504
Langmuir 2005, 21, 504-507
Synthesis of Mesoporous Zeolite A by Resorcinol-Formaldehyde Aerogel Templating Yousheng Tao,*,†,‡,§ Hirofumi Kanoh,† and Katsumi Kaneko*,† Department of Chemistry, Faculty of Science, Chiba University, Chiba 263, Japan, Environmental Science Research Center, Xiamen University, Xiamen 361005, China, and R&D Center of Saline Lake and Epithermal Deposits, Chinese Academy of Geological Sciences, Beijing 100037, China Received September 16, 2004. In Final Form: November 20, 2004 Mesoporous zeolite A has been synthesized by using a template method with resorcinol-formaldehyde aerogels having three-dimensional mesopores. It was characterized with X-ray powder diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption/desorption. The pore size distribution calculated from the nitrogen adsorption isotherm is a bimodal distribution with micropores and mesopores. Field emission scanning electron micrograph observations confirm the presence of mesopores.
Introduction Zeolites, microporous crystalline solids consisting of tetrahedral primary building blocks linked through their oxygen atoms, are three-dimensional networks containing channels and cavities of molecular dimensions. They are probably the solids most widely used in catalysis as well as in separation and purification. However, their intricate pore and channel systems in the molecular size range (0.3 to ∼1.5 nm) not only impose diffusion limitations on reaction rates but also impose a high backpressure on flow systems.1 In the past decade, therefore, there have been a considerable number of attempts to improve the micropore diffusion in zeolites by adjusting their crystal sizes to nanoscale ranges.2 However, filtering the colloidal particles is not easy, which severely hinders their practical applications. Moreover, the volume and surface area of the micropores in zeolites have been reported to decrease because the ordering of the three-dimensional network of the zeolite particles deteriorates when the crystal size decreases to the nanocrystalline region (i.e., below 100 nm).3,4 Several dual templating methods for the preparation of mesoporous zeolite materials have thus been proposed recently. One is macrotemplating, with carbon black particles for preparing ZSM-5 of a wide pore size distribution (10-100 nm) and with monodisperse polystyrene spheres for preparing macroporous silicates averaging 250 nm in diameter.5,6 Another is nanocasting, * Corresponding author. E-mail: pchem2.s.chiba-u.ac.jp. † Chiba University. ‡ Xiamen University. § Chinese Academy of Geological Sciences.
kaneko@
(1) (a) Herrmann, C.; Haas, J.; Fetting, F. Appl. Catal. 1987, 35, 299. (b) vanDonk, S.; Broersma, A.; Gijzeman, O. L. J.; van Bokhoven, J. A.; Bitter, J. H.; de Jong, K. P. J. Catal. 2001, 204, 272. (c) Pe´rez-Ramı´rez, J.; Kapteijn, F.; Groen, J. C.; Dome´nech, A.; Mul, G.; Moulijn, J. A. J. Catal. 2003, 214, 33. (2) (a) Madsen, C.; Jacobsen, C. J. H. J. Chem. Soc., Chem. Commun. 1999, 673. (b) Zhan, B. Z.; White, M. A.; Lumsden, M.; Jason, M. N.; Robertson, K. N.; Cameron, T. S.; Gharghouri, M. Chem. Mater. 2002, 14, 3636. (c) Li, Q.; Creaser, D.; Sterte, J. Chem. Mater. 2002, 14, 1319. (d) Holmberg, B. A. W. H.; Norbeck, J. M.; Yan, Y. Microporous Mesoporous Mater. 2003, 59, 13. (3) Camblor, M. A.; Corma, A.; Valencia, S. Microporous Mesoporous Mater. 1998, 25, 59. (4) Nhut, J. M.; Pesant, L.; Tessonnier, J. P.; Wine, G.; Guille, J.; Cuong, P. H.; Ledoux, M. J. Appl. Catal., A 2003, 254, 345. (5) Jacobsen, C. J. H.; Madsen, C.; Houzvicka, J.; Schmidt, I.; Carlsson, A. J. J. Am. Chem. Soc. 2000, 122, 7116.
