Synthesis and Study of the First Zeolitic Uranium Borate - Crystal

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Synthesis and Study of First Zeolitic Uranium Borate Yucheng Hao, Vladislav V. Klepov, Philip Kegler, Giuseppe Modolo, Dirk Bosbach, Thomas E. Albrecht-Schmitt, Shuao Wang, and Evgeny V. Alekseev Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01487 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 1, 2017

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Crystal Growth & Design

Synthesis and Study of First Zeolitic Uranium Borate

Yucheng Hao†, Vladislav V. Klepov§, Philip Kegler†, Giuseppe Modolo†, Dirk Bosbach†, Thomas E. Albrecht-Schmittξ, Shuao Wang∆ and Evgeny V. Alekseev†,‡,*

†

Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany ‡

Institut für Kristallographie, RWTH Aachen University, 52066 Aachen, Germany

§

Department of Chemistry, Samara National Research University, 443086 Samara, Russia

ξ

FSU Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306-4390, United States of America ∆

School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China

*contact E-Mail: [email protected]

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Abstract A complex, three-dimensional (3D), open-framework, lead uranyl borate, (H2O)Pb3(UO2)3B14O27, denoted as LUBO, was synthesized via a hydrothermal method. LUBO crystallizes in the hexagonal space group P63/m and exhibits a zeolite-like anionic borate framework (B14O27)12-. The main structural unit of the framework is a tubule consisting of 6-membered B6O18 rings. Each ring is connected to the successive one by three diborate groups, and these tubules propagate along the c axis. The tubules possess 6-membered ring (MR) windows in the axial direction and 8-MR windows on its sides. Interconnection of the parallel tubules, which consist exclusively of BO4 tetrahedra, is provided by triangular BO3 fragments perpendicular to the axis of the tubules. The framework has large pores as well as channels with 8-MR windows extending along the [100], [010] and [110] directions that are consistent with the overall hexagonal symmetry of the structure. The lead cations occupy 8-MR windows and form [Pb3(H2O)] groups with attached water molecules that are located at the center of the tubules. The method of Voronoi-Dirichlet tessellation reveals that the lone pairs of the lead cations are located outside the tubule. Uranyl cations form UO8 coordination polyhedra in the shape of a hexagonal bipyramid. The thermal stability and vibrational spectroscopy of LUBO are also delineated in this work.


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1. Introduction Inorganic open-framework materials and zeolites (microporous oxo-silicate based materials) have attracted considerable attention for decades. These materials are widely used in industry as catalysts, detergents, for ion-exchange/sequestration, and for gas adsorption and separations1-4. Actinide-bearing open-framework materials are particularly relevant in nuclear waste remediation5-8. The most common porous materials fall into the aluminate, silicate, germanate, and phosphates families9-13. While fewer borates adopt porous structures, the complexity of these compounds can rival both silicates and phosphates because boron is found within both BO3 triangles and BO4 tetrahedra in these compounds; whereas silicates and phosphates are restricted to [MO4] building units. Both BO3 and BO4 units have a tendency to polymerize yielding a vast array of extended structures14-17. To date, more than 100 borate minerals and more than 1000 synthetic inorganic borate phases have been characterized18-21. In the borate family numerous classes of ternary borate compounds, such as aluminoborates, borosilicates, borogermanates, and borophosphates have been synthesized through the incorporation of additional types of oxoanions22-26. However, borates with multi-directional open-framework structures are poorly represented thus far. For example, PbB4O7, a non-linear optical material of lead tetraborate, possesses a 3D boron-oxygen open framework structure, in which BO4 tetrahedra are assembled into a simple corner-linked tetrahedral network27. The boron network of PbB4O7 has one 6-membered ring (MR) open tunnel along the a and b axes, with dimensions of 4.5 × 4.5 Å. Pure actinide borates AnO2[B8O11(OH)4] (An = U, Np)28 crystallize into non-centrosymmetric crystals in the polar space group Cc in which four crystallographic unique BO4 tetrahedra and BO3 triangles share corners to form a borate framework with large pores consisting of 9MRs. The borate framework also has three types of channels, but helical nature of polyborate chains and twisting interlayer borate groups makes these channels less open. Another example of an actinide borate framework, Li[(UO2)B5O9](H2O) 29, has a 3D two-directional open-framework structure that is based on a layered motif that is common for the alkali metal uranyl borates family6. BO3 linkers further connect the borate layers to form a 3D borate network structure that includes 9-MR channels along the b-axis and 103 ACS Paragon Plus Environment


