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Ind. Eng. Chem. Res. 2010, 49, 5616–5624

Synthesis of Faujasite Films on Carbon Fibers and Characterization of Their Sorption Properties Alexandra Jakob,† Valentin Valtchev,*,‡ Michel Soulard,† and Delphine Faye§ Equipe Mate´riaux a` Porosite´ Controˆle´e, Institut de Science des Mate´riaux de Mulhouse (IS2M), UniVersite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France, Laboratoire Catalyse & Spectrochimie, ENSICAEN UniVersite´ de Caen - CNRS, 6 bouleVard du Mare´chal Juin, 14050 Caen, France, and Laboratoire de Contamination, Centre National d’Etudes Spatiales, 18, aVenue Edouard Belin, 31401 Toulouse Cedex 9, France

The ultimate goal of this study is the preparation of materials that prevent organic contamination of satellites in low earth orbit. Carbon fibers were chosen as a substrate because of their light weight and high mechanical resistance, along with a highly accessible surface that could optimize the sorption of the contaminants. Further, the active phase (FAU-type zeolite) was synthesized onto support applying the secondary growth method. The zeolite coatings were crystallized under different conditions in order to control the characteristics of the material. Alkalinity of the initial gel, Si/Al ratio, and synthesis time appear to strongly influence the film characteristics. Thus, highly crystalline FAU-type films with different thicknesses and levels of hydrophilicity were grown. The sorption properties of zeolite films and counterpart powders were investigated by thermogravimetric analyses toward several organic adsorbents (cyclohexane, n-hexane, p-xylene, trimethylbenzene, tetramethoxysilane). The obtained data revealed that the adsorbed quantity of organic species increased with the Si/Al ratio of the zeolite framework due to its increased hydrophobocity. Sorption capacities of the films appeared to be slightly lower than those of their powder counterparts, which was attributed to lower crystal accessibility on zeolite layers. Nevertheless, the set of experimental data demonstrated that the obtained materials are fully conversant with the target application. I. Introduction The ordered microporous network of zeolites has raised great interest in preparing films and membranes that present attractive applications in various industrial fields. In recent years, high performance materials have been developed for catalysis,1-5 separations in liquid and gas phases,6-9 and sensing devices.10-13 Due to its large pore volume and wide channels, FAU-type zeolite has been extensively used for all these applications, namely, in the form of supported layers on substrates such as alumina,14,15 silica glass,16,17 and various metals.18-20 Different synthesis routes were employed for the synthesis of zeolite coatings, such as electrophoresis21,22 and laser pulsed deposition,23 but hydrothermal synthesis remained the most frequently used one, being applied on preseeded15,24 or unseeded25,26 substrates. Since the synthesis of FAU-type membrane was studied in detail, a rigorous control of synthesis conditions was achieved for the hydrothermal treatment route. The substrate chemical nature, precursor gel composition, seeding approach, synthesis time, and temperature were found to be the key parameters in order to obtain films with a controlled thickness and homogeneity.26-30 As mentioned, besides the catalytic and sensing applications, FAU-type materials offer great potential as sorption media. For instance, the faujasite also presents excellent sorption properties toward alkanes,31-35 xylenes,36-39 and other aromatic molecules.40 Thermogravimetric analysis has been proven to be a relevant method for investigation of the sorption properties of zeolite materials toward different probe molecules. It has been used under the static mode by Bellat et al. to obtain the adsorption isotherms of xylene isomers on faujasite and hence determine * Corresponding author E-mail: [email protected]. † Universite´ de Haute Alsace. ‡ Universite´ de Caen. § Centre National d’Etudes Spatiales.

the sorption capacities at saturation.33,37 For a NaY zeolite presenting a Si/Al ratio of 2.43, the sorption quantity was assessed to about 2.11 mmol g-1 at saturation. This method can also be used in the dynamic mode, in order to determine the sorption capacity at a given relative pressure and temperature.40 A potentially important application of zeolite films related to their sorption properties, which could be of great interest in the spacecraft industry, is the decontamination of sensitive surfaces of optical instruments. Thus, in low earth orbit, satellites undergo the outgassing of organic pollutants that result in a decrease of their global performances, due to the contamination of key devices such as optical systems and thermal radiators. The chemical nature of these contaminants was investigated by the National Aeronautic and Space Administration (NASA).42 Most of the contaminants derived form polymers and plasticizers; in addition some elastomers could also be found. Consequently, to control this pollution, satellites’ instruments were subjected to different cleaning treatments before flight embarkation. Nevertheless, methods like preflight bakeouts proved to be expensive, time-consuming, and of limited efficiency, since a high number of cleaning cycles is required to achieve a sufficient contamination decrease. As a consequence, a technique enabling molecular contamination control throughout the lifetime of the space mission had to be set up, and zeolites were considered as relevant materials to achieve this goal. Due to their excellent selectivity in terms of size and shape, zeolite can retain efficiently in their pores the organic contaminants. In addition, zeolites present great thermal and mechanical stability and are relatively inexpensive. In order to assess the efficiency of such kinds of materials, zeolite films and powders were used in satellites for low-earth orbit missions.43,44 Barengoltz et al. presented a study that aimed at correcting the optical aberration of the Hubble Space Telescope, which required the design of the Wide Field Planetary Camera 2

