Modifying the Crystal Habit of Zeolite L by Addition of an Organic

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Modifying the Crystal Habit of Zeolite L by Addition of an Organic Space Filler Rhea Brent,† Alan J. W. Lobo,‡ Dewi W. Lewis,‡ and Michael W. Anderson*,† Centre for Nanoporous Materials, School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom, and Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom ReceiVed: July 20, 2010; ReVised Manuscript ReceiVed: September 14, 2010

The crown ether 21-crown-7 was found to be effective as a crystal habit modifier of zeolite L. The function of the template was to facilitate the bridging of cancrinite columns on the {101j0} face and hence increase the crystal diameter while hindering growth in the long axis of the crystal. The rational selection of 21-crown-7, over a choice of other candidate molecules, was carried out by calculating the binding energy of the molecules inside the zeolite L pore cavity using ZEBEDEE. This work is an excellent example of effectively applying understanding obtained from studying the crystal growth of a zeolite to exert control over a particular growth process. Introduction The purpose of this work was to investigate whether a ringshaped organic molecule, such as a crown ether, could have an influence on the crystal habit (or shape) of zeolite L. The role of such a molecule in the synthesis of zeolite L could have a structure-directing or structure-inhibiting effect and, thus, if either of these processes occurs on one crystallographic face in preference to the other, it may be possible to modify the habit of the resultant crystals. In general, a template is an entity that is able to direct the synthesis of a nanoporous material into a particular framework. The template can be inorganic, such as sodium or potassium ions, or it can be organic, with a wide range of shapes and compositions. A very common type of organic template used in zeolite synthesis is the family of quaternary alkyl ammonium salts of which the most commonly used is tetramethylammonium hydroxide (TMA). The presence of such an entity can be useful for obtaining a particular phase of zeolite. These ions are incorporated into the cavities of the framework because of complementarities in charge distribution, size, and shape between the template and the cavity. Such incorporation indicates that building units grow around the template. The use of organic templates has aided an increase in the number of zeolite frameworks discovered, which can often be unique to one type of template.1 An example of this is ZSM-57 which can only be formed in the presence of N,N,N,N′,N′,N′-hexaethylpentamethylenediammonium ions.2 In recent years, there has been a classification of templates that splits their function into the following categories: first, true templates that direct the structure toward a particular geometry unique to one type of template; second, structure-directing agents where several different species direct the formation of a particular framework; third, space fillers that are several different templates able to direct several different types of geometries.3 Supramolecular compounds such as crown ethers have been found to be encapsulated into the structure of zeolites. They * To whom correspondence should be addressed. E-mail: m.anderson@ manchester.ac.uk. † The University of Manchester. ‡ University College London.

are thought to fill the zeolite cavities optimizing the van der Waals contacts between the host and the guest. Other cyclic molecules such as cryptands, cyclodextrins, calixarenes, and cavitands could all potentially have similar space-filling functions.4 A famous particular example of the use of crown ethers as template was in the synthesis of the FAU framework type (zeolite Y), where Delprato et al.5 were able to improve the crystallinity and were able to increase the Si/Al ratio of samples through their incorporation resulting also in enhanced catalytic performance. For the FAU structure, it was found that these circular-shaped molecules had a similar geometry to the faujasite supercage. They tested a variety of crown ether molecules and found that highly siliceous crystals were formed on addition of 15-crown-5 and 18-crown-6 to the synthesis recipe. Furthermore, each crown ether formed a different polymorph of zeolite Y: 15-crown-5 yielding the cubic polymorph (denoted FAU) and 18-crown-6 yielding the hexagonal polymorph (denoted EMT). Both polymorphs were found to have enhanced thermal stability over the nontemplated FAU owing to the increased silica content of the crystals. The mode of the crown ether templation is thought to occur via complexation with sodium ions during the crystallization phase, which are then integrated into the pores; the slightly larger pore in EMT requires the larger 18-crown-6 molecule. The increase in Si/Al was rationalized by the sodium crown ether complexes’ ability to reduce the dissolution of alumina in the crystallizing gel.6 The crown ethers forming zeolite Y polymorphs are an example of space-filling templation. Zeolite L (LTL) is a one-dimensional pore aluminosilicate that forms crystals with a hexagonal cylinder morphology (shown in Figure 1i). For industrial applications, the shape, or habit, of zeolite L crystals is critical. Because the pores run along the long axis of the cylinder, the length of the crystal determines pore length, while its diameter determines the number of pores exposed to the surface. Thus, a low aspect ratio (that is, length/diameter) crystal is preferred for catalytic reactions since the short pore channel length provides a short path for diffusion of reactants and products.7 It is possible to synthesize zeolite L using potassium as the sole templating species; therefore, the formation of its structure is not dependent upon the addition of an organic template. Previous studies investigating factors that modified the crystal habit of zeolite L