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MR tunnels along the a-axis. UO8 hexagonal bipyramids occupy the 9-MR tunnels and leave only one type of channel accessible for migrating species. (C4N2H12)U2O4F6 is the first open framework actinide material that to be reported. Its structure is templated by the piperazinium cation5. Subsequently, a series of organically-templated, porous actinide materials have been reported, such as [NC4H12]2[(UO2)6(H2O)2(SO4)7],30 (NH4)4[(UO2)5(MoO4)7](H2O)5 and other uranyl sulfates and selenites with 18-crown-6 templates31. A remarkable inorganic cationic framework [ThB5O6(OH)6][BO(OH)2]⋅2.5H2O32 was prepared by Wang et al. through a low-temperature flux method. This thorium borate material has a supertetrahedral cationic framework and possesses rare inter-channel borate anions that are readily accessible for replacement by tetrahedral anions, such as chromate

CrO42–

and

pertechnetate33 TcO4–.

The

first

reported

uranyl

borate

phosphate,

Ba5[(UO2)(PO4)3(B5O9)]·nH2O,34 has complex borate nanotubes with a large external diameter of 2 × 2 nm. This compound was obtained as a result of a high-temperature solid state reaction (>1000 ℃); however, its nanotubular fragments are isolated and do not adopt a real 3D open-framework structure. In the uranyl borate system, a plethora of alkali and alkali earth metal uranyl borates have been reported by Wang et al.6 and Wu et al.34, 35 Yet, most of them are layered structures due to the character of uranyl cations; only few of these compounds are 3D frameworks28-37. In the present work we report how a divalent post-transitional metal, Pb2+, can substitute for alkali or alkaline earth metal cations in the uranyl borate system. This material forms a zeolite-like uranyl borate, (H2O)Pb3(UO2)3B14O27, denoted as LUBO that contains multidimensional intersecting channels within a 3D borate framework structure. The synthetic route, zeolite-like structural topology, thermal analysis, and vibrational spectroscopy were investigated in detail.

2. Experiment Section Caution! The UO2(NO3)2·6H2O used in this work contained natural uranium; nevertheless, standard precautions for handling radioactive materials must be followed.


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2.1 Materials and Methods. Uranyl nitrate UO2(NO3)2·6H2O (International Bioanalytical Industries, Inc.), lead nitrate Pb(NO3)2 (Alfa-Aesar, 99.5%), and lithium tetraborate Li2B4O7 (VWR chemicals, 99.0%). 2.1.1 Syntheses of LUBO: The LUBO compound was obtained from a typical hydrothermal reaction. UO2(NO3)2·6H2O (0.0516 g, 0.10 mmol), Pb(NO3)2 (0.0332 g, 0.10 mmol), Li2B4O7 (0.0515 g, 0.30 mmol) and deionized water (0.8 ml), in a ratio of U : Pb : B = 1 : 1 : 12, were sealed into a teflon-lined stainless steel autoclave (23 ml) and then transferred into a box furnace, heated up to 220 °C, held 36 hours and then cooled down to 160 °C at a rate of 3 °C /h. It was then cooled down to room temperature with a cooling rate of 6 °C /h. The resulting products were washed with hot water and then rinsed with ethanol. Large yellow sphere-shaped crystals of LUBO were obtained. The crystals were ~ 0.2 mm large. The crystals with good qualities were collected for further analyses. Energy dispersive X-ray spectroscopy (EDS) elemental analysis (See Figure S1) on several single crystals gave average molar ratios of Pb : U = 1.02 : 1.00 for LUBO, which are in good agreement with those obtained from single crystal X-ray diffraction data analysis. 2.2 Crystallographic Studies and X-ray powder diffraction. Single crystal diffraction data of the title compound were collected on an Agilent Technologies SuperNova diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multi-scan method38. The crystal structure of the title compound was solved by direct methods and refined by a full-matrix least-squares fitting on F2 using SHELX-201439. Its structure was checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm, and no higher symmetry were found40. Crystallographic data and structural refinements for the title compound are summarized in Table 1. More information about the important bond distances and angles are listed in Table S1. X-ray powder diffraction data was measured on a Bruker-AXS D4 Endeavor diffractometer, 40kV and 40mA, in Bragg−Brentano geometry. The diffractometer is equipped with a copper X-ray tube and a primary nickel filter producing graphite monochromized CuKα1, 2 radiation (λ = 1.54187 Å). A linear 5 ACS Paragon Plus Environment


