ARTICLE pubs.acs.org/crystal
Submicrometer Zeolite A Crystals Formation: Low-Temperature Crystallization Versus Vapor Phase Gel Transformation L. Dimitrov,† V. Valtchev,*,‡ D. Nihtianova,† and Y. Kalvachev† † ‡
Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., bl. 107, 1113 Sofia, Bulgaria Laboratoire Catalyse & Spectrochimie, ENSICAEN, Universite de Caen, CNRS, 6 boulevard du Marechal Juin, 14050 Caen, France ABSTRACT: In the present study, low-temperature hydrothermal crystallization and vapor phase gel transformation have been employed to synthesis LTA-type zeolite crystals of submicrometer size. The crystal growth curves under hydrothermal conditions at 35, 50, and 65 °C were obtained. From these curves, the induction periods and the times for reaching maximum crystallinity for respective temperatures were determined. A set of characterization methods, including XRD, FTIR, SEM, TEM, and DLS, was employed to obtain complementary information. The inspection of the crystalline solids obtained under hydrothermal conditions showed that the size of the crystals is a function of the crystallization temperature. The largest crystals (300 300 nm) were obtained at 65 °C, while much smaller particles were synthesized at lower temperatures. Second approach involved vapor-phase transformation of the solid part of initial gel. This approach has also yielded submicrometer-sized zeolite crystallites. The characteristics of crystalline particles synthesized by two methods were compared. Both synthetic methods showed relatively high conversion (70 80%) of the initial gel into LTA-type zeolite, which is substantial advantage in respect to nanozeolite synthesis from clear solutions.
’ INTRODUCTION Zeolites are crystalline microporous high surface area solids that find applications as heterogeneous catalysts or separation media for gases, liquids, and ions.1 Wide application of zeolites in chemical process industry is due to their intrinsic characteristics, namely, high selectivity because of the regular system of microporous and tunable catalytic activity because of the presence of active sites whose force and distribution could be modulated.2 These characteristics make zeolite-type materials indispensable for numerous processes and their uses steadily grow through the years. Lately the application of zeolites exceeded the traditional catalytic and separation applications. Microporous zeolite-type solids showed promising performance in areas as electronics, optics, medicine, and sensing.3 21 Most of these emerging applications are related with the use of smaller zeolite particles, that is, submicrometric (below 1000 nm) and nanosized (below 100 nm). In addition to new applications the submicrometersized zeolites offer an interesting alternative to conventional micrometer sized zeolite in the traditional areas of utilization because of improved reactivity and faster reaction kinetics.22,23 The latter is particularly important for catalytic uses, where the long diffusion pathway is often the reason for blocking and deactivation of the catalyst.24,25 The zeolite nanocrystals are still not readily available due to the some particularities of the synthesis making the large scale production unfavorable from environmental and economic points of view.26 Thus the quest for alternative methods that might provide zeolite nanoparticles remains open and any advances are highly appreciated. For instance, recently Wakihara and co-workers reported the preparation of zeolite nanoparticles by grinding27 followed by secondary growth of bred particles.28 r 2011 American Chemical Society
In general, the zeolite syntheses are performed in closed systems, where the supersaturation leads to spontaneous nucleation.29,30 Upon such conditions, the control of the nucleation allows directing the crystal size. In other words, the nutrient pool is limited and after exhausting of a building component the crystal growth stops. Thus, the increase in the number of nuclei leads to a decrease of the ultimate crystallite size. Hence, the formation of small zeolite crystals requires conditions that favor the nucleation over crystal growth. In contrast, large zeolite crystals could be formed if the nucleation is suppressed and thus a few nuclei are formed and grow in the system. Conventional zeolite syntheses usually provide crystals with a size ranging between one and several micrometers. Variation of the initial gel composition allows the size of zeolite crystals to be controlled to some extent. A recent example for such a study was the work of Leiggener et al. who have carefully studied the influence of different chemical parameters on LTA-type zeolite crystals size.31 Theese authors found that a partial replacement of Na for tetramethylammonium in the initial gel led to substantial reduction of zeolite crystals size. Nanocrystals with a narrow particle size distribution are generally obtained from so-called clear solutions.32 34 The homogeneity of such systems is much higher in respect to the hydrogels conventionally employed in zeolite syntheses. However, the low crystalline yield and the exhaustive amounts of organic structure directing agents used in such systems make their industrial application undesirable. In addition, the formation of zeolite suspensions Received: July 9, 2011 Revised: September 2, 2011 Published: September 07, 2011 4958
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comprising discrete colloidal particles makes the purification and processing of crystalline product particularly difficult. The colloidal suspensions of monodisperse zeolite nanoparticle present interest for some of the advanced applications mentioned above. On the other hand, these materials do not offer substantial benefits in large scale catalytic or separation processes, where aggregated nanocrystals can be used. The objective of the present study is the development of a synthesis procedure that circumvents the above disadvantages and ensures high conversion of the initial reactants into nano- or submicrometer sized zeolites. Two synthesis approaches that favor the nucleation over the growth are tested and the characteristics of synthesized crystalline products compared.
