Article pubs.acs.org/crystal
A Germanate with a Collapsible Open-Framework A. Ken Inge,*,† Kirsten E. Christensen,‡ Tom Willhammar,† and Xiaodong Zou*,† †
Inorganic and Structural Chemistry and Berzelii Center EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden ‡ Department of Chemistry, University of Oxford, Oxford OX1 3TA, United Kingdom S Supporting Information *
ABSTRACT: A novel open-framework germanate, |NC2H8∥N2C6H18| [Ge7O14.5F2]·4H2O denoted SU-65 (SU = Stockholm University), with 24-ring channels and a very low framework density of 8.9 Ge atoms per 1000 Å3 was synthesized under hydro-solvothermal conditions. The framework of SU-65 is built of 5-connected Ge7 clusters decorating the fee net and is a framework orientation isomer to ASU-16. Half of the 8and 12-rings in ASU-16 are instead 10-rings in SU-65 due to the different orientations of half of the clusters in the crystal structure. Flexibility of the frameworks is also influenced by the orientation of the clusters. The unique unit cell angle in SU-65 changes upon heating, unlike ASU-16 which only undergoes changes in unit cell lengths. SU65 undergoes significant structural changes at 180 °C in a vacuum, forming SU-65ht. The crystal structure of SU-65ht was investigated by rotation electron diffraction, X-ray powder diffraction, and infrared spectroscopy. Through these techniques it was deduced that SU-65ht has similar clusters, symmetry, and topology as SU-65, but one of the unit cell lengths is shortened by approximately 5 Å. This corresponds to a 22% decrease in unit cell volume.
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INTRODUCTION Open-framework oxides1 such as zeolites,2 phosphates,3−6 and germanates7,8 are of interest for potential applications in sizeand shape-selective heterogeneous catalysis, separation, and ion exchange.9 While the synthesis and design of materials with novel framework topologies remain as popular research topics, interest has also been focused on the synthesis of families of related framework isomers.10 Zeolite beta has five polytypes, often referred to as the zeolite beta polymorphs. All of these polymorphs can be described by varying the stacking sequence of the same layered building unit. While zeolite beta polymorphs B,11 C,12,13 D,14 and E14 are achiral, polymorph A comprises chiral channels and thus is of interest for enantiomeric selectivity.15 Similarly, SU-15 are SU-32 are silicogermanate zeolites with 3D frameworks related by similar layers.16 Neighboring layers are symmetry-related by an inversion center in SU-15, whereas in SU-32 they are related by either a 6 1 - or 6 5 -screw axis, depending on the enantiomorph, resulting in helical channels. However, controlling the synthesis conditions to yield one pure polytype or framework isomer can be a challenging task as exemplified by the competition of nucleation between SU-15 and SU-32, and the intergrowth of zeolite beta polytypes. Framework isomerism has also been observed in openframework germanates, a class of oxides that often have interesting structural features, such as extra-large pores. The majority of open-framework germanates are constructed of large composite building units (CBUs), such as the Ge7X19 (X = O, OH, F) cluster17−38 (Ge7 cluster) as illustrated in Figure © XXXX American Chemical Society
1. With the availability of large CBUs, open-framework germanates with extra-large pores can be readily constructed
Figure 1. Ge7 cluster in (a) ball-and-stick and (b) polyhedral representations. Tetrahedral, trigonal bipyramidal, and octahedral germanium atoms/polyhedra are shown in green, yellow, and red, respectively. Oxygen/fluorine atoms are shown in white.
