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Hydrothermal synthesis, crystal structure and thermal behaviour of a zincoborophosphate, (H4TETA)1.5[Zn6B6P12O48] · 1.5H2O (TETA = triethylenetetraamine), with a chiral tetrahedral framework (CZP framework type)
Microporous and Mesoporous Materials 78 (2005) 97–102 www.elsevier.com/locate/micromeso
Hydrothermal synthesis, crystal structure and thermal behaviour of a zincoborophosphate, (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (TETA = triethylenetetraamine), with a chiral tetrahedral framework (CZP framework type) Michael Wiebcke b
a,*
, Ansgar Bo¨gershausen b, Hubert Koller
b
a Universita¨t Hannover, Institut fu¨r Anorganische Chemie, Callinstrasse 9, D-30167 Hannover, Germany Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Institut fu¨r Physikalische Chemie, Corrensstrasse 36, D-48149 Mu¨nster, Germany
Received 29 July 2004; received in revised form 27 September 2004; accepted 27 September 2004 Available online 24 November 2004
Abstract By reacting strongly acidic aqueous mixtures of ZnCl2 (or ZnO), H3BO3, H3PO4 and triethylenetetraamine under mild hydrothermal conditions (160 °C, pH < 2) the new zincoborophosphate (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (1) has been obtained. 1 is the first metalloborophosphate with a zeolite tetrahedral framework (CZP framework type) synthesized in the presence of an organic amine and structurally closely related to a known sodium zincoborophosphate hydrate. The non-framework H4TETA4+ cations and water molecules are trapped in helical channels. Upon heating 1 loses all water molecules in the temperature range from 90 to 290 °C with retention of the framework structure. The framework collapses, however, in the course of decomposition reactions of the organic cations at temperatures above 390 °C. 1 was characterized by single-crystal X-ray crystallography, 11B and 13C MAS NMR and FT-IR spectroscopy, thermogravimetry and variable-temperature powder X-ray diffraction. Crystal data for 1: Hexag˚ , c = 14.8879(6) A ˚ (291 K). onal, space group P6522, Z = 1, a = 9.6685(4) A Ó 2004 Elsevier Inc. All rights reserved. Keywords: Zincoborophosphate; CZP framework type; Triethylenetetraamine; Hydrothermal synthesis; Crystal structure
1. Introduction Crystalline open framework and microporous materials such as aluminosilicate zeolites and metal phosphates [1] are of interest due to their established or potential applications in traditional fields (ion exchange, separation, catalysis) [1,2] as well as in new fields (lasers, switches, sensors, etc.) [3,4]. Compared to aluminophosphates [5] and gallophosphates [6] borophosphates are a * Corresponding author. Tel.: +49 (0)511 762 3698; fax: +49 (0)511 762 3006. E-mail address: [email protected] (M. Wiebcke).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.09.020
young class of materials [7,8]. Among the compounds synthesized and structurally characterized so far are the open-framework borophosphates A[B2P2O8(OH)] (A = Rb+, Cs+) [9] and NH4[BPO4F] [10] with 3D interrupted tetrahedral frameworks and, in particular, zincoborophosphates A[ZnBP2O8] (A = NH4+, K+, Rb+, Cs+) [11–13] with 3D fully connected tetrahedral frameworks of feldspar-related and zeolite gismondine (GIS) [14] topology. A series of compounds MIMII(H2O)2[BP2O8] Æ xH2O (MI = Li, Na, K; MII = Mn, Fe, Co, Ni, Zn, Cd; x = 0.5–1.0) [15–17] and related phases containing NH4+, Rb+ or Cs+ and additional transition-metal ions [18–20] have been reported to possess chiral 3D frameworks built from BO4/PO4 tetrahedra and
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MO4(H2O)2 octahedra (CZP topology). Interestingly, for NaZn(H2O)2[BP2O8] Æ H2O reversible dehydration with the formation of microporous Na[ZnBP2O8] Æ H2O (tetrahedral framework) has been demonstrated [21]. Also, a number of diamines have been successfully used as organic structure-directing agents to prepare openframework metal borophosphates [22–25], the 3D frameworks of which are non-tetrahedral (having metal connectivities of 5 or 6) and are interrupted (possess terminal hydroxyl groups). The protonated diamines could not be removed without collapse of the framework structures. During exploratory synthetic work we have now obtained for the first time in the presence of an organic amine, triethylenetetraamine (TETA), a zincoborophosphate with a zeolite tetrahedral framework (CZP framework type). The new compound, (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (1), is structurally closely related to Na[ZnBP2O8] Æ H2O. 1 was characterized by singlecrystal X-ray crystallography, 11B and 13C MAS NMR and FT-IR spectroscopy, simultaneous thermogravimetry (TG)/difference thermal analysis (DTA) and variable-temperature powder X-ray diffraction (XRD). Here, these experiments and the results are reported.
2. Experimental 2.1. Syntheses Hydrothermal reactions were carried out in homemade Teflon-lined stainless-steel autoclaves with 8 mL capacity. 1 was prepared from starting mixtures of approximate molar compositions ZnCl2:H3BO3:H3PO4: TETA:H2O = 1:1:2:0.75:7. A typical experiment was as follows: 2.517 g (85%) H3PO4, 1.000 g H2O, 1.469 g ZnCl2 and 0.654 g H3BO3 were combined. To this, 1.177 g TETA was added dropwise under stirring. Heat evolved and a white solid separated. The mixture (pH 1) was boiled (allowing water to evaporate) until a viscous homogeneous clear gel resulted. The hot gel was transferred into an autoclave (degree of filling 50%) and heated at 160 °C under static conditions and autogeneous pressure for three days. No change of pH was measured after hydrothermal treatment. The solid product was recovered by filtration, washed with water and ethanol and dried in air at 70 °C. Tiny colourless crystals of pure 1 were obtained with a yield of 30% (based on Zn). Elemental analysis, found/calc.: C, 5.92/5.84; H, 2.00/1.96; N, 4.53/4.55; total TG weight loss, found/ calc.: 16.14/16.25; good agreement between experimental and simulated powder XRD patterns; IR (pressed KBr pellet, cm1): 3439m, 3220m, 3201sh, 3052w, 2926w, 2872w, 2826w, 1661w, 1595m, 1522m, 1478w, 1150s, 1114s, 1019vs, 998vs, 966sh, 925m, 845m, 772w, 670m, 641m, 546w, 509s, 451w.
