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Chem. Mater. 1996, 8, 356-359
Hydrothermal Synthesis and Characterization of a New Potassium Phosphatoantimonate, K8Sb8P2O29‚8H2O Yonglin An, Shouhua Feng,* Yihua Xu, and Ruren Xu Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, P.R. China
Yong Yue Wuhan Institute of Physics, Academia Sinica, Wuhan 430071, P.R. China Received April 24, 1995. Revised Manuscript Received November 28, 1995X
A new potassium phosphatoantimonate was hydrothermally synthesized and characterized by means of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), thermogravimetric analysis (TGA), infrared spectroscopy (IR), 31P solid-state NMR, and adsorption isothermal analysis. Results showed that this compound has a layered structure with the formula K8Sb8P2O29‚8H2O. It crystallizes in the monoclinic system with unit-cell parameters a ) 23.95(2) Å, b ) 9.51(5) Å, c ) 8.19(3) Å, and β ) 124.77°. The structure can be intercalated by organic amine molecules and the potassium ions located in between layers can be exchanged by monovalent ions. Factors such as the initial Sb/P molar ratio, pH, reaction temperature, and cations which dominate the hydrothermal synthesis of the product were assessed.
Introduction The solid-state syntheses and characterization of several potassium phosphatoantimonates, including K3Sb3P2O14‚xH2O, KSb2PO8, KSbP2O8, K2SbPO6, and K5Sb5P2O20 were reported by Piffard et al.1-7 in the 1980s. The ion-exchange and ionic conducting properties of these compounds were extensively studied by Piffard and by Greenblatt et al.8,9 The interesting composition dependence on structure exhibits regular variations from three-dimensional framework to layered or chain structures as the composition increases in Sb/P ratio. The potassium ions in these structures can be readily exchanged by other monovalent ions such as Li+, Na+, NH4+, Rb+, Cs+, and H+, and some of these compounds showed good ion-exchange properties and comparable ionic conductivities. By virtue of the above features, many investigations of these compounds or related compounds were caried out over the past few years.10-15 Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Piffard, Y.; Lachgar, A.; Tournoux, M. J. Solid State Chem. 1985, 58, 253. (2) Lachgar, A.; Deniard-Courant, S.; Piffard, Y. J. Solid State Chem. 1988, 73, 572. (3) Piffard, Y.; Lachgar, A.; Tournoux, M. Mater. Res. Bull. 1985, 20, 715. (4) Piffard, Y.; Oyetola, S.; Courant, S.; Lachgar, A. J. Solid State Chem. 1985, 60, 209. (5) Lachgar, A.; Deniard-Courant, S.; Piffard, Y. J. Solid State Chem. 1986, 63, 409. (6) Piffard, Y.; Lachgar, A.; Tournoux, M. Mater. Res. Bull. 1986, 21, 1231. (7) Piffard, Y.; Verbaere, A.; Lachgar, A.; Deniard-Courant, S. and Tournoux, M. Rev. Chim. Miner. 1986, 23, 766. (8) Greenblatt, M.; Wang, E. Chem. Mater. 1991, 3, 542. (9) Greenblatt, M.; Wang, E. Chem. Mater. 1991, 3, 703. (10) Botto, I. L.; Garcia, A. C. Mater. Res. Bull. 1989, 24, 1431. (11) Crosnier, M. P.; Piffard, Y. Eur. J. Solid State Inorg. Chem. 1989, 26, 529. (12) Crosnier, M. P.; Piffard, Y. Eur. J. Solid State Inorg. Chem. 1990, 27, 845. (13) Pagnoux, C.; Verbaere, A.; Kanno, Y.; Piffard, Y.; Tournoux, M. J. Solid State Chem. 1992, 99, 173. X
