pubs.acs.org/Langmuir © 2010 American Chemical Society
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Optical Investigation of the Intergrowth Structure and Accessibility of Brønsted Acid Sites in Etched SSZ-13 Zeolite Crystals by Confocal Fluorescence Microscopy† Linn Sommer,‡,§ Stian Svelle,‡ Karl Petter Lillerud,‡ Michael St€ocker, Bert M. Weckhuysen,§ and Unni Olsbye*,‡
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‡ Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway, §Inorganic Chemistry and Catalysis, Debye Institute for NanoMaterials Science, Utrecht University, 3584 CA Utrecht, The Netherlands, and SINTEF Materials and Chemistry, Department of Hydrocarbon Process Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway
Received April 13, 2010. Revised Manuscript Received May 8, 2010 Template decomposition followed by confocal fluorescence microscopy reveals a tetragonal-pyramidal intergrowth of subunits in micrometer-sized nearly cubic SSZ-13 zeolite crystals. In order to accentuate intergrowth boundaries and defect-rich areas within the individual large zeolite crystals, a treatment with an etching NaOH solution is applied. The defective areas are visualized by monitoring the spatial distribution of fluorescent tracer molecules within the individual SSZ-13 crystals by confocal fluorescence microscopy. These fluorescent tracer molecules are formed at the inner and outer crystal surfaces by utilizing the catalytic activity of the zeolite in the oligomerization reaction of styrene derivatives. This approach reveals various types of etching patterns that are an indication for the defectiveness of the studied crystals. We can show that specially one type of crystals, denoted as core-shell type, is highly accessible to the styrene molecules after etching. Despite the large crystal dimensions, the whole core-shell type SSZ-13 crystal is utilized for catalytic reaction. Furthermore, the confocal fluorescence microscopy measurements indicate a nonuniform distribution of the catalytically important Brønsted acid sites underlining the importance of space-resolved measurements.
Introduction For metal-based catalysts, the influence of defects on their catalytic properties is well documented and has been subject of systematic studies throughout the past four decades.1-5 For zeolites, the situation is quite different. Even though transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and infrared spectroscopy (IR) studies, for example, have demonstrated defects in crystal growth, catalytic sites, and outer surface structures of zeolite and zeotype crystals used in catalytic studies, less attention has been given to † Part of the Molecular Surface Chemistry and Its Applications special issue. *To whom correspondence should be addressed. E-mail: unni.olsbye@ kjemi.uio.no. Telephone: þ47-22855456. Fax: þ47-22855441.
(1) Somorjai, G. A. Chem. Rev. 1996, 96(4), 1223–1235. (2) Somorjai, G. A. Introduction to surface chemistry and catalysis; Wiley: New York, 1994; p 667. (3) Zaera, F.; Somorjai, G. A. Langmuir 1986, 2(5), 686–688. (4) Nørskov, J. K.; Bligaard, T.; Hvolbaek, B.; Abild-Pedersen, F.; Chorkendorff, I.; Christensen, C. H. Chem. Soc. Rev. 2008, 37(10), 2163–2171. (5) Ferrer, D.; Blom, D. A.; Allard, L. F.; Mejia, S.; Perez-Tijerina, E.; JoseYacaman, M. J. Mater. Chem. 2008, 18(21), 2442–2446. (6) Anderson, M. W.; Agger, J. R.; Meza, L. I.; Chong, C. B.; Cundy, C. S. Faraday Discuss. 2007, 136, 143–156. (7) Anderson, M. W.; Pachis, K. S.; Prebin, F.; Carr, S. W.; Terasaki, O.; Ohsuna, T.; Alfreddson, V. J. Chem. Soc., Chem. Commun. 1991, 23, 1660–1664. (8) Yonkeu, A. L.; Buschmann, V.; Miehe, G.; Fuess, H.; Goossens, A. M.; Martens, J. A. Cryst. Eng. 2001, 4(2-3), 253–267. (9) Gonzalez, G.; Stracke, W.; Lopez, Z.; Keller, U.; Ricker, A.; Reichelt, R. Microsc. Microanal. 2004, 10(2), 224–235. (10) Sun, J.; Zhu, G. S.; Chen, Y. L.; Li, J. X.; Wang, L. F.; Peng, Y.; Li, H.; Qiu, S. L. Microporous Mesoporous Mater. 2007, 102(1-3), 242–248. (11) Fickel, D. W.; Shough, A. M.; Doren, D. J.; Lobo, R. F. Microporous Mesoporous Mater. 2010, 129(1-2), 156–163. (12) Bordiga, S.; Roggero, I.; Ugliengo, P.; Zecchina, A.; Bolis, V.; Artioli, G.; Buzzoni, R.; Marra, G.; Rivetti, F.; Spano, G.; Lamberti, C. J. Chem. Soc., Dalton Trans. 2000, 21, 3921–3929.
