Article pubs.acs.org/cm
Intergrown New Zeolite Beta Polymorphs with Interconnected 12Ring Channels Solved by Combining Electron Crystallography and Single-Crystal X‑ray Diffraction Zheng-Bao Yu,†,‡,∥ Yu Han,§ Lan Zhao,§ Shiliang Huang,† Qi-Yu Zheng,*,‡ Shuangzheng Lin,¶ Armando Córdova,¶ Xiaodong Zou,*,† and Junliang Sun*,†,∥ †
Berzelii Center EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, Stockholm SE−106 91, Sweden ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Advanced Membranes and Porous Materials Center & Imaging and Characterization Core Lab, King Abdullah University of Science and Technology, Thuwal 21534, Saudi Arabia ¶ Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm SE−106 91, Sweden ∥ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *
ABSTRACT: Two new polymorphs of zeolite beta, denoted as SU-78A and SU-78B, were synthesized by employing dicyclohexylammonium hydroxides as organic structuredirecting agents. The structure was solved by combining transmission electron microscopy and single-crystal X-ray diffraction. SU-78 is an intergrowth of SU-78A and SU-78B and contains interconnected 12-ring channels in three directions. The two polymorphs are built from the same building layer, similar to that for the zeolite beta family. The layer stacking in SU-78, however, is different from those in zeolite beta polymorph A, B, and C, showing new zeolite framework topologies. SU-78 is thermally stable up to 600 °C. KEYWORDS: porous material, zeolite, zeolite beta, intergrowth, polymorph, crystal structure determination, transmission electron microscopy, electron crystallography
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INTRODUCTION Zeolites are crystalline microporous materials that have shown important applications as adsorbents, ion exchangers, and catalysts.1 Beta is the first example of zeolites with high Si/Al ratio; it was synthesized by Mobil Oil in 1967, using tetraethylammonium hydroxide as an organic structuredirecting agent (OSDA).2 The structure of beta contains three-dimensional (3D) intersecting channels with large pore openings defined by 12 (Si,Al)O4 tetrahedra (12-ring). Beta has shown wide applications in catalysis such as hydrocarbon conversion,3 alkylation,4 and acylation5 of aromatics. The structure elucidation of zeolite beta has been very challenging, because of the intergrowth. Twenty years after its discovery, Newsam et al. and Higgins et al. independently proposed the structure models of beta,6 which is an intergrowth of two closely related polymorphs: polymorph A and polymorph B. Another polymorph in the same beta familybeta polymorph C (BEC)was also proposed, but could only be synthesized 12 years later by the Zou and Corma groups.7 The key parameter for the success of synthesizing beta polymorph C was the incorporation of germanium in the framework to stabilize the double 4-rings (D4Rs) in the structure. The three © 2012 American Chemical Society
beta polymorphs are built from the same building layer, but the layers are stacked in different ways. Zeolites with different channel systems can have different properties. For example, zeolite beta polymorph A is chiral and may display enantioselective properties in sorption, separation, and catalysis,8 while beta polymorph C can greatly enhance the diffusion and shape selectivity for acylation reactions.9 Great efforts were made during the past decade to obtain zeolite betarelated materials or enrich a specific polymorph.10 Materials with intergrowths of zeolite beta polymorph A, B, and C (ITQ16)10e and polymorph B and CH (SSZ-63)10g have been reported, which were synthesized using designed OSDAs. Herein, we report two new members of the zeolite beta family: SU-78A and SU-78B, synthesized using OSDAs with two dicyclohexyl groups. SU-78 is an intergrowth of SU-78A and SU-78B and consists of complex twinning and disorder, making the structure solution challenging. Recently, we reported the structures of several novel zeolitesITQ-37,11a ITQ-38,11b and Received: May 29, 2012 Revised: September 12, 2012 Published: September 14, 2012 3701
dx.doi.org/10.1021/cm301654d | Chem. Mater. 2012, 24, 3701−3706
Chemistry of Materials
Article
ITQ-3911cdetermined by combining electron crystallography and powder X-ray diffraction (PXRD). ITQ-39 is an intergrowth of three different polymorphs, and the structure was solved from domains only a few nanometers in size.11c Here, for the first time, we combine transmission electron microscopy (TEM) and single crystal X-ray diffraction (SXRD) to solve the complex intergrown structure of SU-78. In fact, SU-78A and SU-78B are the remaining two hypothetical polymorphs of zeolite beta, polymorphs D and E, predicted by Burton and coworkers.10g To the best of our knowledge, what we present here is the first report of a real material with the two polymorphs (D and E) of zeolite beta.