10.1021/ie901683y  2010 American Chemical Society Published on Web 05/12/2010

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(WFPC-2). FAU-type films synthesized on cordierite substrates were incorporated into the WFPC-2, and the contamination rate of the satellite was measured. A 40% decrease of the molecular contamination was achieved, showing the potential of zeolites as depollution materials. The shaping of the zeolite was of crucial importance, since powder adsorbent would cause a secondary contamination due to their deposition on sensible surfaces, while the corresponding films did not trigger any further pollution and can be incorporated in appropriate areas during the whole space mission. In order to prepare materials presenting properties compatible with in-orbit use, besides the active part, an appropriate substrate must be chosen. Carbon-based materials are light weight, have high mechanical resistance, and are chemically inert under low temperature nonoxidizing conditions. The compatibility between zeolite films and carbon supports has already been proven. Zeolite films have been synthesized on various carbon materials. Carbon tubes were used as substrates to obtain well intergrown films presenting attractive properties as membranes,45,46 while FAU-type zeolite was prepared on glassy carbon electrodes for cyclic voltammetry.47,48 Very few studies have been conducted concerning the deposition of zeolites on carbon fibers, principally for the preparation of hollow fiber materials.49-51 Valtchev et al. have reported the synthesis of MFI-type zeolite on carbon fibers in order to produce hollow fibers by calcination of the carbon material.52,53 The synthesis procedure was performed in situ on preseeded substrates, and the film thickness was controlled by carefully tuning synthesis parameters. The goal of this work is to achieve highly controlled formation of FAU-type films on carbons fibers in order to face the requirements of the spacecraft industry. Hydrothermal synthesis with preseeding was used, and the influence of synthesis parameters, including Si/Al ratio, on the film morphology was studied. A comparative study of the sorption performances of films and powders with respect to different molecules like aromatics and linear alkanes was also among the goals of the investigation. II. Experimental Section A. Materials. The substrates used in this study were commercial carbon chopped roving fibers (99%, Alfa Aesar) with an average diameter of approximately 10 µm. The substrates were cleaned in an ultrasonic bath by successive 10 min immersions into acetone and ethanol. They were finally removed from the ultrasonic bath, rinsed with distilled water, and airdried. B. Seeds Preparation. Seed crystal preparation involved a precursor solution of molar composition Al2O3/3.1SiO2/ 4.22TMAOH/191H2O. At first, aluminum isopropoxide (98%, Aldrich) was dissolved into water and mildly stirred for 1 h. Tetramethylammonium hydroxide pentahydrate (TMAOH · 5H2O, 97 wt %, Lancaster) was then added and stirred for 0.5 h. Finally, colloidal silica (Ludox HS-40, Aldrich) was poured in the mixture and stirred for 1 h. The resulting precursor solution was transferred into a sealed polypropylene bottle and hydrothermally treated at 100 °C for 6 days. The resulting milky liquid was centrifuged at 22 000 rpm for 40 min and washed with deionized water. The operation was repeated four times. The pH of the colloidal suspension was then adjusted to 9 with a 0.1 M NH3 solution C. Film Synthesis and Post-Treatment. After the substrates had been cleaned, they were placed in a 1 wt % aqueous solution that was obtained by diluting a solution of cationic polymer