via scale chemistry.39 ASU-16 (ASU = Arizona State University) is a remarkable germanate constructed of fiveconnected Ge7 clusters and has flexible 24-ring channels.26 The channels flatten with increasing temperature. The analogous silicogermanate, SU-12, has a similar arrangement of Ge7 clusters that crystallizes in a higher symmetry tetragonal system, compared to the orthorhombic lattice system of ASU-16.28 Hydrogen-bonding between the framework and organic templates, as well as the introduction of silicon, results Received: August 3, 2016 Revised: November 1, 2016 Published: November 11, 2016 A
DOI: 10.1021/acs.cgd.6b01150 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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(0.300 g), distilled water (0.40 mL), dimethylformamide (DMF, 4.00 mL), 1,6-diaminohexane (DAH, 0.8 mL), and 40 wt % hydrofluoric acid (0.50 mL) with a molar ratio of 1:7.7:18.0:3.8:1.7. The mixture was sealed in a 22 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 7 days under autogenous pressure. Small rod-shaped crystals of SU-65 (20 × 4 × 4 μm3) were filtered, rinsed with distilled water, and dried at 60 °C overnight. Structure Determination. Single crystal X-ray diffraction data were collected on a Rigaku Saturn724+ diffractometer at 100 K using synchrotron radiation (λ = 0.6889 Å) at the Beamline I19, Diamond Light Source, Didcot, UK. Data reduction and absorption correction were applied using CrysAlisPro, and the structure was solved and refined using SHELXS and SHELXL, respectively.52 Crystallographic and refinement details are provided in Table S1 in the Supporting Information. Characterization. Scanning electron microscopy was performed on a JEOL JSM-7000F with BSE, EDS, and WDS options. A scanning electron micrograph of SU-65 is shown in Figure S1 in the Supporting Information. Thermogravimetric (TG) analysis was performed on a PerkinElmer TGA7 under nitrogen atmosphere. A sample of SU-65 was placed in a platinum crucible and heated from 50 to 900 °C with a heating rate of 10 °C min−1. In situ X-ray powder diffraction (XPD) was conducted on a PANalytical X’Pert PRO MRD equipped with an Anton-Parr XRK900 reaction chamber using Cu Kα1,2 radiation (λ = 1.5418 Å) and variable slits. The samples were heated from room temperature to 500 °C with a heating rate of 7 °C min−1. Temperature was equilibrated for 2 min prior to each measurement. Separate experiments were run under vacuum, in air, and in nitrogen. Automatic stage height adjustments to compensate for the linear thermal expansion of the stage were applied in the experiment under vacuum. Such adjustments were not available for the experiments in air and nitrogen, so instead a silicon standard was used. The changes to the unit cell parameters were examined by performing Le Bail fits using TOPAS-Academic V4.53 SU-65 was observed to undergo significant structural changes at 180 °C in vacuum, to form SU-65ht. Rietveld refinement on SU-65ht was performed using TOPAS-Academic V4. Infrared (IR) spectroscopy was performed on a Varian 670 to compare Ge−O bands between SU-65 and SU-65ht. Topological analysis was performed with TOPOS 4.054 and Systre.55 Rotation electron diffraction (RED)56,57 data were collected using the software RED data collection on a JEOL JEM-2100 transmission electron microscope equipped with LaB6 filament and operated at 200 kV. The sample was prepared by crushing it in an agate mortar, dispersing it in ethanol, and adding a droplet of the dispersion onto a TEM grid. The RED data were collected by acquiring 70 selected area electron diffraction (SAED) patterns in steps of 1° on a crystal with an arbitrary orientation in the range from −5° to +64°. The electron diffraction patterns were reconstructed into a 3D data set using the software RED data processing.57
in more rigid cylindrical channels in SU-12. The allowed Si− O−Si bond angles are more restricted compared to Ge−O−Ge angles, which influences the geometry and flexibility of the channels. Another family of germanates include the layered 2periodic structures of ASU-20,20 SU-23,21 SU-71,31 |C8H24N4| [NbOGe6O13(OH)2F],34 and |(H3dien)0.5(H2O)2|[Ge7O14F3· 0.5[In(dien)2]],29 all built of four-connected Ge7 clusters arranged in a 44 net. While the underlying nets of these five structures are identical if the Ge7 clusters are represented as single point nodes, these layered structures are unique in terms of the orientation of the Ge7 cluster, giving rise to orientation isomers, neglecting the presence of different guest species. Orientation isomers have the same CBUs but orientated in different ways in the crystal structure.10 