Shape-dependent crystal growth of zeolite L studied by atomic force microscopy Rhea Brent and Michael W. Andersona

aCentre for Nanoporous Materials, School of Chemistry, University of Manchester, M13 9PL, UK.

Abstract The objective of this study is to determine the mechanism of growth of zeolite L and thereby control the crystal habit. AFM was applied to study the surface features on both the (100) side walls, and the (001) hexagonal faces of zeolite L and the likely mechanism of growth was determined. Crystals with modified habits were also studied, and found to exhibit different surface features from one another. Keywords: Zeolite L, Crystal Habit, Crystal Growth, AFM

1. Introduction Crystal habit (or morphology) is an important property for the industrial use of zeolites. Manipulating habit can be used to tailor the number of pores exposed to the surface of a crystal, as well as the length of pore channels. The shape of a crystal can vary because the different faces grow at different rates relative to one another. Zeolite L (LTL) contains one dimensional porosity in the c-direction of the crystal. Crystals of zeolite L are most commonly observed as hexagonal cylinders, the length of which may be varied from short disc to long needle-shaped crystals, and hence the length of the pore channels can be modified. Variations in crystal habit are most conveniently described in terms of their aspect ratio, which is their length/diameter. Previous studies have shown how changes in individual synthetic parameters can modify the habit of hexagonal cylinders of zeolite L crystals [1]. Compositional variations appear to have the most marked affect on the crystal habit, within the ranges for which zeolite L can still be obtained. What is still not adequately understood is how the growth mechanism of the crystal is affected to bring about these habit modifications. Complete control and predictability of crystal shape evolution can only be obtained once the affect of synthetic changes is understood on a mechanistic level. It has been shown that Atomic Force Microscopy (AFM) is an excellent tool for determining crystal growth mechanisms in zeolites [2]. It gives a snapshot in time as to how the shape of a crystal has developed, by imaging nanoscopic surface features. This study attempts to understand the mechanism of growth of zeolite L by observation of both crystallographic faces; the hexagonal face (001) and side walls (100). Our results are discussed in reference to previous Transmission Electron Microscopy (TEM) studies and the likely terminating units and step heights present on each face have been determined [3]. In addition, mechanistic differences between crystals with different aspect ratio are explored.

2. Experimental Crystals of zeolite L with different habits were synthesised based on a preparation described by Lee et al. [1] with molar gel composition:

10.2K2O:1Al2O3:20SiO2:xH2O where x = 800, 1030 and 1200

Zeolites and Related Materials: Trends, Targets and ChallengesProceedings of 4th International FEZA ConferenceA. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.

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Once prepared the gel was stirred for 18 hours 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 with copious amounts of water, before being left to dry at 110°C overnight. SEM was carried out using an FEI QUANTA ESEM. AFM was carried out using a JPK NanoWizard in contact mode.

3. Results and discussion Table 1 shows the dimensions of the zeolite L crystals prepared by varying the water content in the preparation. As the water content in the reactant gel is increased the aspect ratio of crystals was found to increase. Figure 1 shows SEM images of the three types of crystals, from which this increase in aspect ratio with water content can be observed. Table 1: Dimensions of crystals obtained from gel composition: 10.2K2O:1Al2O3:20SiO2:xH2O

Molar Water Content, x

Average Crystal Length L / �m

Average Crystal Diameter D / �m

Average Aspect Ratio L/D

800 2.8 1.9 1.5

1030 3.8 1.5 2.3

1200 7.2 1.5 5.1

Figure 1: SEM images of the three zeolite L preparations with aspect ratio: i) 1.5; ii) 2.3; iii) 5.1.

Figure 2 shows AFM images of both the hexagonal face and side walls of the crystals. Examination of the hexagonal face shows that when the aspect ratio increases the number of terraces decreases significantly, and the average height of terraces observed decreases from 11.2 to 1.5nm. The heights of the lowest aspect ratio crystals ranged from 0.7 to 53.4nm, the range was found to decrease in the largest aspect ratio crystals to 0.83 to 2.06nm. The lowest aspect ratio crystals exhibited irregular-shaped terraces. As the aspect ratio increased the terraces became larger, more concentric and more hexagonal in shape.

