Carbon Membranes

Ind. Eng. Chem. Res. 2007, 46, 3997-4006

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Synthesis and Permeation Properties of Silicalite-1/Carbon Membranes A Ä ngel Berenguer-Murcia,† Leszek Gora,‡ Weidong Zhu,§ Jacobus C. Jansen,‡ Freek Kapteijn,§ Diego Cazorla-Amoro´ s,*,† and A Ä ngel Linares-Solano† Departamento de Quı´mica Inorga´ nica, UniVersidad de Alicante, Ap. 99, San Vicente del Raspeig, 03080 Alicante, Spain, Ceramic Membrane Centre, The Pore, Delft UniVersity of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands, and Catalysis Engineering, DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands

Macroporous carbon disks have been used as supports for the growth of continuous silicalite-1 films by both the direct and secondary (i.e., seeded) growth methodology and at conditions different from those used in a previous study. After calcination, the synthesized zeolite/carbon composites were mounted in a WickeKallenbach cell and the permeation of different gases (pure gases and gas mixtures) were analyzed at different pressures and temperatures. The results show that the prepared materials show acceptable membrane properties in the separation of CO2/N2 and n-C4H10/i-C4H10. Thus, carbon might be envisaged as a possible alternative support to conventional materials such as ceramics or sintered metals because of its interesting properties. 1. Introduction Nowadays, membrane technology is envisaged as a desirable alternative to conventional separation processes such as filtration, distillation, or purification because of its comparatively low energy consumption and compact design. During the last two decades, the field of zeolite membrane preparation and characterization has witnessed an intense research activity, which has resulted in numerous groups having successfully prepared a wide variety of zeolite membranes.1,2 This interest derives mainly from the fact that zeolites are widely used in the preparation of membranes for separation of molecular mixtures because they are crystalline aluminosilicates with a threedimensional framework, which, in general, form uniform-sized channels,3 and thus, these materials present better selectivity, thermal stability, and chemical stability than organic polymeric membranes.4 Since the possibility of self-supported zeolite membranes was almost ruled out because of mechanical stability problems,5 a significant number of papers has appeared in the literature, describing the synthesis of zeolite membranes on a large variety of supports,2,6-8 among which ceramic and metallic supports are those most widely used. However, these supports can suffer from partial dissolution under strong alkaline conditions (as is the case in zeolite synthesis) and negatively influence zeolite film formation.9 Additionally, both materials are also expensive; these drawbacks hinder their extensive use as supports for zeolite membrane preparation. In this sense, carbon materials, which possess low density, great chemical and thermal stability (under nonoxidizing conditions), and a high hydrophobic character,10 are very interesting supports for zeolite membranes. The low thermal expansion coefficient and relatively low capital cost of carbon supports can account for an additional advantage,7 and may even be produced in hollow fiber form with the potential of high membrane area per unit volume. Our research group has already reported the growth of silicalite-1 on different carbon materials,11 and more specifically on macroporous carbon disks7 by direct hydrothermal synthesis * To whom correspondence should be addressed. Tel.: +34 965909350. Fax: +34 965903454. E-mail: [email protected] † Universidad de Alicante. ‡ Ceramic Membrane Center, Delft University of Technology. § Catalysis Engineering, Delft University of Technology.

for the use as composite membranes for separation of gas mixtures. In ref 7, we presented an extensive study on the different synthetic conditions (i.e., hydrothermal synthesis temperature, time, aging time, NaOH content, pretreatment of the support, etc.) that affect the growth of a continuous layer of silicalite-1 on the employed carbon supports. In synthesis terms, the main conclusions that were reached in the aforementioned work were that the use of an aging time, clear synthesis solutions, and an oxidation pretreatment of the supports were very important in zeolite/carbon composites synthesis. Furthermore, we commented on which synthetic parameters should be tuned to optimize zeolite growth on carbon materials. Having said all this, the aim of the present paper is to grow continuous defect-free silicalite-1 layers on a macroporous carbon support by standard hydrothermal treatment using different methodologies along the lines as indicated in our earlier work.7 Furthermore, the performance of these composite silicalite-1/carbon materials is reported in comparison with previous results. The integrity of the as-synthesized materials is tested in a permeation cell. Upon removal of the template, the calcined membranes are placed in a Wicke-Kallenbach cell and their performance in the separation of permanent gases (CO2/N2) and of C4 isomers (n-butane/i-butane) is evaluated. The permeation tests are performed at different gas pressures and temperatures. The results show that, although the process is still in need of fine-tuning, carbon materials are a promising alternative to conventional supports in zeolite membrane preparation. 2. Experimental Section 2.1. Preparation of the Supports. Macroporous carbon disks were cut from a macroporous carbon sheet (thickness ) 0.5 mm, mean pore size ) 0.7 µm) provided by Poco Graphite (DFP-1). These disks were of 13 mm in diameter and 0.5 mm thick. These carbon disks were treated using HNO3 following the procedure described elsewhere.7 2.2. Growth of Silicalite-1 Layers on the Support. Silicalite-1 layers were grown on the carbon supports by standard hydrothermal synthesis as described in ref 7. The silicate solutions were obtained from TEOS (tetraethoxysilane) (Aldrich 13,190-3) or Aerosil 200 (Degussa), adding high-purity NaOH (Aldrich 48,402-4). Tetrapropylammonium hydroxide (TPAOH) (Aldrich 25,453-3) and tetrapropylammonium bromide (TPA-Br) (Aldrich 2,556-8) were used as the templating agents.