Lowering in the reaction mixtures the amount of amine (TETA:ZnCl2 < 0.75) yielded 1 admixed with a-BPO4. The pH value was found to be a crucial parameter. At pH values higher than 2 under otherwise similar conditions a known boron-free zincophosphate, (H4TETA)[Zn2(HPO4)4] [26], was produced. ZnO was also used successfully as a Zn source. A higher amount of H3PO4 was added in order to keep pH below 2. The starting molar compositions were approximately ZnO:H3BO3:H3PO4:TETA:H2O = 1:1:5:1:20. However, such protocols needed longer periods of reaction (5 days, 160 °C) to yield white powder of pure 1. 2.2. Methods of characterization Powder XRD patterns were measured at room temperature on a STOE STADI-P diffractometer using CuKa1 radiation (Ge monochromator, linear positionsensitive detector). For variable-temperature XRD measurements the same instrument was equipped with a STOE heating attachment. A powdered sample of 1 was filled into a silica glass capillary of 0.5 mm diameter which was left unsealed. XRD patterns in the range from 8 to 45° 2H were recorded at various temperature steps between 21 and 890 °C. TG/DTA measurements were performed simultaneously on a Netzsch STA429 thermoanalyser in a flow of pure oxygen (flow rate 160 mL/min) up to 1030 °C at a ramp of 5 °C/min. FT-IR spectra were recorded on a Bruker ISF25 spectrometer in the range from 400 to 4000 cm1. Bruker DSX-500 and DSX-400 spectrometers were used to record 11B MAS and 1H–13C CP/MAS NMR spectra, respectively, both equipped with 4 mm CP/MAS probes. The following parameters were applied: 11B nuclei; spectrometer frequency, 160.48 MHz; pulse width, 4.5 ls (30° flip angle); repetition rate, 1 s; spinning speed, 15 kHz; 13 C nuclei; spectrometer frequency, 100.63 MHz; pulse width 5.5 ls (90°); contact time, 1.5 ms; repetition rate, 15 s; spinning speed: 10 kHz. The chemical shift references are BF3 Æ OEt2 for 11B and liquid TMS for 13C, using adamantane (dCH2 = 38.56 ppm) as a secondary standard. 2.3. Single-crystal X-ray structure analysis A hexagonal prismatic needle (length/diameter: 0.15/ 0.05 mm) was glued to the tip of a glass fibre and mounted on a STOE IPDS image-plate diffractometer for the X-ray measurements (MoKa radiation, graphite monochromator). Absorption corrections based on symmetry-equivalent reflections were applied to the intensity data (MULABS option in the PLATON package) [27]. For structure solution (direct methods) and refinements (F2 values, full-matrix least-squares procedure including all data) the programs SHELXS-97 and SHELXL-97 were used [28]. Refinement of the Flack x
M. Wiebcke et al. / Microporous and Mesoporous Materials 78 (2005) 97–102
parameter indicated correct choice of absolute structure. All framework atoms were refined with anisotropic displacement parameters. Difference Fourier maps revealed that the non-framework species (H4TETA4+ and water) are disordered. Various models of disorder were tested. In the final refinement of the best structure model found (space group P6522) isotropic displacement parameters were used for the C, N and O atoms; H atoms were not considered. Crystal data and details of the structure analysis are summarized in Table 1, while final atomic parameters including statistical occupancy factors are listed in Table 2. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK as supplementary publication no. CCDC-250178 and can be obtained by contacting the CCDC.
Table 1 Crystal data and details of structure analysis for 1 Empirical formula
C9H36B6N6O49.5P12Zn6
Formula weight Crystal system Space group/Z ˚) a (A ˚) c (A ˚ 3) V (A T (K) qcalc (Mg m3) l(MoKa) (mm1) 2Hmax (deg) Total data collected Unique data: I > 2rI/total Refined parameters Flack x parameter Extinction parameter R1: I > 2rI/all data wR2: I > 2rI/all data ˚ 3) Largest diff. peak/hole (e A
1849.16 Hexagonal P6522/1 9.6685(4) 14.8879(6) 1205.26(9) 291 2.55 3.48 60.17 22134 936/1150 70 0.01(4) 0.036(5) 0.057/0.069 0.148/0.154 0.81/0.80a
a
99
3. Results The new open-framework zincoborophosphate 1 has been synthesized hydrothermally from strongly acidic aqueous gels containing ZnCl2 (or ZnO), H3BO3, H3PO4 and TETA (160 °C, pH < 2). 3.1. Crystal structure The 3D anionic framework, 31[Zn6B6P12O48]6, possesses the topology of the chiral zeolite framework-type CZP [14]. The ZnO4, BO4 and PO4 tetrahedra with typical bond lengths and angles (Table 3) share common corners and are strictly ordered. When the 3D framework is viewed along the hexagonal c-axis (Fig. 1), two kinds of helical channel may be identified that are centered, respectively, by 65 and 32 screw axes (in the case of the enantiomorphic form with space group P6522). Each 65 channel is circumscribed by a 1D, ‘‘spiro-type’’, borophosphate ribbon composed of rings of four tetrahedra (four-membered ring, 4MR). As shown in Fig. 2, the helical ribbons, in turn, are interconnected by ZnO4 units into the 3D framework,
Table 3 ˚ ) and angles (deg.) for 1 Selected bond lengths (A Zn–O1 (2x) P–O1 P–O3 B–O2 (2x) O1–Zn–O1 O1–Zn–O3 (2x) O1–P–O2 O1–P–O4 O2–P–O4 O2–B–O2 O2–B–O4 (2x) Zn–O1–P Zn–O3–P
1.954(5) 1.506(5) 1.503(5) 1.472(7) 121.2(3) 108.9(2) 111.0(3) 107.6(3) 109.6(3) 103.9(6) 112.6(3) 130.3(3) 126.4(3)
Zn–O3 (2x) P–O2 P–O4 B–O4 (2x) O1–Zn–O3 (2x) O3–Zn–O3 O1–P–O3 O2–P–O3 O3–P–O4 O2–B–O4 (2x) O4–B–O4 P–O2–B P–O4–B
1.940(4) 1.561(5) 1.553(6) 1.470(7) 97.2(2) 125.5(3) 113.5(3) 105.5(3) 109.6(3) 111.0(3) 105.9(7) 131.0(4) 127.5(4)
The peak and hole are close to Zn and P, respectively.