910 R. Brent and M.W. Anderson

Figure 2: Error signal AFM images of the three different zeolite L preparations where i) and iv) have aspect ratios of 1.5, ii) and v) have aspect ratios of 2.3 and iii) and vi) have aspect ratios of 5.1, and where i)-iii) were taken on the hexagonal (001) face of the crystal and iv)-vi) were taken on a portion of the side walls (100).

Additionally, the lowest aspect ratio crystals exhibited multiple hexagonal-shaped “holes” on the surface, around which the terraces have grown. Fewer holes are observed as the crystals increase in aspect ratio. These results indicate a relationship between defect formation during growth and the ultimate length of the crystal. The growing units on the surface of the crystal appear to be hindered by the presence of such hexagonal shaped holes. Further investigations will be carried out to determine the significance of the defected crystals observed with relation to the mechanism of growth between crystals with different habits. The AFM images from the side wall of the crystal were found to consist of elongated rectangular terraces, where growth has occurred almost entirely in the c-direction of the crystal and lateral spread is minimal. Such terraces often span the entire length of the crystal. These elongated rectangular terraces stack on top of one another to create a corrugated surface, with a curved cross-section. These observations suggest that the mechanism of growth occurs via a series of nucleations, which spread significantly faster in the [001] direction than the [100] direction. As the aspect ratio increases the number of terraces observed decreases, whilst the heights of terraces remain constant. The lateral spread of terraces increases as the aspect ratio increases. A greater number of terrace terminations are also observed when the aspect ratio of crystals increases. The heights of steps on the surface of a crystal give an indication about the way in which units stabilize themselves onto the growing crystal. Height measurements were taken from the AFM images of crystals with aspect ratios of 1.5, the smallest steps observed on the hexagonal face were approximately 0.75nm, whereas those on the side walls were approximately 1.2 nm and more commonly, 1.6 nm. Figure 3 shows the cross- sectional areas of the terraces of interest, and how the heights were determined.

911Shape-dependent crystal growth of zeolite L studied by AFM

Figure 3: Cross-section depicting the smallest steps present on the surface of each face of crystals with aspect ratios 1.5. Where 1i) shows the position of the cross-section, 1ii) shows the cross-section and the corresponding heights and 1iii) shows a schematic of the possible cancrinite unit of attachment onto the hexagonal face. Where 2i) shows the position of the cross-sections, 2ii) and iv) show the cross-sections and the corresponding heights and 2iii) and v) show schematics of the possible cancrinite unit of attachment onto the side walls.

The secondary building units of zeolite L are cancrinite cages and double six rings, the TEM work by Ohsuna et al. [3] determined that the likely terminating units on the hexagonal face are cancrinite cages (with a height of 0.75 nm). From the AFM height information, it seems plausible that the mechanism of growth of zeolite L occurs via the appearance of a single cancrinite cage, on both faces of the crystal. On the side walls, the height of one cancrinite species is approximately 1.2 nm, as subsequent units are incorporated, they attach to the side of the first cage and hence the height of 1.6nm is observed. This is shown schematically in Figure 3.2iii) and iv). This work is discussed in more detail in another publication [4].

4. Conclusion In summary, zeolite L was found to grow via cancrinite cage incorporation, although different mechanisms of incorporation were observed depending on the face of the crystal. When the crystal habit was modified, the features observed on the hexagonal face varied considerably, suggesting growth had been modified. It is possible that this was caused by the occurrence of defects in the crystal.

References [1] Lee, Y.-J.; Lee, J. S.; Yoon, K. B., Microp. Mesop. Mater. 80 (1-3) (2005) 237. [2] Anderson, M. W.; Agger, J. R.; Thornton, J. T.; Forsyth, N., Angew. Chem. Int. Ed. Engl.,

35 (1996) 1210. [3] Ohsuna, T.; Slater, B.; Gao, F.; Yu J.; Sakamoto, Y.; Zhu, G.; Terasaki, O.; Vaughan, D.

E. W.; Qiu S. C.; Catlow, R. A., Chemistry - A European Journal, 10 (2004) 5031. [4] Brent R., Anderson M. W., Angew. Chem. Int. Ed. Engl. Submitted.

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