37 Thus, n-C4H10/i-C4H10 separation selectivi-ties were 30-50 at 365 K, but close to 1 at 473 K whereadsorption coverages are low. In contrast, the maximumselectivity for membrane M12 is 111, which is more than2 times as high and shifted to 60 K higher temperature.This may be due to the combination of preferentialadsorption (since separation selectivity > ideal selectiv-ity) and molecular sieving. At 473 K, the separationselectivities decrease because less n-C4H10 is adsorbedand separation is expected to be mainly by molecularsieving.

For lower quality membranes, no separations areobtained at high temperature, whereas for membranescontaining fewer defects, separation selectivities of 20-36 are observed at high temperature. Figure 14 com-pares n-C4H10 and i-C4H10 permeances as a function of

Table 5. Comparisons of Ideal and Separation n/i-C4H10 Selectivities of MFI-Zeolite Membranes

geometry/zeolite support temp (K)ideal

selectivitysep.

selectivity refs

flat/silicalite stainless steel 300 64 Geus et al.43

523 1.5flat/ZSM-5 R-alumina 303 18.4 Yan et al.25

458 31.1flat/ZSM-5 (posttreatment coking) R-alumina 381 325 Yan et al.24

458 322flat/silicalite R-alumina 298 90 52 Vroon et al.26

373 50 19473 11 11

flat/silicalite stainless steel 295 58 27 Bakker et al.36

403 25 23tubular/silicalite γ-alumina 298 20 Bai et al.21

443 0.6tubular/silicalite γ-alumina 303 7.0 Giror-Fendler et al.44

R-alumina 483 28tubular/silicalite R-alumina 303 7.4 Kusakabe et al.40

372 18tubular/silicalite R-alumina 303 3.0 56 Moueddeb et al.45

373 8.0 9.0473 14 8.0550 11 10

tubular/ZSM-5 γ-alumina 365 33 Coronas et al.22

429 4.5473 1.5

tubular/ZSM-5 γ-alumina 301 7.9 5.5 this work429 ∼6.0 42473 2.0 18

tubular/ZSM-5 R-alumina 301 14 7.7 this work429 ∼7.0 110473 4.2 20

tubular/ZSM-5 stainless steel 301 14 5.5 this work429 40 55473 23 36

Figure 13. n-C4H10/i-C4H10 separation selectivities for ZSM-5membranes prepared under similar conditions with Ludox as theSi source, but on different porous supports, as indicated.

Figure 14. Mixture permeances of n-C4H10 and i-C4H10 formembranes M2 (TEOS, γ-alumina) and M12 (Ludox, R-alumina).


temperature for two membranes. The n-C4H10 per-meances are similar in the two membranes, except athigher temperatures, where the permeance increasesmore for membrane M2. The i-C4H10 permeances havequite different temperature dependencies, however. Inmembrane M2, the i-C4H10 permeance increases almost3 orders of magnitude over the temperature range,whereas the i-C4H10 permeance in membrane M12increases less than 1 order of magnitude. The largeincrease in membrane M2 is probably because somei-C4H10 permeates through nonzeolite pores. Thus,i-C4H10 permeates 100 times faster in M2 (γ-aluminasupport, TEOS) than in M12 (R-alumina, Ludox) at hightemperature, and M12 has much higher selectivity athigh temperature. Vroon et al.26 claimed that the MFI-zeolite membranes with defects showed no separationselectivity at 473 K, and they used a n-C4H10/i-C4H10separation selectivity of 11 at 473 K as proof of defect-free MFI membranes. Thus, membrane M12 apparentlyhas a significantly lower concentration of defects thanM2.

The maximum in separation selectivity that was seenfor all our membranes is probably due to a combinationof effects. At low temperature, n-C4H10 is effective atblocking i-C4H10 permeance in most membranes, thoughi-C4H10 can also slow n-C4H10 permeance some. As thetemperature increases, the n-C4H10 permeance increasesbecause of activated diffusion, but i-C4H10 is still blockedby n-C4H10 so its permeances does not increase as fast.Thus, the selectivity increases. At higher temperature,the n-C4H10 permeance increases less because its cover-age decreases. This lower coverage is less effective atblocking i-C4H10, so its permeance increases signifi-cantly and the selectivity decreases. The i-C4H10 per-meance may also increase because the pore size canincrease with temperature or because nonzeolite poresincrease in size.