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(poly(diallyldimethylammonium) choride), 20 wt % in H2O, Aldrich) to reverse their surface charge. Then, the substrates were immersed in a faujasite colloidal suspension. The preseeded substrates were then treated in two different precursor gels, to obtain zeolites NaX and NaY. Zeolite X was obtained from a gel of molar composition 8Na2O/0.2Al2O3/1SiO2/200H2O. Two solutions (A and B) were first prepared. Solution A was obtained by mixing sodium silicate (26 wt % SiO2, 8 wt % Na2O, 66 wt % H2O, Riedel de Hae¨n) and water. Solution B was formed by dissolving sodium aluminate (54 wt % Al2O3, 37 wt % Na2O, 9 wt % H2O, Carlo Erba) into water. Sodium hydroxide (99.9 wt %, Riedel de Hae¨n) was then added to solution B, which was mixed over 10 min until complete dissolution. The two solutions were then mixed together and stirred at 1000 rpm for 20 min, before being sealed in a polypropylene bottle together with pretreated carbon fibers and being hydrothermally treated for different periods of time at 90 °C. After the synthesis, the so-obtained films were cleaned in an ultrasonic bath for 10 min to remove the loosely attached crystals, rinsed with distilled water, and air-dried. Zeolite Y was prepared with a precursor gel of molar composition 20Na2O/1Al2O3/10.7SiO2/825H2O. Two solutions (A and B) were first prepared. Solution A was obtained by mixing sodium silicate (26 wt % SiO2, 8 wt % Na2O, 66 wt % H2O, Riedel de Hae¨n) and water. Solution B was formed by dissolving sodium aluminate (54 wt % Al2O3, 37 wt % Na2O, 9 wt % H2O, Carlo Erba) into water. Sodium hydroxide (99.9 wt %, Riedel de Hae¨n) was then added to solution B, which was mixed over 10 min until complete dissolution. The two solutions were then mixed together and stirred at 1000 rpm for 12 h, before being sealed in a polypropylene bottle and hydrothermally treated for different periods of time at 90 °C. After the synthesis, the obtained films were cleaned in an ultrasonic bath for 10 min to remove the loosely attached crystals, rinsed with distilled water, and air-dried. D. Characterization. Powder samples were studied using a conventional X-ray diffractometer (PANanalytical, X’Pert Pro MPD), while zeolite/carbon fiber composites were characterized with a STOE STADI-P diffractometer, working in the transmission mode. The zeolite composition was determined by X-ray fluorescence (Philips, MagiX). The size of the zeolite seed crystals was measured by dynamic light scattering (Malvern HPPS-ET). Morphological features of zeolite crystals and films were investigated with a scanning electron microscope (SEM, Philips XL30 FEG). Nitrogen sorption measurements were performed with an ASAP 2040 MP instrument. The specific surface was calculated with the BET equation while the microporous volume was distinguished from the mesoporous one by the t-plot method. Sorption properties of organic molecules on the zeolite films and powders were studied using a SETARAM TGA 92 thermobalance under dynamic conditions. Placed in a platinum crucible, the zeolite samples were first heated up to 200 °C at a rate of 10 °C/min and maintained at this temperature for 2 h to achieve complete dehydration. After cooling down to 25 °C, sorption of organic molecules was isothermally carried out under two nitrogen flows. The first was a reactive one circulating through a cell containing the studied organic liquid. The cell was maintained at 25 °C in a thermostatic bath, which allowed determination of the equilibrium vapor pressure by the “cold point process”. The second nitrogen flow was used as a carrier gas to dilute the reactive one. The P/P0 value, where P was the adsorbate pressure and P0 the equilibrium vapor pressure, was

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Figure 1. Particle size measured by DLS (a) and XRD patterns (b) of faujasite nanocrystals employed for substrate seeding.

Figure 3. XRD patterns obtained on unseeded (a) and seeded fibers after hydrothermal treatment in zeolite X yielding gels at 90 °C for 1 h (b), 2 h (c), and 4 h (d).

Figure 2. SEM micrographs of unseeded (a, d) and seeded (b, c) fibers prior to (a, b) and after (c, d) the secondary growth step at 90 °C for 6 h.

hence set up. Thus, it was possible to isothermally study the quantity of adsorbed organics as a function of time. In these experiments, the sorption properties of zeolite films and powders in respect to n-hexane (99.0 wt %, Fluka), cyclohexane (99.5 wt %, Aldrich), p-xylene (98 wt %, Fluka), trimethylbenzene (98 wt %, Aldrich, TMB), and tetramethoxysilane (98 wt %, Fluka, TMOS) were investigated. The zeolite content deposited on the fibers was estimated by thermogravimetric analysis that consisted of heating up the zeolite/fiber composite to 1000 °C under an air flow at a ramp rate of 2 °C/min to completely eliminate the substrate. Then, the amount of composite was calculated to provide 40 mg of zeolite. III. Results and Discussion A. Effect of Seeding on Zeolite Film Formation. The size distribution of the seed crystals was determined by DLS (Figure 1a), while their crystallinity was determined by XRD (Figure 1b). The average size of the particles was estimated to be 88 nm, which is suitable for seeding purposes. A standard FAUtype XRD pattern was obtained, showing a slight broadening of the diffraction peaks due to the small size of the crystals. SEM inspection of carbon fibers prior to and after the seeding step was performed. The fibers had a diameter of approximately 10 µm (Figure 2a) and after seeding was comprised of random faujasite nanocrystals attached to their surface (Figure 2b). The surface was not uniformly covered by the seeds crystals, but the seeding step was sufficient to enable the zeolite growth as it is presented in Figure 2c. On the other hand, the hydrothermal treatment of unseeded fibers did not lead to a zeolite film formation (Figure 2d).