silicon strip LynxEye detector (Bruker-AXS) was used. Data were recorded in the range of 2θ = 10 − 80° with 10 s/step and a step width of 0.02°. The aperture of the fixed divergence slit was set to 0.2 mm and the receiving slit with 8.0 mm, respectively. The discriminator of the detector was set to an interval of [0.16 - 0.25 V]. 2.3 Scanning Electron Microscopy (SEM)/Energy-dispersive X-ray Spectroscopy (EDS) Analysis. For inspection of the crystals surface and elemental analysis, scanning electron micrographs and energydispersive X-ray spectroscopy (SEM/EDS) data were collected on a FEI Quanta 200F Environment Scanning Electron Microscope with a low-vacuum mode at 0.6 mbar. SEM/EDS results were given in Supporting Information (Figure S1). 2.4. Raman and IR Spectroscopy. A non-polarized Raman spectrum was recorded on a Horiba LabRAM HR spectrometer using a Peltier cooled multichannel CCD detector. An objective lens with a 50× magnification was linked to the spectrometer, allowing the analysis of samples as small as 2 µm in diameter. The samples were in the form of single crystals. The incident radiation was produced by a HeNe laser line at a power of 17 mW (λ = 632.8 nm). The focal length of the spectrometer was 800 mm, and an 1800 gr/mm grating was used. The spectral resolution was approximately 1 cm-1 with a slit of 100 µm. The spectrum was recorded in the range of 100−4000 cm-1. An infrared (IR) spectrum was measured using a polycrystalline sample mixed with KBr, on a Bruker Equinox 55 FT-IR spectrometer at room temperature. IR data (KBr) for LUBO are: 3435(s), 2924(w), 2857(w), 1629(m), 1384(w), 1380(w), 1344(s), 1118(m), 1024(s), 991(w), 877(s), 832(w), 794(w), 751(w), 725(w), 705(s), 645(w) 596(s), 565(w), 519(w), 445(s) cm−1. 2.5. Thermal Analysis (TG-DSC Experiments). The thermal behavior of polycrystalline LUBO in the range from room temperature to 1200 °C was studied by differential scanning calorimetry (DSC) analysis coupled with thermogravimetry (TG) in dry air at a heating rate of 10 °C/min using a Netzsch STA 449C Jupiter apparatus. The sample (10.5 mg) was loaded in a platinum crucible, which was covered by a platinum lid. an air flow of 20-30 mL/min was applied during the measurements.


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2.6. Bond-Valence Analysis. As a semi-empirical method for the approximate determination of the valence states, bond valence sums (BVSs) of all atoms in the lead uranyl borate phase were calculated and agree well with corresponding formal values of the oxidation states. The bond-valence parameters for U(VI)-O, Pb(II)-O, and B(III)-O were used according to Brese and O’Keeffe41-42. Whereas the bondvalence parameter of U(VI)-O was according to Burns43.

