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Directing the distribution of potassium cations in zeolite-LTL through crown ether addition Hae Sung Cho, Adam R. Hill, Minhyung Cho, Keiichi Miyasaka, Kyungmin Jeong, Michael W. Anderson, Jeung Ku Kang, and Osamu Terasaki Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00832 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Crystal Growth & Design

Directing the distribution of potassium cations in zeolite-LTL through crown ether addition

Hae Sung Choa,†, Adam R. Hillb,†, Minhyung Choa, Keiichi Miyasakaa, Kyungmin Jeonga, Michael W. Andersonb,*, Jeung Ku Kanga,* and Osamu Terasakia,c,d,* a

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea b Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom c

Department of Materials and Environmental Chemistry, Stockholm University, Stockholm

SE-10691, Sweden d

School of Physical Science and Technology, Shanghai Tech, 319 Yueyang Road, Shanghai

200031, China †

Contributed equally to this work

We discover that a crystal morphology of zeolite-LTL could be modified by crown ether (21-crown-7, CE), where CE decreases the aspect ratio of zeolite-LTL while increasing the nucleation of domains on the (0001) face and hindering their growth along the c-axes. Moreover, the study using the scanning electron microscopy supports that the ratio between the rates for generation of cancrinite columns and bridging cancrinite columns on the {10-10} face remains constant among the LTL frameworks with different amounts of CE molecules. In addition, X-ray diffraction analysis shows that potassium cations re-distribute into pore cavities (t-lil) from cancrinite cages (t-can) and t-ste cages by the strong interactions between potassium and CE as the amount of CE molecules is increased. Additionally, Monte-Carlo simulations clarify that stabilization of the t-lil cage via the re-distribution of potassium cations at high CE concentration is attributed to the dominant effect in the crystal morphology changes observed.

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To understand the catalytic and adsorption properties of zeolites, it is important to investigate their structure/property relationships. Especially, studying the morphology of an anisotropic zeolite crystals has been of great interest because of the strong influence on controlling its properties. Thus, morphology control of the material with particular crystallographic direction is highly desirable to obtain maximum properties for applications. Zeolite-LTL, referring to an anisotropic crystalline aluminosilicate with the hexagonal space group (SG) of P6/mmm and a one-dimensional pore-channel along the c-axis (Figure 1) [1] has been used for many applications as catalysts [2-5] and membranes [6]. Cancrinite cages (t-can) are linked via double six-ring (t-hpr) forming columns along the c-axis resulting in one dimensional 12-membered ring pores. The typical crystal morphology of zeolite-LTL is a hexagonal prism with extension along the c-axis. As a consequence, it is important to control the aspect ratio of the crystal, generally shortening the c-axis and increasing the aand b-axes for its pore geometry, to acquire the desired intracrystalline diffusion lengths for guest species. [7-12] In general, modification of the crystal morphology for zeolite-LTL has been achieved by changing chemical compositions of alumina, silica, potassium hydroxide, and water for the initial synthesis gel [7-11]. Atomic force microscopy (AFM) studies of zeolite-LTL indicated a relationship between its crystal growth mechanism and morphology [12]. Recently, zeoliteLTL frameworks with different crystal habits were synthesized by changing the amount of an organic additive [13]. Furthermore, the crystal length and diameter of zeolite-LTL could be controlled by loading of crown ether (21-crown-7, CE) in the synthesis medium without its chemical composition. Our attention in the present work focuses on a detailed investigation for the crystal morphology and structure of the zeolite-LTL framework with different amounts of CE. In previous work, Brent et al. used AFM and scanning electron microscopy (SEM) analyses to determine the effect of CE on the crystal growth of zeolite-LTL. Zeolite-LTL with different loading of CE showed a similar lateral spread of terraces, even though zeolite-LTL with various water contents exhibited different heights and widths of terraces on the side wall. Despite the previous work, it is still unclear where the CE molecule is located within the zeolite-LTL framework and how CE affects its crystal morphology. Hence, a detailed structural investigation on the CE with the zeolite-LTL framework is required to fully understand the effect of CE on its crystal structure. For this study, high resolution (HR) SEM has been carried out to study the fine structure of zeolite-LTL with different CE loadings. The