The SEM observation was confirmed by the XRD study. After the hydrothermal treatment, the seeded substrates exhibited the typical FAU-type materials XRD pattern (Figure 3d), while under a similar treatment, the unseeded material remained amorphous (Figure 3a). B. Synthesis of Zeolite X Films: Growth Kinetics. The Si/ Al ratio of the coating was estimated to be 1.21, confirming that zeolite NaX was prepared. In order to study the growth kinetics of the faujasite materials on the substrate, preseeded carbon fibers were immersed in a FAU-type precursor solution at 90 °C for 1, 2, 3, 4, 5, and 6 h. The kinetics of film growth was followed by XRD. Figure 3 (b-d) showed the patterns obtained after different periods of time. After 1 h of hydrothermal treatment, the material did not exhibit the XRD pattern of FAU-type zeolite (Figure 3b). The sequence of diffraction peaks (2θ ) 9.00°, 2θ ) 18.08°, and 2θ ) 30.69°) suggested that a lamellar-type crystalline solid was deposited on the fibers. It was impossible to identify more precisely the structure of the lamellar phase. Figure 3c shows that FAU-type zeolite appeared after 2 h of hydrothermal treatment, coexisting with the unknown lamellar material. After 4 h of hydrothermal treatment (Figure 3d), the crystallinity of the faujasite increased substantially, while the lamellar phase was not detectable anymore. No significant changes were observed upon the prolongation of the synthesis time to 6 h (XRD pattern not shown); that is, a pure FAU-type zeolite was deposited on the fibers. The crystal orientation appeared to be random, as the XRD patterns did not show any modification of the peak intensities. The obtained results show that 6 h is the optimum time for the synthesis of zeolite X film, when the nutrient pull is already exhausted. Further prolongation of the synthesis time may lead to film dissolution and the formation of undesired phases. SEM views at different magnifications are represented in Figure 4 for various synthesis times. After 1 h of hydrothermal treatment (Figure 4a and b), a material with morphological features different from FAU-type zeolite was deposited on the carbon fibers, which was in good agreement with XRD data. When the substrate was treated for 2 h (Figure 4c and d), an intergrown layer of isometric crystals with a thickness of about 0.7 µm appeared. Careful inspection of the zeolite film along

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Figure 4. SEM low (a, c, e, g) and high (b, d, f, h) magnification views of zeolite X films deposited on carbon fibers after hydrothermal treatment at 90 °C for 1 h (a, b), 2 h (c, d), 4 h (e, f), and 6 h (g, h).

the length of the fibers revealed that fiber coverage is not uniform due to the many entanglements in the fiber layout. The prolongation of the synthesis time resulted in thicker films (Figure 4e and f), and after 6 h of hydrothermal treatment, a 3.5-µm-thick FAU-type monolayer was obtained (Figure 4g and h). Intergrown crystallites were faceted and showed an octahedral shape typical of faujasite crystals. Crystals in the bulk powder were found to be of larger size (5-6 µm), highlighting steric hindrance that limited crystal growth into the films. Side view analysis of the zeolite film after 6 h of hydrothermal treatment showed that it was only made of zeolite crystals, confirming the disappearance of the lamellar phase. At this stage some loosely attached crystals were present at the film surface. However, further extension of synthesis time did not provide thicker films. All of the obtained films were firmly attached to the substrate, since it was impossible to detach them by a 10 min ultrasonic treatment.

Figure 5. Film growth kinetics of zeolite X films on seeded carbon fibers from a gel with composition 8 Na2O/0.2 Al2O3/1.0 SiO2/200 H2O.

Figure 5 presents the evolution of film thickness as a function of synthesis time for hydrothermal treatment at 90 °C. The thickness assessment was based on the SEM study of the crosssectional views of zeolite films. Prior to the SEM inspection, the fibers were ground in order to detach some parts of the film.

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Figure 6. XRD patterns of zeolite films obtained at 90 °C for 6 h from gels with a Na2O/SiO2 ratio of 2 (a), 4 (b), 6 (c), 8 (d), and 10 (e).

C. Synthesis of Zeolite X Films: Effect of Alkalinity. The influence of the precursor solution alkalinity on the film

morphology was studied. Carbon fibers were immersed in a solution of molar composition xNa2O/0.2Al2O3/1SiO2/200H2O, with x ) 2, 4, 6, 8, and 10. The pH remained at 14 for all the gel compositions. XRD patterns of treated fibers are represented in Figure 6. The XRD pattern of the sample obtained at a Na2O/ SiO2 ratio of 2 comprised the most intense peak (2θ ) 6.09°) of the FAU-type zeolite. The peak intensity was very weak, suggesting that a low quantity of zeolite was deposited on the substrate. On the other hand, the XRD pattern of the corresponding bulk powder (not shown here) was completely amorphous. These results revealed that the composition of the precursor gel was out of the crystallization field of faujasite, and only the presence of seed crystals induced the crystallization on the fibers’ surface. The crystal growth process, however, rapidly declined under this condition, and a film with low thickness was obtained. The higher intensity of the faujasite peaks obtained from a gel with a Na2O/SiO2 of 4 pointed at the

Figure 7. SEM low (a, c, e, g) and high (b, d, f, h) magnification views of zeolite X films deposited on carbon fibers after hydrothermal treatment at 90 °C for 6 h with a Na2O/SiO2 ratio of 2 (a, b), 4 (c, d), 8 (e, f), and 10 (g, h).