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studies on how these defects influence the catalytic properties of those important catalytic materials.6-17 This is not least due to the difficulty of rationally synthesizing batches of microporous materials with only one parameter variation. However, the understanding of the internal structures of zeolite crystallites and the nature and accessibility of the catalytically active sites is crucial in order to design optimized zeolitic catalyst materials. During recent years, the focus of research activities has been drawn more and more toward the observation of molecular processes occurring on the subcrystal level.18-20 Often, it is taken for granted that nicely grown large crystals are perfectly ordered, defect-free single crystals.21 However, especially in the context of diffusion studies in large crystals, the complex morphologies, intergrowth structures, and defects in zeolites have been discussed.22-24 (13) Bordiga, S.; Ugliengo, P.; Damin, A.; Lamberti, C.; Spoto, G.; Zecchina, A.; Spano, G.; Buzzoni, R.; Dalloro, L.; Rivetti, F. Top. Catal. 2001, 15(1), 43–52. (14) Pascale, F.; Ugliengo, P.; Civalleri, B.; Orlando, R.; D’Arco, P.; Dovesi, R. J. Chem. Phys. 2002, 117(11), 5337–5346. (15) Sokol, A. A.; Catlow, C. R. A.; Garces, J. M.; Kuperman, A. J. Phys. Chem. B 2002, 106(24), 6163–6177. (16) Weidenthaler, C.; Fischer, R. X.; Shannon, R. D.; Medenbach, O. J. Phys. Chem. 1994, 98(48), 12687–12694. (17) Brabec, L.; Kocirik, M. Mater. Chem. Phys. 2007, 102(1), 67–74. (18) Weckhuysen, B. M. Angew. Chem., Int. Ed. 2009, 48(27), 4910–4943. (19) Schoonheydt, R. A. Angew. Chem., Int. Ed. 2008, 47(48), 9188–9191. (20) Roeffaers, M. B. J.; Hofkens, J.; De Cremer, G.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Sels, B. F. Catal. Today 2007, 126(1-2), 44–53. (21) Seebacher, C.; Rau, J.; Deeg, F. W.; Br€auchle, C.; Altmaier, S.; J€ager, R.; Behrens, P. Adv. Mater. 2001, 13(18), 1374–1377. (22) Lehmann, E.; Chmelik, C.; Scheidt, H.; Vasenkov, S.; Staudte, B.; K€arger, J.; Kremer, F.; Zadrozna, G.; Kornatowski, J. J. Am. Chem. Soc. 2002, 124(29), 8690–8692. (23) Geier, O.; Vasenkov, S.; Lehmann, E.; K€arger, J.; Schemmert, U.; Rakoczy, R. A.; Weitkamp, J. J. Phys. Chem. B 2001, 105(42), 10217–10222. (24) Tzoulaki, D.; Heinke, L.; Schmidt, W.; Wilczok, U.; K€arger, J. Angew. Chem., Int. Ed. 2008, 47(21), 3954–3957.
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Recent developments in the field of microspectroscopic imaging allow the detailed investigation of structural subunits in zeolite catalysts, and the spatial distribution of guest molecules within individual crystallites can be observed.18,20,22,23,25-28 The intergrowth structures and defects in several zeolitic materials have been unraveled by techniques, such as fluorescence microscopy and interference microscopy,22,23,29 representing much more powerful tools compared to traditional techniques. Karwacki et al.29 introduced a new approach for exploring intergrowth structures of zeolite crystals by fluorescence microscopy. Briefly, this method utilizes the presence of organic template molecules in the as-synthesized zeolite crystals. When template molecules are removed by calcination, fluorescent intermediates are formed in the heating process. The spatial distribution of these compounds throughout individual crystals can be visualized by confocal fluorescence microscopy, as the emanating fluorescence patterns provide 3-D information about the structural subunits.29 Furthermore, Kox et al.26,30 and Stavitski et al.25,31 demonstrated how the catalytic activity can be mapped by creating fluorescent tracer molecules inside the pore system of the zeolite via an acid catalyzed reaction. In their studies, the fluorescent species were formed by the styrene oligomerization reaction in large ZSM-5 crystals. Also other reactions that create stable carbocations inside the zeolite pores can be used for imaging catalytic activity. For example, Roeffaers and coworkers20,28 followed the dehydration of 1,3-diphenyl-1,3-propanediol and the oligomerization of furfuryl alcohol inside a mordenite crystal by fluorescence microscopy, while Kox and co-workers32,33 have investigated the cracking and oligomerization of thiophene derivatives in large ZSM-5 crystals. The detailed and idealized intergrowth models reported so far in literature were developed from