The orientations of the clusters are influenced by hydrogen-bonds with various structure directing agents (SDAs), such as solvent molecules and ammonium species. Although the majority of porous materials are often regarded as rigid frameworks, a number of porous oxides and inorganic− organic hybrid framework materials have flexible frameworks that adapt to physical or chemical stimuli. The unit cell parameters and volume of a flexible crystalline framework material can vary upon changes in temperature, pressure, or exchange/removal of guest species in the pores. While many metal−organic frameworks such as MIL-5340 and MIL-8841 are well-known for their flexible frameworks,42,43 large framework flexibility was only reported in a few inorganic oxides. Zeolites of the ABW,44 NAT,45 and RHO46 framework types can exhibit profound changes to their unit cell lengths and volumes. The unit cell volume of zeolite RHO increases by 25% from 2720 Å3 (cubic, a = 13.96 Å) to 3443 Å3 (cubic, a = 15.10 Å) when calcium ions in the pores are replaced by hydrogen ions.47,48 Dehydration and ion exchange of RHO leads to significant distortion of the framework, altering not only the unit cell parameters but also the symmetry of the crystal structure. When heated the unit cell volume of natural natrolite (NAT), one of the first zeolite structures solved in 1930 by Linus Pauling, decreases by 21% resulting in metanatrolite.49,50 In yet another example, the unit cell volume of zeolite ABW decreases by 23% upon dehydration, flattening the 8-rings.51 Zeolites with the GIS, LTA, and FAU framework types also show framework flexibility although to lesser extents (ΔV < 10%).50 Here we report a novel open-framework germanate, SU-65, formulated as |NC2H8∥N2C6H18|[Ge7O14.5F2]·4H2O with Ge7 clusters decorating the fee net. The uninodal 5-connected fee net is built from the stacking of planar 4.82 nets. ASU-16 and SU-12 also have Ge7 clusters decorating the fee net. While differences between these two previously known structures involve bond angles, symmetry, and chemical composition, the unique orientations of the clusters in SU-65 give rise to an entirely different orientation framework isomer.10 Control over the shape of the framework and its dynamics may allow finetuning of host−guest interactions important in specific sizeand shape-selective applications. Herein we discuss how the template interactions direct the formation of certain structural features, and describe the unusual structural changes of SU-65 when subjected to heating in various environments. Such significant structural changes have not been reported in the related orientation isomers.
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RESULTS AND DISCUSSION SU-65 has a framework built entirely of Ge7 clusters. The Ge7 cluster consists of four tetrahedral GeX4 (where X = O or F), two trigonal bipyramidal GeX5, and one octahedral GeX6 (Figure 1). In the structure of SU-65 all four tetrahedra and one of the two trigonal bipyramids in each Ge7 cluster connect to neighboring clusters resulting in five-connected clusters in the T4P connection mode.18 8-ring and extra-large 24-ring channels are aligned parallel to the a-axis, with rings built of four and eight Ge7 clusters, respectively. The framework can be described as layers of Ge7 clusters in the bc-plane (Figure 2) stacked and connected along the a-axis. Two layers exist in each unit cell and neighboring layers are related by inversion symmetry. The 24-ring channels are interconnected by 10-ring windows, whereas the 8- and 24-rings channels share 8, 10, and 12-ring windows, resulting in a 3D channel system.
EXPERIMENTAL SECTION
Synthesis of SU-65. SU-65 was prepared under hydrosolvothermal conditions from a mixture of germanium dioxide B
DOI: 10.1021/acs.cgd.6b01150 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. Polyhedral representation of a layer of Ge7 clusters in the bcplane in SU-65 viewed along [100].
The Ge7 clusters in SU-65 decorate the fee net as in ASU-16 and SU-12. The three phases however vary in symmetry and lattice parameters (Table 1). The framework structures of SUTable 1. Symmetry and Unit Cell Parameters of SU-65, ASU16, and SU-12a space group lattice system a/Å b/Å c /Å β/° Framework atoms
SU-65
ASU-16
SU-12
I2/a (15) monoclinic 16.788(3) 25.811(5) 29.117(6) 94.34(3) Ge, O, F
I222(23) orthorhombic 16.9109(8) 24.267(2) 30.210(3) 90 Ge, O, F
I42̅ m (121) tetragonal 27.646(2) 27.646(2) 17.089(1) 90 Ge, Si, O, F
a
Due to the standard settings of the tetragonal lattice, the 24-ring channels in SU-12 run parallel to the c-axis rather than the a-axis as in SU-65 and ASU-16.
