J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) 11th International Congress on Catalysis - 40th Anniversary
Studies in Surface Science and Catalysis, Vol. 101 1996 Elsevier Science B.V.
Characterization of a Zeolite Membrane for Catalytic Membrane Reactor Application Anne Giroir-Fendler, J6r6me Peureux, Henri Mozzanega and Jean-Alain Dalmon Institut de Recherches sur la Catalyse, 2 av. Albert Einstein, 69626 Villeurbanne Cedex France
Abstract This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported.
1. INTRODUCTION One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs. In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [ 1-3 ]. Following this principle, when using a membrane, lower temperatures can be used leading to a longer catalyst lifetime and energy and cost saving. However such a membrane should be stable at high temperature (ca 500~ highly permselective (loss of reactant should be of course avoided) and permeable enough (the permeation rate should be in the same range as the reaction rate). Dense Pd-based membranes have been first used for CMRs applications . They are indeed highly selective for H 2 permeation but are expensive, sensitive to ageing and poisoning and are strongly limited by their low permeabilities. Classical commercial ceramic porous materials, as those obtained via sol-gel processes, generally have adequate permeabilities but could present some drawbacks. They indeed have a limited thermal stability and are generally not permselective enough: their pores are in the mesoporous range and maximum separation factors correspond to Knudsen diffusion mechanisms. To ensure a better separation, molecular sieving will act much better. This size exclusion effect will require an ultramicroporous (i.e. pore size D < 0.7 nm) membrane. Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as D 2 and D respectively). The presence of large pores will therefore cancel any sieving effect. A zeolite membrane, where the pores originate from the structure, presents only one type of (ultramicro)pore and therefore seems to be a good candidate for CMRs application. Moreover the structural origin of the pores should induce a much better thermal stability of the
128 membrane when compared to sol-gel systems where only the texture originates the pores and controls size evolutions. The preparation of defect-free zeolite membranes is the subject of a recent and intensive research for gas separation and CMRs applications . Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications , like selective oxidation and oxidative dehydrogenation of hydrocarbons; they may also act as active contactor in gas or gas-liquid reactions. This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Results in CMRs applications are also briefly given.
2. EXPERIMENTAL 2.1 Preparation of the zeolite membrane Commercial porous ceramic tubes (SCT/US Filter Membralox T1-70 ) were used in this study as support for the zeolite material. They are made (Figure 1) of three consecutive layers of macroporous a-A1203 with average pore sizes decreasing from the external to the internal layer. A thin toplayer made of mesoporous y-Al20 3 was also present in some samples. For gas permeability, gas separation and catalytic measurements the tubes were first sealed at both ends with an enamel layer before zeolite synthesis. Tubes with porous lengths up to 20 cm were used in this study.
Figure 1: Schematic of a cross-section of a commercial SCT tube used as support. Layers 1, 2 and 3 are made of ~-AI203 and have respective thicknesses of (Bm): 1500, 40, 20 and average pore sizes of (~tm): 12, 0.9, 0.2. Layer 4 (optional) is made of ~/-AI203 and has a thickness of 34 pm and average pore size of 4.5 nm.
The silicalite-alumina membrane was prepared after adding a solution containing the silicalite precursor (i.e. silica + template) to the above-mentioned porous tube (hereafter called support) and a specific hydrothermal treatment performed ; under the chosen conditions no material is formed in the absence of the porous support. The tube is then calcined at 673 K for removing the template. 2.2 Characterization of the zeolite membrane SEM an& SEM-EDX analyses have been used in order to observe how and where the new material forms on the alumina support. XRD and 29Si MASNMR studies have been performed for its identification. Porous characteristics of the composite material have been explored using N 2 adsorption-desorption experiments (Micromeritics ASAP 2000M). Transport properties have been studied before and after Si deposition using a rig similar to the one for catalytic testings (Figure 2). Pure gas permeabilities (H2, He, N2, normal and isobutane) were studied by measuring the flux passing though the membrane as a function of temperature and pressure for a constant transmembrane differential pressure (no sweep gas).
