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FEATURE ARTICLE Nanocrystalline Zeolites and Zeolite Structures: Synthesis, Characterization, and Applications Sarah C. Larsen* Department of Chemistry and Nanoscience and Nanotechnology Institute, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: June 26, 2007; In Final Form: August 30, 2007
Nanocrystalline zeolites are porous nanomaterials with crystal sizes of less than 100 nm that possess unique external and internal surface reactivity. Nanocrystalline zeolites, such as silicalite, ZSM-5 and Y, were synthesized and extensively characterized by powder X-ray diffraction, nitrogen adsorption isotherms, dynamic light scattering, and electron microscopy. Spectroscopic characterization of the nanocrystalline zeolites by FTIR and solid-state NMR provided detailed structural information about internal and external surface sites. The nanocrystalline zeolites were also used as building blocks to form larger, hollow zeolite structures with encapsulated metal or organic species. The surface properties of nanocrystalline zeolites and hollow zeolite structures were tailored through functionalization of surface silanol groups. Applications of nanocrystalline zeolites and zeolite structures in the selective catalytic reduction of NOx and the photoreduction of Cr(VI) to Cr(III) in aqueous solution were investigated. The unique properties and reactivity of nanocrystalline zeolites and the potential for future applications of these materials will also be discussed.
1. Introduction Zeolites are aluminosilicate molecular sieves with pores of molecular dimensions. Due to the microporosity, zeolites have very high surface areas and have been widely used in applications such as catalysis, ion exchange, and separations.1 Specific zeolites can be identified by chemical composition (Si/Al), pore size, and pore topology. Two common zeolites, ZSM-5 with the MFI structure and Y with the faujasite structure, are shown in Figure 1. Typically, zeolites are industrially manufactured with micron-sized crystals or crystal aggregates. Recently, there have been efforts to synthesize nanocrystalline zeolites, zeolites with crystal sizes of less than 100 nm.2 Several groups have developed synthetic methods for preparing nanocrystalline zeolites,2,3 such as silicalite,4-7 ZSM-5,8-11 Beta,12,13 mordenite,14 and faujasite.15-19 The scaling down of zeolite crystals from the micrometer to the nanometer scale leads to enhanced zeolite properties such as increased surface area and decreased diffusion path lengths. As the crystal size is decreased below 100 nm, the zeolite external surface area, which is distinct from the internal pore surface and is negligible for micron-sized zeolites, increases dramatically, resulting in zeolites with over 25% of the total surface area on the external surface.6 If active sites are incorporated onto the external surface, high surface reactivity results, leading to zeolites with improved catalytic properties. Another advantage of nanocrystalline zeolites is the decreased diffusion path length relative to micron-sized zeolites and the optical transparency of films produced from nanocrytalline zeolites. The improved properties of nanocrystalline zeolites for * To whom correspondence should be addressed.
adsorption and intracrystalline diffusion afford many potential opportunities for their application in environmental catalysis, environmental remediation, decontamination, and drug delivery. In this feature article, the synthesis and characterization of nanocrystalline zeolites (silicalite, ZSM-5, and Y) will be described with emphasis on the physicochemical properties of these porous nanomaterials. Strategies for spectroscopic characterization of the external surface and for tailoring the properties of the external surface through functionalization will be presented. Nanocrystalline zeolites have also been used as porous building blocks for hollow zeolite structures that can encapsulate transition metals and organic functional groups. Applications of nanocrystalline zeolites and zeolite structures for the selective catalytic reduction of NOx and the photoreduction of Cr(VI) in aqueous solution will be described. The unique properties and reactivity of nanocrystalline zeolites and the potential for future applications of these materials in environmental protection and drug delivery will be discussed. 2. Synthesis and Characterization of Nanocrystalline Zeolites a. Synthesis of Nanocrystalline Silicalite, NaZSM-5, and NaY. Zeolites silicalite (purely siliceous form of ZSM-5), ZSM5, and Y were synthesized with discrete crystal sizes of 15-25 nm as described in the literature3,4,8,15,20 and by us.6,9,21,22 The synthetic methods involve a template molecule, such as tetrapropylammonium hydroxide (TPAOH) for silicalite and ZSM-5 and tetramethylammonium hydroxide (TMAOH) for zeolite Y. The template molecule acts as an organic structure-directing agent and can be removed postsynthesis by high-temperature calcination in air or oxygen. The crystallization conditions are
10.1021/jp074980m CCC: $37.00 © 2007 American Chemical Society Published on Web 11/07/2007
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Sarah C. Larsen received her BA degree from Bowdoin College in 1986 and her PhD in Chemistry from Harvard University in 1992. She was awarded a Department of Energy Distinguished Postdoctoral Fellowship (1993-1995) to conduct postdoctoral research in the Department of Chemical Engineering, University of California, Berkeley and the Lawerence Berkeley Laboratory. Professor Larsen joined the faculty of the Department of Chemistry at the University of Iowa in 1995 and is currently an Associate Professor. She received a Faculty Scholar Award (2004-2007) at the University of Iowa, and she is the Associate Director of the Nanoscience and Nanotechnology Institute at the University of Iowa (NNI@UI) which is focused on the environmental and health aspects of nanoscience and nanotechnology. She has research interests in nanocrystalline zeolites and mesoporous materials and their applications in heterogenous catalysis, decontamination, remediation, and drug delivery. She is also interested in the characterization of these materials by magnetic resonance spectroscopy (solid state NMR and EPR) and in calculations using DTF methods.