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Figure 8. Film thickness as a function of the Na2O/SiO2 ratio in the initial gel.

presence of a larger amount of zeolite deposited on the substrate. A further increase of the XRD pattern intensity was observed at a Na2O/SiO2 equal to 6. No significant changes in peak intensities were observed at a Na2O/SiO2 ratio of 8 (Figure 6c). The variation of the alkalinity of the synthesis solution resulted in changes in the film thickness. It was found that the average crystal size increased with the precursor gel alkalinity, rising from 0.5 to 3.5 µm for the gels with a Na2O/SiO2 ratio of 2 and 8, respectively (Figure 7). As a consequence, the film thickness increased. All obtained films were continuous, compact, and firmly attached to the carbon substrate. Figure 8 presents the film thickness as a function of the Na2O/SiO2 ratio, showing that it was possible to control the thickness between 0.5 and 3.5 µm. It is worth mentioning that, in the case of fibers immersed in a gel presenting a Na2O/SiO2 ratio of 10, no zeolite film was obtained. A possible reason might be the dissolution of seed crystals in strongly alkaline media, thus preventing the crystal growth on the fiber surface. Hence, the diffraction peaks observed on the corresponding XRD pattern (Figure 6e) are obviously emanating from zeolite crystals attached to the fiber layouts or to a few large zeolite aggregates attached to some of the fibers. D. Growth of Zeolite Y Films. Zeolite Y films were synthesized in order to study the influence of the Si/Al ratio on the sorption capacity. Further sorption properties of zeolite Y (Si/Al ) 1.91) and zeolite X (Si/Al ) 1.21) were compared. Both materials exhibited the XRD patterns typical of FAU-type materials, and no extra peaks were observed. A slight shift to higher than 2θ values was observed when the Si/Al was increased (Si/Al ) 1.9), which is most probably a consequence of decreasing unit cell parameters. On the basis of the difference of the intensity of diffraction peaks, one can say that the mass of zeolite X deposited on the substrate is higher than zeolite Y. The size of zeolite Y crystals was about 1 µm. The coverage of the zeolite fibers was not homogeneous, as can be seen in Figure 9a. The presence of the bold area is most likely a consequence of a long induction period that may result in dissolution or detachment of the seeds that induced the growth on the substrate surface. In contrast, rapid growth of zeolite X provided uniform and thicker films.

Figure 10. Nitrogen adsorption (filled symbols)/desorption (open symbols) isotherms at -196 °C for zeolite X films obtained after 1 (square), 4 (triangle), and 6 (circle) h of hydrothermal treatment at 90 °C.

E. Sorption Properties of FAU-Type Films and Powders: Nitrogen Sorption Properties. The specific surface area of the nontreated fibers was estimated to be 7.3 m2 g-1. Nitrogen sorption tests were performed on the samples obtained during the investigation of growth kinetics of zeolite X films. After 1 h of hydrothermal treatment, the prepared material exhibited a type IV adsorption/desorption isotherm that is typical of a mesoporous material. A nitrogen uptake at high pressures comprised a hysteresis loop which was attributed to the textural porosity of the material (Figure 10). Meanwhile, the adsorbed quantities remained low, since material with low specific surface area crystallized on the support surface. As reported above, the prolongation of synthesis time resulted in the formation of a FAU-type film. Consequently, a type I isotherm was recorded, where the steep increase of the nitrogen uptake at low relative pressures corresponded to the filling of the micropores. Table 1 reports the specific surface area and the microporous volume of the films and powders after different periods of hydrothermal treatment. It can be clearly seen that the presence of the seeds had a strong influence on the specific surface area of the materials, especially at shorter crystallization. After 1 h of hydrothermal synthesis, the fibers exhibit a specific surface area (SBET ) 31.1 m2g-1) that is superior to that of the corresponding powder (SBET ) 18.1 m2g-1). This result is not surprising since the solid formed in the bulk is amorphous, while the carbon fibers contain zeolite seeds and the lamellar phase described above. After 2 h of hydrothermal treatment, the specific surface area of the composites (SBET ) 32.2 m2 g-1) is still higher than that of the material formed in the bulk (SBET ) 18.8 m2 g-1), suggesting that the zeolite growth on the fibers overcomes the one in the bulk. After this stage, the micropore volume and specific surface area of zeolite powders are superior to those of corresponding zeolite-fiber composites, which is due to the contribution of the low-surface-area carbon substrate. The results of nitrogen sorption tests performed on zeolite Y films are presented in Table 2.