’ EXPERIMENTAL SECTION Synthesis. The chemicals used in the present study were: sodium silicate (Merck, 28 wt % SiO2, 8% Na2O), sodium aluminate (Riedel de Haien, Al2O3 54 wt %, Na2O 43%), sodium hydroxide pellets (SigmaAldrich), and distilled water. The reactants were mixed in ratios providing a gel with following molar composition: 6 Na2O/0.505 Al2O3/1.0 SiO2/ 150 H2O. The procedure includes dissolution of the sodium aluminate and maior part of the required sodium hydroxide into distilled water until clear solution is obtained. If necessary the mixture is heated at 60 °C for complete dissolution of the sodium aluminate. The rest of sodium hydroxide and water are added to the water glass solution. In a typical preparation, 20 g of H2O were mixed with 2.81 g of NaAlO2 and 8 g of NaOH. The second mixture comprised 37.3 g of H2O, 0.93 g of NaOH, and 5 g of sodium silicate solution. The sodium aluminate solution is gradually added to water glass solution in a polypropylene (PP) vessel at stirring speed 8000 rpm using a Heidolph Silent Crusher mixer. After 15 min of mixing, the PP vessel is placed into oven at temperatures 35, 50, and 65 °C for crystallization. After certain periods of time, the reactant mixture is manually homogenized and aliquots collected for analysis of solid product. The vapor phase gel transformation was performed after aging of the gel at 50 °C. After this stage, the gel was centrifuged at 12 000 rot/min for 10 min. The remaining water content was evaluated by drying the separated solid phase at 100 °C for 5 h and calcination at 600 °C for 5 h. The molar composition of the gel separated by centrifugation was 1.63 Na2O/0.52 Al2O3/1.0 SiO2/13.15 H2O. The gel was placed in a basket positioned in the upper part of the reaction vessel and subjected to treatment at 100 °C for 2, 4, and 17 h in atmosphere rich of water vapors. Typically 5 g of gel were treated in a 500 mL autoclave without adding additional water. Thus, the total amount of water in 500 mL autoclave was provided by the gel and corresponded to 5.4 mL. After synthesis, the sample was dispersed in distilled water by ultrasonic treatment, washed several times by consecutive steps of centrifugation and redispersion in distilled water and then dried at 50 °C overnight. Characterization. The X-ray powder diffraction patterns were recorded on diffractometer D2 Phaser (Bruker) with CuKα radiation, working at acceleration 30 kV and current 10 mA. The 2θ scanned range was 4 40° with step of 0.05° min 1 and 1 s acquisition time. FTIR spectra were taken on a Bruker Tensor 37 spectrometer using KBr pellet technique. For each sample, 64 scans were collected at a resolution of 2 cm 1 over the wavenumber region 4000 400 cm 1. The scanning electron microscope (SEM) analyses were obtained on Philips 515 aparatus, working at 20 kV accelerating voltage. The samples were covered with gold before investigation. The high-resolution transmission electron microscopy (HRTEM) images were obtained with JEOL 2100 equipped with LaB6 electron source at accelerating voltage 200 kV.
Figure 1. Crystal growth kinetics curves according to XRD analysis of LTA-type materials synthesized at 35 (a), 50 (b), and 65 °C (c).