10.1021/ie060688+ CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2006

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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007

Table 1. Overview of the Membranes Prepared in This Study hydrothermal treatment synthesis

composition

time (h)

temperature (°C)

seeding

figure

(1) (1) (2) (3) (4) (5) (6) (6)

1SiO2‚4EtOH‚0.33TPAOH‚120H2O 1SiO2‚0.33TPAOH‚120H2O 2SiO2‚8EtOH‚0.33 TPA-OH‚120H2O 3SiO2‚12EtOH‚0.33TPA-OH‚120H2O 1SiO2‚4EtOH‚0.30TPA-OH‚15H2O 1SiO2‚4EtOH‚0.33TPA-OH‚0.15NaOH‚15H2O 1SiO2‚4EtOH‚0.62TPA-OH‚0.62TPA-Br‚142H2O 1SiO2‚4EtOH‚0.62TPA-OH‚0.62TPA-Br‚142H2O

24 24 24 24 24 24 18 18

180 180 180 180 150 150 180 180

no no no no no no no yes

1A 4 1B, 6B 1C, 6C 3A 3B, 6A 5A 5B

The carbon supports were held vertical inside the autoclaves, by means of a Teflon piece inserted inside the autoclave, to reduce sedimentation of zeolite crystals on the support. The synthesis solution was poured inside the autoclave with the disk in it, and the hydrothermal synthesis was carried out, after an aging time of 6 h, under static conditions. Thermal treatment was performed at temperatures ranging from 150 to 180 °C for 24 h. After the synthesis, the composites were washed and tested for mechanical stability in an ultrasonic bath for 15 min. The resulting materials were dried at 398 K for 30 min. In the case of samples synthesized by the secondary (i.e., seeded) growth method, we prepared a batch of silicalite-1 seeds following the procedure described by Persson et al.12 by hydrothermal synthesis of a clear solution at 333 K for 2 weeks with the synthesis solution composition 1 SiO2‚4 EtOH‚1TPAOH‚49 H2O. 2.3. Characterization. The prepared materials were all first screened by scanning electron microscopy (SEM) using a Philips XL20 microscope operating at 15 kV. The crystalline phases were analyzed by means of X-ray diffraction (XRD) using a Seifert 2002 diffractometer. Before performing any other experiments on the synthesized composites, the as-synthesized membranes were assembled in a permeation cell so as to analyze if the layer grown on the macroporous support was continuous. A pressure of 3 bar was applied to the feed side of the membrane (i.e., the side of the membrane where the zeolite layer is grown), and vacuum was applied to the other side. Lack of permeation on the permeate side is a clear indication of the zeolite layer being continuous. 2.4. Template Removal. The selected materials (those that presented continuous layers of zeolite) were calcined in a horizontal furnace. The heat treatment performed on the samples was selected considering a previous study by Geus and van Bekkum:13 • Heating up of the sample up to 673 K in a N2 atmosphere (flow ) 100 mL/min) at a rate of 0.5 K/min. • Maintaining that temperature with the same furnace atmosphere for 4 h. • Switching the gas flow from N2 to synthetic air (flow ) 100 mL/min). • Maintaining that temperature under the new atmosphere for 16 h. • Cooling down the sample to room temperature at a rate of 0.5 K/min. It must be noted that, although this thermal treatment differs markedly from the one reported by our research group, the calcination treatment employed in this study does not produce burnoff of the carbon support (see Figure 10 of ref 7). 2.5. Permeation Experiments. Helium, nitrogen, carbon dioxide, and hydrocarbons were introduced from cylinders in the gas mixing section of the setup. The gas pressure was regulated from 100 to 200 kPa by means of back-pressure controllers (BPC), and the flow rates of the gases were regulated