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were evaporated completely and a dry gel was formed. Quantitative water then was added into the gel to give a batch composition of 0.6SiO2:0.4GeO2:0.5R1OH:30H2O. The final mixture was heated in an autoclave at 175 °C under static conditions for 7 days. The solid product was recovered by filtration, washed with distilled water, and dried in air. Attempts were made to synthesize SU-78 using three other RnOH OSDAs (n = 2−4) with structural motifs similar to R1OH. The synthesis batch composition and synthesis parameters were kept the same as those using R1OH. Characterization. Powder X-ray powder diffraction (PXRD) was performed on a PANalytical X’Pert PRO diffractometer equipped with a Pixel detector and using Cu Kα1 radiation (λ = 1.5406 Å). In situ PXRD data were collected from room temperature (RT) to 600 °C under vacuum on a PANalytical X’Pert PRO MPD diffractometer equipped with an Anton-Parr XRK900 reaction chamber, using Cu Kα radiation (λ = 1.5418 Å). The heating rate was 2 °C/min and the temperature was equilibrated for 5 min prior to each data collection. Thermogravimetric analysis (TGA) was performed in air from RT to 780 °C with a heating rate of 8 °C/min, using a high-resolution thermogravimetric analyzer (Perkin−Elmer, Model TGA 7). Thin cross sections of SU-78 crystals were fabricated on an FEI Helios Model NanoLab 400S FIB/SEM dual-beam system, using Pt/C deposition for sample protection. Scanning electron microscopy (SEM) was also performed on the same FIB/SEM dual-beam system. SAED patterns and HRTEM images were acquired on an FEI Titan ST electron microscope (Cs = 1.2 mm; Cc = 1.5 mm; 300 kV). Synchrotron single-crystal X-ray diffraction (XRD) data were collected at the beamline I19, Diamond Light Source, U.K. (λ = 0.68890 Å). The programs ELD13 and Trice14 were used for processing and indexing the SAED patterns, and for determining the unit cell, respectively. Crystallographic image processing on HRTEM images was carried out using the program CRISP.15
EXPERIMENTAL SECTION
Chemicals and Materials. The chemicals of dicyclohexyamine, Nmethyl-cyclohexylamine, iodomethane, iodoethane, 2-iodopropane, iodobutane, and tetraethyl orthosilicate (TEOS) were purchased from Alfa Aesar and used without any modifications. Synthesis of OSDAs. Four different organic structure-directing agents RnOH (n = 1−4) were tested (Figure 1). The synthesis of
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RESULTS AND DISCUSSION Powder X-ray diffraction (PXRD) shows that pure SU-78 phase was obtained using R1OH as the OSDA. We found that, while R2OH was effective to direct the formation of SU-78, R3OH decomposed during the synthesis and resulted in only GeO2. Using R4OH with only one cyclohexyl group led to ZSM-11. These results indicate that the synthesis of SU-78 may need structure-directing agents (SDAs) with two dicyclohexyl groups. In situ PXRD shows that SU-78 was stable up to 600 °C (see Figure S1 in the Supporting Information). Thermogravimetric analysis (TGA) shows that the OSDA could be removed at 400 °C (see Figure S2 in the Supporting Information). These results indicate that permanent pores of SU-78 could be generated by calcination. The framework was stable in a moisture-free environment after calcination, but collapsed after being exposed in air for 1 h. The chemical formula of SU-78, as calculated according to the results obtained from the inductively coupled plasma−mass spectrometry (ICP-MS) and TGA, is |(CH3)(C2H5)N(C6H12)2OH|·[Si0.71Ge0.29O2]16. Scanning electron microscopy (SEM) shows that all SU-78 crystals have similar morphologies with serious twinning (Figure 2a). Each SU-78 crystal is, in fact, a twin of two rhombic prismatic crystals that are rotated by 90°, with respect to each other, around the long rhombic axis and grow into each other. A schematic drawing of the twinning is given as an insert in Figure 3. The PXRD pattern (Figure 2b) shows not only sharp peaks but also broad ones at, for example, 7.7°, 20.1°, and 28.7°, indicating that the crystals contain disorder or defects. Because of the twinning and disorder, it was difficult to interpret the single-crystal X-ray diffraction (SXRD) data and determine the unit cell of SU-78. TEM, combined with focused ion beam (FIB) analysis, was thus applied to investigate the