’ RESULTS AND DISCUSSION In the course of this study two approaches, low-temperature hydrothermal syntheses and crystallization in vapor phase, have been applied to obtain zeolite A crystals of submicrometric size. Low-Temperature Synthesis. Generally, zeolite A syntheses are performed in the temperature range 90 100 °C for periods of time ranging between several tens of minutes and several hours. It is well-known, however, that the aging under ambient conditions has a pronounced effect on the subsequent crystallization process. Several research groups suggested that the aging step results in the formation of viable nuclei that induce the crystallization upon raising the temperature.35 37 The moderate temperature favor the nucleation over the growth and thus the number of nuclei per unit gel is higher. Once zeolite nuclei formed under ambient conditions, there are neither thermodynamic nor chemical constrains that should interfere with the subsequent growth of the crystallites. On the basis of these considerations we have performed the synthesis of zeolite A synthesis at three different 4959
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Crystal Growth & Design temperatures, which are substantially below the temperature employed in its production (∼95 °C). The kinetics of crystal growth was followed by X-ray diffraction analysis, as a reference sample was employed a solid obtained from the same initial gel at 100 °C for 4 h. Figure 1 shows the changes of the crystallinity as a function of the synthesis time at 35, 50, and 60 °C. It is noteworthy that at all three temperatures zeolite A with 100% crystallinity is obtained. As can be expected, the time for reaching maximum crystallinity was longer with the decrease of the temperature. For instance, the reference samples crystallized for 4 h, while 9, 20 and 25 h of hydrothermal treatment were necessary for the samples synthesized at 65, 50, and 35 °C, respectively. After reaching the maximum crystallinity, a down turn of the crystallization curves was observed. The latter suggested a phase transformation, in other words dissolution and transformation of zeolite A into other material.
Figure 2. Comparative XRD-IR study of crystal growth kinetics of zeolite A at 35 °C.
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However, the XRD study did not reveal the presence of other crystalline phase for studied periods of time. The series of samples was further examined by IR spectroscopy, thus complementing the information of the long-range (XRD) with short-range (IR) order analysis. The band at around 560 cm 1, characteristic of external linkages of double fourmember ring vibrations, was employed to evaluate the crystallinity of the material.38 40 The appearance of this band was associated with the beginning of the crystallization process, that is, the appearance of secondary building units in the LTA-type structure. To avoid the dependence of intensity from the sample weight, the ratio of the bands intensity at 556 and 1009 cm 1, I556/I1009, was used. As can be seen from the Figure 2, the IR spectroscopy is more sensitive to the early stages of zeolite formation. The latter requires existence of least several unit cells to reveal the formation of studied material. Thus, the IR analysis revealed that X-ray amorphous materials possess a certain level of crystalline order. The crystallization process begins after 6 h at 50 °C and after 18 h at 35 °C. Figure 2 depicts the kinetics of crystallization at 35 °C according to the XRD and IR studies results. The results of the study at low temperature was further used to promote the formation of large number of nuclei at low temperature that were subjected to treatment at higher temperature in order to decrease the overall synthesis time. The SEM inspections of the samples showed that the materials obtained at lower temperatures are substantially smaller in size in respect to reference sample (Figure 3). Under conventional conditions crystals with size 3 4 μm were obtained (Figure 3a) At least 1 order of magnitude smaller crystals were obtained at the syntheses performed at lower temperatures, as the smallest particles were obtained at 35 °C (Figure 3b). The latter particles were isometric without defined crystal features. In contrast, wellshaped cubic crystals of about 300 300 nm size were synthesized at 65 °C (Figure 3c). The TEM investigation of the
Figure 3. SEM micrographs of LTA-type zeolite crystals synthesized at 100 °C for 4 h (a), 35 °C for 25 h (b), 50 °C for 17 h (c), and 65 °C for 10 h (d). Scale bar = 1 μm. 4960
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Figure 4. TEM micrographs of fully crystalline LTA-type materials obtained at 35 (a), 50 (b), and 65 °C (c). Scale bar = 200 nm.
Figure 5. Powder XRD patterns of the solids obtained in vapor phase at 100 °C after 2 (a), 4 (b), and 17 h (c) synthesis.
samples confirmed the results of SEM study (Figure 4). The TEM inspection revealed that the material synthesized at 35 °C was fairly inhomogeneous. The major fraction in the solid was 30 100 nm nanoparticles. The sample contained also much larger (200 300 nm) zeolite A crystals. At higher temperatures, much more uniform samples were obtained as the major fraction synthesized at 50 and 65 °C was about 200 nm and 300 400 nm, respectively. The formation of more uniform in size materials at elevated temperatures was attributed to the Ostwald ripening, that is, after exhausting of nutrient pool the larger zeolite crystals continue growing at the expense of smaller and more unstable crystallites. In other words, the smaller crystallites were dissolved and the aluminoslilicate species transported via solution to the growing crystals. To limit this characteristic for hydrothermal crystallization process a second synthetic approach was employed. Namely, the solid part of a preaged gel was separated from mother liquor and subjected to crystallization in atmosphere of water vapors. Vapor Phase Gel Transformation. Vapor phase syntheses were performed to decrease the ultimate zeolite crystal size without a substantial increase of crystallization time. This approach could be situated between hydrothermal synthesis and solid-state transformation. The excess of water in the initial gel was eliminated by high speed centrifugation. Nevertheless, the initial gel comprised sufficient amount of moisture to saturate the autoclaves with water vapor that is indispensable for completing zeolite crystallization. Vapor phase crystallization allowed to limit the longrange transport of nutrients and thus the dissolution of smaller and less stable crystallites. The XRD diffraction patterns of the samples subjected to 2, 4, and 17 h vapor phase treatment are shown in Figure 5. According
Figure 6. SEM micrographs of gel preaged for 6 h at 50 °C (a) and zeolite A crystals synthesized by vapor phase crystallization at 100 °C for 4 h (b). Particle size distribution according to DLS analysis (c) of the pretreated gel (solid line) and zeolite crystals (dotted line). Scale bar = 10 μm.