65 and ASU-16 appear similar down the 24-ring channels, but a close examination of the walls of the channels reveals significant structural differences. Unlike the framework of the silicogermanate SU-12 which is related to the germanate ASU-16 by variations in bond angles and element type, the orientations of the Ge7 clusters are unique in SU-65. Bonds between clusters would have to be broken, the clusters rotated and then reattached to transform between the frameworks of ASU-16 and SU-65. The differences in the orientations of the Ge7 clusters are more easily visualized by representing Ge7 clusters as rectangles (Figure 3).21 The protruding vertex on each of the four GeX4 tetrahedra mark a corner of the rectangle. The rectangles are colored red if the octahedral germanium atom points up or yellow if the trigonal bipyramids instead point up toward the viewer. Half of the Ge7 clusters in SU-65 have the same orientation as those in ASU-16/SU-12, while the other half are rotated as shown in Figure 3. 8-rings are formed by four Ge7 clusters perpendicular to the bc-plane. The sequence of long (L) and short (S) edges of the rectangles that circumscribe the 8-rings are LLSS in ASU-16/SU-12 and SLSL in SU-65. A 3D simplification with trigonal prisms representing Ge7 clusters instead of rectangles is also presented in Figure S2 in the Supporting Information.
Figure 3. Simplification of Ge7 clusters in ASU-16/SU-12 and SU-65. The clusters are represented as rectangles colored in accordance with whether the octahedral site points up (red) or down (yellow). The Ge7 clusters forming the 8-ring are arranged in the sequence LLSS in ASU-16/SU-12 and SLSL in SU-65.
Due to the different orientations of the Ge7 clusters, half of the 8- and 12-rings found in ASU-16 become 10-rings in SU-65. Projections perpendicular to the 24-ring channels also show clear discrepancies in framework symmetry between SU-65 and ASU-16 (Figure S3). The projection along [010] in ASU-16 shows overlapping rings with similar size. In fact, the rings stacked along the projection are related by a twofold axis. In SU-65 rings with varying sizes overlap in the projection (i.e., 12-rings overlap with 8-rings), creating a unique 3D channel system compared to ASU-16. Depending on how nodes are defined the underlying nets of ASU-16 and SU-65 can either be topologically identical or distinctly different. It should be noted that ASU-16 and SU-12 are topologically indistinguishable regardless of how nodes are C
DOI: 10.1021/acs.cgd.6b01150 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. Hydrogen-bonds between SDAs and the frameworks of ASU-16, SU-12 and SU-65. (a−c) SDAs in the 24-rings. (d−l) SDAs in rows of Ge7 clusters viewed perpendicular to the 24-ring channels. Carbon, nitrogen, and oxygen atoms of the guest species are colored black, blue, and red, respectively. Hydrogen-bonds are shown as dashed magenta lines.