10.1021/jp1067445  2010 American Chemical Society Published on Web 10/05/2010

Designing Zeolite Habit Modifiers

Figure 1. Zeolite L. (i) A scanning electron micrograph of a zeolite L crystal and (ii) schematics of the structure in the [0001] and [1000] directions. (The green boxes outline the cancrinite cage.)

Figure 2. (A) Atomic force micrograph of the {101j0} face of zeolite L showing typical examples of cancrinite columns and shoulder surface features. (B) Schematics showing the stabilization of cancrinite columns on the {101j0} face. (i) A single cancrinite column protruding from the surface equal to 1.2 nm; addition of a subsequent column over a ditch site cannot occur. (ii) Two adjacent cancrinite columns attaching at growth sites on the crystal. (iii) A bridging cancrinite column can incorporate over two adjacent columns giving a terrace height of 1.6 nm.

showed that varying the proportions of alumina, silica, potassium hydroxide, and water in the preparation had the most marked effect on crystal habit.8-10 The crystal structure of zeolite L is shown in Figure 1ii, where the secondary building units are the cancrinite cage (highlighted by the green squares) and the double six ring. These units stack on top of one another to form columns (termed cancrinite columns) shown in the [100] projection in Figure 1ii. The crystal growth mechanism of zeolite L has been studied using atomic force microscopy.11 Observation of the nanoscale features on the {101j0} face of the crystal showed that growth was dominant in the c-direction of the crystal illustrated by elongated, narrow-shaped terraces (shown in Figure 2A). The evidence of narrow terraces suggests that growth in the a-direction of the crystal is severely frustrated. The narrowest terraces observed were found to have a step height of 1.2 nm,

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18241 which suggests that a single cancrinite column is stabilized on the surface of the crystal (shown in Figure 2Bi). Wider terraces were found to consistently have a step height of 1.6 nm, which corresponds to the stacking of three cancrinite columns to form the 12-membered pore structure (shown in Figure 2Biii). For the crystal to spread laterally, and therefore to form terraces with a step height of 1.6 nm, a bridging cancrinite column must link on top of two adjacent 1.2 nm columns (shown in Figure 2Bii). Since growth in the lateral direction is frustrated, linking of the bridge onto a single cancrinite column cannot occur (Figure 2Bi). It has been shown that modifying the crystal habit of zeolite L can affect the crystal growth features observed on the {101j0} surface. Low aspect ratio crystals present very narrow terraces; however, when the aspect ratio is increased, the lateral spread of terraces increases and single cancrinite columns are rarely observed. In this work, the addition of a space-filling molecule such as a planar cyclic molecule such as a crown ether of a size comparable to the zeolite L pore ring was proposed. It was hypothesized that the space-filling molecule may enable the bridging of adjacent cancrinite columns to form the pore structure and hence to facilitate growth in the lateral, a-direction of the crystal. This could modify the crystal habit by increasing the diameter of the crystals in preference to their length. The size of the crown ether is clearly important in relation to the pore size of the zeolite of interest. Therefore, for this study, the crown ethers 18-crown-6, 21-crown-7, and 24crown-8 were chosen for investigation. In addition, some other common templatessthe tetra alkyl ammonium cations tetraethylammonium (TEA), tetrapropylammonium (TPA), and tetrabutylammonium (TBA)swere also evaluated as well as a cyclodextrin that has a cyclic “bung”-like structure. The rationale for testing R-cyclodextrin molecules was to explore the opposite templating functionality to the crown ether molecules, that is, to hinder growth in the c-direction rather than to facilitate growth in the orthogonal a-direction. It was envisaged that this molecule may dock onto the pores on the hexagonal face of zeolite L and cause a blockage due to their large size thereby hindering the addition of further growth units onto this face. This would theoretically have a similar effect on the resultant crystal habit modification of zeolite L as the crown ether facilitating a-directional growth. Figure 3 shows the structures and approximate outer diameters of these molecules. These molecules were chosen because they are approximately of a similar size to fit into the undulating zeolite L pore channel which measures 11.0 Å at its widest point. Ideally, the space-filling template should interact strongly with the framework without being distorted from its own equilibrium conformation thus maximizing the binding of the molecule with the framework. The selection of molecules with the most favorable interactions can be calculated using ZEBEDDE (zeolites by evolutionary de novo design) developed by Lewis et al.12,13 Originally devised to build templates within the cavities and pores, the code can also be used to dock and then optimize the position, using Monte Carlo and energy-minimization methods, of adsorbed molecules within such cavities. Once the most stable sites are identified, the binding energy (the difference between the energy of the adsorbate/pore and the isolated adsorbate and pore) can be used as indicative of the “templating ability” of the molecules.14 The ZEBEDDE code has also been modified to carry out Monte Carlo simulated annealing (MCSA) surface adsorption calculations.15 The methodology used in these calculations is discussed later.