Figure 9. SEM low (a) and high (b) magnification views of zeolite Y films deposited on carbon fibers after hydrothermal treatment at 90 °C for 1 day.

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Table 1. Specific Surface Areas and Microporous Volumes of Zeolite X Films and Powders Prepared at 90 °C for Different Periods of Time synthesis time (h)

specific surface area of the zeolite X films (m2 g-1)

microporous volume of the zeolite X films (cm3 g-1)

specific surface area of the zeolite X powders (m2 g-1)

microporous volume of the zeolite X powders (cm3 g-1)

1 2 4 6

31.1 32.2 402.7 630.3

0.005 0.02 0.16 0.25

18.1 18.8 485.8 722.3

0.003 0.01 0.16 0.25

Table 2. Specific Surface Areas and Microporous Volumes of Zeolite Y Films and Powders Prepared at 90 °C for Different Periods of Time synthesis time (h)

specific surface area of the zeolite Y films (m2 g-1)

microporous volume of the zeolite Y films (cm3 g-1)

specific surface area of the zeolite Y powders (m2 g-1)

microporous volume of the zeolite Y powders (cm3 g-1)

3 6 15 24

17.7 53.6 46.9 121.3

0.002 0.02 0.02 0.05

86.3 322.3 447.4 681.7

0.02 0.12 0.18 0.28

Table 3. Adsorbed Amounts on Zeolite X Films and Powders at P/P0 ) 0.5 adsorbate

kinetic diameter (Å)

zeolite X film nads (mmol/g)

zeolite X powder nads (mmol/g)

nads (film)/nads (powder)

% nads (film)

% nads (powder)

nitrogen water n-hexane p-xylene cyclohexane TMB TMOS

3.5 2.65 4.3 5.8 6.0 7.3 8.9

9.16 11.8 1.44 1.31 1.58 1.23 0.8

9.16 12.3 1.54 1.50 1.89 1.39 1.12

1.0 0.96 0.93 0.87 0.84 0.88 0.71

63.7 59.3 50.8 61.2 53.9 38.9

66.9 59.9 58.9 65.0 61.5 53.2

In comparison to zeolite X, the specific surface areas and micropore volumes of the zeolite Y composites were significantly inferior to their counterpart powders, which is most probably due to the lower coverage of the carbon substrate. Nevertheless, the adsorption isotherms (not shown) were type I, confirming that a microporous material was deposited on the fibers. F. Sorption Properties of FAU-Type Films and Powders: Sorption Capacity and Kinetics of Zeolite X Films and Powders. An issue that has to be addressed prior to the presentation of sorption properties of zeolite films and powders is the reliability of the experimental results. Sorption of molecules with appropriate dimensions is an intrinsic property of zeolites, which is not influenced by the size of employed crystals. Sorption studies performed on very large zeolite crystals showed that the micropore volume is fully accessible at standard temperature and pressure.54,55 Zeolite films did not show particular behavior for the adsorption of inert gases and organic molecules; that is, saturation of a zeolite molecular sieve without applying additional pressure or specific temperature was observed.56,57 Hence, the experiments in the present study were performed under standard conditions as the goal was to compare the capacity and the kinetics of adsorption of synthesized zeolite films and their powder counterparts. Sorption capacities of the zeolite powders and films were studied toward organic molecules presenting substantial differences in kinetic diameter and shape. The characteristics of molecules employed in the present study are summarized in Table 3. As known, a sorption phenomenon is strongly influenced by the affinity of sorbates toward the zeolite framework. In this study, it has been found that alkanes presented better affinity and compacting efficiency toward the faujasite cavity than aromatic molecules. Concerning larger molecules like TMB and TMOS, the kinetic diameter was found to be the predominant parameter. Thus, these two species presented a sorption rate that was significantly lower than those measured for smaller ones. The latter was attributed to a hindered diffusion into the zeolite cavities.

The adsorbed quantities on zeolite films were compared to their powder counterparts. It was found that the zeolite films presented a sorption capacity of about 90% of the corresponding zeolite powders for small molecules. Substantially reduced sorption efficiency on the faujasite films with respect to TMOS was observed. This phenomenon was obviously due to the large kinetic diameter, which is close to one of the FAU-type cavities.