Figure 1. Organic structure-directing agents (OSDAs) employed in the synthesis of SU-78, in which R1, R2, and R3 have two cyclohexyl groups and R4 has only one cyclohexyl group. R1OH is as follows. First of all, dicyclohexyamine and iodoethane were dissolved in methanol (MeOH) with a dicyclohexyamine:iodoethane molar ratio of 1:1.2. The solution was kept stirring for 48 h at ambient temperature. The solvent was then evaporated under reduced pressure, and the residual was washed with ethyl acetate (EtOAc) several times to afford a white solid after drying. The solid powder was dissolved in water and neutralized by excess of sodium hydroxide. Afterward, the solution was extracted by CH2Cl2 for three times. The combined organic layer was dried by anhydrous MgSO4, and the solvent was evaporated under reduced pressure to afford a slightly yellow liquid (N-ethyl dicyclohexylamine). In the second step, iodomethane was added to a solution of N-ethyl dicyclohexylamine in MeOH with a Nethyl dicyclohexylamine:iodomethane molar ratio of 1:1.2 and the mixture was stirred at ambient temperature for 48 h. After removal of the solvent, the residual was washed by EtOAc several times to afford a white solid with a yield of ∼80%. The iodide was converted to hydroxide (R1OH) by using an ion-exchange resin in a batch overnight. The synthesis procedure of R2OH,12 R3OH, and R4OH are similar to that of R1OH, except that iodomethane was used for R2OH; 2-iodopropane and iodomethane were used, respectively, for R3OH; N-methyl-cyclohexylamine and iodobutane were used as the substrates for R4OH. Synthesis of Zeolites Using RnOH as an OSDA. The synthesis of SU-78 was carried out in an OH− media. Typically, GeO2 was first added into a solution of N-ethyl−N-methyl-dicyclohexylammonium hydroxide (R1OH) in water under stirring. After the GeO2 was completely dissolved, a silica precursor tetraethyl orthosilicate (TEOS) was added. The mixture was kept stirring until the water and ethanol 3702
dx.doi.org/10.1021/cm301654d | Chem. Mater. 2012, 24, 3701−3706
Chemistry of Materials
Article
similar phenomenon was also observed in the disordered zeolite beta. The space group and the unit-cell parameters of SU-78B (one of the polymorphs of SU-78) were determined from this tilt series to be P2/m and a = 12.6 Å, b = 12.8 Å, c = 13.8 Å, β = 108°, using the programs ELD13 and Trice.14 All of the sharp peaks in both the PXRD and SAED patterns could be indexed as hkl with h = 3n using this unit cell (see Figure S4 in the Supporting Information). After finding the twin laws and unit-cell parameters, it becomes possible to interpret the SXRD data. The unit-cell parameters could also be determined from the SXRD data. The precession image along the [100] (Figure S5 in the Supporting Information) reconstructed from the SXRD data of a SU-78 crystal is similar to the SAED pattern taken at the twin boundary (see Figure 3b) from a thin section of the SU-78 crystal. This further confirms that each SU-78 crystal contains two twin domains, which are rotated by 90° around the c*-axis. Most sharp diffraction spots can be indexed as 0kl from one twin domain, while the rest of the sharp spots can be indexed as h0l (h = 3n) from the other twin domain. The streaks are along the c*-axis and can be indexed as h0l (h = 3n ± 1). In the entire reciprocal space, the streaks run along the c*-axis for reflections hkl with h = 3n ± 1, which is consistent with the monoclinic unit cell. Reflection intensities were extracted from the SXRD data. Apparently, the intensities at the streaks are not reliable, so we applied a two-dimensional (2D) charge-flipping algorithm16 only to the sharp 0kl reflections, to first determine the structure projection along the [100]. The most frequently obtained electron density map with a low R-value is shown in Figure 4a, from which Si/Ge atoms could be easily located and
Figure 2. (a) SEM image and (b) powder X-ray diffraction (PXRD) pattern of the as-synthesized SU-78, which show twinning morphology and disorder, respectively.
Figure 3. SAED patterns of SU-78 from (a) a single domain and (b) a twin boundary. The thin section cut from the middle of the crystal is marked in periwinkle color, and the areas where the SAED patterns were taken are marked in red in the schematic representation of the SU-78 crystal. The SAED pattern in panel a was taken along the a-axis of SU-78 and resembles the SAED pattern of beta polymorph C along the a-axis. The SAED pattern in panel b resembles that of zeolite beta with intergrown beta polymorphs A and B and is, in fact, a superposition of the SAED patterns of SU-78 along the [001] from one twin component and along the [010] from another twin component of SU-78.
twinning and disorder in the crystal. We applied the FIB technique to fabricate thin cross sections in order to study the orientations in specific regions of the SU-78 crystals by selected-area electron diffraction (SAED). The schematic drawing in Figure 3, together with Figure S3a in the Supporting Information shows a thin cross section (thickness of