carefully controlled so that nucleation occurs more readily than crystal growth. This is generally accomplished through lower synthesis temperatures that favor nucleation over crystal growth but also result in slow nucleation and crystal growth and, thus, prolonged synthesis time and low yields. The resulting colloidal solutions of zeolite nanocrystals are separated from the synthesis solution by high-speed centrifugation since conventional filtering methods will not work for these very small nanocrystals. The detailed synthesis conditions including gel composition, synthesis temperature, and synthesis time for the synthesis of nanocrystalline silicalite, NaZSM-5, and NaY are compiled in Table 1. For silicalite, the crystal size was systematically varied from 20 to 39 nm by incrementally increasing the synthesis time with a constant synthesis temperature of 60 °C. For silicalite crystals with sizes ranging from 58 to 74 nm, the temperature was increased to 70 and then 80 °C, as indicated in Table 1. Recycling methods for increasing the yield were implemented as described in the next section. b. Synthetic Method for Increasing the Yield of Nanocrystalline Zeolites. The main strategy of many of the methods for synthesizing nanocrystalline zeolites is to terminate the synthesis process while the zeolite crystals are still in the nanometer size range, thus stopping further crystal growth.2 Typical product yields for nanocrystalline zeolites reported in the literature1-8 and described above are less than 10% based on the synthesis gel composition, as compared to nearly 100% yields for conventional micron-sized zeolites. Nanocrystalline zeolites are present in colloidal suspensions and are recovered from solution by centrifugation with the remaining synthesis solution typically being discarded, resulting in the disposal of valuable chemical materials, such as unused organic template. The method of recycling the synthesis solution after zeolite crystals are recovered was previously reported for micron-sized zeolite synthesis23-27 and has recently been applied to nanocrystalline zeolites.3,28
Figure 1. Two common zeolite structures: (a) Y and (b) ZSM-5.