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Brent et al. TABLE 1: Calculated Binding Energies of Organic Molecules inside the Zeolite L Pore Cavity organic molecule (positioned inside pore) 24-crown-8 21-crown-7 18-crown-6 TEA TPA TBA R-cylcodextrin

Figure 3. Organic compounds investigated for space-filling inside the cavities of zeolite L. The approximate outer diameter of each is shown.

Results and Discussion Before any experimental work was undertaken, computational work using ZEBEDEE was carried out to determine the possible position of the crown ethers inside the structure of the zeolite. Figure 4 shows the most likely positions of the molecules within the pore channels of zeolite L. The structures obtained by ZEBEDEE showed that the 18-crown-6 and 21-crown-7 molecules could sit in either the horizontal, vertical, or diagonal position with respect to the {0001} face. The calculated binding energies for the crown ether molecules and for the other molecules, for comparison, are shown in Table

position horizontal vertical diagonal horizontal vertical diagonal

calculated binding energy (kJ mol-1) +674 -122 +161 +118 -135 +108 +41 -50 -71 -89 +2000

1. A negative figure for the binding energy represents a favorable interaction, whereas a positive energy is unfavorable. For 21crown-7, it seems that the horizontal position is the most likely to occur, and the least likely to occur is the vertical position. The same appears to be true for 18-crown-6; however, on applying simulated annealing, we found that the smaller molecule can take a large number of different configurations suggesting (as does the initial binding energy) that the “fit” is poorer than that for 21-crown-7. The computational results for 24-crown-8 show that the molecule is too large to sit in a planar position inside the cavities of zeolite L; instead, it prefers to curl up into a twisted configuration resulting in a very strained structure. The binding energy given in Table 1 is for the 24crown-8 sitting in a horizontal configuration; it can be seen that the energy is extremely unfavorable. This suggests that 24crown-8 may not have a habit-directing influence on zeolite L. The binding energy calculated for R-cyclodextrin was the energy required to bind the molecule onto the exposed pores on the hexagonal face of zeolite L (see Figure 1 in the Supporting Information) and was found to be extremely unfa-

Figure 4. Molecules bound in zeolite L: (a) TEA, (b) TPA, (c) TBA, (d) 18-crown-6, (e) 21-crown-7, and (f) 24-crown-8 showing the most favorable bound positions of the molecules within the structure in the [0001] direction of the crystal. (g-i) The three positions tested in ZEBEDEE calculations for 21-crown-7 docked within the zeolite L cavity shown in the [1000] direction (see Table 1).