with colloid-imprinted carbons as templates for preparing nanosized ZSM-5 crystals with some interparticle mesopores.7 Jacobsen et al. prepared mesoporous ZSM-5 with pores 12-30 nm in diameter by impregnating the synthesis gel components with multiwall carbon nanotubes, and it contained iron oxide particles that had been encapsulated as impurities in the carbon material.8 Although the exploitation of templates in designing mesoporous zeolites has clearly been receiving a great deal of attention and has become the main objective of those working in template synthesis,9 a postsynthesis hydrothermal dealumination treatment has been used to produce defect domains 5-50 nm in size (which are attributed to mesopores) in the faujasites, mainly zeolite Y. This treatment, however, cannot provide zeolite Y having mesopores uniform in size and lattice position. Such inhomogeneities are even more pronounced in a higher-temperature stream-deactivated commercial USY cracking catalyst.10 We have very recently prepared well-crystalline mesoporosity-donated ZSM-5 and zeolite Y by using carbon aerogel templating,11,12 but it was not easy to synthesize mesoporous zeolite A even when using carbon aerogel templating. Here, we report that zeolite A with mesoporous channels (meso-NaA) can be synthesized by using resorcinol-formaldehyde (RF) aerogels as templates. This aerogel is a low-density, open-cell foam derived from the polycondensation of resorcinol with formaldehyde.13 In (6) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308. (7) Kim, S. S.; Shah, J.; Pinnavaia, T. J. Chem. Mater. 2003, 15, 1664. (8) (a) Boisen, A.; Schmidt, I.; Carlsson, A.; Dahl, S.; Brorson, M.; Jacobsen, C. J. H. Chem. Commun. 2003, 958. (b) Schmidt, I.; Boisen, A.; Gustavsson, E.; Stahl, K.; Pehrson, S.; Dahl, S.; Carlsson, A.; Jacobsen, C. J. H. Chem Mater. 2001, 13, 4416. (9) (a) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (b) Go¨ltner, C. G. Angew. Chem., Int. Ed. 1999, 38, 3155. (c) Schu¨th, F.; Schmidt, W. Adv. Mater. 2002, 14, 629. (10) (a) Lynch, J.; Raatz, F.; Dufresne, P. Zeolites 1987, 7, 333. (b) Choi-Feng, C.; Hall, J. B.; Huggins, B. J.; Begerlein, R. A. J. Catal. 1993, 140, 395. (c) Sasaki, Y.; Suzuki, T.; Takamura, Y.; Saji, A.; Saka, H. J. Catal. 1998, 178, 94. (11) Tao, Y.; Kanoh, H.; Kaneko, K. J. Am. Chem. Soc. 2003, 125, 6044. (12) Tao, Y.; Kanoh, H.; Kaneko, K. J. Phys. Chem. B 2003, 107, 10974. (13) (a) Pekala, R. W. J. Mater. Sci. 1989, 24, 3221. (b) Pekala, R. W.; Alviso, C. T.; Kong, F. M.; Hulsey, S. S. J. Non-Cryst. Solids 1992, 145, 90.
