DOI: 10.1021/cg100964j
Coaxial Core Shell Overgrowth of Zeolite L - Dependence on Original Crystal Growth Mechanism
2010, Vol. 10 5182–5186
Rhea Brent,† Sam M. Stevens,†,‡ Osamu Terasaki,‡,§ and Michael W. Anderson*,† †
Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Manchester M13 9PL, U.K., ‡Department of Structural Chemistry, Stockholm University, S-10690, Stockholm, Sweden, and §Graduate School of EEWS, KAIST, Yuseong, Daejeon, Korea
Received July 21, 2010; Revised Manuscript Received October 1, 2010
ABSTRACT: In this study, the first reported core-shell material combining zeolite L (LTL) and barium-exchanged zeolite L (Ba-LTL) is described. The use of atomic force microscopy to probe the LTL surface growth mechanism was employed to understand the growth pattern of the resultant Ba-LTL coaxial overgrowth layer. Additionally, the dependence of the original crystal surface on the resulting habit of the Ba-LTL core-shell layer is explained. High-resolution scanning electron microscopy (HRSEM) was employed to observe the fine detail of the Ba-LTL layer, and cross-sectional polishing was used to view the boundary between the two crystalline materials.
Introduction Core-shell zeolite composites are comprised of a single zeolite crystal surrounded by a polycrystalline shell of another zeolite phase. Overgrowth occurs between the two zeolite phases because of differences either structurally or compositionally between the two layers. The shell layer can be grown in an orientated fashion around the original seed crystal to give favorable properties. For instance, the BEA-MFI coreshell composite was found to combine the high absorption capacity of zeolite beta (BEA) and the high separation power of silicalite-1 (MFI) for use as a catalyst.1 The first study of these types of hybrid zeolites was by Goossens and co-workers,2,3 in which they synthesized an oriented film of small octahedral crystals of FAU (the cubic polymorph of zeolite Y) on larger hexagonal platelet crystals of EMT (the hexagonal polymorph of zeolite Y). They were synthesized by first preparing the EMT crystals and using them as seeds in the FAU-synthesis gel, where a secondary nucleation mechanism caused the FAU crystals to grow around the seed crystal. In their original preparation, they observed that the FAU crystals completely covered the surfaces of the EMT crystal core. By increasing the number of seeds in the FAU synthesis gel, they observed a decrease in the coverage of the shell layer on the EMT surface. In these conditions, the FAU layer showed a preference to grow on the corners and edges of the hexagonal plates rather than the flat hexagonal face of the crystal. They explained that this was likely to be caused by the increase in anchorage points for the growing layer at these positions. Recent work by Tian et al.4 studied the core-shell properties of two metal substituted aluminophosphate-5 (AFI) materials. The crystals consisted of a Ti-AFI shell surrounding a Cr-AFI core; they observed that the shell grew coaxially around the hexagonal-shaped Cr-AFI crystals orienting themselves in the same crystallographic direction. By studying the crystallization of Ti-AFI on Cr-AFI over time, they were able to observe that the shell material grows faster on the *To whom correspondence should be addressed. E-mail: m.anderson@ manchester.ac.uk. pubs.acs.org/crystal
Published on Web 11/11/2010
{1010} face than the hexagonal {0001} face of the core material. This was likely to result from differences in surface roughness of the two crystallographic faces, yielding anisotropy in the number of secondary nucleation sites. It is noteworthy that in a control experiment using unsubstituted AFI crystals as the core material, they found no preference for growth on a particular crystal face. The formation of hybridlike zeolite materials in other systems such as SOD-LTA, BEA-LTA, FAU-MFI, and MFI-BEA has also been investigated.5 Zeolite L (LTL) was first prepared by Breck and Acara6 in the late 1950s using potassium as the cationic species. Later it was found by Baerlocher and Barrer7 that an LTL framework material, in which some of the potassium cations were substituted for barium, could be synthesized. In addition to containing barium, the resultant crystals contained a Si/Al ratio of 1, rather than the more typically reported Si/Al=3 for K-LTL. Both K-LTL and Ba-LTL crystals grow with hexagonal cylinder morphology; however, the size of crystals differs on addition of barium to the synthesis. K-LTL crystal length varies between 1-7 μm, whereas Ba-LTL crystals are typically less than a few hundred nanometers long and tend to form as aggregates, shown in Figure 1. The hexagonal face is denoted as the {0001} face and the side walls of the crystal are the {1010} faces. The habit, which is governed by the crystal aspect ratio, can be modified by variations in experimental synthesis conditions.8 By modifying the length of LTL crystals, the habit can vary from short cylinders to long needles. Atomic force microscopy (AFM) has been utilized to study the crystal growth of zeolitic materials.9 The nanometer resolution of the instrument can probe the surface topography of a crystal to identify key crystal growth features such as the growth mechanism and defect formation.10 Recently, the crystal growth of K-LTL was studied using this technique, and differences in growth mechanism were reported when the crystal habit was modified by observing the differences in the crystal topography.11 High-resolution scanning electron microscopy (HRSEM) has also proven to be a complementary technique for observing the surface growth mechanism of zeolites.12 r 2010 American Chemical Society
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Figure 1. (A, B) Scanning electron micrographs of barium-zeolite L crystals.