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HRSEM study enables us to examine the effect of CE on the crystal growth and nucleation of zeolite-LTL. In addition, by using the synchrotron powder XRD experiment with Rietveld refinements, we have determined the detailed positions of CE molecules in the zeolite-LTL framework. Furthermore, we have used Monte-Carlo simulations to discover how CE affects the crystal growth of zeolite-LTL. The zeolite-LTL frameworks with different CE loadings were synthesized by using the following synthesis gel composition; 10.2 K2O : 1 Al2O3 : 20 SiO2 : 1030 H2O : [0 – 4] 21crown-7. The resultant zeolite products are denoted as LTL-n, where ‘n’ denotes the 21crown-7 molar fraction in the zeolite-LTL synthesis gel. The formula of LTL-0, 1, 2 and 4 is K9.8Si28Al8O72, K9.8Si28Al8O72, K9.52Si28Al8O72, K9.24Si28Al8O72, representatively. Also, the effect on the crystal morphology has been studied through a detailed SEM investigation at different CE loadings. Figure 2 (top row) shows the changes in the cylindrical single crystal morphology by the addition of CE. The crystal length along the c-axis and diameter of (0001) face of LTL-0 zeolite crystal is 3.8 and 1.4 µm, respectively. With increasing CE loading, the length of the zeolite crystals shortens to 1.6 µm and the diameter increases to 2.1 µm. The aspect ratio, defined as the length / diameter of a zeolite particle, was found to be gradually decreased from 2.7 to 0.8 with the increased loading of CE (See supplementary information, Table S1). Low magnification SEM micrographs with energy dispersive spectroscopy confirm that all zeolite-LTL crystals with the same amount of CE exhibit similar aspect ratios and Si/Al ratio (Figure S1). The SEM images taken on the (0001) faces of LTL-n zeolite crystal showed further how the surface nucleation events have been affected by addition of CE (Figure 2, middle row). For zeolite-LTL frameworks synthesized without CE, 1.6 nucleation sites on the domain area of 103 nm2 were found on (0001) face (Table S1). With increasing amount of CE in the synthesis medium, the number of zeolite nucleation sites was also determined to be increased while the average size of nucleation domains was decreased. The SEM images of LTL-4 also show 19 nucleation sites on the 226 nm2 domain area (Table S2). These results indicate that CE molecules within zeolite-LTL facilitates nucleation events, while they hinder the growths in the lateral side and the c-axis of the individual zeolite domains. These results indicate that the aspect ratio of zeolite-LTL is decreased by adding CE molecules due to the multiple nucleation, while the growths along the diameter and the length of individual zeolite domains are delayed. Additionally, high resolution SEM images for the side-wall of LTL-n give more information for the effect of CE on the lateral spread of terraces (Figure 2, bottom row).