The synthesis of nanocrystalline zeolites in high yield is enabled by periodically removing nanocrystals from the synthesis solution and reusing the synthesis solution as shown schematically in Figure 2.28 The rationale to reuse the clear solution is that (1) based on the low yield of nanocrystalline zeolite product, only a small portion of nutrients in the synthesis solution are consumed and the clear solution composition is very close to the original synthesis solution composition and (2) only zeolite crystals that are heavy enough are recovered by centrifugation, so that very small zeolite crystals (10 nm or less) are still present in the clear solution. These small crystals serve as nucleation sites that directly grow into larger crystals, eliminating the long nucleation and growth time originally required. By repeatedly reusing the clear synthesis solutions after periodically removing zeolite nanocrystals, significantly higher product yields (6 and 10 fold) were achieved for nanocrystalline silicalite-1 and Y zeolites, respectively.28 For NaY, additional NaOH was added to the recycled synthesis solution. The crystal sizes of zeolites synthesized using recycled synthesis materials show some variation. The size control could potentially be improved by in situ monitoring of crystal size using dynamic light scattering methods, but this has not been implemented yet. The Si/Al ratios of the different batches of Y zeolites do not vary much as shown in Table 2 suggesting that the silicon and aluminum sources are consumed uniformly throughout the synthesis. Using this method, high quality, monodisperse, nanocrystalline zeolites, such as silicalite-1 and Y, can be
18466 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Larsen
TABLE 1: Synthesis Conditions for Nanocrystalline Zeolites (Silicalite, NaZSM-5, and NaY) gel composition
vessel
T(°C)
time (hrs)
crystal sizea, nm
9TPAOH:0.16NaOH:25Si:495H2O: 100EtOH
reflux flask
60
(a) 240, (b) 288, (c) 360
(a) 20, (b) 25, (c) 39
5,6
autoclave
70 80 165
240 96 120
58 74 15
8,9
autoclave
165
120
60
9
95
84
23 (stirring)
15,22
zeolite silicalite
NaZSM-5b
NaYc
9TPAOH:0.16NaOH:Al:25Si: 495H2O:100EtOH 9TPAOH:0.16NaOH:Al:25Si: 300H2O 0.07Na:2.4TMAOH:1.0Al:2.0Si: 132H2O:3.0i-PrOH:8.0EtOH
reflux flask
ref
50 (no stirring) a
Crystal size determined from scanning electron microscope images. Si/Al ) 20. Si/Al ) 1.8. b
c
Figure 2. Flow chart indicating the recycling procedure for the high yield synthesis of nanocrystalline silicalite or NaY.
TABLE 2: Characterization of Multiple Batches of Silicalite and NaY Synthesized Using the Recycling Methoda sample
batch
Si/Al
crystal sizeb (nm)
product yieldc (%)
silicalite-1 silicalite-1 silicalite-1 silicalite-1 silicalite-1 NaY NaY NaY NaY
1 2 3 4 5 1 4 7 10
NA NA NA NA NA 1.87 1.74 1.80 1.83
20 20 20 22 26 26 31 20 25
6.8 13.6 22.4 32.5 41.8 4.0 15.8 28.8 43.3
Data from ref 28. b Crystal size calculated from external surface area as described previously.6 c Cumulative product yield.
Figure 3. XRD patterns and SEM images of two representative silicalite samples with different crystal sizes. Adapted with permission from ref 6. Copyright 2004. American Chemical Society.
synthesized more efficiently. From an environmental perspective, this process is extremely beneficial because the amount of waste produced is minimized by the reuse of starting products. c. Characterization of Nanocrystalline Zeolites. The nanocrystalline zeolites are extensively characterized by powder X-ray diffraction (XRD) to assess crystallinity, inductively coupled plasma/optical emission spectroscopy (ICP/OES) to determine the elemental composition, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to measure crystal size and morphology, and dynamic light scattering (DLS) to measure particle size distribution and nitrogen adsorption by the BET method to determine surface area. Figure 3 shows representative XRD patterns and SEM images of silicalite samples synthesized in our laboratory. The SEM
images clearly show the size and crystal morphology of the synthesized silicalite-1 samples with 20 and 1000 nm crystal sizes. The crystal sizes and size distributions can be obtained by manually measuring crystals (25-50 crystals averaged) in the SEM images and taking the average as being representative of the zeolite particle size. As shown in the SEM images in Figure 3 (top), the nanocrystalline silicalite particles are present as discrete, uniform zeolite nanocrystals. The crystallinity and zeolite identity are verified using powder X-ray diffraction through a comparison of the XRD pattern with zeolite standard patterns. The XRD patterns are used as a fingerprint to confirm the zeolite structure. The linewidths observed in the XRD powder patterns broaden as the crystal size decreases according to Scherer’s equation. The XRD peaks for 20-nm silicalite are
a
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J. Phys. Chem. C, Vol. 111, No. 50, 2007 18467
Figure 4. Distribution of particle sizes for silicalite-1 measured using DLS. The average particle size from DLS is 102 nm. Inset shows an SEM image of the silicalite-1 sample (scale bar ) 500 nm).