Designing Zeolite Habit Modifiers

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18243 TABLE 2: Crystal Dimensions Taken from Scanning Electron Micrographs of 10 Zeolite L Crystals with 21-Crown-7 Addeda amount of crown ether/ crystal crystal aspect Si/Al K/Si mol equivalents length/µm diameter/µm ratio (average) (average) 0 1 2 4 1 + Cs 0 1 2 4

3.8 (0.2) 2.0 (0.3) 1.8 (0.1) 1.8 (0.2) 3.0 (0.4) 3.8 2.0 1.8 1.8

1.5 (0.1) 1.6 (0.1) 1.5 (0.2) 2.3 (0.2) 1.2 (0.2) 1.5 1.6 1.5 2.3

2.5 (0.1) 1.3 (0.2) 1.2 (0.2) 0.8 (0.2) 2.5 (0.4) 2.5 1.3 1.2 0.8

3.5 (0.1) 3.5 (0.1) 3.5 (0.2) 3.5 (0.1) 3.4 (0.2) 3.47 3.50 3.49 3.50

0.4 (0.1) 0.4 (0.1) 0.3 (0.1) 0.3 (0.1) 0.2 (0.1) 0.35 0.35 0.34 0.33

a The standard deviations are shown in brackets. 1 + Cs is 1 mol of 21-crown-7 pre-bound with cesium cations.

Figure 5. Scanning electron micrographs of zeolite L crystals synthesized with varying amounts of 21-crown-7, where (i, ii) 0 mols, (iii, iv) 1 mol, (v, vi) 2 mols, and (vii, viii) 4 mols.

vorable. This result suggested that the hypothesis regarding the functionality of this molecule to poison the growth of zeolite L in the c-direction of the crystal was unlikely. On inspection of the binding energies for the tetra alkyl ammonium molecules TEA, TPA, and TBA, we find that while these molecules are well bound at specific sites within the zeolite L cavity, their interactions with the framework are significantly less favorable than for the 18-crown-6 and 21-crown-7 molecules. Figure 4 shows the positions of the alkyl ammonium ions within the zeolite L structure. TEA fits very loosely into the cavity with the fit becoming increasingly better as the molecules become larger with TBA appearing to have the best fit. This is evidenced by the increase in favorability of the binding energies in Table 1. We concluded from this computational work that the two crown ethers, 18-crown-6 and 21-crown-7, may well have an effect on the crystal growth of zeolite L. It was therefore decided to undertake the synthesis of zeolite L from standard preparation methods but with the addition of these two crown ethers to determine if they do indeed bring about a modification in the crystal habit. Using a zeolite L gel composition, which usually produces long cylinders of zeolite L (of the type shown in Figure 1i), addition of 1 mol equivalent of 18-crown-6 to the reaction mixture resulted in no zeolite L crystals being formed. Instead, an unknown crystalline phase was obtained. In contrast, when experiments containing 1 and 2 and 4 mol equivalents of 21crown-7 were carried out, crystalline zeolite L was obtained. Scanning electron microscopy (SEM) images are shown in Figure 5, where the crown ether samples are compared against the original samples synthesized without crown ether. Inspection

of the crystal dimensions, shown in Table 2, shows that the crystal habit was modified by the addition of 21-crown-7. The aspect ratio decreased from 2.5 to 0.8 on addition of 4 mol equivalents of 21-crown-7, and the length of the crystals decreased with the diameter increasing as more crown ether was added. Analysis of the chemical composition of the resultant crystals showed that there was no significant variation in either Si/Al or K/Si ratios obtained on addition of 21-crown-7. This could suggest that the observed habit modification arises from a steric modification rather than from a chemical modification in structure, which is also known to modify crystal habit in zeolites. The observation of a change in crystal habit could be caused by a templating or “poisoning” effect of the crown ether as we hypothesized. However, it is possible that the presence of ether in the synthesis mixture could have affected solubility or viscosity, and this could have caused the modifications. (Similarly, this may be the reason that the presence of 18-crown-6 results in no crystallization of zeolite L. The gel is now modified so as not to be conducive to zeolite L formation.) To establish whether the crown ether was indeed responsible for modifying the habit, it was necessary to first determine whether it was present in the resultant crystals. If the crystals were found to contain ether, then it is more likely that they are responsible for the shape modification; however, if they were not involved, the ether would have been removed on filtering and washing. To determine if 21-crown-7 was present in zeolite L, thermogravimetric analysis (TGA) and solid-state carbon-13 NMR were carried out. The results from the TGA analysis of the crystals obtained from these experiments are shown Table 1 of the Supporting Information. Both NMR (shown in Figure 2 of the Supporting Information) and TGA analysis were able to confirm the presence of 21-crown-7 in the zeolite L structure corroborating the strong binding energy suggested by the calculations. The percentage weight loss (determined by TGA) associated with the decomposition of 21-crown-7 yielded an approximation of the relative amount of 21-crown-7 incorporated into the zeolite L structure. For 1 mol of 21-crown-7 added to the synthesis recipe, the ratio of moles of 21-crown-7 to 1 zeolite L unit cell was calculated as 0.05. Similarly, for 2 mols of 21crown-7 the ratio was 0.18, and for 3 mols the ratio was 0.76 (these calculations assume that in the two higher heating steps both template and strongly bound water is removed and that the amount of strongly bound water is determined from the sample with no crown ether). Since there is one available ring site for 21-crown-7 to be positioned per unit cell, the crystals with the highest loading of crown ether were found to be threequarters filled with crown ether. This proves that a significant