10.1021/la047686j CCC: $30.25 © 2005 American Chemical Society Published on Web 12/17/2004
Letters
Langmuir, Vol. 21, No. 2, 2005 505
the synthesis of meso-NaA (see the Experimental Section), the structure-directing agent tetramethylammonium hydroxide (TMAOH) templates the micropores within the crystals, while the mesostructural RF aerogel templates the mesopores that could be within the crystals and within the aggregates. To the best of our knowledge, this is the first preparation of well-crystalline mesoporous zeolite A. Since zeolite A nanocrystal synthesis is known to be noticeably difficult to handle, there are only a few reports concerning the preparation of nanosized zeolite A as well as zeolite A films14,15 and there are no reports of the successful preparation of mesoporous zeolite A. Experimental Section Materials. The following materials were used as received without further purification: resorcinol (C6H4(OH)2, min 99.0%, Wako), formaldehyde solution (HCHO, 36.0-38.0%, Wako), sodium carbonate (Na2CO3, min 99.5%, Wako), aluminum isopropoxide (Al(iPrO)3, min 95.0%, Wako), silica sol (SiO2, 30%, Aldrich), tetramethylammonium hydroxide pentahydrate (TMAOH‚5H2O, ICN), and sodium hydroxide pellets (NaOH, min 96.0%, Wako). Preparation of RF Aerogels. The RF aerogels used as templates in the experiments were prepared, following published procedures,13 by starting with a solution containing resorcinol, formaldehyde, and sodium carbonate in ion-exchanged water and performing sol-gel polymerization of resorcinol and formaldehyde first at 323 K for 24 h and then at 363 K for 72 h. The sodium carbonate was used as a catalyst, and the molar ratio of resorcinol to catalyst (R/C) was held at 200. The residual water was removed from the resorcinol-formaldehyde sol-gel by multiple exchanges with fresh acetone. The RF aerogels were finally obtained in monolithic form by drying the RF sol-gels at 313 K and ca. 10 MPa for 4 h with supercritical CO2, for which the critical temperature and pressure are 304.1 K and 7.4 MPa. Synthesis of meso-NaA. Synthesis gels were typically prepared according to the method described by Mintova et al.16 in the following way: A reaction mixture with a composition (molar basis) of 0.42 Na2O/11.21 SiO2/1.84 Al2O3/13.79 (TMA)2O/ 587 H2O was prepared by combining silica sol with a mixture of Al(iPrO)3, TMAOH‚5H2O, and sodium hydroxide solution under vigorous stirring at room temperature in order to obtain a clear aluminosilicate solution. When the reaction mixture was transferred to a reaction cell containing RF aerogels, which had been evacuated at 383 K and 1 mPa for 2 h prior to use, the zeolite precursor was introduced into the mesopores of RF aerogels. After aging for 9 days at room temperature, the reaction cell was transferred into a stainless steel autoclave and heated in an oven at 353 K for 1 week. The autoclave was then cooled to room temperature. After the product was washed and dried, the RF aerogels as well as any organics were removed by pyrolysis (under flowing oxygen diluted with argon) at 773 K for 8 h in an electronic muffle furnace. Finally the mesoporous zeolite A was obtained. Zeolite A (NaA), a reference material, was synthesized in bulk solution by following the above-mentioned procedures in the absence of RF aerogels. Characterization. The powder X-ray diffraction (XRD) patterns were acquired on a M18XHF X-ray automatic diffractometer (MacScience Co.) using a monochromatized X-ray beam from nickel-filtered Cu KR (0.154 050 nm) radiation and operated at 40.0 kV and 200.0 mA. The nature of the phases present in the sample was checked using the database of the Joint Committee on Powder Diffraction Standards (JCPDS). The crystallite size was calculated from the XRD line broadening (14) (a) Rakoczy, R. A.; Traa, Y. Microporous Mesoporous Mater. 2003, 60, 69. (b) Wang, H.; Huang, L.; Holmberg, B. A.; Yan, Y. Chem. Commun. 2002, 1708. (15) (a) Hedlund, J.; Schoeman, B.; Sterte, J. Chem. Commun. 1997, 1193. (b) Lovallo, M.; Tsapatsis, M. Chem. Mater. 1996, 8, 1579. (c) Matsukata, M.; Nishiyama, N.; Ueyama, K. Stud. Surf. Sci. Catal. 1994, 84, 1183. (d) Masuda, T.; Hara, H.; Kouno, M.; Kinoshita, H.; Hashimoto, K. Microporous Mater. 1995, 3, 565. (16) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958.