TABLE 3: External and Total Surface Areas of Nanocrystalline Zeolites
zeolite
crystal size (nm)a
silicalite6 silicalite6 NaZSM-59 NaY22
20 1000 15 23
Si/Al
external SA (m2/g)b
total SA (m2/g)c
% SA on external surfaced
20 1.8
174 2 208 178
506 343 556 584
34 490 nm) of ZSM-5 tubes: (a) iron-encapsulated ZSM-5 tubes and (b) iron-exchanged ZSM-5 tubes. The experiments were conducted using 5 mg of ZSM-5 tubes, 1.25 mL of sodium dichromate (10-4 M), and 1.25 mL of sodium tartrate (3.28 × 10-2 M). Reproduced with permission from ref 84.
the Cr(VI) was completely reduced after approximately 2.25 h. For iron-exchanged ZSM-5 tubes, the Cr(VI) was completely reduced in 35 min. The iron-exchanged ZSM-5 tubes are more active initially with a rate approximately four times faster than the iron-encapsulated ZSM-5 tubes and this is attributed to the different accessibility of iron in the two samples. Diffusion of Cr(VI) to the interior of the ZSM-5 tubes is required for the iron-encapsulated ZSM-5 tubes, whereas the iron exchanged into
Feature Article the zeolite shell is more accessible in the iron-exchanged ZSM-5 tubes. Rate constants are not published for Cr(VI) reduction on ferritin,83 but the time scale for the photoreduction is qualitatively comparable to what is observed for the iron-containing zeolites. The used zeolite was recovered at the end of the run and reused so that the performance of the material in sequential catalyst tests could be evaluated. A decrease in the iron concentration of the used zeolite from 6.1 to 4.7 wt % indicated that the iron in the iron-exchanged ZSM-5 tubes leached into solution. The iron concentration in the iron-encapsulated ZSM-5 tubes did not undergo leaching and remained constant at 7.9 wt %. These results are consistent with the hypothesis that the iron encapsulated species are located on the interior of the ZSM-5 tube and are thus harder for the Cr(VI) to access but are also less likely to leach out of the zeolite into solution relative to the iron-exchanged ZSM-5 tubes. 6. Summary and Future Directions Nanocrystallline zeolites (Y, ZSM-5, and silicalite-1) with crystal sizes of 50 nm or less have been synthesized with uniform, homogeneous crystal size distributions. An environmentally friendly method for synthesizing nanocrystalline zeolites that includes removing zeolite nanocrystals and recycling the synthesis solution and unused reagants was developed and has led to increased yields after several recycle steps. Extensive physicochemical characterization of the nanocrystalline zeolites has been conducted to determine the unique properties of these porous, zeolitic nanomaterials. Nanocrystalline zeolites have also been used as building blocks for larger hollow zeolite structures. The hollow structures have a zeolite shell such that diffusion to the interior of the zeolite structure is governed by the pore structure of the zeolite and a hollow interior that can be used to encapsulate active species. The nanocrystalline zeolites and zeolite structures were tailored for specific applications through surface functionalization. The nanocrystalline zeolites have significantly increased external surface areas that account for up to 30% of the total zeolite surface area. The external surface sites have been characterized by FTIR and solid-state NMR spectroscopy of the framework atoms and of adsorbed probe molecules. Specific signals in the hydroxyl region of the FTIR spectrum have been assigned to silanol groups and to hydroxyl groups near aluminum sites which are both located on the external zeolite surface. The reactivity of these external surface sites can be investigated using in situ FTIR spectroscopy. Nanocrystalline NaY exhibited enhanced reactivity for SCR of NO2 relative to commercial NaY zeolites with larger crystal sizes. The faster reaction rates and decreased production of undesirable surface species were attributed to the unique properties of nanocrystalline zeolites. Nitrate species formed on the external surface sites of nanocrystalline NaY and under SCR conditions; the external surface nitrate species reacted rapidly. This is the first example in the literature demonstrating enhanced external surface reactivity for nanocrystalline zeolites. There are many other potential catalytic applications for nanocrystalline zeolites. We have also studied the decomposition reactions of chemical warfare agent simulants, such as DMMP (dimethylmethylphosphonate) and 2-CEES (2 chloroethyl sulfide), on nanocrystalline NaY and NaZSM-5.85,86 The external surface sites are implicated in the reactions of DMMP and 2-CEES on nanocrystalline zeolites. Our work is the first to show that the increased external surface area of nanocrystalline zeolites can be utilized as a