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There were no indications of single t-can columns for zeolite-LTL with different amounts of CE, as confirmed using AFM on the previous study [13]. For a lateral growth, bridging t-can columns should require two t-can columns to be linked at growth sites on the crystal (Figure S2). This reveals that the addition of CE plays to maintain the ratio between the rates of generating single t-can columns and bridging t-can columns. In addition, to understand the effect of CE molecules on the growth of a zeolite-LTL crystal, we have performed the powder XRD measurements capable of giving information for the crystal structure with different amounts of CE (Figure S3). The Lebail refinement was made by adopting the P6/mmm SG as a model for finding the unit cell parameters. It is discovered that the parameters a, c and the calculated unit cell volume differ by the very small amounts in zeolite-LTE crystals with varying quantities of CE molecules. This indicates that CE does not lead to large changes in the dimensions of the zeolite framework. Rietveld refinement was also performed to determine the atomic structural differences between the samples on different loadings of CE molecules. Previous studies have shown six cation sites in the zeolite-LTL framework: site A at the center of t-hpr, site B at the center of t-can, site C at the center of 8-membered ring between t-cans (t-ste), site D in the eightmembered rings inside t-lil, site E at the center of t-kaa, and site F at the center of t-lil [13, 18]. Fourier analysis of LTL-0 shows that the potassium cations are located at site B (K1), C (K2) and D (K3) (Figure 3, S3-S4, Table 1, S3) with an average of 9.8 potassium cations per unit cell, as shown by elemental analysis [13]. Potassium cations are not found in the center of t-lil for zeolite-LTL without CE. These results are in agreement with previous work [1417], which suggested that potassium cations tend to be located in t-can rather than inside the t-lil. When CE molecules are incorporated inside t-lil, the experiments show that some potassium cations at site F (K4) begin to be observed. For LTL-1, 0.09 potassium cations are re-distributed to the center of t-lil per unit cell from other cationic sites (Table S4). As the increasing amount of CE molecules is incorporated, the occupancy of potassium cations at site F increases up to 0.75 while their occupancies in sites B, C and D decrease to 0.79, 0.81 and 0.75, respectively (Table S5-S6). The effect of changing potassium cation distribution in t-can, t-ste and t-lil on crystal growth along the a- and c-directions has been also explored, along with the effect on nucleation of zeolite-LTL. The SEM images of zeolite-LTL with CE demonstrate that potassium cations inside the CE rings of t-lil can promote nucleation of a zeolite crystal even when the potassium content is not large enough to occupy the center of t-lil (Figure 2f and g).


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Moreover, the results (Figure S5, Table 1) show that the increase of potassium cations in the large pore cavity causes the decrease of potassium in both the t-can and t-ste cages for zeolite-LTL with CE. This leads to suppression not only of a lateral spread along the a direction by decreasing generation of t-can columns and bridging them (t-ste), but also of tcan column growth in the c direction. These results propose that the re-distribution of potassium cations from t-can and t-ste to t-lil by addition of CE inhibits growths in the a and c–axis while it facilitates nucleation, thereby causing the aspect ratio to be decreased on the increasing loading of CE molecules. In order to explain the experimental observations, it is also necessary to understand the likely effect of the growth of the LTL structure on both the side-walls and the basal facets as a result of incorporation of the CE. This can be achieved by considering the stabilization/destabilization of different parts of the LTL structure due to the presence of CE molecules and potassium cations. Here, we have employed the Monte Carlo code CrystalGrower and the methodology outlined recently [19], where the structure is partitioned into five metastable and rate-determining elements corresponding to the natural tiling of zeolite-LTL: t-can, t-hpr, t-kaa, t-lil and t-ste (Figure 1) [18]. In particular, it is of the most interest to understand a stabilizing/destabilizing effect of the CE on the t-lil cage. This effect is computed using CrystalGrower and the results have been illustrated in the sequence of images shown in Figures 4, 5, S6 and S7. This method has been also extended to the cages originally containing potassium ions before re-distribution to t-lil: t-can and t-ste, as demonstrated in Figures S8, S9, S10, and S11. From previous work, it is known that the condensation of one Si-O-Si bond from solution account for a ∆G contribution of < 5 kcal mol-1 [19]. In this light, in order to test the effect of stabilization/destabilization of the t-lil, t-can and t-ste cages relative to the other cages, we have fixed the condensation of Si-O-Si bonds in all cages to 3 kcal mol-1 and then systematically varied the ∆G corresponding to the condensation for the cage. Simulations have been performed under the low supersaturation condition: ~ 3 kcal mol-1 (Figures 4-5, S9, and S11), and the equilibrium condition (Figures S6, S7, S8, and S10). The aspect ratio of the LTL structure is strongly controlled by the relative stabilization of the t-lil tile. Our results show that stabilization of the cage initially leads to an increase in the aspect ratio of zeolite-LTL to a maximum when the energy for t-lil is set as 2.0 kcal mol-1 (Figure 4), resulting in the crystals similar in appearance to those grown at the low concentrations of CE molecules (Figure 2a, b, e and f). Meanwhile, upon increasing the