Figure 2. Rig for gas transport and catalytic measurements Gas separation performances (H2/n-butane , n-hexane/2-2 dimethylbutane) have been measured using a sweep gas (countercurrent mode) in order to increase the permeation driving force (no differemial pressure was used); permeate and retentate compositions (see Figure 2) were analysed using on line gas chromatography. Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation  was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-O mixed oxide  was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor).
3.1 Morphology studies The robe has been first characterised just after the hydrothermal treatment step (i.e. before the calcination). The N 2 isotherm is typical of macroporous materials (Figure 3, curve 1) and the tube is gas-tight.
Figure 3. N 2 isotherms before (curve I) and after (curve 2) template removal
130 All the other characterization studies have been performed after the calcination step. XRD ext?eriments have shown that the material formed during the synthesis has the MFI structure. 29Si and 27A1 MASNMR spectra indicated that this phase has a Si/AI ratio varying between 550 and 30 as a function of the sample prepared and also that no extra framework AI is present. SEM micrographs (Figure 4) reveal the presence of small crystallites (Figure 4, C) deposited on the large ot-Al20 3 particles of the thicker layer of the support (layer n~ Figure 1) and of larger ones on the external surface of the ot-Al20 3 layer n~ (Figure 4, B). However, when using as starting material a ceramic tube with a ,/-AI203 toplayer (Figure 1), the formation of crystallites at the external surface is not observed.
Fi~mare4. SEM micrographs of the silicalite-alumina composite material. A: cross-section of the tube. B and C: magnifications of the inner surface of the tube and of the first otAI203 layer.
131 Crosswise SEM-EDX analyses show that the global Si/AI ratio (zeolite + support) varies in a very different way (Figure 5) according to the presence or absence of the ~/-AI203 toplayer in the support.
1 SilAI 0.8
30 L (pm)
Figure 5. Crosswise SEM-EDX analysis in the composite membranes (L, radial distance from the inner surface of the tube). Curve 1 (e) corresponds to support without the ~,AI203 toplayer, curve 2 (r-l) with the 7-A1203 toplayer. N 2 adsorption leads to a type I isotherm (Figure 3, curve 2), typical of microporous systems. The corresponding pore size distribution, as calculated using the Horvath-Kawazoe equation [ 11] is given in Figure 6. A sharp maximum, near 0.6 nm is observed. Other methods of isotherm analysis, such the DFT method [ 12], lead to very similar results in the microporous range, but also reveals the mesoporous domain and part of the macroporous one (pore diameters < 200 nm). Very few pores are present in the mesoporous range. Hg porosimetry experiments completed this characterization and have shown [ 13] that pores corresponding to the intermediate ot-Al203 layers (n ~ 2 and 3, see Figure 1) almost completely disappeared after the zeolite synthesis.
volume 0.002 (cm3/g)
Figure 6. Pore size distribution (m) and cumulative pore volumes (..-). Microporous domain, Horvath-Kawazoe equation.
3.2 Transport studies The zeolite-alumina tube is no more gas-tight after the thermal treatment. The presence or absence of the y-A1203 toplayer in the starting support does not influence the gas transport properties of the final zeolite-alumina tube. For similar temperature conditions the permeance for nitrogen varies with the applied pressure in a different way according to nature of the sample (i.e. the two starting supports or the zeolite-alumina composite, Figure 7).
Internal pressure (1# Pa)
Figure 7. N2 permeances at 30~ in the tx-Al203 support alone (1, O), in the ot-Al203 support plus the ,/-AI203 toplayer (2, II) and in the zeolite-support composite (3, A). In the zeolite-alumina composite the behaviour of different gases (permeance of pure gases as a function of the temperature, Figure 8) behave very differently from those predicted by ideal Knudsen diffusion processes.
10- ~ c L " ' ~ ' c ~ . . . . . 5 3 . . " Permeance ~ (lO'7mole/s.Pa. 2 )" w _
200 300 Temperature ('C)
Figure 8. Permeances in the zeolite-support composite for H2 (1, r'l), N 2 (2, A), He (3, II), n-butane (4, O), isobutane (5, o). Intemal pressure 1.2 105 Pa. Gas separations also show non-Knudsen behaviors. In the case of the H2/n-butane mixture, the temperature has a drastic effect on the main permeating gas, at low temperature almost only butane, the heavier component, permeates (Figure 9).