The gas permeances were measured for our mem-branes using a pressure drop method because that iscloser to the conditions that might be applied to themembranes. Most studies of zeolite membranes byothers used a sweep gas on the permeate side or on bothsides of the membrane. Thus, for comparison to otherstudies, n-C4H10 and i-C4H10 permeances were alsomeasured for membrane M14 with a sweep gas on thepermeate side. As shown in Figure 15, at 300-373 K,n-C4H10 and i-C4H10 permeances are similar for the twomethods, whereas at 430-473 K, n-C4H10 permeances

measured by the sweep gas method are slightly lowerthan those measured by the pressure drop method andi-C4H10 permeances are higher. The lower n-C4H10permeance is probably due to back permeation ofhelium. The higher permeance for i-C4H10 presumablyresults because of an increase in the driving forcebecause the i-C4H10 coverage is lower on the permeateside. Though these differences do not appear to be large,the n-C4H10/i-C4H10 separation selectivities measuredby the sweep gas method are approximately half thosemeasured by the pressure drop method. Thus, whenmembranes are compared, a factor of 2 difference inselectivity may just be due to different measurementmethods.

Many studies noted the lack of correlation betweensingle-gas and mixture behavior22,26,36,45 due to prefer-ential adsorption in mixtures. This effect is predominantat low temperature (300-400 K) where coverage is highand is less important at higher temperature. Thus,n-C4H10/i-C4H10 ideal and separation selectivities cor-relate at 473 K in the results obtained by Vroon,26

Moueddeb,45 and ourselves. The n-C4H10/i-C4H10 idealand separation selectivities at 473 K may be a betterindicator of membrane quality than selectivities meas-ured at room temperature. At lower temperatures (300-400 K), adsorbed coverages are high, and defects canbe blocked by adsorption species.

Good membranes were not obtained every time onR-alumina supports, presumably because of differencesin the properties of the alumina structure and poredistribution. For example, when two R-alumina tubeswere filled with the same synthesis solution, the syn-thesis solution went through one tube but not the other.For two other supports, the synthesis solution did notgo through either tube at room temperature, but aftercrystallization, one tube was still filled with liquid andthe other tube was not. In contrast, for stainless steelsupports reproducibility was higher.

Conclusions

Alkali-free, ZSM-5 zeolite membranes with high se-lectivities were successfully synthesized on porousR-alumina, γ-alumina, and stainless steel tubular sup-ports.

Membrane properties (location of the zeolite, idealselectivities, permeances, separation selectivities, andtemperature dependencies of selectivities) depend on thepreparation procedure. A procedure that uses a smallamount of synthesis gel, which only contacts the innersurface of the tubular support but is allowed to pen-etrate into the support before crystallization, producesmembranes with fewer defects and high ideal andseparation selectivities.

Membrane properties depend on gel compositions andsynthesis conditions. Higher temperature, longer times,fewer layers, and more concentrated gels yield betteralkali-free ZSM-5 membranes.

The nature of the Si source affects the location,morphology, and thickness of zeolite crystals and con-sequently causes a large difference in separation selec-tivities. The highest separation selectivities were ob-tained for membranes prepared using silica sol (LudoxAS40).

The pore size and material properties of the supportaffect the separation selectivities. The highest separa-

Figure 15. Mixture permeances of n-C4H10 and i-C4H10 measuredby using pressure drop and sweep gas methods for membrane M14,which is supported on stainless steel.


tion selectivities for n/i-C4H10 are observed on R-aluminasupports, whereas the highest selectivity at 473 K isobtained on a stainless steel support.

Ideal n/i-C4H10 selectivities do not correlate withseparation selectivities at 300-400 K because of pref-erential adsorption in mixtures, but they correlate at473 K, where adsorption coverage is low. Thus, selec-tivities at measured higher temperatures may be betterindicators of MFI membrane quality.

Acknowledgment

We gratefully acknowledge support by the NewEnergy and Industrial Technology Development Organ-ization (NEDO) of Japan.

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Received for review December 30, 1998Revised manuscript received April 12, 1999

Accepted April 28, 1999

IE980808G


alkali-free zsm-5 membranes: preparation conditions and separation performance

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