DOI: 10.1002/chem.201405449

Communication

& Porous Materials

A 3D 12-Ring Zeolite with Ordered 4-Ring Vacancies Occupied by (H2O)2 Dimers Jie Liang,[a, b] Jie Su,[b] Yingxia Wang,*[a] Yanping Chen,[a] Xiaodong Zou,[b] Fuhui Liao,[a] Jianhua Lin,*[a] and Junliang Sun*[a, b] Abstract: A germanate zeolite, PKU-14, with a threedimensional large-pore channel system was structurally characterized by a combination of high-resolution powder X-ray diffraction, rotation electron diffraction, NMR, and IR spectroscopy. Ordered Ge4O4 vacancies inside the [46.612] cages has been found in PKU-14, in which a unique (H2O)2 dimer was located at the vacancies and played a structure-directing role. It is the first time that water clusters are found to be templates for ordered framework vacancies.

Zeolites are interesting due to their uniformly sized pores of molecular dimensions and wide industrial applications in catalysis, adsorption, and separation.[1–5] Because the properties of zeolites are heavily influenced by their structures, much effort has been focused on preparing zeolites with new framework types and characterizing their structures.[6–8] So far, 225 types of zeolite frameworks have been approved by the Structure Commission of the International Zeolite Association (IZA-SC).[9] Among them, most are 3D four-connected frameworks in which each tetrahedral center is fully surrounded by another four TO4 tetrahedra.[10] Some interrupted structures with threeconnected tetrahedra are also included as zeolites due to their novel porous structures. These interrupted zeolites may have interesting properties, such as better selectivity between adsorbed molecules and framework defects.[11] The improved hydrophilicity of interrupted zeolites by terminal hydroxyl groups may also increase the possibility to adsorb water molecules in the cavities.[12] To date, twelve interrupted zeolite frameworks (-CHI, -CLO, -IRY, -ITV, -LIT, -PAR, -RON, -SVR, -WEN, *-ITN,[13] ITQ-43[14] and *-SSO[15]) with terminal hydroxyl groups are known with three-connected nodes. For most of them, the hy[a] Dr. J. Liang,+ Prof. Y. Wang, Y. Chen, F. Liao, Prof. J. Lin, Prof. J. Sun College of Chemistry and Molecular Engineering Peking University, Beijing 100871 (P. R. China) E-mail: [email protected] [email protected] [email protected] [b] Dr. J. Liang,+ Dr. J. Su,+ Prof. X. Zou, Prof. J. Sun Berzelii Center EXSELENT on Porous Materials Department of Materials and Environmental Chemistry Stockholm University, Stockholm 10691 (Sweden) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405449. Chem. Eur. J. 2014, 20, 16097 – 16101

droxyl groups are always pointing to the framework channels, except those of -SVR. In the cavity of -SVR, four terminal oxygen atoms are located around a space similar to that occupied by a Si atom, which can be viewed as one Si vacancy.[11] Such a unique structural feature may endow SSZ-74 (-SVR) with a better selectivity and catalytic performance.[16] Studies on the ordered vacancies of zeolites are important, but till now, the only zeolite that contains ordered vacancies in the structure is SSZ-74 (-SVR). Thus, more zeolites with ordered vacancies are desirable. Germanate-based zeolites favor the formation of such frameworks, because Ge not only tends to form four-connected zeolites, but also Ge O Ge bonds can easily be broken and generate three-connected nodes.[17, 18] However, unless they crystallize in a single-crystal form, the structural elucidation of these polycrystalline germanium-based zeolites is challenging. The combination of high-resolution powder X-ray diffraction (PXRD) and TEM is an effective way to solve the structures of polycrystalline materials, and they are successfully applied to SSZ-74,[11] ITQ-43,[14] ITQ-37,[19] TNU-9,[20] IM-5,[21] and ITQ-40.[22] However, the structure determination of germanate-based zeolites is still confronted by other challenges. Besides the peak overlap in PXRD patterns caused by the large unit cell, the instability of germanate-based zeolites under the electron beam brings extra difficulties in solving structures. Therefore, apart from PXRD and TEM, some complimentary techniques are very helpful for the local structural information, such as IR and solid-state NMR spectroscopy. Here, by combining these aforementioned techniques, we solved the structure of the germanate zeolite PKU-14, which possesses ordered Ge4O4 four-ring vacancies. PKU-14 was synthesized by using dicyclohexyldimethylammonium hydroxide (M2Cy2N + OH ) as the organic structure-directing agent (OSDA). A mixture with a molar ratio of 1.0 GeO2 :0.5 SDA:0.5 HF:2.5 H2O was sealed in a 3 mL Teflon-lined autoclave and kept at 150 8C for 14 days with a rotating speed of 15 rpm. After washing, colorless cuboid crystals with the size of ~ 20 mm were obtained. Synchrotron single-crystal X-ray diffraction was first applied for structure determination. However, the direct indexing of the data resulted in unreliable unit-cell parameters and many unindexed reflections. This indicates the presence of severe twinning in the crystal, which can also be evidenced by the Rubik’s Cube-like crystal from the SEM image (Figure S1, Supporting Information). To avoid the twinning, PKU-14 was re-examined by using the rotation electron diffraction (RED) method,[23, 24] which can collect 3D electron diffrac-