Table 2 Atomic coordinates, statistical occupancy factors and equivalent/ ˚ 2) for 1 isotropic displacement parameters (A Atoma
x
y
z
Occ.
Ueq/Uiso
Zn P B O1 O2 O3 O4 N C1 C2 O5
0.51002(6) 0.7614(2) 0.670(1) 0.6142(6) 0.7790(6) 0.2925(5) 0.7442(5) 0.361(1) 0.334(4) 0.226(4) 0.076(3)
0.48998(6) 0.6066(2) 0.8353(5) 0.4796(6) 0.7749(6) 0.3786(6) 0.5648(6) 0.1806(7) 0.069(4) 0.040(4) 0
1/12 0.0736(1) 1/12 0.0269(3) 0.0604(3) 0.0384(3) 0.1752(4) 1/12 0.004(2) 0.033(2) 0
1 1 1 1 1 1 1 1 0.5 0.25 0.25
0.0197(3) 0.0173(4) 0.019(2) 0.025(1) 0.0214(9) 0.024(1) 0.021(1) 0.041(2) 0.075(8) 0.031(6) 0.032(7)
a
Atoms on special Wyckoff sites: Zn, 6b; B, 6b; N, 6b; O5, 6a.
32 b a
65
Fig. 1. 3D framework of 1 in polyhedral representation as viewed along the c-axis. 65 and 32 scew axes are located, respectively, at 0, 0, z and 2/3, 1/3, z. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4.
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c
Fig. 2. Two helical tetrahedral borophosphate ribbons interconnected by ZnO4 tetrahedra. Additionally, on the right-hand side is shown a helix of disordered non-framework species, while on the left-hand side is shown a chain built from alternating H4TETA4+ cations and water molecules. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4. Black balls, C atoms; white balls, N atoms; grey balls, water O atoms. Hydrogen bonds are represented by dotted lines.
b c
a 65
32
C2 O3
O1
(a)
(b) Fig. 3. (a) A 12-membered ring with one N and two C positions of a H4TETA4+ cation. O1 and O3 atoms in Zn–O–P linkages are indicated. (b) A 65 and a 32 channel with the location of the nonframework atoms as seen along the c-axis. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4. Black balls, C atoms; white balls, N atoms; grey balls, water O atoms.
thereby further 4MRs and 8MRs are formed. In addition, highly puckered, oval-shaped 12MRs of C2 point group symmetry may be identified, each being passed by a 65 and a 32 axis (Fig. 3a). The 8 MRs define the 32 channels.
The 32 channels are empty, while the 65 channels are occupied by disordered non-framework amine and water molecules (Figs. 2 and 3b). For host–guest charge balance the organic species have to be fully protonated. The resulting H4TETA4+ cations reside in the free space left in the thread of a borophosphate ribbon. The water molecules are located, outside the thread, near a channel axis. The structure analysis suggests that H4TETA4+ cations and water molecules alternate along a 65 channel. It should be noted that the hexagonal c-parameter corresponds to 1.5 repeats of such a cation–water chain. This can be seen in Fig. 2 (left-hand side). Therefore, any structure model with complete ordering of the non-framework species requires at least a doubling of the c-axis (besides a reduction of symmetry). Corresponding weak superstructure reflections were not observed at room temperature neither with the single-crystal studied nor on the powder XRD patterns measured. However, such reflections are seen on certain high-temperature XRD patterns (see below). Disorder of the non-framework species is likely the result from lack of periodic ordering of the cation–water chains located in different channels. Positional disorder of the methylene groups within a given chain may also exist. Independent from the causes of disorder, it is obvious that the flexible H4TETA4+ cations adopt a helical conformation that allows a tight fit in the thread of a borophosphate ribbon. A reasonable bond sequence along the nitrogen–carbon backbone is N–C1–C2–N– C1–C1–N–C2–C1–N with the corresponding torsion angles of 153°, 142°, 66°, 156°, 66°, 142° and 153°. The cations and water molecules are fixed by N–H O and O–H O hydrogen bonds as is suggested by the (typical) N O and O O distances ranging from 2.71(2) ˚ ; the closed approach between methylene to 2.943(6) A ˚ . It should be C and framework O atoms is 3.02(4) A noted that all the mentioned close N O and C O guest–host distances of a H4TETA4+ cation involve exclusively the (expectedly) most negatively charged framework O atoms in Zn–O–P linkages (O1 and O3), and that every N atom is located approximately in the centre of a 12MR between the 65 and 32 channels (Fig. 3). The structure model described is consistent with chemical analysis and spectroscopic data. The IR spectrum confirms the presence of an amine and water. The characteristic bands for stretching modes of O–H, N–H (at wavenumbers P 3052 cm1) and C–H groups (2926–2826 cm1) as well as bending modes of O–H (1661 cm1), N–H (1595–1522 cm1) and C–H (1478 cm1) groups are seen. The 13C NMR spectrum (Fig. 4) exhibits three, partly overlapping signals. This is expected for intact H4TETA4+ cations and suggests that three distinct crystallographic carbon sites occur. These are approximately modelled by the C1 and C2 positions and their statistical occupancy factors (ratio 2:1; Table
50
45
40
35
30
δ /ppm Fig. 4.