large zeolite crystals with sizes from 10 up to 200 μm.20,29,34 Those crystals showed a rather homogeneous and flawless appearance, and according to Roeffaers et al.34 about 90% of the investigated crystals behaved as described, making the samples ideal model systems. However, hydrothermal zeolite synthesis often yields batches of crystals that show some variations in size, morphology, and defectiveness. Observed bulk properties and reactivities observed in conventional catalytic tests arise from this rather heterogeneous mixture of crystals. In addition, it has been shown that the developed idealized intergrowth models for ZSM-5 vary within the batch of (25) Stavitski, E.; Kox, M. H. F.; Weckhuysen, B. M. Chem.;Eur. J. 2007, 13(25), 7057–7065. (26) Kox, M. H. F.; Stavitski, E.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2007, 46(20), 3652–3655. (27) Roeffaers, M. B. J.; Ameloot, R.; Baruah, M.; Uji-i, H.; Bulut, M.; De Cremer, G.; M€uller, U.; Jacobs, P. A.; Hofkens, J.; Sels, B. F.; De Vos, D. E. J. Am. Chem. Soc. 2008, 130(17), 5763–5772. (28) Roeffaers, M. B. J.; Ameloot, R.; Bons, A. J.; Mortier, W.; De Cremer, G.; de Kloe, R.; Hofkens, J.; De Vos, D. E.; Sels, B. F. J. Am. Chem. Soc. 2008, 130(41), 13516–13517. (29) Karwacki, L.; Stavitski, E.; Kox, M. H. F.; Kornatowski, J.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2007, 46(38), 7228–7231. (30) Kox, M. H. F.; Stavitski, E.; Groen, J. C.; Perez-Ramirez, J.; Kapteijn, F.; Weckhuysen, B. M. Chem.;Eur. J. 2008, 14(6), 1718–1725. (31) Stavitski, E.; Kox, M. H. F.; Swart, I.; de Groot, F. M. F.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2008, 47(19), 3543–3547. (32) Kox, M. H. F.; Domke, K. F.; Day, J. P. R.; Rago, G.; Stavitski, E.; Bonn, M.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2009, 48(47), 8990–8994. (33) Kox, M. H. F.; Mijovilovich, A.; S€attler, J. J. H. B.; Stavitski, E.; Weckhuysen, B. M. ChemCatChem 2010, published online April 16, http://dx.doi. org/10.1002/cctc.200900329. (34) Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; Blanpain, B.; L’ho€est, P.; Jacobs, P. A.; De Schryver, F. C.; Hofkens, J.; De Vos, D. E. Angew. Chem., Int. Ed. 2007, 46(10), 1706–1709. (35) Karwacki, L.; Kox, M. H. F.; de Winter, D. A. M.; Drury, M. R.; Meeldijk, J. D.; Stavitski, E.; Schmidt, W.; Mertens, M.; Cubillas, P.; John, N.; Chan, A.; Kahn, N.; Bare, S. R.; Anderson, M.; Kornatowski, J.; Weckhuysen, B. M. Nat. Mater. 2009, 8(12), 959–965.
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material used.27,35 Therefore, Schoonheydt19 points out that these models are oversimplifications and that every ZSM-5 crystal might be unique. On this base, it is important to investigate not only the spatial heterogeneity within one individual crystal but also the heterogeneity within a batch of crystals, especially when less homogeneous samples are studied. In the present study, we have investigated a zeolite SSZ-13 sample with CHA topology that is rather heterogeneous in crystal size distribution, defectiveness, and intergrowth features. We have chosen the approach developed by the Weckhuysen group, namely, to study individual zeolite crystals in a batch by the template removal technique introduced by Karwacki et al.29 The result is compared to the idealized tetragonal-pyramidal intergrowth model that was revealed for the SAPO analogue of zeolite SSZ-13, SAPO-34.29 In addition to the investigation of assynthesized crystals, as previously published for MFI, AFI, and CHA topologies,29 we subjected the batch to postsynthesis treatment. In order to probe the defectiveness of the crystals, the calcined H-SSZ-13 sample was treated with a low concentration NaOH solution. Thereby, preferably defect-rich areas of the crystals were attacked.36,37 Subsequently, these defective areas could partially be visualized by applying the styrene reaction as developed by Kox et al. and Stavitski et al. to the etched H-SSZ-13 crystals.25,26 The treatment affects various crystals to a different extent, which probably is correlated to crystal defects in the parent crystals. The accessibility of the intracrystalline zeolitic pore system is not solely dependent on the crystal size. Interestingly, herein we demonstrate that also relatively large zeolite crystals might be utilized in their full volume for catalysis, depending on their intergrowth structure and the applied etching procedure.