to the XRD study the samples synthesized for 2 and 4 h were highly crystalline LTA-type materials, while Sodalite was the dominant phase after 17 h vapor treatment. Obviously, at longer crystallization time, a phase transformation took place no matter of fact that the synthesis was performed in vapor phase and thus the long-range transport in the system limited. On Figure 6, SEM micrographs of the preaged at 50 °C initial gel together with the zeolite A crystals obtained after 4 h vapor phase syntheses are 4961
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Crystal Growth & Design presented. The zeolite product is homogeneous with particle size below 1 μm. Particle size distribution for aged at 50 °C gel and ultimate crystalline product was determined by dynamic light scattering analysis. Prior to analysis the colloidal suspensions were kept at room temperature for 24 to sediment very large aggregates. The analysis showed slightly larger particle size distribution of the crystalline product. The maximum of the peak shifted form 690 nm for the aged gel to 730 nm for crystalline particles (Figure 6c). Thus, the DLS data supported the SEM study and confirmed the efficiency of vapor phase crystallization. It is important to mention that the conversion of the initial gel into crystalline LTA-type material was 75%. This value is lower in repect to the reference sample (95% conversion) synthesized at 100 °C, but still in the range of the conversions observed at low temperature hydrothermal treatment (70 80 wt %). Thus, vapor phase synthesis of submicrometer zeolites is an alternative of hydrothermal crystallization and certainly deserves further consideration in the quest for more efficient crystallization methods.
’ CONCLUSIONS A comparative study of the products of low temperature hydrothermal crystallization and vapor phase transformation of LTAtype zeolite yielding gels was performed. Both approaches provided submicrometer sized zeolite A crystals. The obtained crystalline particles were at least 1 order of magnitude smaller in size in respect to the reference sample prepared under conventional hydrothermal conditions employing the same initial gel. It was found that the size of zeolite nanoparticles obtained under hydrothermal conditions was a function of the crystallization temperature. Thus, the main fraction of zeolite A particles obtained at 35, 50, and 65 °C was 50 100, 200, and 300 nm, respectively. Zeolite crystallites obtained by vapor phase synthesis were larger in size (∼700 nm). In the latter case, the zeolite particles size was related with the size of gel particles subjected to vapor phase crystallization. Both approaches resulted in relatively high gel conversion (70 80%) that is substantially higher in respect to the conversion of clear solutions into LTA-type zeolite.26 ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: + 33231452733. Fax: + 3321452822.
’ ACKNOWLEDGMENT The authors acknowledge the financial support of Bulgarian National Science Fund under contract DTK 02-47. ’ REFERENCES (1) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974. (2) Karge, H. G., Weitkamp, J., Eds. Molecular Sieves: Science and Technology; Springer: Berlin, 1999. (3) Mintova, S.; Mo, S.; Bein, T. Chem. Mater. 2001, 13, 901. (4) Wang, Z.; Larsson, M. L.; Grahn, M.; Holmgren, A.; Hedlund, J. Chem. Commun. 2004, 2888. (5) Kornic, S.; Baker, M. Chem. Commun. 2002, 1700. (6) Zhang, Y.; Chen, F.; Shan, W.; Zhuang, J.; Dong, A.; Cai, W.; Tang, Y. Microporous Mesoporous Mater. 2003, 65, 277.