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Communication tion data from nano- or submicron-sized crystals. The 3D recipTherefore, the real structure of PKU-14 may adopt a lower symrocal lattice of PKU-14 was reconstructed as shown Figure S2a, metry. Supporting Information, in which 85 % of reflections can be inTo check the possible lower symmetry, synchrotron PXRD dexed by a tetragonal unit cell (a = 19.2 and c = 26.9 ), which was collected for the Rietveld refinement at the beamline is consistent with the one obtained from synchrotron singleID31, ESRF. As expected, the first strong peak with indices of crystal X-ray diffraction data. The possible space groups could (110) from in-house data was split into two peaks as shown in be I4/mcm, I-4c2, and I4-cm, as indicated by the reflection conFigure S6, Supporting Information, which indicated that the g ditions from the 2D reciprocal space slices cut from the 3D angle might deviate from 908. The profile fitting of PKU-14 was RED data (Figure S2b–d, Supporting Information). Since most significantly improved after using a new monoclinic unit-cell zeolite structures are centrosymmetric, attention was first paid with a = 19.8075(7), b = 26.7538(7), c = 19.8127(7)  and b = to the centrosymmetric space group I4/mcm. 90.4810(5)8 with the space group I2m. Rietveld refinement was Initial attempts to solve the structure of PKU-14 from inthen performed with soft geometric restraints for the bond dishouse PXRD data or RED data using direct methods failed. tances and angles of the framework. Since the OSDA was Considering that germanium prefers to form several composite intact in the final product, as confirmed by the CHN analysis building units (CBUs) with oxygen, direct-space methods were and solid-state 13C NMR spectrum (Figure S7, Supporting Inforapplied that may overcome these difficulties by introducing mation), the structure of OSDA excluding H atoms was introthe geometry of the germanium CBUs to supplement the difduced as a rigid body inside the pores and refined subsefraction data. Previous IR studies on germanates revealed that quently. The final refinement of this model was improved sigthe bands in their IR spectra may help for the identification of nificantly and converged with Rp = 8.52 %, Rwp = 11.30 %, and the CBUs in structures.[25] The IR spectrum of PKU-14 (Figure S3, Rexp = 4.90 % (Figure 1). Further crystallographic details are proSupporting Information), with bands in the range of 400 ~ vided in the Supporting Information. 1000 cm 1, is similar to that of a zeolitic germanate ASU-7 and With the monoclinic symmetry, the framework of PKU-14 quartz-type GeO2,[25] which indicates that PKU-14 might have contains 16 crystallographically independent Ge atoms and 36 a zeolitic structure constructed with GeO4 tetrahedra. Thus, the parallel tempering algorithm implemented in the program FOX[26] was applied to the inhouse PXRD data. The centrosymmetric space group I4/mcm was employed, and four independent GeO4 tetrahedra, as estimated by the symmetry and unit-cell volume, were input with random positions and orientations in the asymmetric unit as predefined building units. After an approximately tenminute running, a promising structural model was obtained. Finally, four symmetry-independent framework Ge atoms and ten O atoms in the framework Figure 1. Rietveld refinement of powder X-ray diffraction for as-synthesized PKU-14. The curves from top to bottom are simulated, observed, and difference profiles, respectively; the bars below curves indicate peak were located. positions. Rietveld refinement of PKU-14 against the in-house PXRD data, O atoms, whereas four oxygen atoms are presented as OH however, gave a high R-value (Rwp = 19 %). The high R value is groups as confirmed by the strong OH vibration (a peak at mainly due to the poor profile-fitting caused by the mismatch 3528 cm 1) in the IR spectrum (Figure S3, Supporting Informabetween the calculated and experimental peaks in the highangle region (Figure S4, Supporting Information). We also notion). The framework of PKU-14 is built with interrupted [46.612] ticed that there are only two symmetry-independent doublecages, which are further linked to six neighboring cages four-rings (d4rs) in the structure model with the space group through d4rs (Figure 2 a) following the primitive cubic net. I4/mcm. Because fluorine ions often reside inside d4rs, we Thus, the whole framework structure can be described by would expect two symmetry-independent fluorine positions. using two tiles [46.612] (in yellow) and [46] (in green) as shown 19 However, the F MAS NMR spectrum of the as-synthesized in Figure 2 b. The 3D 12-ring channel system surrounded by these two tiles can be presented by two other tiles: a large tile PKU-14 (Figure S5, Supporting Information) showed four sig[126] (in blue) corresponds to the large cavity and a [44.64.126] nals between d = 7–13 ppm with the ratio of about 2:2:1:1. Chem. Eur. J. 2014, 20, 16097 – 16101