Experimental Section Postsynthesis Modifications. The as-synthesized SSZ-13 material was provided in house. The synthesis procedure is based on ref 38. The molar composition of the gel was 100SiO2/ 26Al2O352Na2O/10TMAda-OH/2240H2O (with TMAda-OH/ N,N,N-trimethyl-1-adamantammonium hydroxide). The template was removed by calcination in oxygen at 450 °C overnight followed by 550 °C for 24 h. Protonated H-SSZ-13 was obtained by 3-fold ion exchange with 1 M NH4NO3 (20 mL/g) for 2 h at 75 °C; the samples were washed with deionized water and calcined in air at 550 °C for 2 h. A ramp of 5 °C/min was used. In order to reveal defective areas and eventually to introduce some mesoporosity, the H-SSZ-13 crystals were washed with 0.05 M NaOH. The alkaline treatment was carried out at 75 °C for 1 h using 20 mL of alkaline solution per gram of zeolite. Afterward, the crystals were washed with water until pH neutral and dried overnight at 100 °C. In order to obtain the protonated zeolite, the ion exchange and calcination procedures were applied again as described above. Confocal Fluorescence Microscopy. The fluorescence microscopy studies were performed on a Nikon Eclipse LV150 upright microscope with a 100, 0.7 NA dry objective lens. For fluorescence microphotographs, 510-560 nm light from a mercury source was used. The confocal images were collected with a Nikon D-Eclipse C1 head connected to the laser light sources (405, 488, and 561 nm). A photomultiplier tube in the range of 575-635 nm was used to detect the emission. The reaction was performed in an in situ cell (FTIR 600, Linkam Scientific Instruments) equipped with a Linkam TMS 93 temperature controller. (36) Bjørgen, M.; Joensen, F.; Holm, M. S.; Olsbye, U.; Lillerud, K. P.; Svelle, S. Appl. Catal., A 2008, 345(1), 43–50. (37) Holm, M. S.; Svelle, S.; Joensen, F.; Beato, P.; Christensen, C. H.; Bordiga, S.; Bjørgen, M. Appl. Catal., A 2009, 356(1), 23–30. (38) Eilertsen, E. A.; Nilsen, M. H.; Wendelbo, R.; Olsbye, U.; Lillerud, K. P. Stud. Surf. Sci. Catal. 2008, 174A, 265–268.
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The template decomposition in order to reveal the intergrowth structure was carried out as follows: The as-synthesized SSZ-13 crystals were heated to 600 °C with a heating rate of 10 °C/min. An oxygen flow of 50 mL/min was applied. The decomposition reactions were quenched as soon as 600 °C was reached, and subsequent confocal scans of the crystals were performed at room temperature. The styrene reaction was performed in the same cell as the template decomposition. Styrene derivatives (Arcos Organics and Aldrich) were used as received. The catalyst crystals were heated to reaction temperature (200 °C), and liquid styrene compounds were added. After 5 min, the samples were cooled to room temperature and confocal scans of the crystals were performed. Some of the crystals, marked with an asterisk (/), were irradiated with a neutron flux of 1013 n/cm-2 s-1 at the facilities of IFE, Kjeller, Norway prior to the study. This was done in context with a different study that has not come to publication yet due to a patent application.39 The effect of the irradiation is not considered as relevant for the description of the H-SSZ-13 intergrowth structure and further results presented in this study. All features described were also found for the nonirradiated material. However, since some of the clearest and most informative fluorescence pictures were obtained from the irradiated sample, these results are included in this paper. In order to keep the paper concise, only few exemplary crystals are shown. A broader overview is provided in the Supporting Information, Part I. Computational Details. The proton affinities (PAs) of the four probe molecules employed for confocal microscopy were determined by quantum chemical calculations performed with the Gaussian 03 program.40 Accurate thermochemical data were calculated using the composite G3MP2 method.41 The G3MP2 method yields an average absolute deviation of 5.4 kJ/mol for the 299 energies from the G2/97 test set and 4.3 kJ/mol for the subset of eight proton affinities.
Figure 1. Representative SEM overview picture of the investigated zeolite SSZ-13 sample.
Results and Discussion Template Removal Process Reveals SSZ-13 Intergrowth Structure. In bright-field illumination and scanning electron microscopy (SEM) images, most of the investigated SSZ-13 crystals appear as nearly cubic crystals with a particle size distribution between 5 and 20 μm. The SEM image shown in Figure 1 gives a representative overview picture of the sample under study. In order to visualize structural subunits within an individual SSZ-13 crystal, the generation of fluorescent marker molecules throughout the whole crystal is necessary. This was achieved by heating the as-synthesized material to 600 °C in pure oxygen and thereby initializing the decomposition of the organic template. Before heating, no fluorescence signal was obtained from the as-synthesized material. Weak fluorescence was observed when the temperature exceeded 400 °C due to the starting (39) Sommer, L.; Svelle, S.; Olsbye, U.; Lillerud, K. P. U.K. Patent Application No. 1005916.0, 2010. (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2004. (41) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1999, 110(10), 4703–4709.