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Figure 6. Atomic force micrographs of zeolite L {101j0} side wall synthesized with (i) 0 mols, (ii) 1 mol, and (iii) 2 mols 21-crown-7 (4 mols was not analyzed because of the small size of the {101j0} face on these crystals).

quantity of crown ether can be incorporated into the zeolite L structure. However, we are not able to, as yet, confirm the actual positioning of the ether within the material. Atomic force microscopy (AFM) was carried out on the {101j0} face of the crystals, and the images obtained are shown in Figure 6. The addition of 21-crown-7 does not appear to modify the lateral spread of terraces despite the change in crystal habit in contrast to the narrow features observed when the aspect ratio is decreased by changing the water concentration as described in earlier work.11 This gives some evidence that the crown ether is facilitating the bridging of cancrinite columns causing fewer narrow terraces to be observed. The above results showed that the addition of 21-crown-7 to the synthesis of zeolite L had an influence on the resultant crystal habit. Crown ethers are known to favorably bind to inorganic cations with 21-crown-7 reported to bind favorably with both potassium and cesium ions but with the strongest preference for cesium ions.16 The determination of the location of cations within the zeolite L framework by Burton and Lobo17 suggested that although potassium is observed inside the pore cavity of the crystal, the most favorable positions are in the smaller units within the structure, such as the cancrinite cages. If some of the potassium ions during our synthesis of zeolite L became bound to 21crown-7 molecules, these would of course now be located in the pore since this is the only position large enough in the zeolite framework to accommodate the large crown ether molecule. The increase in potassium ions within this position of the framework, and the concomitant decrease within the cancrinite cages, could also be responsible for the modifications in crystal habit observed. Thus, an experiment was designed to investigate this hypothesis by prebinding the 21-crown-7 with cesium before its addition to the zeolite synthesis. Since the crown ether binds preferentially to cesium, the molecule should not “mop up” the potassium ions in the synthesis gel. First, 1 molar equivalent of cesium chloride (0.29 g) was added to 1 mole equivalent of 21-crown-7 (0.54 g) and was stirred for 6 hours at room temperature. After this time, the solid material was filtered, and the filtrate was added to the zeolite L synthesis (0.36 g, 1 molar equivalent). The crystals formed from this preparation are shown in Figure 7. Comparing these crystals with the original zeolite L crystals with no crown ether (Figure

Figure 7. Scanning electron micrographs of zeolite L with (i) no 21crown-7 and (ii) 1 mol of 21-crown-7 bound to cesium.