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Figure 2. a) The framework structure of as-synthesized PKU-14 along the [010] direction. b) The tiling of PKU-14.

tile (in pink) is the tile for the windows. It is noted that with the underlying topology sqc12267, PKU-14 has an optimized cubic Pm-3m symmetry with the a-axis ~ 13.5 . In the as-synthesized PKU-14, the 12-ring channel system is occupied by three half-occupied OSDAs (M2Cy2N + cations) as located by the simulated annealing algorithm (Figure S9, Supporting Information). All OSDAs are connected to the framework by weak hydrogen-bonds (C H···O in the range of 2.721– 3.245 ) and play a structure-directing role. These OSDAs also balance the negative charges raised by the F anions in the d4rs. Additionally, the arrangement of the M2Cy2N + and F ions resembles the positive/negative charge distribution in the nbo net as shown in Figure S10, Supporting Information, and such a charge distribution may further account for the stabilization of the framework. The most interesting feature in PKU-14 is the interrupted [46.612] cage. Although similar cages have been reported in other zeolite structures, such as ITQ-21[27] and ITQ-26,[28] both of them have a four-ring located in the centre of the cage to stabilize the cage (Figure 3 c and d). In PKU-14, instead of a four-ring, eight terminal hydroxyl groups are located in the [46.612] cage and point to the centre, thus a large void was formed inside of the cage (Figure 3 a). Since this void used to be occupied by a T4O4 four-ring (T = Si or Ge) in ITQ-21 and ITQ-26, it can be considered as an ordered Ge4O4 vacancy. Compared with the ordered Si vacancy in SSZ-74, such a large Ge4O4 vacancy in PKU-14 is unprecedented and energetically unfeasible. Through the careful PXRD refinement, a unique (H2O)2 dimer was found in the Ge4O4 vacancy of PKU-14 (Figure 3 a). The water molecules in the (H2O)2 dimer are located on the two-fold rotation axis with the Ow1···Ow2 distance of 2.775 , which is comparable to the corresponding distance of 2.759  in ice Ih at 90 8C.[29] Consequently, the (H2O)2 dimer is hydrogen-bonded to the eight Ge OH terminals inside the [46.612] cage with the Ow···O distances of 2.753–2.770 . The 1H MAS NMR spectrum of PKU-14 (Figure S11, Supporting Information) confirms these hydrogen-bonds, in which the peak at d = 3.0 ppm corresponds to the hydrogen atoms in water molecules, and the band at d = 13.8 ppm is from the hydrogen atoms in terminal hydroxyl groups.[30] These strong hydrogen bonds indicate that the water dimer plays a structure-directing role in the formation of the framework and stabilizes the interrupted [46.612] cage in PKU-14. Similar water clusters were reported in some other inorganic open-framework structures, Chem. Eur. J. 2014, 20, 16097 – 16101