3. Results and Discussion 3.1 Synthesis. While investigating the Pb-U-B-O system under hydrothermal conditions, we obtained a novel lead uranyl borate with zeolite-like structure. It is important to note that this is the first actinide borate that obtained from a real hydrothermal reaction with a large water volume. Previously, most of the actinide borates were prepared via H3BO3 mild temperature flux reactions6, 19. The initial molar ratio of the reagents of 1 : 1 : 3 for UO2(NO3)2·6H2O, Pb(NO3)2, and Li2B4O7, respectively, was used with addition of 0.8 ml of water. This leads to the formation of single crystalline samples of LUBO. It is noteworthy that the reactions in similar synthetic conditions with slightly altered initial molar ratios, viz. 1:1:1, 1:1:2, and 1:1:4, did not lead to the formation of this compound. It implies that the initial ratios of the reagents are of great importance for the preparation of the final phases. The highest yield of the reaction is ~55% (based on U), which was achieved by increasing the reaction time to four days. The purity of the resulting phase was confirmed by powder XRD diffraction (See Figure S2). Further increase of the reaction time does not provide any reasonable improvement of the yield. It is interesting that we have used similar methods in the alkaline earth metal uranyl borates system with Ca, Sr and Ba but no zeolite-like phases were obtained. We suppose that the Pb2+ cation with its lone electron pairs has played a template role for the formation of the structure. The role of lead is similar to that of organic templates in synthesis of zeolites and other types of microporous structures30-31. As mentioned above, the synthetic method in the present work is different from low-temperature boric acid flux method used by Wang et al.6, 7

, high-temperature/high-pressure (HT/HP) hydrothermal syntheses by Wu et al.35, and slow evaporation



method implemented by Zhang et al44.. We have provided another new facile route for preparation of actinide borates which was not previous used for synthesis in this system. 3.2. Crystal Structure and Topology Description. LUBO crystallizes in the hexagonal space group P63/m (No. 176). The asymmetric unit contains single U and Pb atoms, three independent boron and seven O atoms. The main structural unit in this compound is a novel (B14O27)12– open borate framework that incorporates uranyl and lead cations. The uranium atoms have UO8 coordination polyhedra geometry forming hexagonal bipyramid. 3D anionic borate framework, (B14O27)12–, contains both BO3 triangles and BO4 tetrahedra (see Figure 1). The uranyl borates with alkaline and alkaline earth cations reported so far generally contain molecular, chain, or layer structures, with only few exceptions of 3D frameworks6, 19. In LUBO, the borate framework can be described as tubules connected through BO3 triangular borate groups. The tubule consists of 6-MRs formed by six BO4 groups (see Figure 2a), each ring is linked then to the successive one by three diborate B2O7 groups (see Figure 2b, Figure S3b). The tubules are extending along the c direction. The tubules are connected by the triangular borate groups, which share an oxygen atom with the bridging B2O7 dimers (see Figure 2c and 2d). Therefore, the framework contains channels with 6-MR windows along the c translation, and three 8-MR channels along the [100], [010], and [110] directions. A simplified anionic representation of the borate framework is shown on Figure 2a'–2d'. In the structure of LUBO, the BO3 triangles are parallel forming pairs of stacked BO3 groups. Although the exact nature of these contacts is unclear, the presence of an interaction between the boron atoms between the BO3 groups is confirmed by Voronoi-Dirichlet polyhedra (VDP) method45 (see Figure 3). According to this method, each atom of a crystal structure is represented as a polyhedron in which each inner point is closer to the selected atom (B3 here) than to any other atom of the structure46-48. The VDP solid angle corresponding to the B···B contact is equal to 6.5% (for comparison, the faces that correspond to the B-O bonds are 28.7% each), the distance between the boron atoms is ~2.96 Å, which is ~0.86 Å lesser than the doubled boron van der Waals radius according to Alvarez49. The BO3 groups have the shape of almost regular triangles with B-O bond lengths of 1.371(8) Å and O–B–O bond angles of 119.98(6)°. The remaining two crystallographic sorts of boron atoms, B(1) and B(2), form slightly 8 ACS Paragon Plus Environment