1i), it is evident that the aspect ratio of the crystals is unchanged on addition of cesium-bound 21-crown-7. This indicates that the binding of potassium by the crown ether was also responsible for the crystal habit modification rather than the directing role of the ether alone. The most likely role of the crown ether is to increase the number of cations present in the pore cavity potentially facilitating the bridging of cancrinite columns and giving favorable growth in the usually frustrated a-direction of the crystal. Energy-dispersive X-ray spectroscopy was used to determine the chemical composition of the cesium-bound crown ether synthesis of zeolite L and gave evidence that the Si/Al ratio of the crystals remained the same while their K/Si ratio decreased (results are shown in Table 2). The decrease in the potassium content may have been caused by a substitution of cesium ions into the structure. Conclusion In conclusion, it has been found that 21-crown-7 has an effect on the crystal habit of zeolite L. It appears to poison growth in the c-direction of the crystal and to facilitate growth in the a-direction (with the highest diameter of zeolite L crystal measured in this work from 4 mols of 21-crown-7). Its action as a crystal habit modifier does not appear to modify the chemical composition of the resultant crystals. TGA and NMR were able to confirm the presence of the ether within the composition of the crystals. Calculations show that the crown ether is most likely to sit horizontally inside the pore channel. Addition of 18-crown-6 failed to result in the crystallization of zeolite L, and calculations showed that the binding of this

Designing Zeolite Habit Modifiers molecule in the zeolite L channels is not as strong as that of 21-crown-7. The larger 24-crown-8 would need to contort its cyclic planar configuration in order to fit inside zeolite L pores and, therefore, it is unlikely to be effective as a crystal habit modifier. It is proposed that the role of the 21-crown-7 is to position more cations into the pore cavity hence providing a template in which cancrinite columns can preferentially incorporate in the normally frustrated a-direction of the crystal. The success of using and selecting an organic molecule to modify the crystal habit of zeolite L demonstrates that by using an understanding of the crystal growth mechanism of a zeolite it is possible to impart control on its crystal properties. Experimental Section Crystals of zeolite L with different habits were synthesized by modifying a preparation described by Lee et al.10 with molar gel composition

10.2K2O:1Al2O3:20SiO2:1030H2O:xtemplate where x ) 18-crown-6 or 21-crown-7. 18-Crown-6 was purchased from SigmaAldrich. 21-Crown-7 was synthesized using a procedure by Ziafati et al.:18 33.67 g of potassium hydroxide was dissolved in 30.00 g of distilled water and then 5.61 g of 1,2-bis(2-chloroethoxy)ethane and 5.82 g of tetraethlylene glycol were added. This mixture was heated to boiling under reflux for 3 h until a yellow oil formed on the surface of the water. After cooling, dichloromethane was added and the organic layer was separated from the aqueous layer using a separating funnel. The organic layer was dried over anhydrous magnesium sulfate and then was concentrated using a rotary evaporator forming an oil. The oil was then purified using vacuum distillation, which yielded a yellow oil. The product was characterized using carbon-13 and proton NMR and mass spectrometry and was found to be 21crown-7. For the zeolite preparation with 1 mole of 21-crown-7, 25.00 g of reactant was prepared. Potassium hydroxide (1.56 g) was dissolved in distilled water (10.00 g). To this solution, aluminum sulfate octadecahydrate (Al2(SO4)3 · 18H2O) (0.79 g) was added, and the solution was stirred for 10 minutes until a clear solution was obtained. A siliceous solution was prepared separately by adding Ludox (HS-40 colloidal silica) (3.47 g) to distilled water (8.83 g). The silica solution was then added to the alumina solution under vigorous stirring. Finally, 0.36 g of 21-crown-7 was added. The mixture was then left to stir for 18 h at room temperature by which time a turbid gel resulted. The gel was 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. X-ray diffraction (XRD) was carried out using the Philips X’Pert diffractometer between 2 and 60 degrees 2θ. Crystals were ground and compacted into a sample holder to minimize the risk of preferred orientation. The crystal dimensions were determined from SEM by taking average measurements of a statistically relevant amount of crystals. Energy-dispersive spectroscopy (EDS or EDAX) was used to determine the approximate composition of crystals. This