J. Phys. Chem. C, Vol. 111, No. 50, 2007 18473 reactive surface with unique active sites for functionalization, catalysis, and adsorption. In the future, we envision nanocrystalline zeolites as new bifunctional catalyst materials with the active sites on the external surface playing an important role in the activity of the material and ideally coupling to internal site reactivity. Acknowledgment. The author acknowledges Professor Vicki Grassian for her important contributions to this work. Many co-workers have contributed to this work including the following: Dr. Weiguo Song, Dr. Gonghu Li, Dr. James Woodworth, Ramasubramanian Kanthasamy, Karna Barquist, Anton Petushkov, Rachelle Justice, and Matthew Johnson. This material is based on work supported by the Environmental Protection Agency through EPA Grant No. R82960001, NSF (CHE0639096), and PRF (44756-AC5). This work was initiated with seed grants from the University of Iowa’s Carver research grant program and Center for Health Effects of Environmental Contamination (CHEEC) at the University of Iowa. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry, and Use; Wiley-Interscience: New York, 1974. (2) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494. (3) Larlus, O.; Mintova, S.; Bein, T. Microporous Mesoporous Mater. 2006, 96, 405. (4) Li, Q.; Mihailova, B.; Creaser, D.; Sterte, J. Microporous Mesoporous Mater. 2000, 40, 53. (5) Li, Q.; Creaser, D.; Sterte, J. Microporous Mesoporous Mater. 1999, 31, 141. (6) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Langmuir 2004, 20, 4696. (7) Schoeman, B. J.; Sterte, J.; Otterstedt, J.-E. Zeolites 1994, 14, 110. (8) Van, Grieken, R.; Sotelo, J. L.; Menendez, J. M.; Melero, J. A. Microporous Mesoporous Mater. 2000, 39, 135. (9) Song, W.; Justice, R. E.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Langmuir 2004, 20, 8301. (10) Reding, G.; Maurer, T.; Kraushaar-Czarnetzki, B. Microporous Mesoporous Mater. 2003, 57, 83. (11) Kim, S. S.; Shah, J.; Pinnavaia, T. J. Chem. Mater. 2003, 15, 1664. (12) Camblor, M. A.; Corma, A.; Mifsud, A.; Perez Pariente, J.; Valencia, S. Synthesis of nanocrystalline zeolite Beta in the absence of alkali metal cations. Prog. Zeolite Microporous Mater., Parts A-C 1997, 105, 341. (13) Camblor, M. A.; Corma, A.; Valencia, S. Microporous Mesoporous Mater. 1998, 25, 59. (14) Hincapie, B. O.; L. J., G.; Zhang, Q.; Sacco, A.; Suib, S. L. Microporous Mesoporous Mater. 2004, 67, 19. (15) Li, Q. H.; Creaser, D.; Sterte, J. Chem. Mater. 2002, 14, 1319. (16) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem.-Int. Ed. 1999, 38, 3201. (17) Zhu, G.; Qui, S.; Yu, J.; Sakamoto, Y.; Xiao, F.; Xu, R.; Terasaki, O. Chem. Mater. 1998, 10, 1483. (18) Holmberg, B. A.; Wang, H.; Norbeck, J. M.; Yan, Y. Microporous Mesoporous Mater. 2003, 59, 13. (19) Castagnola, N. B.; Dutta, P. K. J. Phys. Chem. B 1998, 102, 1696. (20) Morales-Pacheco, P.; Alvarez-Ramirez, F.; Angel, P. D.; Bucio, L.; Dominguez, J. M. J. Phys. Chem. C 2007, 111, 2368. (21) Song, W.; Kanthasamy, R.; Grassian, V. H.; Larsen, S. C. Chem. Commun. 2004, 1920. (22) Song, W.; Li, G.; Grassian, V. H.; Larsen, S. C. EnViron. Sci. Technol. 2005, 39, 1214. (23) Eapen, M. J.; Reddy, K. S. N.; Shiralkar, V. P. Zeolites 1994, 14, 295. (24) Fitoussi, M.; Korngold, A. Process for production of ZSM-5 zeolites using large amount of recycled templating agent. European Patent No. 602450; Super Industry Ltd.: Israel, 1995; p 7. (25) Muller, U.; Voss, H.; Schubert, E.; Hill, F.; Petersen, H. Production of a zeolite-containing solid. U.S. Patent 2004014591A1; BASF Aktiengesellschaft: U.S., 2004.
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