13
101
38.1
44.5 43.9
M. Wiebcke et al. / Microporous and Mesoporous Materials 78 (2005) 97–102
Fig. 6. TG/DTA curves for 1. Flowing oxygen atmosphere.
C CP/MAS NMR spectrum of 1. Spinning speed, 10 kHz.
2); small peaks in the final difference electron-density map near the C1 atom point to a higher degree of disorder which could not be reasonably modelled. The chemical shift values at 38.1 ppm (–CH2–NH3) and 43.9 and 44.5 ppm (–CH2–NH2–CH2–) are typical for methylene C atoms in primary and secondary amines and their protonated forms [29]. Similar shift values are reported for related protonated amines enclosed in some openframework gallium phosphates [30,31]. The single narrow line seen in the 11B NMR spectrum at 2.3 ppm (Fig. 5) implies that the electric field gradient at the 11 B nulcei (I = 3/2) is small and confirms a tetrahedral environment [32,33]. 3.2. Thermal behaviour
-2.3
TG/DTA curves are displayed in Fig. 6, and selected powder XRD patterns measured at various temperature steps are displayed in Fig. 7. The first weight loss of 1.3% seen on the TG curve between 90 and 290 °C (no event seen on the DTA trace) corresponds to the release of all non-framework water molecules (calc.: 1.5%). The
20
10
0
-10
-20
-30
δ /ppm Fig. 5.
11
B MAS NMR spectrum of 1. Spinning speed, 15 kHz.
Fig. 7. Selected powder XRD patterns of 1 measured at various temperature steps. A weak superstructure reflection in the pattern at 290 °C is marked by *. CuKa1 radiation.
framework structure is preserved during dehydration. This is demonstrated by the XRD pattern of the dehydrated phase at 290 °C which compares well with the pattern of 1 at 90 °C, apart from the (expected) shift of diffraction lines due to a shrinkage of the cell volume ˚ 3 (0.7%) and one additional reflection of very of 8.2 A low intensity seen at 8.9° 2H. This superstructure reflection (see above) indicates a doubling of the hexagonal cparameter. Then, the reflection can be cleanly indexed as 0 0 3. On further heating, the weak reflection disappears (see XRD pattern at 390 °C) while the cell volume increases. The second weight loss step of 9.3% occurring between 390 and 670 °C (endothermic DTA peak at 466 °C) probably arises from decomposition and release of most of the organic cations (calc.: 11.9%). The processes are accompanied by a progressive collapse of the framework structure to yield amorphous material as is revealed by the XRD pattern at 490 °C. Later, crystalline material appears which on the XRD pattern at 690 °C is identified as a mixture of dense a-BPO4 (ICDD
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data base; card, 34–132) and c-Zn2P2O7 (ICDD; 39– 711). Weight loss continues up to 935 °C (main exothermic DTA peak at 686 °C); probably the remaining part of the organic and water stemming from amine protons and framework oxygens (calc.: 2.9%) escapes. The total weight loss (16.1%) is in good agreement with the calculated ones (16.2%). The XRD pattern at 890 °C originates from as yet unidentified crystalline material.
4. Discussion 1 is the first metalloborophosphate with a zeolite tetrahedral framework synthesized in the presence of an organic amine. As may be inferred from the unit cell parameters, the host structure of 1 is an isotype of Na[ZnBP2O8] Æ H2O (unit cell formula: Na6[Zn6B6˚ ), which has P12O48] Æ 6H2O; a = 9.5404, c = 14.7780 A been recently prepared by Boy et al. [21] by (reversible) thermal dehydration of as-synthesized NaZn(H2O)2[BP2O4] Æ H2O (octahedral–tetrahedral framework) and the structure of which has been determined from powder XRD data. The dehydrated sodium compound encloses chains of alternating Na+ and H2O species in the 65 ˚ ) which, channels (nearest Na Na distance at 4.056 A respectively, correspond to the N atoms and ethylene groups/H2O molecules in 1 (nearest N N distance at ˚ ). Clearly, the covalent H4TETA4+ cations more 3.912 A effectively fill the void space (free thread of the borophosphate ribbons) and by combined (ionic) N–H O and C–H O interactions with the negatively charged ZnO4 tetrahedra as mentioned above (Fig. 3a) exclude additional water molecules from the coordination sphere of the Zn2+ centres. With regard to the (unknown) mechanism of phase formation it is of interest that 1 could only be prepared under conditions (strongly acidic media) that are very similar to the conditions in the syntheses of the rather large number of purely inorganic metal borophosphates with CZP topology [14–21]. Thus, it is likely that the H4TETA4+ cations (high charge density, flexible geometry) function in a very similar way as hydrated inorganic cations. Finally, we note that an amine-containing manganese(II) gallophosphate, (H3DETA)[Mn(H2O)2GaP2O8] (DETA = diethylenetriamine), with an octahedral–tetrahedral framework of CZP topology has been recently reported [34].