Figure 2. Scanning electron micrographs in which (A) shows short cylinders of the original K-LTL crystals and (B and C) show the resulting core-shell K-LTL/Ba-LTL crystals. (D) shows long cylinders of the original K-LTL crystals, and (E and F) show the resulting core-shell barium zeolite L crystals. (G) shows needles of the original K-LTL crystals and (H and I) show the resulting core-shell K-LTL/Ba-LTL crystals.
Experimental Section Seed crystals of K-LTL with different habits were synthesized by modifying a preparation described by Lee et al.,13 with molar gel compositions: 10:0 K2 O : 1 Al2 O3 : 20 SiO2 : 800 H2 O short cylinders ðaspect ratio ¼ 1:3Þ 10:2 K2 O : 1 Al2 O3 : 20 SiO2 : 1030 H2 O long cylinders ðaspect ratio ¼ 2:5Þ 10:2 K2 O : 1 Al2 O3 : 20 SiO2 : 1200 H2 O needles ðaspect ratio ¼ 5:1Þ To prepare short cylinders; potassium hydroxide (3.85 g) was dissolved in distilled water (30.00 g). To this solution, aluminum sulfate octadecahydrate (Al2(SO4)3 3 18H2O) (1.98 g) was added and stirred for 10 min until a clear solution was obtained. A siliceous solution was prepared separately by adding Ludox (HS-40 colloidal silica) (8.76 g) to distilled water (5.40 g). The silica solution was then added to the alumina solution under vigorous stirring. Once
prepared, the gel was stirred for 18 h at room temperature and then transferred into a Teflon-lined stainless steel autoclave. Synthesis took place at 180 °C for 3 days, after which the reaction was quenched by plunging the autoclave into cold water. The resulting crystals were filtered and washed with copious amounts of distilled water, before being left to dry at 110 °C overnight. The synthesis of Ba-LTL was carried out using the procedure outlined by Burton,14 in which the synthesis gel contained the following composition: 1:00 MTK : 4:46 KOH : 0:25 BaðOHÞ2 : 292:00 H2 O MTK (metakaolin, SiAlO7/2) was first prepared by heating kaolin clay (5.00 g) in a furnace at 800 °C overnight. To prepare 100.00 g of gel, potassium hydroxide (5.13 g) was dissolved in water (91.54 g). Barium hydroxide was added to the basic solution and stirred vigorously until completely dissolved. Finally, MTK (1.96 g) was added and the mixture was stirred for 10 min. 5.00 g of the Ba-LTL gel was transferred into a polypropylene bottle, along with 0.20 g of K-LTL seed crystals; the bottle was then sealed and placed in an oven at 85 °C for two days. The reaction was then quenched by plunging the bottle into cold water. The resulting crystals were filtered and
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Figure 3. (I) Atomic force micrographs of the original K-LTL crystals where (A and D) are short cylinders, (B and E) are long cylinders, and (C and F) are needles. The hexagonal {0001} faces are shown in images A-C and the side walls {1010} are shown in images D-F. Vertical and horizontal scales equal unless explicitly shown. (II) Schematic showing the dependence of the original K-LTL crystal surface on the coaxial overgrowth of Ba-LTL. The blue structure represents the original K-LTL on the {1010} face and the green represents Ba-LTL. washed with copious amounts of distilled water, before being left to dry at 110 °C overnight. Scanning electron microscopy (SEM) was carried out using an FEI QUANTA ESEM, in low vacuum. Crystals were dispersed onto an aluminum stub covered in an adhesive carbon tape. The adhered crystals were then covered in a thin layer of gold to minimize the effect of charging of the material. High-resolution scanning electron micrographs were collected using the JEOL JSM-7401F fitted with a cold field emission electron source. Cross-sectional polishing was performed using a JEOL JSM09010 cross section polisher by first embedding the zeolite L crystals into carbon glue. A beam of focused argon ions was then bombarded at the material, causing attrition of both the glue and material. By interruption of the attrition process using an argon beam resistance mask, a smooth and contamination-free cross-section was produced.15
AFM was carried out using a JPK NanoWizard in contact mode. The AFM tips used were silicon nitride with a force constant of 0.58 N m-1 (supplied by Veeco Probes NP). Crystals were fixed in place by embedding in thermoplastic.