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With an aim to develop new routes to access these types of compounds, we began a systematic investigation into the hydrothermal system K2O-Sb2O5-P2O5H2O and found that several phases such as K3Sb3P2O14‚5H2O (A ) P, As) can be synthesized via a hydrothermal route under mild conditions and moreover some new phases were prepared.16-19 Also the hydrothermal synthesis route brings a new insight to the synthetic aspects of novel phosphatoantimonates that might not be obtained by solid-state reactions. In this paper, we present the hydrothermal synthesis and structural characterization of a novel potassium phosphatoantmonate, K8Sb8P2O29‚8H2O. Experimental Section The typical synthesis procedure for crystalline K8Sb8P2O29‚8H2O was as follows: 4.95 g of antimonyl potassium tartrate (analytical grade) was dissolved in deionized water, and then 1.67 mL of H2O2 (30 wt %, analytical grade) was added dropwise to the solution with stirring magnetically. H3PO4 (0.35 mL, 85%) and KOH (1.94 g, analytical grade) were added in turn. The resulting solution, having a molar composition 7K2O‚3Sb2O5‚P2O5‚450H2O, was sealed with 80% fill in a PTFE-lined stainless steel autoclave with volume of 25 cm3, and crystallized under autogenous pressure at 200 °C for 8 days. After autoclave cooling to room temperature, the product was filtered, washed with deionized water, and dried at ambient temperature. For the intercalation of amine: 0.2 g of product was mixed in 15 mL of 0.02 N n-octadecylamine hydrochloride solution (14) Taulelle, F.; Sanchez, C.; Livage, J.; Lachgar, A.; Piffard, Y. J. Phys. Chem. Solids 1988, 49, 299. (15) Husson, E.; Genet, F.; Lachgar, A.; Piffard, Y. J. Solid State Chem. 1988, 75, 305. (16) An, Y.; Feng, S.; Xu, Y.; Xu, R.; Yue, Y. J. Mater. Res. 1994, 11, 2745. (17) An, Y.; Feng, S.; Xu, Y.; Xu, R.; Yue, Y. J. Mater. Chem. 1994, 6, 985. (18) An, Y.; Feng, S.; Xu, Y.; Xu, R.; Yue, Y. J. Mater. Chem. 1995, 5, 773. (19) An, Y. Ph.D. Thesis, Jilin University, 1994.
© 1996 American Chemical Society
A New Potassium Phosphatoantimonate
Figure 1. Reaction composition diagram for the K8Sb8P2O29‚ 8H2O. (water serves as solvent). The intercalation reaction was carried out in a thermostatic bath at 65 °C for 36 h. The product was separated by centrifugation, washed with ethanol and water in turn, and dried at ambient temperature. Chemical composition analysis was carried out as described previously.18 The molar composition corresponds to 4K2O‚4Sb2O5‚P2O5‚8H2O. The water content was obtained via TGA. Powder XRD was carried out on a Rigaku D/MAX-IIIA X-ray diffractometer with Cu KR radiation. The data were collected by step scanning mode with a step length of 0.02° and a scanning rate of 0.2°/min. The TREOR program was used for the indexing of XRD pattern. Observation of the crystallites was performed on a Hitachi X-650 scanning electron microscopy. IR spectra were obtained with a Nicolet 5DX spectrometer using the KBr pellet technique. 31P MAS NMR study was conducted on a Brucker-MLS-400 spectrometer. The resonance frequency was 162 MHz; spinning rate 6 kHz; 90° pulse length, 3.5 µs. 85% H3PO4 solution was used as the external reference. DTA and TGA studies were made on a PerkinElmer DTA-7000 differential thermal analyzer and a TG-7 thermogravimetric analyzer with a heating rate of 10 °C/min, respectively. A N2 atmosphere was used with the flow rate of 40 mL/min. XPS research was carried out on a VG Scientific Escalab Mark-II spectrometer with Mg KR radiation, and Cls ) 285.00 eV was used as reference. The adsorption was conducted on a Cahn-2000 Vacuum Electrobalance system.