Acknowledgment We are grateful to B. Beiße and G. Wildermuth (Universita¨t Konstanz) for performing TG/DTA measurements.
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Hydrothermal synthesis, crystal structure and thermal behaviour of a zincoborophosphate, (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (TETA = triethylenetetraamine), with a chiral tetrahedral framework (CZP framework type) Michael Wiebcke b
a,*
, Ansgar Bo¨gershausen b, Hubert Koller
b
a Universita¨t Hannover, Institut fu¨r Anorganische Chemie, Callinstrasse 9, D-30167 Hannover, Germany Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Institut fu¨r Physikalische Chemie, Corrensstrasse 36, D-48149 Mu¨nster, Germany
Received 29 July 2004; received in revised form 27 September 2004; accepted 27 September 2004 Available online 24 November 2004
Abstract By reacting strongly acidic aqueous mixtures of ZnCl2 (or ZnO), H3BO3, H3PO4 and triethylenetetraamine under mild hydrothermal conditions (160 °C, pH < 2) the new zincoborophosphate (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (1) has been obtained. 1 is the first metalloborophosphate with a zeolite tetrahedral framework (CZP framework type) synthesized in the presence of an organic amine and structurally closely related to a known sodium zincoborophosphate hydrate. The non-framework H4TETA4+ cations and water molecules are trapped in helical channels. Upon heating 1 loses all water molecules in the temperature range from 90 to 290 °C with retention of the framework structure. The framework collapses, however, in the course of decomposition reactions of the organic cations at temperatures above 390 °C. 1 was characterized by single-crystal X-ray crystallography, 11B and 13C MAS NMR and FT-IR spectroscopy, thermogravimetry and variable-temperature powder X-ray diffraction. Crystal data for 1: Hexag˚ , c = 14.8879(6) A ˚ (291 K). onal, space group P6522, Z = 1, a = 9.6685(4) A Ó 2004 Elsevier Inc. All rights reserved. Keywords: Zincoborophosphate; CZP framework type; Triethylenetetraamine; Hydrothermal synthesis; Crystal structure
1. Introduction Crystalline open framework and microporous materials such as aluminosilicate zeolites and metal phosphates [1] are of interest due to their established or potential applications in traditional fields (ion exchange, separation, catalysis) [1,2] as well as in new fields (lasers, switches, sensors, etc.) [3,4]. Compared to aluminophosphates [5] and gallophosphates [6] borophosphates are a * Corresponding author. Tel.: +49 (0)511 762 3698; fax: +49 (0)511 762 3006. E-mail address: [email protected] (M. Wiebcke).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.09.020
young class of materials [7,8]. Among the compounds synthesized and structurally characterized so far are the open-framework borophosphates A[B2P2O8(OH)] (A = Rb+, Cs+) [9] and NH4[BPO4F] [10] with 3D interrupted tetrahedral frameworks and, in particular, zincoborophosphates A[ZnBP2O8] (A = NH4+, K+, Rb+, Cs+) [11–13] with 3D fully connected tetrahedral frameworks of feldspar-related and zeolite gismondine (GIS) [14] topology. A series of compounds MIMII(H2O)2[BP2O8] Æ xH2O (MI = Li, Na, K; MII = Mn, Fe, Co, Ni, Zn, Cd; x = 0.5–1.0) [15–17] and related phases containing NH4+, Rb+ or Cs+ and additional transition-metal ions [18–20] have been reported to possess chiral 3D frameworks built from BO4/PO4 tetrahedra and
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MO4(H2O)2 octahedra (CZP topology). Interestingly, for NaZn(H2O)2[BP2O8] Æ H2O reversible dehydration with the formation of microporous Na[ZnBP2O8] Æ H2O (tetrahedral framework) has been demonstrated [21]. Also, a number of diamines have been successfully used as organic structure-directing agents to prepare openframework metal borophosphates [22–25], the 3D frameworks of which are non-tetrahedral (having metal connectivities of 5 or 6) and are interrupted (possess terminal hydroxyl groups). The protonated diamines could not be removed without collapse of the framework structures. During exploratory synthetic work we have now obtained for the first time in the presence of an organic amine, triethylenetetraamine (TETA), a zincoborophosphate with a zeolite tetrahedral framework (CZP framework type). The new compound, (H4TETA)1.5[Zn6B6P12O48] Æ 1.5H2O (1), is structurally closely related to Na[ZnBP2O8] Æ H2O. 1 was characterized by singlecrystal X-ray crystallography, 11B and 13C MAS NMR and FT-IR spectroscopy, simultaneous thermogravimetry (TG)/difference thermal analysis (DTA) and variable-temperature powder X-ray diffraction (XRD). Here, these experiments and the results are reported.
2. Experimental 2.1. Syntheses Hydrothermal reactions were carried out in homemade Teflon-lined stainless-steel autoclaves with 8 mL capacity. 1 was prepared from starting mixtures of approximate molar compositions ZnCl2:H3BO3:H3PO4: TETA:H2O = 1:1:2:0.75:7. A typical experiment was as follows: 2.517 g (85%) H3PO4, 1.000 g H2O, 1.469 g ZnCl2 and 0.654 g H3BO3 were combined. To this, 1.177 g TETA was added dropwise under stirring. Heat evolved and a white solid separated. The mixture (pH 1) was boiled (allowing water to evaporate) until a viscous homogeneous clear gel resulted. The hot gel was transferred into an autoclave (degree of filling 50%) and heated at 160 °C under static conditions and autogeneous pressure for three days. No change of pH was measured after hydrothermal treatment. The solid product was recovered by filtration, washed with water and ethanol and dried in air at 70 °C. Tiny colourless crystals of pure 1 were obtained with a yield of 30% (based on Zn). Elemental analysis, found/calc.: C, 5.92/5.84; H, 2.00/1.96; N, 4.53/4.55; total TG weight loss, found/ calc.: 16.14/16.25; good agreement between experimental and simulated powder XRD patterns; IR (pressed KBr pellet, cm1): 3439m, 3220m, 3201sh, 3052w, 2926w, 2872w, 2826w, 1661w, 1595m, 1522m, 1478w, 1150s, 1114s, 1019vs, 998vs, 966sh, 925m, 845m, 772w, 670m, 641m, 546w, 509s, 451w.