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Figure 2. (A) Representative optical micrographs before and after template removal. (B) Confocal fluorescence images of the top, the horizontal middle plane, and a horizontal plane closer to the bottom of a crystal after template decomposition and a schematic indication about where the slices are taken from.
decomposition of the template molecules. When the final temperature of 600 °C was reached, the crystal exhibited strong fluorescence due to decomposed template in the zeolite voids. The heating process led to darkening of the crystals due to coke formation, as also reported by Karwacki et al.29 (Figure 2A). In order to examine the spatial distribution of the fluorescent species within the zeolite crystals, the sample was cooled down to room temperature and a confocal fluorescence scan of individual crystals was performed. Figure 2B shows optical slices taken from the top, the horizontal middle plane, and a horizontal plane closer to the bottom of a crystal with dimensions of 19 19 19 μm3. It is schematically indicated from which part of the crystal the slices are taken. Because of the optical properties of the material, the fluorescence signal becomes weaker and the structural features are less resolved when taking confocal slides from the deeper parts of the SSZ-13 crystal. Although the crystals appeared nearly cubic in the overview pictures (see Figure 1) and the surface of the investigated crystal appears flat under the optical microscope, the fluorescence image corresponding to the crystal surface reveals that the surface is not perfectly flat grown. The crystal rather seems to have a tapered surface, which would be consistent with Langmuir 2010, 26(21), 16510–16516
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the development of the (-110), (-10-1), (-101), and (-1-10) facets. The crystal morphology depends on the growth rate of the faces, which can be imitated with a simple model in the Materials Studio program (further explanations are given in the Supporting Information, Part I: Different growth faces and the resulting morphology in the CHA topology). However, we see that the morphology of a SSZ-13 crystal that obviously contains intergrowth units can be imitated by applying the model of a defectfree single crystal. This underlines the difficulty of distinguishing intergrowth crystals from real single crystals, and the results of the model can be questioned. Hence, these results confirm the importance of confocal fluorescence measurements for the investigation of intergrowth structures in zeolites. The confocal image obtained from the horizontal middle plane reveals the same tetragonal-pyramidal shaped subunits as found for SAPO-34, which also possesses CHA topology.29 However, in contrast to the SAPO-34 crystals described by Karwacki et al.,29 the SSZ-13 crystals used in this work are less regular grown than the SAPO-34 crystals and the tetragonal-pyramidal subunits are less symmetrical. Furthermore, a shell giving high fluorescence intensity is very pronounced in SSZ-13 crystals. Between this shell and the inner part of the crystal, an intensity gap is observed, indicating an inhomogeneous distribution of the fluorescent species. Within the as-synthesized material, an even distribution of the template molecules can be assumed. Hence, it must be easier to remove template molecules from the darker spots in the pattern, probably due to differences in diffusivity in the different crystal units. Probing the Pore Accessibility of Etched SSZ-13 Crystals with the Styrene Oligomerization Reaction. The treatment of zeolites with alkaline solution is a well-known technique that often is applied in order to obtain combined micro- and mesoporosity in zeolites.36,42,43 Additional mesoporosity is considered to have a favorable effect on the surface area and diffusion properties of those materials. The alkaline solution acts as etching agent and dissolves selectively silicon from the framework. However, when applying low concentrated etching solutions, this will not necessarily lead to extended mesopore formation or structural damage, but it will rather reveal defect-rich areas in the etched material as the etching solution will attack weakly crystalline areas first. Overall, etching is a widely used method in material sciences, when defective materials are investigated.17,24,44,45 Subsequent to etching, the sample is exposed to fluorogenic molecules. The micropore system of zeolites with CHA topology is accessible through 8-ring windows that are too narrow to be entered by styrene derivatives. Thus, defective areas can partially be visualized with the help of fluorescent marker molecules that are created by styrene oligomerization reaction at catalytically active sites located in these areas. This concept has been demonstrated already for mesoporous H-SSZ-13.46 In order to investigate the defectiveness of the H-SSZ-13 crystals in this study, the calcined H-SSZ-13 crystals were etched with a 0.05N NaOH solution for 1 h at 75 °C. Subsequently, the etched crystals were exposed to various styrene derivatives at 200 °C. When 4-methoxystyrene was applied, the sample turned (42) Le Van Mao, R.; Le, S. T.; Ohayon, D.; Caillibot, F.; Gelebart, L.; Denes, G. Zeolites 1997, 19, 270–278. (43) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Perez-Ramirez, J. Microporous Mesoporous Mater. 2004, 69(1-2), 29–34. (44) Wloch, J. Microporous Mesoporous Mater. 2003, 62(1-2), 81–86. (45) Kortunov, P.; Vasenkov, S.; Chmelik, C.; K€arger, J.; Ruthven, D. M.; Wloch, J. Chem. Mater. 2004, 16(18), 3552–3558. (46) Sommer, L.; Mores, D.; Svelle, S.; St€ocker, M.; Weckhuysen, B. M.; Olsbye, U., Microporous Mesoporous Mater. 2010, published online March 23, http://dx.doi.org/10.1016/j.micromeso.2010.03.017.