Results and Discussion Crystals of K-LTL with different habits were prepared in which the aspect ratio (that is, the length/diameter) was varied. The three types of crystals produced are described as short cylinders, long cylinders, and needles with increasing aspect ratio, SEM micrographs of which are shown in Figure 2A,D,F. The core-shell materials were prepared by using these three different types of crystals as seeds in the synthesis of Ba-LTL. The same experimental conditions were
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Figure 4. High resolution scanning electron micrographs of Ba-LTL seeded with short cylinders of K-LTL, where (A) shows the {1010} face and (B and C) show the {001} face. Ba-LTL can clearly be observed growing around the terraces of the original crystal in image (C), shown by the green line. Micrographs recorded on JEOL JSM-7401F HRSEM. (A) Everhart-Thornley (ETD) detector; working distance = 8 mm; column energy = landing energy = 1 keV. (B, C) In-lens detector; working distance = 2.5 mm; column energy = 2.5 keV; landing energy = 1 keV (stage bias = 1.5 kV).
Figure 5. High resolution scanning electron micrographs of Ba-LTL seeded with short cylinders of K-LTL, where cross sections of the crystals have been made (A) and (B) are in the [100] and (C) is in the [001] direction. Micrographs recorded on JEOL JSM-7401F HRSEM: in-lens detector; working distance = 3.0 mm; landing energy = 1 keV; column energy = 2 keV; (stage bias is therefore = 1 kV); cross-section polishing conditions, see main text.
employed to prepare each core-shell material; the only variable was the habit of the original seed crystals. The resultant core-shell K-LTL/Ba-LTL materials are shown in Figure 2B, C,E,F,H,I. It appears that the Ba-LTL has overgrown coaxially around the K-LTL crystals; the degree of coverage seems dependent upon the original seed crystal used. For the short cylinder seed crystals, the surfaces appear to be completely covered by the Ba-LTL crystal layer, whereas for long cylinder and needle-shaped seed crystals, the coverage of Ba-LTL seems to occur preferentially on the {1010} face. The longest needles show no coaxial overgrowth on the {0001} face. This observation is similar to the results of Tian and co-workers,4 in which they found preferential growth of Ti-AFI on the {1010} face as opposed to the {0001} faces of hexagonal cylinders of Cr-AFI. This suggests that some inherent property of the seed crystal is responsible for the preferred growth of Ba-LTL on the {1010} face. The most likely explanation is the prevalence of terraces on the surface of the seed crystal, able to facilitate the secondary nucleation of the new layer. As shown in Figure 3(I), as the crystal habit is modified, so too are the surface features on the crystal. From inspection of the AFM images for short cylinders Figure 3(I)A,D, it is observed that the {1010} face contains narrow elongated terraces, while the {0001} face is made up of high terraces, forming circularlike layers. On the core-shell material the Ba-LTL is orientated on the crystal following the elongated c-directional topography of the {1010} face and the circular-shaped terraces observed on the {0001} face (shown in excellent detail in the HRSEM images in Figure 4). This suggests that the
Ba-LTL crystals nucleate at step sites on the original crystal; since there is a high incidence of steps on both faces of the crystal, the surface is completely covered with new material. For the longer crystals, however, the {0001} face shows relatively fewer surface steps, and it follows that nucleation of Ba-LTL did not occur on this face (aside from positions where defects were known to occur). However, the surface steps on the {1010} face still enabled the nucleation of Ba-LTL due to its surface roughness. The habit of Ba-LTL crystals attached to the surface appeared to differ depending on the habit of the original zeolite L crystals. When the seeds were short cylinders, long cylinders of Ba-LTL are obtained; conversely, when the seeds are long or