Results and Discussion 1. Syntheses and Affecting Factors. Figure 1 shows the reaction composition diagram. The compound can be synthesized only in a very narrow region indicated in Figure 1. The reaction composition diagram was determined at a constant crystallization temperature of 200 °C for 8 days, with a H2O/Sb molar ratio of 300. We found the following factors sensitive to the outcome of the hydrothermal synthesis. (a) Sb/P ratio: The Sb/P ratio was a dominating factor for the synthesis. When the Sb/P ratio was in the range 1-3, the title compound can be synthesized in weakly basic media (pH ) 8). Generally, a large Sb/P ratio was favorable for the crystallization, but Sb/P ratios greater than 3 resulted in the formation of hydrated potassium antimonate. Not all of the H3PO4 reacts with Sb2O5 in the hydrothermal system, so appropriate excess H3PO4 was necessary for the protection of the product from forming hydrated potassium antimonate in weakly basic medium. Sb/P ratios smaller than 1 are not conducive to high crystallinity of the product. Studies on the polymerization between H3PO4 and antimonic acid
Chem. Mater., Vol. 8, No. 2, 1996 357
showed that the molecular weight of the resulting polymer increased with increasing Sb/P ratio. However, in the presence of excess phosphoric acid, polymers with high molecular weight were not formed.20 Therefore, a small Sb/P ratio is certainly unfavorable for the crystallization of the desired product. However, the synthesis can be carried out at high pH in the system with a small Sb/P, because in the system with high pH the Sb/P ratio is no longer a dominating factor. (b) pH: KOH was either a reactant or a mineralizer in the hydrothermal crystallization. Usually, higher pH values (e.g., 9-10) accelerates the reaction, but extra high pH (e.g., 13) may result in the formation of hydrated potassium antimonate, especially in the systems with larger Sb/P (e.g., 3) or at a high crystallization temperature. The pH associated with the crystallization temperature influences the crystallization. For example, at 160 °C the K8Sb8P2O29‚8H2O was formed at pH ) 12 in the system of Sb/P ) 1, but at 250 °C a hydrated potassium antimonate was formed at pH g 9 in the system with the same Sb/P ratio. From this result it can be concluded that the hydrated potassium antimonate is a more stable phase compared to K8Sb8P2O29‚8H2O under the hydrothermal conditions. Low-temperature and small Sb/P allow the use of a high pH for the synthesis of the phosphatoantimonate. (c) Crystallization temperature: The product can be crystallized in the temperature range 160-250 °C. High temperatures can accelerate the crystallization; however at low temperatures, the crystallization can be promoted by increasing the pH in the hydrothermal system. (d) Starting materials: Many Sb compounds were used as Sb sources including Sb2O3, SbCl3, K2H2Sb2O7‚4H2O, and Sb2O5‚4H2O; among them SbCl3 and antimonyl potassium tartrate are the most active. When these were used as starting materials, the initial reaction mixtures were homogenous solutions and the crystallization was enhanced. The P source can be H3PO4 or various potassium phosphates. The effect of P sources on the synthesis was not evident. (e) Cations: It was interesting to observe the effect of cations under the same synthetic conditions; hydrated sodium antimonate always formed in the presence of sodium ions, showing that the sodium antimonate phase is very stable. Lithium and cesium phosphatoantimonates were never formed under our hydrothermal synthetic conditions, which may result from the lattice mismatching of the sizes of the Li and Cs ions. The rubidium phosphatoantimonate can be synthesized with poor crystallinity. These phenomena also were observed in solid-state reactions where most phases of phosphatoantimonates were the potassium forms. This showed that potassium ions favor the formation of phosphatoantimonates. The results obtained by Jolivet et al.21 showed that the presence of potassium ions even in small amounts benefits to the formation of phosphatoantimonate in solution. (f) F- ions: It is well-known that F- ion is an effective mineralizer in both aqueous and nonaqueous systems.22-24 The presence of F- decreases considerably (20) Jolivet, J. P.; Lefebvre, J. Bull. Soc. Chim. Fr. 1977, 1-2, 34. (21) Jolivet, J. P.; Lefebvre, J. Bull. Soc. Chim. Fr. 1977, 1-2, 43. (22) Guth, J. L.; Kessler, H.; Wey, R. Proceedings of the 7th International Zealite Conference, Tokyo, 1986, 121. (23) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merrouche, A.; Kessler, H. Nature 1991, 352, 320.
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Figure 2. SEM photograph of the K8Sb8P2O29‚8H2O.
An et al.
Figure 4. 31P static (a) and MAS NMR spectra (b) of the K8Sb8P2O29‚8H2O. Table 1. Powder X-ray Data of the K8Sb8P2O29·8H2O h 1 2 3 -2 -1 0 4 1 -5 -6 -5 -7 6 -3 5 -5 -1 1 -6 -8
Figure 3. Powder XRD patterns of as-synthesized compound (a) and the amine-intercalated compound (b).
the crystallization temperature, and some new phases were synthesized from F- containing systems under mild conditions. However, in the K2O-Sb2O5-P2O5H2O systems, the mineralizing effect of F- was not observed. On the contrary, F- ions retarded the crystallization to some extent; the potassium phosphatoantimonate was not obtained from F--containing systems. 2. Characterization of K8Sb8P2O29‚8H2O. The SEM morphology of the resulting K8Sb8P2O29‚8H2O is shown in Figure 2. All the crystallites have regular shapes of thin hexagonal sheets, which indicated the phase is reasonably pure. The valence state of Sb atoms in the product was detected by the XPS technique. XPS results showed that the binding energy (3d5/2) of Sb is 530.85 eV, corresponding to the Sb (V) state. The XRD pattern of K8Sb8P2O29‚8H2O (Figure 3a) is unique and can be indexed in the monoclinic system with cell parameters a ) 23.95(2) Å, b ) 9.51(5) Å, c ) 8.19(3) (24) Huo, Q. Ph.D. Thesis of Jilin University, 1992.