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Figure 3. Overview optical micrograph (A) and wide-field fluorescence micrograph (B) after the oligomerization of 4-methoxystyrene over etched H-SSZ-13 crystals.
pink due to the oligomerization reaction. However, closer examination of the crystals under an optical microscope revealed that only some of the SSZ-13 crystals developed a pink color, whereas others remained colorless (Figure 3A, overview picture, optical microscope bright-field illumination). Thus, the coloring after styrene oligomerization reveals heterogeneity of the reactivity within the sample batch. Figure 3B shows a second overview picture taken after styrene oligomerization, this wide-field fluorescence micrograph demonstrates the difference in fluorescence intensity emanating from the different SSZ-13 crystals. Some do not show fluorescence at all, indicating that they are free of defects and therefore completely unreactive in styrene oligomerization. Others show intense fluorescence, indicating a high catalytic activity for the styrene oligomerization reaction. Variety in Fluorescence Patterns: Difference in the Amount of Defects in the Parent Crystals. Also among the reactive SSZ-13 crystals, differences in fluorescence intensity are found in the wide-field fluorescence micrograph. Therefore, confocal scans of individual crystals were performed, providing 3-D information about the accessibility of active sites located in the defective areas in high spatial resolution. Five types of SSZ-13 crystals can be distinguished due to their distinct fluorescence patterns. Figure 4 shows representatively one crystal for each type. The crystal dimensions are about 7 7 7 μm3 for the crystals I, III-V and 15 15 15 μm3 for crystal II. The first type of crystals appears as “perfect” because they are not attackable. This indicates a low defectiveness of the crystals, but it DOI: 10.1021/la101454v
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Figure 5. Confocal fluorescence patterns corresponding to the middle horizontal plane of etched crystals after the oligomerization of 4-chlorostyrene (A, B) and 4-fluorostyrene (C*, D*) over etched H-SSZ-13 crystals. (A) and (C*) were measured under identical conditions as the crystals shown in Figure 4. In (B) and (D*), the intensity was boosted in order to highlight details. Figure 4. Confocal fluorescence patterns of crystals with different degrees of defectiveness: (I) nearly unaffected by etching, (II) cross, (III) cloudy, (IV) core-shell, and (V) nonetched crystal for comparison. Crystals marked with an asterisk (/) were subjected to neutron irradiation prior to this study.
can not unequivocally be related to a lack of intergrowth. The second type shows fluorescence intensity at the surface and a crosslike pattern in the middle of the crystal. Here, the etching solution penetrated the crystals along the cross-like intergrowth boundaries which also were revealed by the template removal method. The pyramidal intergrowth units are not attacked, as no fluorescence signal emanates from that area. A third type of SSZ13 crystals which is even more affected by the etching procedure shows a cloudy, nondirected fluorescence pattern without any symmetrical elements or boundaries. For the fourth type of crystals, fluorescence is observed from all crystal areas, meaning that the amount of defects is so high that etching affects even the most interior parts of the crystals. Compared to the third type, the etching pattern is highly symmetrical, reflecting the crosslike intergrowth structure. This type of SSZ-13 crystals is further denoted as core-shell type. It is notable that the most defective crystal type has the highest catalytic activity. As the fluorescence signal depends on the protonation of the 4-methoxystyrene molecules, it is clear that many active sites must be located in the defective areas, also after etching. Hence, the etching leads to full utilization of the large crystal. Furthermore, the fluorescence pattern of the core-shell type crystal suggests an uneven distribution of acid sites, and thus a varying Si/Al ratio within the crystal. The core shows significantly higher fluorescence intensity compared to the shell, indicating a depletion of Al in the shell and a higher acid site density in the core. It is unclear whether this is an effect of the etching treatment or an inherent material property. The fifth type, representative of the nonetched crystals, is shown 16514 DOI: 10.1021/la101454v
Table 1. G3MP2 Proton Affinities (25 °C) in kJ/mol compound
PA G3MP2 (kJ/mol)
4-chlorostyrene 4-fluorostyrene 4-methoxystyrene 4-ethoxystyrene
837 839 899 906