Figure 1. (A) X-ray powder diffraction patterns of (a) mesoNaA and (b) NaA; (B) IR spectra of (a) NaA and (b) meso-NaA. (full width at half-height), corrected for the instrumental broadening, by using the Scherrer equation. Because of the zeolite crystallite size anisotropy and stacking faults, the crystallite sizes are given as intervals estimated from different XRD diffraction peaks. The Fourier transform infrared (FT-IR) spectra in the zeolite framework vibration region were obtained with a FT/IR-410 spectrometer (JASCO Co.) using the KBr pellet technique. Each spectrum was acquired in a wavenumber range between 400 and 1500 cm-1 at a 2 cm-1 resolution. The Raman spectra were obtained with a NRS-1000 laser Raman spectrophometer (JASCO Co.) controlled by a computer. Sample excitation was done with the 532 nm line of a Spectra-Physics LD laser, and the scattered light was detected with a LN/CCD576E GaAs PM tube. The thermogravimetric analysis (TGA) of the samples was performed on a SII TG/DTA 6200 up to 1073 K at a heating rate of 1 K/min in a 200 mL/min flow of an oxygen and nitrogen mixture. The field emission scanning electron micrographs of the samples were obtained on a JEOL JSM-6330F scanning electron microscope using conventional sample preparation and imaging techniques without Au sputtering or any metal coating. The nitrogen adsorption isotherms were measured at 77 K using a gas sorption analyzer (Quantachrome Autosorb1). Zeolite specimens were activated by removing the water of hydration at 623 K for 12 h under 10-4 Pa before the adsorption measurement. RF aerogel specimens were activated under the same pressure but at 383 K for 2 h.
Results and Discussion The powder XRD patterns for meso-NaA and the NaA reference are shown in Figure 1A. The identical peak positions in the XRD patterns show that meso-NaA has pure Linde Type A (LTA) crystalline structure, but the slight broadening of the diffraction lines for meso-NaA indicates that the crystallites in meso-NaA are smaller than those in NaA. The sizes of NaA and meso-NaA crystallites were respectively estimated from the XRD peak broadening to be ∼40 and ∼30 nm by using the Scherrer equation and Bragg angle 2θ values of 7.1° [200] and 30.1° [820].
506
Langmuir, Vol. 21, No. 2, 2005
Letters
Figure 2. Thermogravimetry and differential thermogravimetry curves for a NaA/RF composite under a mixture of flowing oxygen and nitrogen.
FT-IR spectra for NaA and meso-NaA samples are shown in Figure 1B. Peak positions and heights are nearly the same for NaA and meso-NaA. The peak at 464 cm-1 is the structure-insensitive internal TO4 (T ) Si or Al) tetrahedra bending peak of zeolite A. The peak at 570 cm-1 is attributed to the double ring external linkage peak assigned to zeolite A. This chain frequency band is characteristic of zeolite A, and its frequency depends on the Si/Al ratio and crystal structure of the zeolite. A decrease in the intensity of this band is associated with crystallographic decomposition.17 The peak at 660 cm-1 is assigned to external linkage symmetrical stretching. The peaks at 1050 and 1090 cm-1 are assigned to internal tetrahedra asymmetrical stretching and external linkage asymmetrical stretching, respectively. All these bands in the FT-IR spectra match well with the characteristic zeolite A absorption peaks reported in the literature.18,19 On the other hand, Dutta et al. observed a peak at 860 cm-1 in the FT-IR spectra of zeolite A samples with Si/Al ratios equal to 1.4 and 2.7 and assigned it to the S-O stretching mode of terminal silicon atoms in the zeolite crystals.20 This peak is also evident in the spectra of our NaA and meso-NaA samples. As the Raman frequency around 500 cm-1 is sensitive to the framework T-O-T angle in zeolite A, the meso-NaA sample was further examined with Raman spectroscopy. The Raman frequency for meso-NaA is observed at 490 cm-1, the same frequency reported for NaA with an average T-O-T angle of 148.3°.19,21 Overall, the complementary FT-IR and Raman studies indicate that meso-NaA has the structural characteristics of zeolite A frameworks without crystalline defects. The TGA data for a NaA/RF aerogel composite show that most of the weight loss occurred between 523 and 773 K (Figure 2) and that decomposition occurred in two steps as the temperature was raised. The weight loss in the first step may have been associated with the desorption of water and residual organic precursors.22 The weight loss in the second step was most likely due to the decomposition of RF aerogels. There was no weight loss (17) (a) Flanigen, E. M. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; p 80. (b) Wolf, F.; Fuertig, H. Tonind, Ztg. Keram. Rundschau 1966, 90, 310. (18) (a) Flaningen, E. M.; Khatami, H.; Szymanski, H. A. Adv. Chem. Series 1971, 101, 201. (b) Smirnov, K. S.; Maire, M. L.; Bremard, C.; Bougeard, D. Chem. Phys. 1994, 179, 445. (19) Szostak, R. In Handbook of Molecular Sieves; Van Nostrand Reinhold: New York, 1992; p 266. (20) Dutta, P. K.; Barco, D. B. J. Phys. Chem. 1988, 92, 354. (21) Dutta, P. K.; Shieh, D. C.; Puri, M. Zeolites 1988, 8, 306. (22) (a) Lin, C.; Ritter, J. A. Carbon 1997, 35, 1271. (b) MaldonadoHodar, F. J.; Ferro-Garcia, M. A.; Rivera-Utrilla, J.; Moreno-Castilla, C. Carbon 1999, 37, 1199.
Figure 3. Field emission scanning electron micrographs of (A,B) meso-NaA samples and (C) a RF aerogel template.
above 773 K, suggesting that the RF aerogel template was completely removed under the experimental conditions. Typical field emission scanning electron micrograph (FE-SEM) images of meso-NaA and RF aerogel templates are shown in Figure 3. The significant mesoporosity of the mesoporous zeolite A samples is evident in the micrograph in Figure 3A. The agglomerates of spheres are intergrown, affording that aggregated particles have large domain sizes of 30-100 microns (Figure 3B). Mesopores with an average size of ∼30 nm are evident in the RF aerogels (Figure 3C). The mesopores and the walls between them are reported to develop three-dimensionally,13 providing a mold for a replica. Adsorption and desorption isotherms of N2 at 77 K on RF aerogels, meso-NaA, and NaA are shown in Figure 4.
Letters
Langmuir, Vol. 21, No. 2, 2005 507
Figure 4. Adsorption and desorption isotherms of nitrogen at 77 K on (A) RF aerogels and (B) NaA (a) and meso-NaA (b). The inserts show the respective DH mesopore size distributions of RF aerogels and meso-NaA. Table 1. Pore Structural Parameters of the Samples microporosity
mesoporosity
samples
SBET, cm2/g
volume, cm3/g
diameter, nm
volume, cm3/g
meso-NaA NaA RF aerogels
472 502 889
0.20 0.21 0.32
∼0.4 ∼0.4 1.2
0.43
15 ( 5
4.3
30 ( 10
diameter, nm
RF aerogels have an explicit adsorption hysteresis (Figure 4A), indicating that RF aerogels have predominant mesopores (see Table 1). The mesopore size distribution of RF aerogels derived from the adsorption branch of isotherms using the Dollimore and Heal (DH) method23 is shown in Figure 4A (inset) and is in good agreement with the mesopore sizes estimated from FE-SEM images of RF aerogels. Accordingly, RF aerogels can supply mesopore structures suitable for the template synthesis. Nitrogen adsorption (Figure 4B) revealed that the pores of meso-NaA and NaA were accessible to nitrogen at 77 K. This accessibility could be due to the low aluminum content of the samples and thus to there being only a small number of sodium cations in the eight-memberedring pore openings, allowing nitrogen to enter.14,24 The N2 adsorption isotherm of the NaA reference sample, included in Figure 4B for comparison, is basically an IUPAC type I isotherm. Predominant adsorption ended below P/P0 ) 0.02, which is a characteristic of uniform microporous solids. This result is in accordance with the literature.14 The steep uptakes in the N2 adsorption isotherms of both meso-NaA and NaA samples below P/P0 ) 0.02 indicate complete filling of uniform micropores. The Saito-Foley method was used to determine the micropore size distributions (PSDs) of the samples, and all the PSD peaks (23) Dollimore, D.; Heal, G. R. J. Colloid Interface Sci. 1970, 33, 508. (24) Reed, T. B.; Breck, D. W. J. Am. Chem. Soc. 1956, 78, 5972.