Lowering in the reaction mixtures the amount of amine (TETA:ZnCl2 < 0.75) yielded 1 admixed with a-BPO4. The pH value was found to be a crucial parameter. At pH values higher than 2 under otherwise similar conditions a known boron-free zincophosphate, (H4TETA)[Zn2(HPO4)4] [26], was produced. ZnO was also used successfully as a Zn source. A higher amount of H3PO4 was added in order to keep pH below 2. The starting molar compositions were approximately ZnO:H3BO3:H3PO4:TETA:H2O = 1:1:5:1:20. However, such protocols needed longer periods of reaction (5 days, 160 °C) to yield white powder of pure 1. 2.2. Methods of characterization Powder XRD patterns were measured at room temperature on a STOE STADI-P diffractometer using CuKa1 radiation (Ge monochromator, linear positionsensitive detector). For variable-temperature XRD measurements the same instrument was equipped with a STOE heating attachment. A powdered sample of 1 was filled into a silica glass capillary of 0.5 mm diameter which was left unsealed. XRD patterns in the range from 8 to 45° 2H were recorded at various temperature steps between 21 and 890 °C. TG/DTA measurements were performed simultaneously on a Netzsch STA429 thermoanalyser in a flow of pure oxygen (flow rate 160 mL/min) up to 1030 °C at a ramp of 5 °C/min. FT-IR spectra were recorded on a Bruker ISF25 spectrometer in the range from 400 to 4000 cm1. Bruker DSX-500 and DSX-400 spectrometers were used to record 11B MAS and 1H–13C CP/MAS NMR spectra, respectively, both equipped with 4 mm CP/MAS probes. The following parameters were applied: 11B nuclei; spectrometer frequency, 160.48 MHz; pulse width, 4.5 ls (30° flip angle); repetition rate, 1 s; spinning speed, 15 kHz; 13 C nuclei; spectrometer frequency, 100.63 MHz; pulse width 5.5 ls (90°); contact time, 1.5 ms; repetition rate, 15 s; spinning speed: 10 kHz. The chemical shift references are BF3 Æ OEt2 for 11B and liquid TMS for 13C, using adamantane (dCH2 = 38.56 ppm) as a secondary standard. 2.3. Single-crystal X-ray structure analysis A hexagonal prismatic needle (length/diameter: 0.15/ 0.05 mm) was glued to the tip of a glass fibre and mounted on a STOE IPDS image-plate diffractometer for the X-ray measurements (MoKa radiation, graphite monochromator). Absorption corrections based on symmetry-equivalent reflections were applied to the intensity data (MULABS option in the PLATON package) [27]. For structure solution (direct methods) and refinements (F2 values, full-matrix least-squares procedure including all data) the programs SHELXS-97 and SHELXL-97 were used [28]. Refinement of the Flack x
M. Wiebcke et al. / Microporous and Mesoporous Materials 78 (2005) 97–102
parameter indicated correct choice of absolute structure. All framework atoms were refined with anisotropic displacement parameters. Difference Fourier maps revealed that the non-framework species (H4TETA4+ and water) are disordered. Various models of disorder were tested. In the final refinement of the best structure model found (space group P6522) isotropic displacement parameters were used for the C, N and O atoms; H atoms were not considered. Crystal data and details of the structure analysis are summarized in Table 1, while final atomic parameters including statistical occupancy factors are listed in Table 2. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK as supplementary publication no. CCDC-250178 and can be obtained by contacting the CCDC.
Table 1 Crystal data and details of structure analysis for 1 Empirical formula
C9H36B6N6O49.5P12Zn6
Formula weight Crystal system Space group/Z ˚) a (A ˚) c (A ˚ 3) V (A T (K) qcalc (Mg m3) l(MoKa) (mm1) 2Hmax (deg) Total data collected Unique data: I > 2rI/total Refined parameters Flack x parameter Extinction parameter R1: I > 2rI/all data wR2: I > 2rI/all data ˚ 3) Largest diff. peak/hole (e A
1849.16 Hexagonal P6522/1 9.6685(4) 14.8879(6) 1205.26(9) 291 2.55 3.48 60.17 22134 936/1150 70 0.01(4) 0.036(5) 0.057/0.069 0.148/0.154 0.81/0.80a
a
99
3. Results The new open-framework zincoborophosphate 1 has been synthesized hydrothermally from strongly acidic aqueous gels containing ZnCl2 (or ZnO), H3BO3, H3PO4 and TETA (160 °C, pH < 2). 3.1. Crystal structure The 3D anionic framework, 31[Zn6B6P12O48]6, possesses the topology of the chiral zeolite framework-type CZP [14]. The ZnO4, BO4 and PO4 tetrahedra with typical bond lengths and angles (Table 3) share common corners and are strictly ordered. When the 3D framework is viewed along the hexagonal c-axis (Fig. 1), two kinds of helical channel may be identified that are centered, respectively, by 65 and 32 screw axes (in the case of the enantiomorphic form with space group P6522). Each 65 channel is circumscribed by a 1D, ‘‘spiro-type’’, borophosphate ribbon composed of rings of four tetrahedra (four-membered ring, 4MR). As shown in Fig. 2, the helical ribbons, in turn, are interconnected by ZnO4 units into the 3D framework,
Table 3 ˚ ) and angles (deg.) for 1 Selected bond lengths (A Zn–O1 (2x) P–O1 P–O3 B–O2 (2x) O1–Zn–O1 O1–Zn–O3 (2x) O1–P–O2 O1–P–O4 O2–P–O4 O2–B–O2 O2–B–O4 (2x) Zn–O1–P Zn–O3–P
1.954(5) 1.506(5) 1.503(5) 1.472(7) 121.2(3) 108.9(2) 111.0(3) 107.6(3) 109.6(3) 103.9(6) 112.6(3) 130.3(3) 126.4(3)
Zn–O3 (2x) P–O2 P–O4 B–O4 (2x) O1–Zn–O3 (2x) O3–Zn–O3 O1–P–O3 O2–P–O3 O3–P–O4 O2–B–O4 (2x) O4–B–O4 P–O2–B P–O4–B
1.940(4) 1.561(5) 1.553(6) 1.470(7) 97.2(2) 125.5(3) 113.5(3) 105.5(3) 109.6(3) 111.0(3) 105.9(7) 131.0(4) 127.5(4)
The peak and hole are close to Zn and P, respectively.