up to 500 mL/min by means of mass flow controllers (MFC). In all cases, helium was used as the sweep gas with a flow rate of 100 mL/min. The membrane cell was placed in an oven with temperaturecontrol programs. The permeation tests were carried out between 30 and 150 °C. Permeation measurements are performed by the Wicke-Kallenbach (WK) method. The membrane was mounted in a stainless steel flange and sealed by means of a polymeric O-ring to avoid leakage problems. The flange is connected to the membrane cell by means of two Teflon O-rings, and the whole cell was tested for leaks before beginning the experiments. The leakage tests were performed by flowing He through the setup, pressurizing the cell section, and checking with a Helium analyzer so as to detect any possible leaks. In the WK method, both the retentate and the permeate streams are analyzed with a mass spectrometer (LedaMass Quadrupole mass analyzer) or a gas chromatograph (Chrompack CP 9001 using a CP sil-5 (Al2O3/KCl) column) with a flame ionization detector (FID). 3. Results and Discussion 3.1. Support Preparation. As other authors have noted before,14 the support on which the zeolite layer growth is intended is of capital importance, because not only the smoothness of its surface but also its surface functionality plays a key role in both the crystallization of zeolite crystals and their orientation in the final layer. In a previous communication,7 we have already noted the particularities of our carbon support, such as its porous texture and surface roughness. Furthermore, considering the fact that carbon is a strongly hydrophobic material, its interaction with the zeolite synthesis solution (which uses water as solvent) will be considerably poor. In fact, the contact angle of a drop of water placed on top of a carbon disk was higher than 90°, indicating that the support was not wetted by water. Additionally, it has been reported that the presence of strong acidic groups on the surface of the support favors the anchoring of the sol aggregates that form on the support before crystal formation in the presence of Na+ ions.15 Therefore, the carbon disks were immersed in boiling concentrated HNO3 for 12 h to generate as many surface oxygen groups as possible. This process reduced the water contact angle to 65°, which is a significant improvement in terms of interaction between the surface of the support and the synthesis solution. The oxidation treatment significantly increases the amount of surface oxygen groups, although it does not alter the porous structure of the carbon support (despite the strong oxidation procedure) due to its being highly graphitic. 3.2. Growth of Silicalite-1 Layers on the Support. At this point, it is important to remark on our previous results on the growth and operational testing of silicalite-1/carbon composites for membrane applications. In our first work,7 we explored some

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 3999

Figure 2. XRD spectrum of the composite shown in Figure 1B. Peaks marked with an asterisk (*) correspond to graphite.

3.2.1. Effect of the Silica Content. This particular effect was not addressed earlier. Several zeolite/carbon composites were prepared by hydrothermal treatment at 180 °C for 24 h (aging time 6 h). The composition of the synthesis solutions was

Figure 1. SEM images of a silicalite-1/carbon composite prepared at 180 °C for 24 h using (a) synthesis solution 1, (b) synthesis solution 2, and (c) synthesis solution 3.

of the basic synthetic parameters that strongly influence the growth of the aforementioned zeolite on the macroporous carbon support, such as the importance of improving the wettability of the support, the hydrothermal synthesis time and temperature, the aging time, etc. Therefore, in this work we focus on new aspects that were not dealt with previously and attempt to deepen our insight into some of the parameters discussed in our previous work. For comparison and clarity purposes, Table 1 presents all the experiments presented in this communication, including the synthetic conditions and their corresponding figures.

1SiO2‚4EtOH‚0.33TPA-OH‚120H2O

(1)


(2)


(3)