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stability of t-lil to condensation values below 1 kcal mol-1, it is discovered that the aspect ratio is decreased markedly to form the puck-shaped crystals. This is attributed to the stabilization of {10-11} faces at the expense of the basal {0001} faces. Further, we find that stabilization causes the {10-10} faces to be superseded by the {11-20} faces, thus retaining the hexagonal nature of the crystal by exposing different faces. A progressive change can be observed in Figure 5 from {10-10} to {11-20} faces with the interim stabilizations resulting in 12 side wall faces from both {10-10} and {11-20}. This gives the appearance of rounded crystals as the side walls subtend 150o, signaling that there is only a 30o angular difference between side-wall faces. The clearest example of this effect is shown in Figure 5h, where t-lil is stabilized to 0.85 kcal mol-1. These rounded and puck-like crystals have notable similarities to the crystals grown at higher CE concentrations, as seen clearly by comparing the results in Figure 2d and h with those in Figure 5g and h. At the equilibrium supersaturation (Figures S6 and S7), it is determined that the trends in morphology are identical, although the overall morphology and terrace shapes are rougher. This is attributed to Ostwald ripening, where the smaller terraces dissolve back into solution and add onto the single largest growing terrace at the supersaturated conditions. We discover from the t-can simulations mainly that the crystal morphologies become more puck-like as t-can is more stabilized. However, these simulated crystals had larger aspect ratios than those seen with stabilizing t-lil. t-ste showed the opposite trend where the aspect ratio is generally increased as the cage is more destabilized. Further detailed discussion of the simulation results is also found in the Supporting Information. The results of the simulations agree with the conclusion that increasing CE concentration will lead to the stabilization of t-lil at the cost of destabilizing t-can and t-ste via redistribution of potassium cations. These indicate that the stabilization of t-lil has the largest effect on crystal morphology, as observed in experiments that the crystals grow with both the low aspect ratios and rounded morphologies. t-can could also give some contribution to this morphology change due to the decreased aspect ratios of crystals at high energies. No evidence of morphology contributions from t-ste can be observed, or is completely overridden by the effects of t-lil. These results agree with the changes in the potassium occupancy, as shown in Figure S5. It is notable that the simulations do not take into account the defects, which also seem to be present in these crystals and which may also partially contribute to changes in crystal aspect ratio.


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On conclusion, we have presented the effect of CE on the crystal morphology of zeoliteLTL. The structural investigation using the SEM analysis shows that the aspect ratio of the zeolite crystal is decreased by adding CE molecules, while the chemical composition of final products are similar. The HRSEM study on (0001) face of the zeolite crystal indicates the influence of CE on the number of nucleation sites. The terrace with a similar width on the side wall of zeolite-LTL on different amounts of CE indicates that CE has a role in keeping the ratio between the rate of generating single t-can columns and bridging t-can columns constant. The position of CE determined by Rietveld refinement is at the center of t-lil. Due to the high affinity between CE and potassium, the potassium cations within t-can and t-ste cages re-distribute to t-lil by increasing the amount of CE. From the results, we conclude that the crystal morphology change of zeolite-LTL is attributed to the re-distribution of potassium cations from t-can and t-ste to t-lil by the addition of CE. Simulations clarifies further that stabilization of the cage containing CE (t-lil) and destabilization of the t-can cage are key descriptors in controlling the overall shape and morphology of the crystal. Consequently, we expect that the present study would be helpful to understand the crystallization mechanism of zeolite-LTL and design other inorganic materials possessing diverse compositions and crystalline structures.

Associated content Supporting Information. Experimental details, low magnifided SEM micrographs of zeolite LTL with different amounts of CE, figures for the framework structure of zeolite-LTL and schematics of growth processes on the {10-10} face, and tables for the atomic structural information of zeolite-LTL with different amounts of CE, simulation results for t-lil performed at the equilibrium supersaturation conditions with the supersaturation values of 3.0 kcal mol-1 previously shown, simulation results for t-ste and t-can performed at both the equilibrium and low (3.0 kcal mol-1) supersaturation conditions.