for comparison. It is notable that this variety of etching patterns and defectiveness is found within one batch of H-SSZ-13 crystals. These results strongly indicate that observed bulk properties only poorly reflect the heterogeneity of the catalyst batch synthesized. Reactivity of Other Styrene Derivatives. All fluorescence patterns described so far were obtained after the oligomerization reaction of 4-methoxystyrene. In addition to 4-methoxystyrene, also other styrene derivatives were applied. The oligomerization of 4-ethoxystyrene resulted in a comparable fluorescence intensity as was found for 4-methoxystyrene; for some crystals, the intensity was somewhat higher (shown in the Supporting Information, Part II). However, after the oligomerization of 4-chlorostyrene and 4-fluorostyrene, much weaker fluorescence patterns are observed. More precisely, when measured under the same conditions as the 4-methoxystyrene samples, the only fluorescence that could be observed emanated from core-shell type crystals in the etched samples (Figure 5). The crystal dimensions are about 7 7 7 μm3 for all SSZ-13 crystals shown. The observed fluorescence intensities for the different styrene derivatives can be rationalized when their proton affinities are taken into account. The proton affinities of the four styrene derivatives at 25 °C, as calculated using the G3MP2 composite method, are listed in Table 1. The compounds fall into two groups. The calculations predict similar PAs for 4-chlorostyrene and Langmuir 2010, 26(21), 16510–16516
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4-fluorostyrene, and similar, but substantially higher, PAs for 4-methoxystyrene and 4-ethoxystyrene. Given that the PA is an indicator for the reactivity, the calculations conform well to the experimentally observed differences in fluorescence intensity. In contrast to 4-methoxystyrene and 4-ethoxystyrene, 4-chlorostyrene and 4-fluorostyrene form less stabilized cations under protonation, resulting in a lower reactivity. Generally, the observed fluorescence intensities are related to the acid strength, the density, and the accessibility of catalytically active sites within the individual crystals. The finding that observed fluorescence after the acid-catalyzed 4-chlorostyrene and 4-fluorostyrene oligomerization emanated from the coreshell type crystals suggests that crystals with this fluorescence pattern have a higher ability to convert the less reactive 4-chlorostyrene and 4-fluorostyrene compared to other crystal types within the investigated H-SSZ-13 batch. It is remarkable that the most defective crystals are found to be the most reactive crystals for the styrene oligomerization reaction. Possibly, the core-shell type crystals have a higher acid strength than other crystal types, leading to this difference in observed fluorescence intensity. Another explanation would be a higher density of accessible active sites. In that case, more fluorescent molecules are generated, leading to higher observed fluorescence intensity. This investigation discusses the variation in reactivity of the styrene derivatives over one specific zeolite sample. However, reversibly, it can also be suggested to investigate a constant set of styrene derivatives with known PAs under identical experimental conditions over various zeolites. Such a study might contribute to solve the problem of comparing the acid strength among different zeolitic materials. Relevance of the Results to Heterogeneous Catalysis. The importance of studying catalytic systems under as realistic conditions as possible is frequently pointed out.18-20,27-29,47,48 Hydrothermal synthesis of zeolites gives in most cases a particle size distribution instead of a perfectly uniform particle size. Furthermore, also a certain variety in the crystal morphology can be expected. As shown in this work, the heterogeneity within one zeolite batch can influence the catalytic activity as the bulk observation of styrene oligomerization emanates from only a minor crystal fraction, whereas the majority of the crystals do not take part in the reaction. Hence, bulk information and bulk properties can be misleading. This should be taken into account for the interpretation of bulk analyses. The observation of fluorescence signal reflects two parameters: First, the accessibility of the crystal for styrene molecules is probed. The accessibility depends on voids that are large enough for the styrene molecules to enter, that is, pores larger than the 8-ring window apertures. In this case, voids have to be created by postsynthesis etching, and thus, the defectiveness of the crystal is probed. Second, also the distribution of catalytically active sites within the accessible area is reflected by the intensity pattern. The intensity patterns suggest inhomogeneous acid site distribution (Figures 4 and 5). A study of the methanol-to-olefin (MTO) reaction over SAPO-34, a SAPO analogue of SSZ-13, indicates that the reaction mainly takes place in the outer cages of the crystals when micrometer sized crystals are applied.49 However, our results show that the whole crystal can be made accessible for reactants, depending on the defects in the parent material. If the synthesis could be triggered toward the core-shell type of (47) Somorjai, G. A.; Park, J. Y. Angew. Chem., Int. Ed. 2008, 47(48), 9212– 9228. (48) Somorjai, G. A. J. Phys. Chem. B 2000, 104(14), 2969–2979. (49) Hereijgers, B. P. C.; Bleken, F.; Nilsen, M. H.; Svelle, S.; Lillerud, K. P.; Bjørgen, M.; Weckhuysen, B. M.; Olsbye, U. J. Catal. 2009, 264(1), 77–87.