Figure 1 shows the SEM images corresponding to the composites synthesized using the aforementioned synthesis solutions after hydrothermal treatment at 180 °C for 24 h. As a direct consequence of the greater supply of silica in the reaction mixture, the crystals have grown appreciably in size (from ∼5 µm in Figure 1A to 20-30 µm along the b axis in Figure 1 parts B and C). The resulting materials show excellent surface coverage except for the synthesis with the lowest silica content. According to the presented SEM images, we can conclude that higher silica contents in the zeolite precursor solution favor faster crystal growth rates, which result in larger crystal sizes and a more continuous zeolite layer as the amount of TEOS is increased. According to the classical nucleation theory, an increase in the silica concentration in the synthesis solution should enhance both nucleation and crystal growth, but from our SEM results, the increase in the size of the MFI crystals instead of the formation of a larger amount of smaller crystals suggests a greater enhancement in the rate of crystal growth versus the nucleation rate. This increase in crystal size, however, reaches an upper limit once the amount of silica species reaches a threshold concentration, so that further addition of silica does not produce bigger crystals. Our results agree with those reported by Wong et al.16 in which they used a hydrophilic support (Vycor glass). Thus, when using synthesis solution 3, the zeolite crystal size of the continuous layer is ∼20-30 µm, which is practically the same size as the one obtained when synthesis solution 2 is used under the same conditions. In this sense, by varying the amount of silica that is present in the synthesis solution, we may substantially change the size of the crystals forming the zeolite layer and, thus, the zeolite layer thickness. For comparison with previous work,7 the three synthesis compositions were also performed by hydrothermal treatment at 150 °C, although only synthesis solution 3 yielded a continuous zeolite layer as observed by SEM. The desired crystalline phases were confirmed by XRD. Figure 2 shows, as an example, the diffractogram corresponding to Figure 1B, containing three characteristic peaks at 2θ ) 2325° corresponding to the most intense reflections of zeolite

4000


Figure 4. SEM images of the carbon support after hydrothermal treatment at 180 °C for 24 h using solution 1 and a carbon disk (silica source: Aerosil 200).

Figure 3. SEM images of different composites prepared using (a) synthesis solution 4, (b) synthesis solution 5. Both composites were synthesized by hydrothermal treatment at 150 °C for 24 h.

MFI.17 The sharp, very intense peak appearing at 26° and the broad peak at 43° correspond to the graphite support. Although the thickness of the synthesized zeolite layer is considerable (∼25 µm), the most intense peak is still the one corresponding to graphite. As a result, for thin zeolite layers, the peaks corresponding to the zeolite layer were much less intense and, in some cases, became indistinguishable from the background. 3.2.2. Effect of the Presence of Na+ Ions. In a previous study by Jansen et al.,15 there is a very good exemplification of how alkaline ions affect the nucleation and crystal growth of supported MFI zeolites. This study, however, was applied only to hydrophilic supports, and as a consequence, the results and conclusions reached might not be applicable to the supports employed in this paper. Thus, we carried out the growth of silicalite-1 on our carbon supports in the presence and absence of Na+ ions performing the hydrothermal treatment at 150 °C for 24 h (with 6 h of aging time) using the following synthesis compositions:


(4)

1SiO2‚4EtOH‚0.33TPA-OH‚0.15NaOH‚15H2O

(5)

The reason for this more-concentrated synthesis solution to be analyzed is the direct comparison with our previous results.7 On the other hand, when the synthesis is carried out at 180 °C (in order to better compare the results with the previous section), although the zeolite crystals are smaller, the zeolite layer does not cover the totality of the support surface for this temperature. Figure 3 shows the SEM images of the carbon disks after the hydrothermal synthesis under the conditions described above;

the insets include the XRD data obtained for those materials. Since Na+ ions enhance both the nucleation and crystal growth rate15 (i.e., crystallization kinetics), a bigger crystal size should be expected. This seems to be the case for the carbon support used in this study. When using the carbon disk, even in the absence of Na+ ions, the support is completely covered with very small coffin-shaped MFI crystals. The kinetics enhancement brought forth by the added Na+ ions seems to yield bigger crystals with the same surface coverage, which would indicate that the enhancement in crystal growth rate is more significant than that of nucleation. Although Na+ ions enhanced the crystallization rate, an excessive concentration of such ions is to be avoided, since it provokes the appearance of other crystalline phases such as quartz in the resulting material.7 3.2.3. Effect of the Silica Source. Although the effect of the silica species used in the synthesis solution composition has been studied thoroughly by other authors,18 this study pioneers in the growth of zeolite on carbon materials, and thus, this effect should be carefully studied, for it was not addressed in our previous works.7,19,20 For this reason, the hydrothermal treatment on our carbon support was done using synthesis solution composition 1 at 180 °C for 24 h, but using fumed silica instead of TEOS as the silica source. Figure 4 shows the SEM images of the obtained composites using Aerosil 200 (fumed silica) as the silica source. Comparing Figure 4 with Figure 1A, it is clear that, when using a monomeric silica species (TEOS), the obtained zeolite crystals are smaller in size than those synthesized when using fumed silica. In our example, the increase in synthesized crystal size has caused the zeolite layer to cover the totality of the support surface. 3.2.4. Effect of Seeding the Support. Driven by the challenge of growing continuous zeolite layers, we have pointed out the usefulness of the secondary (i.e., seeded) growth method for the synthesis of thin layers of zeolite on macroporous carbon disks.19,20 This methodology can be adapted to substrates with rather different characteristics, making the synthesis of thin, oriented layers of zeolite crystals possible. The main aim of the seed film method, attempted by many different approaches, is to systematically cover the surface of the support with a layer of zeolite crystals so that, according to the classical theory of crystal formation, the nucleation step is, thus, skipped. The advantages brought forth by this method are that shorter synthesis times are needed for a continuous zeolite layer to grow