When introducing a mixture of n-hexane and 2-2 dimethylbutane (45/55 molar ratio), almost only n-hexane permeates: the permeate contains up to 99.5% of the linear isomer (Figure 10).
Flux 100(10 mole/h)
Figure 9. H 2 (D) / n-butane ( t ) separation with the composite zeolite-alumina membrane (fluxes in the permeate as a function of the temperature). A mixture of hydrogen, n-butane and nitrogen (12 : 14 : 74) was fed in the tube (Fig. 2) with a flow rate of 4.8 l/h. Sweep gas (N2), countercurrent mode, flow rate 4.3 l/h.
100 Temperature (~
Figure 10. N-hexane (rl) / 2-2 dimethylbutane (m) separation with the composite zeolite-alumina membrane (fluxes in the permeate as a function of the temperature). A mixture of n-hexane, 2-2 dimethylbutane and nitrogen (5 : 6 : 89) was fed in the tube (Fig. 2) with a flow rate of 2 l/h. Sweep gas (N2), countercurrent mode, flow rate 0.5 l/h.
3.3 Catalytic studies Most of the results have been already partly presented in  (isobutane dehydrogenation) and  (propane oxidative dehydrogenation). Let us recall that the membrane presented in this paper has been associated with a fixed bed catalyst placed within the tube. In the isobutane dehydrogenation the catalytic membrane reactor allows a conversion which is twice the one observed in a conventional reactor operating under similar feed, catalyst and temperature conditions (and for which the performance corresponds to the one calculated from thermodynamics) . In the propane oxidative dehydrogenation, where the membrane separates the two reactants, a 20% increase in the yield was observed with respect to a conventional reactor working at isoconversion [ 10].
134 4. DISCUSSION
4.1 Morphological properties The starting alumina support (Figure 1) is either macroporous or, in the presence of the y-Al203 toplayer, mesoporous. These two materials are highly permeable (Figure 7). The fact that the tubes become completely impervious to any gas aiter the hydrothermal synthesis suggests that the material originated from the synthesis has plugged up the support, resulting in a defect-free gas-tight tube (no cracks or pin-holes). The presence of macropores (Figure 3) suggests however that the porosity (non-passing through pores) has not completely vanished. After the calcination step, experimental data (XRD, 29Si MASNMR) show that a zeolite with the silicalite structure has been formed. 29Si MASNMR indicates for the zeolite material a Si/A1 ratio depending on the sample prepared: it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. SEM micrographs (Figure 4) show the deposition on the ct-Al203 grains of small crystaUites with the typical hexagonal shape of silicalite. The pore size distribution, as deduced from N 2 adsorption, presents a very narrow peak centred on 0.5 nm, also in good agreement with the pore diameter of silicalite-type zeolites. All these data confirm that a well-defined zeolite silicalite-type crystalline phase has been formed in the presence of the alumina porous tube (which seems indispensable for the zeolite synthesis, as no material is formed in its absence). SEM-EDX analyses (Figure 5) underline the difference of the final material whether the support has a y-Al203 toplayer or not. In its absence, the large zeolite crystals growing on the inner surface of the tube (Figure 4) result in high surface Si/Al ratios. In the porous domain corresponding to layers 2 and 3, the Si/Al ratio is more or less constant and could correspond to a complete filling up of the porous volume (layers 2 and 3 have similar porosities). When analysing the much thicker layer n~ where SEM micrographs show that the zeolite does not fill up the pores, the Si/Al ratio decreases. When using a support with the ~/-AI203 toplayer, the absence of zeolite crystals on the surface, as observed by SEM, is confirmed by the EDX analysis: the Si/Al ratio strongly decreases at the surface. This means that the separative zeolite layer has been here formed only in the bulk of the porous support and not mainly deposited on the outer surface of the support (some infiltration in the support being possible), as is generally the case in zeolite membranes described in the literature [ 14-19]. The presence of the zeolite only within the support could lead to some advantages as far as mechanical and thermal stabilities are concerned. Indeed the separative layer is here protected by the resistant t~-AI203 (thus avoiding damage when introducing for instance catalyst pellets in the tube). The thermal stability of the present zeolite membrane has been also checked: after calcination at 700~ the microporous character remains [ 13 ] and transport properties are not significantly modified after oxidative dehydrogenation of propane at 600~ [ 10] and also after several hours at 850~ under air/steam mixtures . Let us also underline that the zeolite membrane described here is prepared in only one hydrothermal synthesis step. This is not always the case for other preparations reported in the literature for which several successive zeolite synthesis could be needed with the same support to suppress defects.