Figure 11. Sorption kinetics of cyclohexane (diamond), n-hexane (square), p-xylene (circle), TMB (cross), and TMOS (triangle) on zeolite films (a) and counterpart powders (b). Table 4. Comparison of the Average Adsorption Kinetics Towards N2 and Organics for Zeolite X Film and Powder adsorbate n-hexane p-xylene cyclohexane TMB TMOS

Vads(X)(powder) Vads(X)(film)/ Vads(X)(film) (10-5mmol g-1 s-1) (10-5mmol g-1 s-1) (Vads(X)powder) 3.16 7.95 5.31 2.1 4.55

3.17 12.87 5.61 4.53 7.86

1 0.62 0.95 0.46 0.58

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Table 5. Sorption Rates of Zeolite X and Y Films and Powders in Respect to Different Adsorbates adsorbate

nads(X)(film) (mmol g-1)

nads(Y)(film) (mmol g-1)

nads(X)(film)/nads(Y)(film)

nads(X)(powder) (mmol g-1)

nads(Y)(powder) (mmol g-1)

nads(X)(powder)/nads (Y)(powder)

nitrogen water n-hexane p-xylene cyclohexane TMB TMOS

9.16 11.81 1.44 1.31 1.58 1.23 0.8

8.10 6.75 1.52 1.34 1.80 0.90 0.85

1.13 1.75 0.95 0.98 0.88 1.37 0.94

9.16 12.32 1.54 1.50 1.89 1.39 1.12

8.10 11.20 2.12 1.56 2.62 1.50 1.31

1.13 1.1 0.73 0.96 0.72 0.92 0.85

In other words, the intergrowth between zeolite crystals building the film created additional diffusional barriers into the zeolite layer in comparison to the corresponding powders. This result is in line with other studies devoted to the adsorption properties of intergrown zeolite films.56,57 Another topic of interest dealt with the filling of the zeolite porosity, which was calculated according to eq 1

%nads

nads(organic) × M(organic) d(organic) ) × 100 nads(N2) × M(N2)

(1)

d(N2) where n(ads) is the quantity adsorbed in mol, M is the molecular weight in g mol-1, and d is the density expressed in g cm-3. The last two columns of Table 3 express the ratio between the adsorbed amounts of organics and nitrogen in percent. Small organic molecules filled about 60% of the available pore volume of faujasite, cyclohexane being the best packed one, due to its strong interactions and high piling into the zeolite cages. For the sake of comparison, the water adsorption was also measured and showed a pore filling of about 60% in both films and powders. The latter result confirmed that, under the employed conditions, even water molecules could not fill entirely the porosity of a hydrophilic zeolite. The kinetics of adsorption were also determined through thermogravimetric analysis (Figure 11). It was found that the equilibrium was reached first on zeolite films, as lower quantities or organics were adsorbed on their cavities. The kinetics experiments were performed under a nitrogen flow at a relative pressure of P/P0 ) 0.5, which may explain the slow equilibria. The fastest kinetics observed for p-xylene were attributed to its polarity, which induced a high affinity for the zeolite framework. For alkanes (n-hexane and cyclohexane), sorption kinetics appeared to be comparable on films and counterpart powders. On the other hand, aromatics and molecules of large kinetic diameter exhibited slower kinetics on the faujasite films in comparison to powders (Table 4). Again, the abundant intergrowth of the zeolite films is considered responsible for the significant decrease of the kinetics, slowing down the organic molecule diffusion into the zeolite porosity.

G. Sorption Properties of FAU-Type Films and Powders: Influence of the Si/Al Ratio on the Sorption Capacities. The effect of the Si/Al ratio on the sorption properties of zeolite-carbon composites was also addressed in the present study. Table 5 compares the sorption rates of zeolites X and Y. An increase of the Si/Al ratio in the faujasite framework strongly favored the adsorption of alkanes (cyclohexane and n-hexane), since the sorption capacity of zeolite Y powders was about 1.25 times higher than the zeolite X ones for these two molecules. The phenomenon was less significant for aromatics, in which the sorption rates of NaY zeolite were about 1.1 higher than those of its NaX counterpart. These data confirmed the enhanced affinity of more siliceous faujasites for organic molecules, while the polarity of the adsorbate also appeared to be a relevant parameter in the sorption phenomena. Besides the Si/Al ratio, the sorption properties of zeolite X are also influenced by the larger presence of sodium cations that may decrease the sorption capacity in respect to organic molecules. In general, zeolite Y films showed higher (about 1.1 times) sorption capacities compared to the corresponding zeolite X films, which was in good agreement with results obtained on zeolite powders. In this study, hydrophobic/hydrophobilic interactions could be considered as the dominating factor since both zeolite X and Y were highly crystalline. Filling rates of the NaX and NaY microporosities are represented in Figure 12. For both zeolites, cyclohexane was the best packed molecule into zeolite cavities, while large molecules like TMOS and TMB could not diffuse well into zeolite cavities. The filling efficiency of the zeolite Y films was seen to be lower compared to the corresponding powder. As zeolite Y powders were shown to present higher sorption rates in respect to alkanes, further optimization allowing the synthesis of thicker zeolite Y films will have to be performed. Another issue of interest deals with the performances of zeolite films in comparison to their counterpart pellets that are shaped with plasticizers. The advantage of an intergrown zeolite coating with respect to the corresponding pellets is that surfaces that are integrated parts of the satellite can be used as substrates. Thus, additional weight can be avoided. Moreover, the intergrown films do not comprise a binder, in contrast to the zeolitecontaining pellets.