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4001

Figure 5. SEM images of a carbon supports after hydrothermal treatment at 180 °C for 18 h using synthesis solution 6 and (A) an unseeded carbon disk, (B) a seeded carbon disk.

on the support, preferential orientation is more easily achieved, and thinner continuous sections of zeolite can be prepared. For this reason, we selected synthesis conditions with which it was not possible to obtain a continuous layer of silicalite-1 by direct hydrothermal treatment in order to ascertain the utility of the secondary growth methodology. In this sense, and as an extension of previous communications19,20 in which seeding was performed by means of electrochemical methods (electrophoretic deposition (EPD)), we performed the seeding of the carbon supports using a batch of 700 nm zeolite seeds (1% weight suspension), by applying vacuum to one of the side of the support. Then, the crystalline nuclei were applied and hydrothermal treatment was carried out at 180 °C for 18 h (6 h aging time) using the following synthesis composition:

1SiO2‚4EtOH‚0.62TPA-OH‚0.62TPA-Br‚142H2O (6) Figure 5 shows the SEM images corresponding to the obtained composites using the seeded and unseeded supports. Upon first observation, the seeding of the supports has yielded a better support coverage after hydrothermal treatment (a high magnification image of Figure 5A corresponding to the unseeded support revealed pinholes and defects), thus indicating the great enhancement in crystallization that takes place when seeding nuclei are anchored to the support surface. In this respect, the inset in Figure 5B shows the cross section of the prepared zeolite layer, corroborating its continuity. The effect of seeding the support also proves to be a great help toward the crystallization, since the induction time for the formation of silicalite nuclei is systematically eliminated, shortening the synthesis times, although this parameter has not

been further studied here. Seeding, however, decreases the silicalite-1 crystal size from 30 to 10 µm in the zeolite layer (Figure 5). The application of the zeolite seed layer apparently results in more growing nuclei than that without seeds and a smaller average crystal size. 3.3. Overview of Silicalite-1 Growth on Carbon Materials. Combining the results obtained here and from previous studies7,19,20 yields some observations that should be highlighted in terms of zeolite layer growth on carbon materials: (i) In the case of direct hydrothermal treatment, and considering the hydrophobic character of our support (in contrast with hydrophilic ceramic supports), the oxidation of the support and the aging time are crucial parameters in order to achieve a sufficient surface coverage with silicalite-1 crystals. (ii) The growth of thin silicalite-1 layers (that is, the growth of small crystalline nuclei) is favored under high pH values, using clear, concentrated synthesis solutions, and at short synthesis times. (iii) High hydrothermal synthesis temperatures only yield significant growth of silicalite-1 crystals when dilute synthesis solutions are used. In the case of concentrated synthesis solutions, high synthesis temperatures hinder crystal growth. (iv) The presence of Na+ ions in the reaction medium enhances crystallization kinetics, thus increasing the crystal size in comparison with alkali-free conditions. Nevertheless, when excessively high alkaline concentrations are used, crystal phase transformations might take place, causing the formation of quartz phases.7 (v) The use of alkaline metal free synthesis solutions results in continuous zeolite layers when concentrated synthesis solutions are used. (vi) The use of monomeric silica species (i.e., TEOS) as the silica source results in smaller crystal sizes, whereas the use of fumed silica results in larger silicalite-1 crystals with a significant degree of intergrowth under the same synthetic conditions. (vii) The secondary (i.e., seeded) growth methodology involves a significant enhancement of the crystallization kinetics, allowing for the possibility to avoid the nucleation step. By using this methodology (through its different approaches), it is possible to grow continuous layers of silicalite-1 on carbon materials using conditions under which such results are not possible following the direct hydrothermal treatment. Furthermore, zeolite layers with thicknesses below 1 µm can be prepared following this methodology. Thus, it is possible to tune the crystal size and, therefore, the zeolite layer thickness by controlling the hydrothermal synthesis conditions, as Figure 6 shows. By using different synthesis solution compositions, the thickness of the zeolite layer can be varied from ∼2 µm (Figure 6A) (highly concentrated synthesis solution with Na+ ions present in the reaction medium), ∼12 µm (dilute synthesis solution) to ∼18 µm (Figure 6C) (dilute synthesis solution with a high concentration of silica precursor). Concentrated synthesis solutions favor higher oversaturation degrees, which in turn cause the formation of a large amount of small crystalline nuclei. On the other hand, more dilute synthesis solutions, although with high silica supplies (solution 3), cause the growth of large MFI crystals. In this sense, the importance of the continuity and thickness of the zeolite layer in its behavior as a membrane must be highlighted, for the final objective in zeolite membrane preparation is the use in some kind of separation and/or reaction. 3.4. Permeation Studies. Table 2 summarizes the main characteristics of the membranes and permeation setup reported