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(7) Walcarius, A.; Ganesan, V.; Larlus, O.; Valtchev, V. Electroanalysis 2004, 16, 1550. (8) Castagnola, N. B.; Dutta, P. K. J. Phys. Chem. B 1998, 102, 1696. (9) Mintova, S.; De Waele, V.; Schmidhammer, U.; Riedle, E.; Bein, T. Angew. Chem., Int. Ed. 2003, 42, 1611. (10) Mintova, S.; De Waele, V.; H€olzl, M.; Schmidhammer, U.; Mihailova, B.; Riedle, E.; Bein, T. J. Phys. Chem. A 2004, 108, 10640. (11) Megelski, S.; Calzaferri, G. Adv. Funct. Mater. 2001, 11, 277. (12) Calzaferri, G.; Br€uhwiler, D.; Megelski, S.; Pfenninger, M.; Pauchard, M.; Hennessy, B.; Maas, H.; Devaux, A.; Graf, U. Solid State Sci. 2000, 2, 421. (13) Wang, Z.; Wang, H.; Mitra, A.; Huang, L.; Yan, Y. Adv. Mater. 2001, 13, 746. (14) Wang, Z.; Mitra, A.; Wang, H.; Huang, L.; Yan, Y. Adv. Mater. 2001, 13, 1463. (15) Li, S.; Li, Z.; Yan, Y. Adv. Mater. 2003, 15, 1528. (16) Li, Z.; Li, S.; Luo, H.; Yan, Y. Adv. Funct. Mater. 2004, 14, 1019. (17) Mintova, S.; Bein, T. Adv. Mater. 2001, 13, 1880. (18) Larlus, O.; Mintova, S.; Valtchev, V.; Jean, B.; Metzger, T. H.; Bein, T. Appl. Surf. Sci. 2004, 226, 155. (19) Mitra, A.; Cao, T.; Wang, H.; Wang, Z.; Huang, L.; Li, S.; Li, Z.; Yan, Y. Ind. Eng. Chem. Res. 2004, 43, 2946. (20) Platas-Iglesias, C.; Elst, L. V.; Zhou, W.; Muller, R. N.; Geraldes, C. F. G. C.; Maschmeyer, T.; Peters, J. A. Chem.—Eur. J. 2002, 8, 5121. (21) Bresinska, I.; Balkus, K. J., Jr. J. Phys. Chem. 1994, 98, 12989. (22) Hoang, T. Q.; Zhu, X.; Lobban, L. L.; Resasco, D. E.; Mallinson, R. G. Catal. Commun. 2010, 11, 977. (23) Bi, J.; Guo, X.; Liu, M.; Wang, X. Catal. Today 2010, 149, 143–147. (24) Larsen, S. C. J. Phys. Chem. C 2007, 111, 18464. (25) Petushkov, A.; Yoon, S.; Larsen, S. C. Microporous Mesoporous Mater. 2011, 137, 92. (26) Tosheva, L.; Valtchev, V. Chem. Mater. 2005, 17, 2494. (27) Wakihara, T.; Sato, T.; Inagaki, S.; Tatami, J.; Komeya, K.; Meguro, T.; Kubota, Y. Appl. Mater. Interfaces 2010, 2, 2715. (28) Wakihara, T.; Ichikawa, R.; Tatami, J.; Endo, A.; Yoshida, K.; Sasaki, Y.; Komeya, K.; Meguro, T. Cryst. Growth Des. 2011, 11, 955–958. (29) Cundy, C. S.; Cox, P. A. Microporous Mesoporous Mater. 2005, 82, 1. (30) Corma, A.; Davis, M. E. ChemPhysChem 2004, 5, 304. (31) Leiggener, C.; Currao, A.; Calzaferri, G. In Materials Syntheses, A Practical Guide; Schubert, U., H€using, N.,; Laine, R., Eds.; Springer: Wien, 2008; p 21. (32) Schoeman, B. J.; Sterte, J.; Otterstedt, J.-E. Zeolites 1994, 14, 208. (33) Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J.-E. Zeolites 1994, 14, 557. (34) Li, Q.; Creaser, D.; Sterte, J. Chem. Mater. 2002, 14, 1319. (35) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press, London, 1982. (36) Subotic, B.; Graovac, A. Stud. Surf. Sci. Catal. 1985, 24, 199. (37) Gora, L.; Streletzky, K.; Thompson, R. W.; Phillies, G. D. J. Zeolites 1997, 18, 119. (38) Szostak, R. Hand Book of Molecular Sieves; Van Nostrand Reinhold: New York, 1992. (39) Kosanovic, C.; Bosnar, S.; Subotic, B.; Svetlicic, V.; Misic, T.; Drazic, G.; Havancsak, K. Microporous Mesoporous Mater. 2008, 110, 177. (40) Lyer, K.; Singer, S. J. Phys. Chem. 1994, 98, 12679.
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