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such as the triple helix (H2O)7 cluster in the aluminophosphate zeolite VPI-5,[31] the (H2O)17 clusters with pentagonal water arrangements in the zinc phosphate MIL-74,[32] the dodecahedral (H2O)20 cluster in zeolite NaA,[33] and the cubic-like (H2O)16 clusters in a pseudo-zeolite Alike silicate[34] and a nanotube germanate JLG-5.[35] In these reported structures, the inorganic

Figure 3. The transformation scheme between a) the monoclinic interrupted [46.612] cage in PKU-14 and b) the cubic interrupted [46.612] cage in dehydrated PKU-14. The different orientation of the four-ring inside the [46.612] cage caused by the symmetry: c) ITQ-26 (tetragonal) and d) ITQ-21 (cubic). The water dimer is shown in blue, and the terminal oxygen atoms are shown in red. The hydrogen bonds between the water dimer are marked as the blue dotted lines.

hosts provide big voids to encapsulate the water clusters of different geometries; on the other hand, these water clusters stabilize the frameworks by forming hydrogen bonds with the inorganic hosts and are believed to play a templating or structure-directing role in the formation of the cavities or channels (Zn-MIL-74 is an exception, because the inorganic host imprints its shape to the (H2O)17 clusters). Because none of these structures possess ordered vacancies in their cavities, PKU-14 is the first one presenting the templating role of (H2O)2 in the formation of ordered framework vacancies. In addition to stabilizing the framework, the water dimer and its interaction with the internal OH groups are also the causes for the [46.612] cage distortion. Along the b-axis in which the (H2O)2 dimer is located, the cages are squashed, which not only creates the weak hydrogen-bonding between the internal OH groups (3.0–3.1  for O O distances), but also induces a shorter cage diameter of 6.9 , as compared to

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Communication the 8.3 and 8.1  along other directions (assuming that the van der Waals radius of oxygen is 1.35 ). We suppose that after the removal of the (H2O)2 dimer, the distortion of the [46.612] cage would be released. To confirm this assumption, the PKU-14 sample was heated at 180 8C to remove the (H2O)2 dimer, and the structural changes were monitored by in situ PXRD as shown in Figure S13, Supporting Information. It is noteworthy that the water molecules can pass through the sixrings of the [46.612] cage in some distorted way since the Ge O bond distances (1.74 ) are larger than that of the Si O distances (1.61 ), and this is also confirmed by the TG analysis of the heated sample (Figure S12, Supporting Information). At 180 8C, the release of water results in a phase transition to the so-called dehydrated PKU-14, and its PXRD patterns can be indexed by a cubic unit-cell (Fm-3c) with a = 27.7683(8) . The Rietveld refinement in Figure S14 (Supporting Information) confirms that without the water dimer dragging from inside of the cage, the [46.612] cage is released to show a high cubic symmetry, different from the squashed-shape in the as-synthesized PKU-14 (Figure 3 b). Interestingly, after re-hydration for seven days in air, the dehydrated PKU-14 can transform back to the original monoclinic PKU-14 (Figure S13, Supporting Information), as evidenced by its unit cell (I2m, a = 19.7345(8), b = 27.112(1), c = 19.7101(8) , and b = 90.567(8)8) which is similar to that of as-synthesized PKU-14 (the small differences might be due to the partial rehydration). The reversible phase transformation between the monoclinic PKU-14 and dehydrated cubic PKU-14 proves that the water dimer plays an important role in the distortion of the [46.612] cage. Such a reversible phase transformation was first found in VPI-5 (VFI, P63/mcm), which can transfer to AlPO-8 (AET, Cmc21) upon heating in the presence of moisture, and then transform back to VPI-5 upon cooling.[36] Similar to that in PKU-14, the water-based triple helix in VPI-5 played a primary role in the phase transformation. In some natural zeolites, reversible-phase transformation was also reported. For example, stellerite-Ca (STI, Fmmm) changed to phase B (Amma) at 400 8C and then rehydrated to the original stellerite after one year,[37] and levyne–Ca transformed to a new levyne B phase after breaking the T O T bond at 308 8C and then partially transformed back to levyne.[38] In these natural zeolites, the reversibility of the dehydration is mainly due to the interactions between extra-framework cations (Na + , Ca2 + ) and the framework oxygen atoms. Due to the different arrangements inside the [46.612] cages, different zeolites exhibit different symmetries. In ITQ-26 (Figure 3 c), all the inner four-rings are oriented normal to the caxis, so the symmetry of the [46.612] cage is tetragonal and the cage diameter in the c-direction is shortened. In ITQ-21, the central four-ring has a random orientation along three directions (Figure 3 d), so the refined structure is averaged which makes the [46.612] cage and framework cubic. Similar to that of ITQ-26, in PKU-14, the insert of the (H2O)2 dimer along the baxis results in a tetragonal [46.612] cage, together with a shorter cage diameter along the b-axis. The further reduced monoclinic symmetry of the interrupted [46.612] cage in PKU-14 might be due to the weak interaction between the OSDA and the cage. After the release of the (H2O)2 dimer, the interrupted Chem. Eur. J. 2014, 20, 16097 – 16101