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distorted BO4 tetrahedra with B-O distances in the range of 1.452(15)-1.509(15) Å and O–B–O angles varying from 101.6(9)° to 114.0(10)° (see Table S1), which is consistent with the previously reported borate structures34-37. Calculated bond valence sum values for B(1), B(2) and B(3) are ca. 3.02, 2.96 and 3.03, respectively, which are agree well with the formal +3 charge of boron atoms. It is worth to compare the borate open framework of LUBO with the actinide free lead tetraborate PbB4O7, because both of them possess hexagonal tunnels in their 3D borate frameworks (see Figure S5). The 3D borate framework in PbB4O7 is based on B3O9 trimers (FBBs). The B(2)O4 tetrahedra share corners to form a chain along the c-axis, whereas B(1)2O7 dimers link the boron chains, completing the 3D borate open framework of PbB4O7. Both of the B6O6 6-MR B-rings in LUBO and PbB4O7 are plotted in Figure S6. The 6-MR in LUBO is a regular hexagon with a B-B distance of ca. 2.5 Å and B-B-B angle of 119.07°, whereas in PbB4O7 the channels are slightly distorted, and B-B distances are ca. 2.5 Å, 2.7 Å and 2.6 Å and B-B-B angles are 123.04°, 108.56° and 120.31° (see Figure S6). We presume that the uranyl group plays an important template role in formation of the regular hexagons within LUBO borate framework. The 6-MR channels form equilateral triangular windows with the edge size of ~3.4 Å, measured as the distance between oxo atoms in the vertices of the triangle ( see Figure 4c). The size of the 6-MR channel is ~3.4 Å × 4.2 Å, which is slightly smaller than those of the 6-MR channels observed in beta-eucryptite50. We speculated that, after a proper activation of the channels and removing the water molecules, this compound may be a good candidate for using as a molecular sieve for some small ions and molecules (such as H2, O2, N2 and NH3). The walls of the 6-MR channels contain 8- and 3-MR windows, in which the 8-MR windows are composed of eight BO4 tetrahedra, perpendicular to the 6-MRs (see Figure 4a). The edge size of triangular 3-MR windows is ~2.4 Å (see Figure 4e). The hexagonal LUBO has a 63 screw axis. It generates three symmetrically equivalent 8-MR trapezoid channels along [100], [010], and [110] with size ~ 3.6 Å × 4.6 Å .This channels are occupied by Pb2+ cations (see Figure 4f). BO3 groups link the described tubules into a framework to form 8-MR channels that are occupied by uranyl cations. The channels have rectangular shape with oxygen atoms in the vertices, and their size is 9 ACS Paragon Plus Environment