needle-shaped, disk-shaped Ba-LTL crystals were obtained (shown most clearly in Figure 2C,F). This is a curious result, and to the authors’ knowledge, a reciprocal relationship between the habit of the core and the habit of the shell has not been observed before. One possibility is that the ability of the Ba-LTL to spread laterally once it has nucleated at a step site is affected by the topography of the {1010} face. On short crystals the narrow, closely arranged terraces (Figure 3(I)D) are likely to hinder the spread of the Ba-LTL in the lateral a-direction and favor growth in the long c-direction of the crystal, hence forming cylinder-shaped crystals. Conversely, the lateral spread of steps on the {1010} face of the long cylinder seed crystals is greater (Figure 3(I)F), and so the growing Ba-LTL is able to form a disk-shaped morphology. This difference in growth mechanism is shown schematically in Figure 3(II). The short cylinder crystals were placed into a crosssectional polishing apparatus so that the inner structure of
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the crystals could be observed. The cross sections were taken in the [001] and [100] directions, and HRSEM images are shown in Figure 5. The images show a contrast line where the boundary between the original crystal and the Ba-LTL layer is present. The overgrowth is coaxial confirmed by the lack of dark contrast which would denote a space or vacancy between the two crystal systems.16 Conclusions Hybrid-like LTL materials have been synthesized by seeding K-LTL with Ba-LTL. The mode of secondary nucleation and growth of the Ba-LTL crystals was found to be dependent upon the surface topography of the seed K-LTL crystals. The attachment of the coaxial overgrowth layer occurred at step sites on the original crystal surface; therefore, the rougher the surface layer, the greater the coverage of Ba-LTL crystals. Additionally, the habit of the Ba-LTL crystals was affected by the surface topography on the {100} face of K-LTL. Low lateral spreading, consistent with short cylinder seed crystals, produced long cylinders of Ba-LTL, whereas a high degree of lateral spreading, observed on long cylinders and needles of K-LTL seeds, formed disk-shaped Ba-LTL crystals. Acknowledgment. The authors would like to thank the EPSRC, ExxonMobil Research and Engineering and The Knut and Alice Wallenberg Foundation for funding this project.
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References (1) Bouizi, Y.; Diaz, I.; Rouleau, L.; Valtchev, V. P. Adv. Funct. Mater. 2005, 15, 1955. (2) Goossens, A. M.; Wouters, B. H.; Buschmann, V.; Martens, J. A. Adv. Mater. 1999, 11, 561. (3) Goossens, A. M.; Wouters, B. H.; Grobet, P. J.; Buschmann, V.; Fiermans, L.; Martens, J. A. Eur. J. Inorg. Chem. 2001, 1167. (4) Tian, D.; Yan, W.; Wang, Z.; Wang, Y.; Li, Z.; Yu, J.; Xu, R. Cryst. Growth Des. 2009, 9, 1411. (5) Bouizi, Y.; Rouleau, L.; Valtchev, V. P. Chem. Mater. 2006, 18, 4959. (6) Breck, D. W.; Acara, N. A. U.S. Patent no. 711,565, 1958. (7) Baerlocher, C.; Barrer, R. M. Z. Kristallogr., Kristallgeom., Kristallphys., Kristallchem. 1972, 136, 245. (8) Larlus, O.; Valtchev, V. P. Chem. Mater. 2004, 16, 3381. (9) Agger, J. R.; Hanif, N.; Cundy, C. S.; Wade, A. P.; Dennison, S.; Rawlinson, P. A.; Anderson, M. W. J. Am. Chem. Soc. 2003, 125, 830. (10) Meza, L. I.; Anderson, M. W.; Agger, J. R.; Cundy, C. S.; Chong, C. B.; Plaisted, R. J. J. Am. Chem. Soc. 2007, 129, 15192. (11) Brent, R.; Anderson, M. W. Angew. Chem., Int. Ed. 2008, 47, 5327. (12) Stevens, S. M.; Cubillas, P.; Jansson, K.; Terasaki, O.; Anderson Michael, W.; Wright Paul, A.; Castro, M. Chem. Commun. 2008, 3894. (13) Lee, Y.-J.; Lee, J. S.; Yoon, K. B. Microporous Mesoporous Mater. 2005, 80, 237. (14) Burton, A.; Lobo, R. F. Microporous Mesoporous Mater. 1999, 33, 97. (15) Erdmann, N.; Campbell, R.; Asahina, S. Microscopy Today 2006, 14, 22. (16) Lopez-Noriega, A.; Ruiz-Hernandez, E.; Stevens, S. M.; Arcos, D.; Anderson, M. W.; Terasaki, O.; Vallet-Regi, M. Chem. Mater. 2009, 21, 18.