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[46.612] cage in dehydrated PKU-14 exhibits a cubic symmetry, which also resembles the cubic cage in ITQ-21. It is noted that the structure of dehydrated PKU-14 and as-synthesized ITQ-21 is difficult to be distinguished by PXRD patterns owing to their same symmetry and similar unit-cell parameters, and the lack of the inner four-rings in dehydrated PKU-14 only slightly induces relative intensity changes. The symmetry comparison of ITQ-26 and ITQ-21, and PKU-14 and dehydrated PKU-14 indicates the flexibility of the [46.612] cage. The in-situ PXRD study (Figure S15, Supporting Information) indicates that PKU-14 is thermally stable until 150 8C. Between 150 and 200 8C, the original PKU-14 was gradually transferred to the dehydrated PKU-14 due to the destruction of the hydrogen bonds between water molecules and framework oxygen atoms. The new phase was stable up to 350 8C, and then collapsed after the removal of the OSDAs. Because of the poor thermal stability of PKU-14, the ozone treatment was used to remove the OSDAs in the framework. The preliminary result of the N2 isotherm measurements of PKU-14 after the ozone treatment shows a BET surface area of 347 m2 g 1 (Figure S16, Supporting Information). In summary, we have successfully synthesized a novel zeolite PKU-14, which contains a 3D 12-ring channel system. Ordered Ge4O4 vacancies inside the [46.612] cage were found in PKU-14, in which a unique (H2O)2 dimer is located at the vacancy and plays a structure-directing role. For the first time, we found the water dimer plays a templating role in the formation of the framework vacancy. PKU-14 also shows a reversible phase transformation with the removal and resorption of water. Once again, the key to the structure determination of complex zeolite frameworks proved to be the combination of PXRD, RED, NMR, and IR spectroscopy.

Acknowledgements This work is supported by the National Natural Science Foundation of China (21171009, 11275012, 21471009), the State Science and Technology Commission of China (2012CB224802, 2010CB833103), the Swedish Research Council (VR) and the Swedish Governmental Agency for Innovation Systems (VINNOVA) through the Berzelii Center EXSELENT and Rçntgen-ngstrçm Cluster. The structure characterization by TEM was supported by the Knut & Alice Wallenberg Foundation through a grant for purchasing the TEM and the project grant 3DEMNATUR. Keywords: electron microscopy · porous materials · structure elucidation · zeolite analogues · X-ray diffraction [1] D. W. Breck, Zeolite Molecular Sieves: Structure, Chemistry, and Use, Wiley, New York, 1974. [2] R. M. Barrer, Hydrothermal Chemistry of Zeolite, Academic Press, London, 1982. [3] J. V. Smith, Chem. Rev. 1988, 88, 149 – 182. [4] A. Corma, Chem. Rev. 1995, 95, 559 – 614. [5] C. S. Cundy, P. A. Cox, Chem. Rev. 2003, 103, 663 – 701. [6] R. M. Barrer, Zeolites 1981, 1, 130 – 140.

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A 3D 12-ring zeolite with ordered 4-ring vacancies occupied by (H2O)2 dimers.

A germanate zeolite, PKU-14, with a three- dimensional large-pore channel system was structurally characterized by a combination of high-resolution po...
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