in the 10-ring windows that connect the 24-ring channels with one another (Figure 4i). Each nitrogen atom forms hydrogenbonds to four framework oxygen atoms. DMA+ was found in a separate row of 10-rings (Figure 4l) that connect the 8- and 24ring channels. Part of another H2DAH2+ (one nitrogen atom and the adjacent carbon atom) was located in the center of an 8-ring forming hydrogen-bonds to six oxygen atoms across four Ge7 clusters (Figure 4f). The presence of DMA+ likely plays a crucial role in the formation of SU-65. Rows of 10-rings are found in SU-65 (Figure 4l), instead of the alternating 8- and 12-rings in ASU-16 (Figure 4j) and SU-12 (Figure 4k). DMA+ cations keep the ring-size and shape consistent in all of these 10-rings of SU-65. Hydrogens on the parent nitrogen atom form hydrogen-bonds to the framework, in this case four, while those on parent carbon atoms are positioned further away from framework atoms. The planar arrangement of the two carbon atoms and nitrogen atom of the DMA+ cation is approximately perpendicular to the plane of the ring (Figure S4). 10-rings containing DMA+ and formed by four Ge7 clusters were also discovered in another open-framework germanate, ASU-12.17 In both ASU-12 and SU-65 two Ge7 clusters form hydrogenbonds with DMA+ and these clusters each contribute three germanium atoms to the 10-ring while the remaining two clusters that do not form hydrogen-bonds to DMA+ contribute only two germanium atoms to the ring resulting in teardrop shaped rings. 1,4-Diaminobutane (DAB) was the only organic SDA used to prepare ASU-16, and 1-aminopropane (AP) for SU-12. HAP+ was found in the 8-rings in SU-12 with no ordered SDAs located in the center of the 12-rings (Figure 4e and k). Two
defined. If the Ge7 clusters are simplified as single nodes, then topologically SU-65 and ASU-16/SU-12 become indistinguishable as the clusters in all three structures decorate the fee net. However, information regarding the orientation of the cluster is entirely lost in such representations. In order to take into account the orientation of the cluster the individual germanium atoms should be selected as nodes, as one would in order to determine the underlying net of a zeolitic structure. Two distinctly different nets are obtained from SU-65 and ASU-16/ SU-12 when the germanium atoms are instead selected as nodes. Submitting the net obtained by ASU-16 (space group I222) or SU-12 (I4̅2m) to Systre indicates that the intrinsic symmetry of the underlying net is that of the crystal structure of SU-12 (I4̅2m). However, submitting the net of SU-65 into Systre indicates that the intrinsic symmetry of the underlying net is that of the crystal structure of SU-65 (I2/a). Thus, SU-65 crystallizes in the highest possible symmetry for its framework, and is in fact topologically distinct from ASU-16/SU-12. On the other hand, ASU-16 and SU-12 are topologically identical, and ASU-16 can be regarded as a lower symmetry variant of SU-12 if framework element type and guest species are neglected. The contrasting orientations of the Ge7 clusters are attributed to the framework interactions with the different SDAs used for synthesis. The positions of organic SDAs in ASU-16, SU-12, and SU-65 were compared to understand the variations in ring size and cluster orientation (Figure 4). Both H2DAH2+ and (CH3)3NH2+ (dimethylammonium cations = DMA+) were found as organic SDAs in the rings of SU-65. The solvent, DMF, converts into DMA+ during hydrothermal treatment under the basic conditions.20 H2DAH2+ was located D
DOI: 10.1021/acs.cgd.6b01150 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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HAP+ cations were also found in every 10-ring connecting the 8- and 24-ring channels (Figure 4h). In ASU-16 H2DAB2+ directs the formation of 10-rings and 12-rings. H2DAB2+ in the 12-rings is spread across in the plane of the ring and the two nitrogen atoms are found on the same face of the ring (Figure 4d). In the 10-rings, H2DAB2+ passes through the ring and the nitrogen atoms form hydrogen-bonds across opposite sides of the ring (Figure 4g). Apart from solvent water molecules no ordered organic SDAs were located in 8- and 12-rings for ASU16 (Figure 4j) in place of the 10-rings in SU-65 (Figure 4l). It should be noted that not all SDAs were crystallographically located in these structures due to positional disorder. Characterization and Phase Transition Studies. CNH analysis indicated weight percentages of 9.49%, 4.13%, and 3.04% for carbon, nitrogen, and hydrogen, respectively, and an atomic ratio of 2.6:1:10.2. The C/N ratio is lower than 3 indicating the presence