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18245 was carried out in high vacuum using the same sample preparation. Quantification was taken by the average of ten measurements of the same sample. Solid-state C-13 NMR and solution-state C-13 and proton NMR were carried out on Bruker AVANCE III 400 and Bruker 400 MHz spectrometers, respectively. For solid-state NMR, ground samples were packed into a sample tube, and 2000 scans were performed (at a spin rate of 5000 Hz). The solution-phase samples were diluted in deuterated chloroform, and 16 scans were performed. TGA was recorded using the TG/DTA220 instrument between 25 and 1000 °C at a heating rate of 5 °C/min. Samples were prepared by weighing approximately 5 mg of sample into an alumina crucible and by crimping a lid on top. AFM was carried out ex situ 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 first preparing a dispersion of crystals in water. A small piece of thermoplastic was placed onto a glass microscope slide and was warmed at approximately 50 °C; the thermoplastic then became soft and was flattened using the end of a spatula. One drop of the crystal dispersion was then placed on top of the thermoplastic and was left to evaporate at 50 °C. This procedure afforded samples where the zeolite L crystals were adequately embedded into the thermoplastic so that crystals remained in place during AFM scanning. Analysis of atomic force micrographs was carried out using JPK image processing software. When required, the images were adjusted using a plane fit algorithm to flatten the image and to carry out cross section analysis. Each molecule was docked into a unit cell of zeolite L using a Monte Carlo method 10 000 times. For each acceptable insertion, where there are no overlaps of van der Waals radii and the interaction energy was below a cutoff, the configuration was optimized to a minimum energy through a combination of Monte Carlo steps and finally energy minimization. For all calculations, the framework is kept fixed, and the interactions are described using the PCFF forcefield.19 The binding energy is calculated as

E(bind) ) E(zeolite+template) - (E(template) + E(zeolite)) where the energies are the energy minima obtained here. Acknowledgment. The authors would like to thank the EPSRC and ExxonMobil Research and Engineering for the funding of this project. Supporting Information Available: A schematic showing the position of R-cyclodextrin bound to a zeolite L and the results from solid-state NMR and thermogravimetric analysis. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lok, B. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. (2) Valyocsik, E. W.; Page, N. M.; Chu, C. T. W. (Mobil Oil Corp., United States). U.S. Patent 4873067, 1989. (3) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756. (4) Sherman, J. Chem. Commun. 2003, 1617. (5) Delprato, F.; Delmotte, L.; Guth, J. L.; Huve, L. Zeolites 1990, 10, 546.

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(6) Wu, C. N.; Chao, K. J. J. Chem. Soc., Faraday Trans. 1995, 91, 167. (7) Verduijn, J. P. (Exxon Research and Engineering Co., United States). Patent WO 9106367, 1991. (8) Ruiz, A. Z.; Bruehwiler, D.; Ban, T.; Calzaferri, G. Monatsh. Chem. 2005, 136, 77. (9) Larlus, O.; Valtchev, V. P. Chem. Mater. 2004, 16, 3381. (10) Lee, Y.-J.; Lee, J. S.; Yoon, K. B. Microporous Mesoporous Mater. 2005, 80, 237. (11) Brent, R.; Anderson, M. W. Angew. Chem., Int. Ed. 2008, 47, 5327. (12) Lewis, D. W.; Willock, D. J.; Catlow, C. R. A.; Thomas, J. M.; Hutchings, G. J. Nature 1996, 382, 604. (13) Willock, D. J.; Lewis, D. W.; Catlow, C. R. A.; Hutchings, G. J.; Thomas, J. M. J. Mol. Catal. A: Chem. 1997, 119, 415.

Brent et al. (14) Lewis, D. W.; Freeman, C. M.; Catlow, C. R. A. J. Phys. Chem. 1995, 99, 11194. (15) Jelfs, K. E.; Slater, B.; Lewis, D. W.; Willock, D. J. Stud. Surf. Sci. Catal. 2007, 170B, 1685. (16) Kimura, K.; Kitazawa, S.; Maeda, T.; Shono, T. Fresenius Z. Anal. Chem. 1982, 313, 132. (17) Burton, A.; Lobo, R. F. Microporous Mesoporous Mater. 1999, 33, 97. (18) Ziafati, A.; Sabzevari, O.; Heravi, M. Phosphorus, Sulfur Silicon Relat. Elem. 2006, 181, 803. (19) Sun, H.; Mumby, S. J.; Maple, J. R.; Hagler, A. T. J. Am. Chem. Soc. 1994, 116, 2978.

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