4002


Figure 6. SEM images of the cross sections of composites synthesized using different synthesis solution compositions: (A) synthesis solution 5, (B) synthesis solution 2, and (C) synthesis solution 3.

by other groups, which have been compared to the materials in this study, for a better understanding of the following discussion. Regarding our results, the quality of the zeolite layer of all the prepared composites was analyzed by checking the permeation prior to calcination. Absence of permeation was considered indicative of a continuous and defect-free zeolite layer. After the preliminary screening of the prepared composites in the permeation cell, the materials synthesized by the secondary growth method were discarded, as well as those prepared using dilute synthesis solutions with low or moderate silica contents (synthesis solutions 1 and 2) and the ones prepared using fumed silica as the silica source, because they showed a noticeable permeation of N2, thus evidencing the discontinuity of the zeolite

layer deposited on the carbon materials. Thus, the membranes selected for the permeation studies were the ones prepared following these synthesis procedures: • Membrane 1: synthesis solution 3; hydrothermal treatment temperature ) 180 °C; hydrothermal treatment time ) 24 h • Membrane 2: synthesis solution 3; hydrothermal treatment temperature ) 150 °C; hydrothermal treatment time ) 24 h • Membrane 3: synthesis solution 5; hydrothermal treatment temperature ) 150 °C; hydrothermal treatment time ) 24 h • Membrane 4: synthesis solution 4; hydrothermal treatment temperature ) 150 °C; hydrothermal treatment time ) 24 h Table 3 shows the thickness of the resulting zeolite layers for the prepared membranes. In the case of the calcined materials, the same pressure of N2 was applied to the feed side and vacuum was applied on the permeate side. The feeding of N2 was then stopped, and the pressure drop between both sides of the membrane was registered versus time in order to plot a permeance vs ∆P graph (see Figure 7). The experiments were performed for the carbon disk without zeolite (blank experiment) and for the prepared composites. In the case of the blank experiment, the permeance through the membrane increased with higher pressure gradients over the membrane, which is indicative of viscous flow, which in turn is an evidence of the existence of large pores (cracks) in the structure of the membrane. Any tested composite that showed a similar behavior was immediately discarded, as was the case of membrane 4. On the other hand, when there were a small number of defects/cracks in the composite, the permeance was constant, a straight line parallel to the x axis, which is indicative of Knudsen dependency. Since N2 does not adsorb on silicalite-1 crystals, it is a result that we should expect for a defect-free zeolite layer. Figure 8 shows the results obtained in the separation of N2/ CO2 mixtures. From the analysis of Figure 8, only membrane 1 would be appropriate for the separation of CO2 from N2, since the separation factors are comparable to those reported in the literature for MFI zeolite21-26 (even for FAU zeolite membranes27,28), although the fluxes observed for membrane 1 in this study (1.8 × 10-3 mol/(s‚m2) for CO2) are lower.21,29 In the case of membrane 2, the separation factors obtained are significantly lower than those found for membrane 1, although the permeation fluxes (8 × 10-3 mol/(s‚m2) for CO2) are better comparable to those reported in the aforementioned references. Finally, the permeation behavior of membrane 3 was deemed inappropriate for further analysis since, although the fluxes going through the membrane were significantly higher than those of membranes 1 or 2, the separation factors obtained for a CO2/ N2 mixture were

Carbon Membranes

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