4.2 Gas transport properties Figure 7 shows that N 2 permeability strongly depends on the pore size. For the macroporous support (curve 1) Poiseuille flow occurs, leading to an increase of the permeance
135 with the pressure. For the mesoporous support (curve 2), Knudsen diffusion rules the transport and permeance becomes pressure independent. When using the microporous zeolite membrane (curve 3) the N 2 permeance decreases when the pressure increases: such a behaviour can be accounted for by activated diffusion mechanisms , which are typical of zeolite microporous systems. In such systems the diffusivity depends on the nature and on the concentration of the diffusing molecule which interacts with the surface of the pore. For gases with low activation energies of diffusion, a decrease of the permeability can be observed . Figure 8 indicates that the permeance of n-butane goes through a maximum when increasing the temperature. This observation agrees with the above-mentioned mechanism. At low temperature the nC 4 concentration is high in the zeolite and permeance is low due to the low probability of finding a free site ensuring the mobility. An increase of temperature will then favour the transport. However, when the temperature is too high the coverage of the interacting species decreases and becomes the limiting factor, leading to a global decline of the transmembrane flux. Such phenomena have been also recently described in other zeolite membranes , [ 18]. The changes in H2/n-butane separation when increasing the temperature (Figure 9) can be explained on the same basis. At low temperature n-butane interacts strongly with the zeolite and owing to its size completely blocks the pores. Hydrogen has no more room to penetrate the pore and only n-butane diffuses. When increasing the temperature, the occupancy of the zeolite porous volume by n-butane progressively decreases and the hydrogen flux increases. However even at high temperature, the observed separation factor is less than the one expected from Knudsen diffusion. These results underline that the experimental separation factor may strongly differ from the one expected from pure gas permeabilities measurements. However this is not always the case, especially when the two components weakly interact with the surface. When using the membrane to separate a H2/isobutane mixture, the permeation of isobutane, due to its size, is restricted over the entire temperature range and the transmembrane fluxes of the two components of the mixture better follow the permeabilities of the pure gases. Separation factors are here much higher (factors up to 80 have been measured). Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-flee.
4.3 Catalytic properties In isobutane dehydrogenation, when using the zeolite membrane associated with a fixed-bed Pt-based catalyst placed in the inner volume of the tube, the yield is twice the one observed in a conventional reactor . This is especially due to the high separative performance of the membrane which selectively removes the produced hydrogen from the reactor. When using a less selective membrane, for instance the starting support with the 7AI203 toplayer (for which only Knudsen separation occurs), the performance of the CMR is limited by the loss of isobutane through the membrane . The zeolite membrane has also been used in the oxidative dehydrogenation of propane. The goal is here to limit the undesirable complete oxidation owing to the controlled addition of oxygen diffusing through the membrane. Following this mode, it is possible to keep a low oxygen/hydrocarbon ratio all along the fixed bed and then limit complete oxidation. Using the
136 zeolite membrane a 20% increase in the propene yield has been obtained [ 10]. Here also, when using only the starting support as diffusion barrier, it is not possible to control the 0 2 addition and the performance is quite similar to the one observed in a conventional reactor [ 10].
CONCLUSIONS The synthesis of a well-crystallised silicate-type zeolite within the porous volume of an ot-Al20 3 tubular support leads to a defect-flee zeolite membrane showing high thermal and mechanical stabilities. In gaseous mixtures, the separative performance of the membrane greatly depends on the temperature and the nature of the components, especially when strong interactions take place. When occurring, molecular sieving leads to high separative performances. These properties, combined with its high thermal stability, suggest that the zeolite membrane is a very promising material for CMR applications.
Acknowledgements We are indebted to SCT - US Filter for providing alumina support tubes.
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