~2.8 × 4.0 Å (see Figure 4d). Uranium atoms occupy a centrosymmetric position in the center of these 8MRs and demonstrate hexagonal bipyramidal environments, sharing the equatorial edges with four different BO4 groups (see Figure S7a, S8). Due to the symmetry, the uranyl cation is linear, whereas its equatorial environment, being predetermined by the borate framework, is not ideally flat. The deviations of the oxygen atoms from the uranyl equatorial plane vary from 0.25 to 0.29 Å. The bond lengths in uranyl groups are equal to 1.793(10) Å and the equatorial U-O distances are in the range of 2.358(8)2.521(8) Å, which is consistent with previously reported uranyl borates14 (see Table S1). Despite the plane-deviation of the equatorial oxo-atoms, the Voronoi-Dirichlet polyhedron volume of the uranium is equal to 9.42 Å3 and agrees well with the average value of 9.3(4) Å for U6+ oxo-phases25. BVS calculations indicate that the uranium cations are U(VI) with a value of ca. 5.9743. The uranium center coordination geometry has a few differences compared to the alkali and alkaline earth metal uranyl borates reported in the literature6, 44. In Na6(UO2(B16O24(OH)8))(H2O)14, the UO8 hexagonal bipyramids are located within the eight boron rings. The 8-MRs are based upon eight corner sharing BO4 tetrahedra, with no BO3 triangles inside. In Wang et al’s investigations,6 a series of M+ (M = Li, Na, K Rb, Cs, Ag, Tl) uranyl borates share a common structural motif consisting of a linear uranyl have been reported. The uranyl cations of M+ uranyl borates are existed as UO8 hexagonal bipyramids surrounded by six BO4 tetrahedra and three BO3 triangles, as B9O9 rings, whereas they are B8O8 rings in the structure of LUBO. Within the other group of compounds, the alkaline earth uranyl borates family, the geometries of uranium centers environment are simpler because uranyl groups are only linked with BO3 triangles36 (see Figure S7 and S9). Lead cations, Pb2+, reside on the side of 8-MR in the borate tubules, completing their walls. Each Pb is eightfold-coordinated as determined by the method of intersecting spheres51. The Pb2+ cations form coordination polyhedra with a shape of bicapped tetragonal prism, where the vertices occupied by oxygen atoms of two edge- and one vertex-sharing BO4 groups, two uranyl oxo atoms, and water molecules. This single water molecule is bonded to three lead cations. Although the planar fragment Pb3O alludes to a metal-oxo-cluster, the oxygen atom most likely belongs to a water molecule. The presence of a water 10 ACS Paragon Plus Environment

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molecule in LUBO is supported by Raman spectroscopy and differential scanning calorimetry along with crystallographic data, i. e. Pb–O (~2.78 Å) bonds in Pb3(H2O) fragment is elongated as compared to the µ3-OPb3 oxo-clusters reported in CSD and ICSD so far (2.15-2.22 Å)52-53. The Pb-O bond lengths are in a wide range from 2.426(9) to 3.000(5) Å and the VDP solid angles corresponding to Pb-O bonds are in a range of 9.15–13.91% (see Figure S10 and Table S1). The Pb atoms have a stereochemically active electron lone pair, which is supported by the Da value showing the deviation of Pb atoms positions from the center of its VDP ~ 0.25 Å. It is in good agreement with the average 0.3(2) Å found for Pb atoms in oxo-environment54. The channels and cavity systems of the boron framework in LUBO can be illustrated using natural tiling by tracing the colors of the tiles clearly55 (see Figure 5). The boron framework is constructed by three types of tiles with [36•63•89], [63], and [36•62•83] face symbols. [36•62•83] cages connected with each other by a propeller shaped [63] t-kah unit (see Figure 5d), which is then linked to a larger cage [36•63•89] (see Figure 5b). The six and eight-MR channels can be seen more clearly with the tiling construction process (see Figure S11, S12 and S13). Uranyl cations reside in the six of nine 8-memebered rings. The remaining three 8-membered rings, which are shared with [36•62•83] tiles, are occupied by Pb2+ cations. The detailed tiling structures are given in Supporting Information. 3.3. Thermal Analysis. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis were performed in the range of 50 - 1200 °C and summarized on figure S14. The TG curve (black solid line) exhibits three prominent weight losses. The first one (0.65%) occurs at ca. 200 °C, and attributed to the loss of water (calculated 0.9%). The water loss at the temperature of approx. 200 °C implies that the water molecules are from the hexagonal channels. The second weight loss is accompanied by a strong endothermic peak at 696 °C on the DSC curve (blue solid line), which corresponds to a partial decomposition of the uranyl borate framework in LUBO. Taking into account the fact that overall weight loss is rather small, we can conclude that this compound and its boron framework in particular are very thermally stable and robust at least up to 696 °C. To confirm this, a polycrystalline sample of LUBO was calcinated at 696 °C for 10 h, resulting in the formation of a yellow solid powder. The PXRD pattern for 11 ACS Paragon Plus Environment