of organic guest species besides H2DAH2+ in accord with the X-ray diffraction data. CNH analysis suggests the presence of two DMA+ and two H2DAH2+ counterions per asymmetric unit, which also includes two Ge7 clusters each with a charge of −3 (calc. CNH ratio 9.51, 4.16, 3.59 wt %). EDS analysis indicated a ratio of Ge:O:F as 13.88:35.37:3.79 which agrees well with the calculated ratio of 14:35:4 and indicates that all terminal species bonded to Ge are F rather than OH. TG analysis in nitrogen atmosphere (Figure S5) indicates a weight loss of 7.1% between 50 and 150 °C attributed to the loss of eight water molecules per asymmetric unit (calc. 7.1%). Between 250 and 370 °C a 13.1% loss occurred, comparable to the expected value for two H2DAH2+ counterions (calc. 11.6%). This was then followed by another loss of 6.5% from 370 to 550 °C which was attributed to the loss of two DMA+ ions (calc. 4.5%). Based on the results from X-ray diffraction, EDS, CHN, and TG analyses the chemical formula of SU-65 was determined as |NC2H8∥N2C6H18|[Ge7O14.5F2]·4H2O. In situ XPD patterns of SU-65 recorded in vacuum, air and nitrogen, shown in Figures 5, S6, and S7 respectively, revealed varying behavior. In air SU-65 remains stable until 220 °C. At higher temperatures crystallinity severely degrades and the material becomes X-ray amorphous. Similar observations were made in nitrogen atmosphere. From 20 to 220 °C the a-axis of the sample heated in air decreases steadily from 16.7 to 16.5 Å while the unique angle β decreases from 94.6° to 93.0°. The
changes in unit cell lengths are subtle when compared to the changes observed in ASU-16. In comparison, previous studies on ASU-16 indicated shortening of the c-axis and lengthening of the b-axis by over 2 Å upon heating, but no changes in unit cell angles were reported. Under vacuum the a-axis and β-angle of SU-65 have smaller values compared to the respective parameters in air at 30 °C (approximately 0.2 Å and 1.2° smaller). The values of these two parameters decrease with increasing temperature, while the other lattice parameters exhibit differing behavior at elevated temperatures (Figure S8). In stark contrast to the experiments performed in air and in nitrogen, in situ XPD of SU-65 under vacuum revealed significant and sudden changes at 180 °C. Bragg reflections corresponding to SU-65 diminished while a new set of reflections emerged. The phase formed at elevated temperatures under vacuum, denoted SU-65ht, was stable until 380 °C. SU-65ht retained its structure after cooling to room temperature and exposure to air for several days. Crystallinity was observed to degrade after 3 weeks at room temperature in air, and a sample that was kept in air for two years was found to be X-ray amorphous, unlike a sample of as-synthesized SU-65, which retained crystallinity during this time. The phase transformation was not found to be reversible when immersing the material in various solvents and mixtures. The unit cell parameters of SU-65ht could not be determined from the XPD data alone due to the widening and overlapping of reflections. Sufficient single crystal X-ray diffraction data for structure determination or refinement were not obtained. SU-65ht was subsequently studied by the rotation electron diffraction (RED) method56,57 to exploit the possibility to study very small single crystals (Figure S9). Although SU-65ht was beam sensitive, a data set of 70 SAED patterns was collected from one crystal and the 3D reciprocal lattice was reconstructed. Based on the reconstruction the unit cell parameters of SU-65ht in the TEM were successfully determined as a = 15.5 Å, b = 21.7 Å, c = 28.6 Å, α = 90°, β = 90°, and γ = 90°. These unit cell parameters were then refined using the Le Bail method against XPD data. The unit cell parameters of SU-65ht at 190 °C under vacuum were refined to a = 16.00 Å, b = 20.70 Å, c = 29.35 Å, α = 90°, β = 90°, and γ = 90°. The XPD data and RED data indicate systematic absences corresponding to the space group I2/a, the space group of SU-65. SU-65ht has a unit cell volume that is 22% smaller than that of SU-65. Discrepancies between the unit cell parameters determined by RED and XPD are in part attributed to the differences in data collection temperatures. Also, deformations in framework structures with organic SDAs can arise due to the influence of the electron beam. We have observed that germanates built of different cluster types can have noticeably different IR spectra.58 The IR spectrum of SU-65ht was recorded and compared to that of SU-65 (Figure S10). The similarities of the two spectra in regions corresponding to Ge−O vibrational modes (