Table 2 Atomic coordinates, statistical occupancy factors and equivalent/ ˚ 2) for 1 isotropic displacement parameters (A Atoma
x
y
z
Occ.
Ueq/Uiso
Zn P B O1 O2 O3 O4 N C1 C2 O5
0.51002(6) 0.7614(2) 0.670(1) 0.6142(6) 0.7790(6) 0.2925(5) 0.7442(5) 0.361(1) 0.334(4) 0.226(4) 0.076(3)
0.48998(6) 0.6066(2) 0.8353(5) 0.4796(6) 0.7749(6) 0.3786(6) 0.5648(6) 0.1806(7) 0.069(4) 0.040(4) 0
1/12 0.0736(1) 1/12 0.0269(3) 0.0604(3) 0.0384(3) 0.1752(4) 1/12 0.004(2) 0.033(2) 0
1 1 1 1 1 1 1 1 0.5 0.25 0.25
0.0197(3) 0.0173(4) 0.019(2) 0.025(1) 0.0214(9) 0.024(1) 0.021(1) 0.041(2) 0.075(8) 0.031(6) 0.032(7)
a
Atoms on special Wyckoff sites: Zn, 6b; B, 6b; N, 6b; O5, 6a.
32 b a
65
Fig. 1. 3D framework of 1 in polyhedral representation as viewed along the c-axis. 65 and 32 scew axes are located, respectively, at 0, 0, z and 2/3, 1/3, z. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4.
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c
Fig. 2. Two helical tetrahedral borophosphate ribbons interconnected by ZnO4 tetrahedra. Additionally, on the right-hand side is shown a helix of disordered non-framework species, while on the left-hand side is shown a chain built from alternating H4TETA4+ cations and water molecules. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4. Black balls, C atoms; white balls, N atoms; grey balls, water O atoms. Hydrogen bonds are represented by dotted lines.
b c
a 65
32
C2 O3
O1
(a)
(b) Fig. 3. (a) A 12-membered ring with one N and two C positions of a H4TETA4+ cation. O1 and O3 atoms in Zn–O–P linkages are indicated. (b) A 65 and a 32 channel with the location of the nonframework atoms as seen along the c-axis. Black tetrahedra, BO4; grey tetrahedra, PO4; white tetrahedra, ZnO4. Black balls, C atoms; white balls, N atoms; grey balls, water O atoms.
thereby further 4MRs and 8MRs are formed. In addition, highly puckered, oval-shaped 12MRs of C2 point group symmetry may be identified, each being passed by a 65 and a 32 axis (Fig. 3a). The 8 MRs define the 32 channels.
The 32 channels are empty, while the 65 channels are occupied by disordered non-framework amine and water molecules (Figs. 2 and 3b). For host–guest charge balance the organic species have to be fully protonated. The resulting H4TETA4+ cations reside in the free space left in the thread of a borophosphate ribbon. The water molecules are located, outside the thread, near a channel axis. The structure analysis suggests that H4TETA4+ cations and water molecules alternate along a 65 channel. It should be noted that the hexagonal c-parameter corresponds to 1.5 repeats of such a cation–water chain. This can be seen in Fig. 2 (left-hand side). Therefore, any structure model with complete ordering of the non-framework species requires at least a doubling of the c-axis (besides a reduction of symmetry). Corresponding weak superstructure reflections were not observed at room temperature neither with the single-crystal studied nor on the powder XRD patterns measured. However, such reflections are seen on certain high-temperature XRD patterns (see below). Disorder of the non-framework species is likely the result from lack of periodic ordering of the cation–water chains located in different channels. Positional disorder of the methylene groups within a given chain may also exist. Independent from the causes of disorder, it is obvious that the flexible H4TETA4+ cations adopt a helical conformation that allows a tight fit in the thread of a borophosphate ribbon. A reasonable bond sequence along the nitrogen–carbon backbone is N–C1–C2–N– C1–C1–N–C2–C1–N with the corresponding torsion angles of 153°, 142°, 66°, 156°, 66°, 142° and 153°. The cations and water molecules are fixed by N–H O and O–H O hydrogen bonds as is suggested by the (typical) N O and O O distances ranging from 2.71(2) ˚ ; the closed approach between methylene to 2.943(6) A ˚ . It should be C and framework O atoms is 3.02(4) A noted that all the mentioned close N O and C O guest–host distances of a H4TETA4+ cation involve exclusively the (expectedly) most negatively charged framework O atoms in Zn–O–P linkages (O1 and O3), and that every N atom is located approximately in the centre of a 12MR between the 65 and 32 channels (Fig. 3). The structure model described is consistent with chemical analysis and spectroscopic data. The IR spectrum confirms the presence of an amine and water. The characteristic bands for stretching modes of O–H, N–H (at wavenumbers P 3052 cm1) and C–H groups (2926–2826 cm1) as well as bending modes of O–H (1661 cm1), N–H (1595–1522 cm1) and C–H (1478 cm1) groups are seen. The 13C NMR spectrum (Fig. 4) exhibits three, partly overlapping signals. This is expected for intact H4TETA4+ cations and suggests that three distinct crystallographic carbon sites occur. These are approximately modelled by the C1 and C2 positions and their statistical occupancy factors (ratio 2:1; Table
50
45
40
35
30
δ /ppm Fig. 4.