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crystals, also relatively large crystals might be utilized for catalysis in their full volume. Hence, depending on the field of application, a certain degree of defects in the crystals can be advantageous. This could present an alternative approach to the synthesis of hierarchical zeolites in which meso- or macroporosity is introduced in an ordered way, for example, by using special templates, in order to make the core of zeolite crystallites easily accessible to reactants.50-55 The crystals investigated in this study are large compared to the submicrometer sized crystals that preferably are applied in catalysis. Now, the question arises whether our findings are transferable to submicrometer sized crystals. In powders, agglomeration of small crystals often is observed, and thus, also here certain heterogeneity can be expected. Furthermore, the crosslike intergrowth pattern that indicates tetragonal-pyramidal shaped subunits in SSZ-13 was observed for 19 19 19 μm3 crystals as well as in 7 7 7 μm3 crystals. As the intergrowth units develop during the whole growth process, intergrowth can be expect in even smaller crystals, although no experimental evidence has been provided yet. According to classical growth models, large crystals grow at the expense of small and defect -rich particles. Hence, the variation in defectiveness might be even higher in small crystals as the growth process is stopped at an earlier stage.
Conclusions For large SSZ-13 zeolite crystals, the same tetragonal-pyramidal intergrowth subunits were found as for large SAPO-34 crystals. H-SSZ-13 crystals etched with diluted alkaline solution were applied in the acid-catalyzed styrene oligomerization reaction. Subsequent confocal fluorescence microscopy measurements revealed a remarkable variety in the amount of defects among different crystals within one SSZ-13 batch, as some crystals were highly active in styrene oligomerization, whereas others did not take part in the reaction. This heterogeneity of defectiveness and reactivity within one batch of catalyst crystals should be taken into account when bulk properties are observed. The crystals in the studied sample could be divided into five types, depending on the observed fluorescence patterns. Here, especially the core-shell type of crystals, being the most defective ones, showed high activity throughout the whole crystal volume. Thus, if the synthesis would focus on this type of crystals, relatively large crystals with high activity throughout the whole crystal might be obtained. Inhomogeneities in fluorescence patterns indicate a variation in acid site density within individual crystals. The reactivities of various styrene derivatives, as reflected in the fluorescence intensity, could be rationalized based on their proton affinities. Acknowledgment. This work was financially supported by the Norwegian Research Council and the KOSK II program. Thanks are due to the Research Council of Norway for the grant of computer time through the NOTUR project (account NN4683K). (50) Jinka, K. M.; Bajaj, H. C.; Jasra, R. V.; Prasetyanto, E. A.; Park, S. E. Top. Catal. 2010, 53(3-4), 238–246. (51) Park, D. H.; Kim, S. S.; Wang, H.; Pinnavaia, T. J.; Papapetrou, M. C.; Lappas, A. A.; Triantafyllidis, K. S. Angew. Chem., Int. Ed. 2009, 48(41), 7645– 7648. (52) Cho, K.; Cho, H. S.; de Menorval, L. C.; Ryoo, R. Chem. Mater. 2009, 21 (23), 5664–5673. (53) Zhou, J.; Hua, Z. L.; Shi, J. L.; He, Q. J.; Guo, L. M.; Ruan, M. L. Chem.; Eur. J. 2009, 15(47), 12949–12954. (54) Harish, R.; Karevski, D.; Sch€utz, G. M. J. Catal. 2008, 253(1), 191–199. (55) Christensen, C. H.; Johannsen, K.; Toernqvist, E.; Schmidt, I.; Topsoe, H. Catal. Today 2006, 128, 117–122.
DOI: 10.1021/la101454v
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We acknowledge E. Eilertsen (University of Oslo) for providing the as-synthesized SSZ-13 material and D. Mores and I. Buurmans (University of Utrecht) for their help with the confocal fluorescence measurements. B.M.W. acknowledges research funding from CW-NWO (Top grant) and NRSC-C.
16516 DOI: 10.1021/la101454v
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Supporting Information Available: Part I: Morphology prediction in Materials Studio. Part II: Catalogue of crystals after oligomerization of 4-methoxystyrene and 4-ethoxystyrene. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(21), 16510–16516