the calcinated sample is identical with the unheated material and reveals the polyborate framework is retained (see Figure S2). 3.4 Vibrational Spectroscopy. The assignment of the peaks in the Raman and IR spectra of LUBO was performed using the literature data35, 36, 44, 56. The Raman spectrum of LUBO contains a strong and sharp peak at 824 cm-1 due to the v1 symmetric stretch mode of uranyl (UO2)2+ units ( Figure 6a). The peaks in the range of 175-210 cm-1 could be assigned to the v2 mode of the uranyl ion. Raman bands in a region of 358–500 cm−1 are attributed to O-B-O symmetric bending ν2 mode in BO4; whereas v4 borate group bending vibration correspond to the peaks in a range of 500-790 cm−1. The Raman spectra show very weak peaks at around the range of (980–1200 cm−1) which have been assigned to O-B-O triply degenerated ν3 asymmetric stretching mode in BO4 tetrahedra. The weak peak at 1363 cm−1 has been attributed to the asymmetrical stretching ν3 mode in BO3 triangles group. Weak peaks corresponding to vibrational modes of water molecules are located at 3530 cm-1 on the Raman spectra. As shown in the IR spectrum on figure 7b, the strong peaks in 3435 cm-1 and 1629 cm-1 demonstrated the presence of H2O in the structure. The bands at 645−850 and 850−1000 cm−1 are attributed to the ν3 antisymmetric stretching vibrations and ν1 symmetric stretching vibrations of (UO2)2+ groups57. It is in good agreement with the bond lengths in uranyl groups to the corresponding positions of the bands58. Bands between 1110 cm-1 and 1390 cm-1 can be assigned to the antisymmetric stretching vibrations of the BO3 groups. The peaks associated with the BO4 groups appeared at around 1024 cm-1. The absorption peaks at the range of 400800 cm-1 can be assigned to the bending vibrations of the BO3 and BO4 units. 4. Conclusion A unique actinide borate LUBO containing a novel 3D borate open framework structure was obtained in mild hydrothermal conditions. It is important to note that this is a first successful attempt to obtain an actinide borate from a real hydrothermal reaction with a large water volume. Previously all of them were made via H3BO3 mild temperature flux reactions. The structure of LUBO was determined by X-ray crystallography and further confirmed by the Raman and IR vibrational spectroscopies. All bands observed in the Raman and IR spectra could be assigned to the corresponding groups in LUBO structure. 12 ACS Paragon Plus Environment

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Its anionic borate framework (B14O27)12- is built of tubules, which are linked through parallel triangular BO3 groups, resulting in multi-directional intersecting channels along the three axes. The simplified net corresponding to the framework of (B14O27)12- is a new 3, 4-coordinated 3-nodal net with a point symbol of {3.6.84}3{32.6.72.8}3{63}. The topology of the covalent boron-oxygen framework is quite complex and rare compared to the previous reported borate network structures. The thermal behavior of LUBO and calcinated PXRD pattern suggest that its borate framework is very robust and can be stable up to temperatures as high as 696 ℃ as demonstrated by the TG-DSC analyses. This fact is supposedly due to abscess of OH group in borate framework which usually leads to the relatively low poly-condensation of the framework with its simultaneous collapse. The Pb2+ cations reside on the side of the tubules and complete its walls with the lone electron pairs outside. The Uranyl groups UO22+ are fitted into windows in the borate framework and have slightly distorted hexagonal bipyramidal environments. The water molecules are located on the axes of the 6-MRs channels. The removal of water from the 6-MRs tubules could produce a potential molecular sieve for small molecules. It is remarkable that the introduction of Pb2+ cations, leads to the formation of novel borate framework with a lack of previously reported analogues. This demonstrated that the Pb2+ cations with lone electron pairs have played a key template role for the formation and stabilization of the (B14O27)12- 3D borate framework. Moreover, the isolation of LUBO has provided a new route for synthesis of the actinide borates with 3D open framework structures. Based on the synthetic condition of LUBO, our future work will focus on the designing and synthesizing actinide borates materials through introducing different polyvalent cations, such as Zn2+, Cd2+, Hg2+, Bi3+ etc. which could generate the high porosity 3D open framework structures. More importantly, these 3D open frameworks can be potential matrixes for cleaning up of the nuclear wastes19.