Author information Corresponding Author Michael W. Anderson. [email protected] Jeung Ku Kang. [email protected] Osamu Terasaki. [email protected]



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Acknowledgment This work was supported by the Global Frontier R&D Program on Center for Hybrid Interface Materials (2013M3A6B1078865), Korea Center for Artificial Photosynthesis, and the BK21+ program. Synchrotron radiation XRD analyses were conducted at SPring-8. The computational section of this work was funded by the EPSRC via a CASE award, along with partial funding by SINTEF, Norway. Simulations were performed using Condor, the University of Manchester high-performance computing pool.

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Figure 1. Unit cell of the LTL framework topology partitioned into natural tiles. t-can is shown in green, t-hpr in pink, t-kaa in purple, t-lil in yellow and t-ste in blue. Unit cell edges are depicted in black. Image produced using 3dt (http://www.gavrog.org).


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Figure 2. SEM micrographs of particles (top row), (0001) face (middle low) and {10-10} side wall (bottom row) of zeolite-LTL samples synthesized with different amounts of CE: (a, e, i) LTL-0, (b, f, j) LTL-1, (c, g, k) LTL-2, and (d, h, l) LTL-4. Circles in the images at the middle row indicate the nucleation domains on the (0001) face.



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Figure 3. Configuration of potassium cation and CE molecule within the 2 2 framework unit cell for zeolite-LTL in the [0001] direction with different amounts of CE: (a) LTL-0, (b) LTL-1, (c) LTL-2, (d) LTL-4. The occupancies of each atom are displayed as circle graphs.


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Figure 4. Simulation results utilizing the Monte-Carlo code CrystalGrower discussed in reference 19 where the LTL structure has been broken down into five natural tiles and simulations were performed at the supersaturation value of ~ 3 kcal mol. From left – right: Results of increasing the destabilization energy for the largest cage / tile (t-lil) from 0.5 kcal mol-1 to 5.0 kcal mol-1 while keeping the energy of all other cages pinned at 3.0 kcal mol-1. The side facets are {10-10}, with the diagonal faces on the top facet {10-11}, the basal face is the {0001} face (not shown). Images were produced using the CrystalGrower visualization package.



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Figure 5. A detailed view of simulations where the destabilization energy for t-lil is varied between 0.50 and 1.00 kcal mol-1 with the energy increasing by 0.05 kcal mol-1 from a – k. Simulations performed at the supersaturation values of ~3 kcal mol-1. Left: A side on view of each tablet shaped crystal, with the end face of the crystal labelled as {0001}, along with the side facets labelled as {10-10}. Also shown are the diagonal facets encroaching on the end face {10-11} and the additional side facets that are rotated by 30° giving the crystal a rounded look: {11-20}. Right: A top-down view of the same simulation results showing, the effect of the growth of these facets on the shape of the crystal. Images were produced using the CrystalGrower visualization package.


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Table 1. Distribution of potassium cations within the zeolite-LTL framework with different amounts of CE Sample LTL-0 LTL-1 LTL-2 LTL-4

Number of potassium cations within LTL zeolite frameworka K1 1.89 1.87 1.80 1.58

K2 2.99 2.94 2.79 2.43

K3 4.92 4.90 4.72 4.48

K4 0 0.09 0.21 0.75

Ktotal 9.8 9.8 9.52 9.24

a) Total number of potassium cations of LTL-n was derived from the TGA. The distribution of potassium cations within the framework of zeolite-LTL was calculated by Rietveld refinement.



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For Table of Contents Only

Title: Directing the distribution of potassium cations in zeolite-LTL through crown ether addition Authors: Hae Sung Cho, Adam R. Hill, Minhyung Cho, Keiichi Miyasaka, Kyungmin Jeong, Michael W. Anderson, Jeung Ku Kang and Osamu Terasaki Table of Contents (TOC)

Synopsis: Zeolite LTL without crown ether shows large aspect ratio with small nucleation domains compared with the zeolite LTL including crown ether. Atomic coordinates analyzed by Rietveld refinement of X-ray diffraction data indicate that re-distribution of potassium cation in center of pore cavity due to strong interaction between crown ether and potassium cation induced different crystal nucleation and growth behavior.


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