were at ∼0.4 nm. By contrast, one notable difference between the nitrogen loadings achieved on NaA and mesoNaA is that an adsorption/desorption hysteresis loop is observed on the meso-NaA at higher loadings. The N2 adsorption isotherm of meso-NaA is a type IV isotherm. This suggests the presence of additional mesopores, which were formed as a result of RF aerogel templating. The mesopore size distribution of meso-NaA is shown in the inset of Figure 4B. The pore size distribution of mesoNaA indicates a maximum at ∼15 nm and a width at half-height of ∼10 nm, corresponding to the thickness of interconnections of RF aerogel clusters. The mesopore size distribution of meso-NaA is much more uniform than that of RF aerogels. The N2 adsorption and FE-SEM results evidence that the meso-NaA mimics the structure of the RF aerogel templates. The pore structural parameters of samples are summarized in Table 1. Overall, analysis of the adsorption isotherms revealed that meso-NaA has a bimodal pore size distribution with micropores of ∼0.4 nm in width and mesopores of 15 ( 5 nm in width, and the total pore volume of meso-NaA is 3 times that of conventional NaA. Donating mesopores (width of 2-50 nm) to zeolites could lead to significantly improved properties as compared to conventional zeolite catalysts, which have very recently been shown in the alkylation of benzene with ethane using mesoporous zeolite single crystals and the NOx conversion over the Co/MFI-type zeolite with the presence of larger pores.25,26 Accordingly, the interconnected mesopore channels in mesoporous zeolites synthesized on RF aerogel templates can reduce mass transfer resistance and enhance diffusion. These zeolites combine the advantages of conventional zeolites and mesoporous molecular sieves; thus we have no doubt that they have opened new fields for future exploration. Indeed, our methodology is not limited to the preparation of mesoporous zeolite A, since it is apparent that the electrostatic interactions between the zeolite clusters and the RF clusters of the template walls, and favorable van der Waals contacts between zeolite nanoparticles, provide the zeolite nanoparticles with enough mobility for the initial migration necessary for nucleation and crystallization within the template mesopores. Since the three-dimensional nanostructures of the RF aerogel template depend on the size and number of RF clusters generated during the sol-gel polymerization, which in turn are determined by the resorcinol-catalyst ratio in a formulation, solution chemistry can theoretically be used to tune the structures of the RF aerogel template at the nanometer level. The organic frameworks are easily removed by burning under mild conditions (low temperature). The successful preparation and utilization of RF aerogels as templates for the synthesis of mesoporous zeolites will thus make it easier to design nanoporous crystals. Acknowledgment. This work was funded by the Grant-in-Aid for Scientific Research S (15101003) from the Japanese Goverment. Professor Zheng Mianping and Professor Zhang Keren of the Chinese Academy of Geological Sciences are gratefully acknowledged for enlightening discussions. LA047686J (25) Christensen, C. H.; Johannsen, K.; Schmidt, I.; Christensen, C. H. J. Am. Chem. Soc. 2003, 125, 13370. (26) Chen, X.; Kawi, S. Chem. Commun. 2001, 1354.