Supporting Information. X-ray crystallographic files in CIF format, selected bond lengths and angles, EDS measurement, XRD patterns, detailed tiling figures.



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Acknowledgments Authors are grateful to Helmholtz Association for funding within VH-NG-815 project. SW is

supported by National Natural Science Foundation of China (21422704).

This work was

supported as part of the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0016568. Authors are grateful for Dr. Klinkenberg, Dr. Schlenz, Mrs. Paparigas for help in Raman, EDS and IR data collection. YH thanks to the support of Chinese Scholarship Council.

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Table 1. Crystal Data and Structure Refinements for LUBO. Formular unit (H2O) Pb3(UO2)3B14O27(LUBO) -1 Formula weight /g mol 2033.04 Space group P63/m a (Å) 10.9331(2) c (Å) 11.9337(3) 3 V (Å ) 1235.36(4) Formula units/cell (Z) 2 λ( Å) 0.71073 F(000) 1728 -3 Dc(g cm ) 5.460 2 GOF on F 1.017 R1 0.0448 wR2 0.1077 R1 = ∑||Fo| -|Fc||/∑|Fo|, wR2 = {∑w[(Fo)2 - (Fc)2]2/∑w[(Fo)2]2}½

Figure 1. View on the structure of LUBO along the c-axis. Uranyl polyhedra, BO3 triangles and BO4 tetrahedra, lead cations, water molecules are shown as yellow, green, blue and pink, respectively.


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Figure 2. Construction of the anionic borate framework [(B14O27)12-]. (a) a B6O24 cluster; (b) B6O24 cluster is connected by B2O7 dimers into a tubule extending along [001] direction; (c) BO3 triangles connect to the tubules (d) a view on 3D anionic borate framework [(B14O27)12-] along the c-axis; (a'-d') corresponding representations of anion topology.



Figure 3. Voronoi-Dirichlet polyhedron of the boron atom of a BO3 group in the structure of LUBO. Boron cations and oxygens are shown as green and red, respectively.


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Figure 4. (a) A six-MRs channel along c-axis with a pink tube model, (b) the six-MRs channel showing with tiling mode along the c-axis; (c) a six-MRs ring on ab plane; (d) an 8-MR a long c-axis; (e) a threeMRs window on the wall the 6-MRs channel; (f) an eight-MRs window along the [100], [010] or [110] direction.



Figure 5. (a) Construction of the anionic borate framework using natural tiling; (b) a new larger cage [36•63•89]; (c) a new cage of [36•62•83]; (d) a [63] t-kah unit.


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Figure 6. Raman shift (a) and FT-IR (b) spectra of LUBO.



For table of content only

Synthesis and Study of First Zeolitic Uranium Borate Yucheng Hao†, Vladislav V. Klepov§, Philip Kegler†, Giuseppe Modolo†, Dirk Bosbach†, Thomas E. Albrecht-Schmittξ, Shuao Wang∆ and Evgeny V. Alekseev†,‡,*

A unique three-dimensional (3D) open framework of lead uranyl polyborate Pb3(UO2)3B14O27(H2O) (LUBO) was synthesized in mild hydrothermal conditions. A novel synthetic method for the actinide borate family was applied. The complex structure, thermal behavior, detailed Raman and FT-IR spectroscopic properties of LUBO are discussed in the manuscript.


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Figure 1 193x132mm (150 x 150 DPI)

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Synthesis and Study of the First Zeolitic Uranium Borate - Crystal

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