13
101
38.1
44.5 43.9
M. Wiebcke et al. / Microporous and Mesoporous Materials 78 (2005) 97–102
Fig. 6. TG/DTA curves for 1. Flowing oxygen atmosphere.
C CP/MAS NMR spectrum of 1. Spinning speed, 10 kHz.
2); small peaks in the final difference electron-density map near the C1 atom point to a higher degree of disorder which could not be reasonably modelled. The chemical shift values at 38.1 ppm (–CH2–NH3) and 43.9 and 44.5 ppm (–CH2–NH2–CH2–) are typical for methylene C atoms in primary and secondary amines and their protonated forms [29]. Similar shift values are reported for related protonated amines enclosed in some openframework gallium phosphates [30,31]. The single narrow line seen in the 11B NMR spectrum at 2.3 ppm (Fig. 5) implies that the electric field gradient at the 11 B nulcei (I = 3/2) is small and confirms a tetrahedral environment [32,33]. 3.2. Thermal behaviour
-2.3
TG/DTA curves are displayed in Fig. 6, and selected powder XRD patterns measured at various temperature steps are displayed in Fig. 7. The first weight loss of 1.3% seen on the TG curve between 90 and 290 °C (no event seen on the DTA trace) corresponds to the release of all non-framework water molecules (calc.: 1.5%). The
20
10
0
-10
-20
-30
δ /ppm Fig. 5.
11
B MAS NMR spectrum of 1. Spinning speed, 15 kHz.
Fig. 7. Selected powder XRD patterns of 1 measured at various temperature steps. A weak superstructure reflection in the pattern at 290 °C is marked by *. CuKa1 radiation.
framework structure is preserved during dehydration. This is demonstrated by the XRD pattern of the dehydrated phase at 290 °C which compares well with the pattern of 1 at 90 °C, apart from the (expected) shift of diffraction lines due to a shrinkage of the cell volume ˚ 3 (0.7%) and one additional reflection of very of 8.2 A low intensity seen at 8.9° 2H. This superstructure reflection (see above) indicates a doubling of the hexagonal cparameter. Then, the reflection can be cleanly indexed as 0 0 3. On further heating, the weak reflection disappears (see XRD pattern at 390 °C) while the cell volume increases. The second weight loss step of 9.3% occurring between 390 and 670 °C (endothermic DTA peak at 466 °C) probably arises from decomposition and release of most of the organic cations (calc.: 11.9%). The processes are accompanied by a progressive collapse of the framework structure to yield amorphous material as is revealed by the XRD pattern at 490 °C. Later, crystalline material appears which on the XRD pattern at 690 °C is identified as a mixture of dense a-BPO4 (ICDD
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data base; card, 34–132) and c-Zn2P2O7 (ICDD; 39– 711). Weight loss continues up to 935 °C (main exothermic DTA peak at 686 °C); probably the remaining part of the organic and water stemming from amine protons and framework oxygens (calc.: 2.9%) escapes. The total weight loss (16.1%) is in good agreement with the calculated ones (16.2%). The XRD pattern at 890 °C originates from as yet unidentified crystalline material.
4. Discussion 1 is the first metalloborophosphate with a zeolite tetrahedral framework synthesized in the presence of an organic amine. As may be inferred from the unit cell parameters, the host structure of 1 is an isotype of Na[ZnBP2O8] Æ H2O (unit cell formula: Na6[Zn6B6˚ ), which has P12O48] Æ 6H2O; a = 9.5404, c = 14.7780 A been recently prepared by Boy et al. [21] by (reversible) thermal dehydration of as-synthesized NaZn(H2O)2[BP2O4] Æ H2O (octahedral–tetrahedral framework) and the structure of which has been determined from powder XRD data. The dehydrated sodium compound encloses chains of alternating Na+ and H2O species in the 65 ˚ ) which, channels (nearest Na Na distance at 4.056 A respectively, correspond to the N atoms and ethylene groups/H2O molecules in 1 (nearest N N distance at ˚ ). Clearly, the covalent H4TETA4+ cations more 3.912 A effectively fill the void space (free thread of the borophosphate ribbons) and by combined (ionic) N–H O and C–H O interactions with the negatively charged ZnO4 tetrahedra as mentioned above (Fig. 3a) exclude additional water molecules from the coordination sphere of the Zn2+ centres. With regard to the (unknown) mechanism of phase formation it is of interest that 1 could only be prepared under conditions (strongly acidic media) that are very similar to the conditions in the syntheses of the rather large number of purely inorganic metal borophosphates with CZP topology [14–21]. Thus, it is likely that the H4TETA4+ cations (high charge density, flexible geometry) function in a very similar way as hydrated inorganic cations. Finally, we note that an amine-containing manganese(II) gallophosphate, (H3DETA)[Mn(H2O)2GaP2O8] (DETA = diethylenetriamine), with an octahedral–tetrahedral framework of CZP topology has been recently reported [34].
Acknowledgment We are grateful to B. Beiße and G. Wildermuth (Universita¨t Konstanz) for performing TG/DTA measurements.
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