Figure 12. Adsorbed quantities of various organic adsorbates on zeolite X and Y films (a) and powders (b).

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IV. Conclusions The possibility to grow zeolite-carbon fiber composites that could be used for molecule decontamination of low earth orbit satellites was studied. Zeolite NaX and NaY films were successfully grown when faujasite nanocrystals were employed for seeding of the substrate. A detailed investigation of growth kinetics revealed the formation of a metastable lamellar phase at the beginning of hydrothermal treatment, which disappeared in the course of reaction. All important factors controlling zeolite film growth, such as the Si/Al ratio, alkalinity, time, and temperature of hydrothermal treatment, were carefully studied. Thus, the synthesis parameters were optimized and films with a controlled thickness between 0.5 and 3.5 µm obtained. Sorption capacities and adsorption kinetics of zeolite films with respect to a series of organic molecules were studied. Both NaX and NaY films showed lower capacities to large molecules with respect to their powder counterparts. The interfaces of interpenetrating zeolite crystals were considered as the most likely reason for the hindered diffusion and packaging ability of large molecules. The level of hydrophobicity was also found important for the sorption behavior of zeolite NaX and NaY; namely, zeolite Y films and powders demonstrated a higher capacity than their zeolite X counterparts. Finally, sorption rates of molecules in zeolite films and powders were studied. Again, the additional barriers due to intergrowth of zeolite crystallites building the film influenced strongly the adsorption of bulkier molecules. In conclusion, the intergrowth of zeolite crystals building the film implies some negative effects on the sorption ability of molecular sieve material. Nevertheless, the obtained zeolitecarbon fiber composites demonstrated satisfactory sorption properties that are relevant to the foreseen application. Acknowledgment The financial support from the CNRS and the CNES is greatly appreciated. Literature Cited (1) Inoue, T.; Nagase, T.; Hasegawa, H.; Kiyozumi, Y.; Sato, K.; Nishioka, M.; Hamakawa, S.; Mizukami, F. Ind. Eng. Chem. Res. 2007, 46, 3743. ¨ hrman, O.; Msimang, V.; van Steen, E.; Bo¨hringer, (2) Hedlund, J.; O W.; Sibya, S.; Mo¨ller, K. Chem. Eng. Sci. 2004, 59, 2647. (3) Bernardo, P.; Algieri, C.; Barbieri, G.; Drioli, E. Catal. Today 2006, 118, 90. (4) Beers, A.; Nijhuis, T.; Kapteijn, F.; Moulijn, J. Microporous Mesoporous Mater. 2001, 48, 279. (5) Ockwig, N. W.; Nenoff, T. M. Chem. ReV. 2007, 107, 4078. (6) Piera, E.; Brenninkmeijer, C. A. M.; Santamaria, J.; Coronas, J. J. Membr. Sci. 2002, 201, 229. (7) Chen, H.; Li, Y.; Yang, W. J. Membr. Sci. 2007, 296, 122. (8) Bowen, T.; Wyss, J.; Noble, R.; Falconer, J. Ind. Eng. Chem. Res. 2004, 43, 2598. (9) Ahn, H.; Lee, H.; Lee, S.; Lee, Y. J. Membr. Sci. 2007, 291, 46. (10) Mintova, S.; Mo, S.; Bein, T. Chem. Mater. 2001, 13, 901. (11) Vilaseca, M.; Coronas, J.; Cirera, A.; Cornet, A.; Morante, J.; Santamaria, J. Catal. Today 2003, 82, 179. (12) Liu, N.; Hui, J.; Sun, C.; Dong, J.; Zhang, L.; Xiao, H. Sensors 2006, 6, 835. (13) Baimpos, T.; Giannakopoulos, I.; Nikolakis, V.; Kouzoudis, D. Chem. Mater. 2007, 20, 1470. (14) Sato, K.; Sugimoto, K.; Sekine, Y.; Takada, M.; Matsukata, M.; Nakane, T. Microporous Mesoporous Mater. 2007, 101, 312. (15) Li, S.; Tuan, V.; Falconer, J.; Noble, R. Microporous Mesoporous Mater. 2002, 53, 59. (16) Osada, M.; Sasaki, I.; Nishioka, M.; Sadakata, M.; Okubo, T. Microporous Mesoporous Mater. 1998, 23, 287.

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ReceiVed for reView October 28, 2009 ReVised manuscript receiVed April 21, 2010 Accepted April 29, 2010 IE901683Y