k l dobs (Å) dcalc (Å) I/I0 0 0 0 1 1 1 0 1 1 1 1 0 0 2 2 2 2 1 2 1
0 19.66 0 9.831 0 6.557 1 6.167 1 6.065 1 5.491 0 4.918 1 4.744 1 4.273 1 3.663 2 3.586 1 3.365 0 3.277 2 3.099 0 3.033 2 3.003 2 2.911 2 2.898 2 2.862 2 2.807
h
k l dobs (Å) dcalc (Å) I/I0
19.68 19 5 0 1 9.837 100 -7 2 2 6.558 2 -9 0 2 6.167 26 1 2 2 6.065 8 8 0 0 5.494 10 -10 1 1 4.919 12 9 0 0 4.750 7 -5 4 2 4.278 3 10 0 0 3.665 6 4 4 1 3.585 6 5 2 2 3.364 13 -12 2 2 3.279 18 -10 2 4 3.100 16 -1 0 4 3.032 14 -10 2 4 3.002 13 -3 5 2 2.913 39 -10 0 5 2.897 36 0 2 4 2.862 7 -11 4 3 2.808 5
2.777 2.688 2.649 2.562 2.459 2.206 2.186 2.027 1.968 1.909 1.879 1.833 1.793 1.764 1.726 1.703 1.624 1.586 1.574
2.777 2.688 2.649 2.562 2.459 2.207 2.186 2.027 1.968 1.908 1.879 1.833 1.793 1.764 1.725 1.703 1.623 1.586 1.574
15 7 7 8 8 3 4 6 3 8 7 4 16 7 4 3 7 5 7
Å, and β ) 124.78 (Table 1). All these results clearly showed that the potassium phosphatoantimonate is a new phase. Figure 4 shows the 31P NMR spectrum of K8Sb8P2O29‚8H2O. It can be seen that the P atoms have the same chemical environments in the structures as there is only one resonance absorption in the spectrum. The isotropic chemical shift is a function of the PO4 connectivity (the number of bridging oxygens in PO4). Normally, the chemical shift at higher field corresponds to a larger connectivity. According to Taulelle,14 an isotropic chemical shift at -6.56 ppm implies PO4 connectivity being smaller than 4. Our static NMR spectrum also evidenced such a structural situation. For a powder sample, the characteristic absorption for all possible random orientations of nuclei with respect to the field will be observed simultaneously, giving a characteristic broad absorption or anisotropy pattern from which the dimensionality of structure can be estimated. Threedimensional networks, where all oxygen atoms in PO4 groups are bonded to Sb atoms, corresponds to small chemical shifts and isotropic absorption, whereas chain and/or layered structures exhibit relative large chemical shifts and anisotropic absorption.14 The static spectrum (anisotropic) was axially symmetric (Figure 4a), indicating that the PO4 ion has C3v local symmetry and the
A New Potassium Phosphatoantimonate
Figure 5. IR spectra of the samples at 25 (a), 50 (b), 100 (c), 150 (d), 200 (e), and 250 °C (f).
connectivity is 3.25 A larger chemical shift anisotropy (σ| ) 12.43 ppm, and σ⊥ ) -43.07 ppm) suggested that the compound has a layered structure, which was confirmed by the intercalation of amine into the compound. From Figure 3b, it can be seen that, after the intercalation, several new peaks at low 2θ appeared. The increase in the d spacing was due to the intercalation of the n-octadecylamine ions located in between the layers. A preliminary study on the cation exchange of this compound showed that the potassium ions can be exchanged by sodium ions. In situ IR spectra of the new potassium phosphatoantimonate at different temperatures are shown in Figure 5. The tetrahedral PO4 ion with Td symmetry has four internal modes of vibrations, i.e., symmetric and antisymmetric stretching modes (ν1 ) 938 cm-1, ν3 ) 1017 cm-1) and bending modes (ν2 ) 420 cm-1, ν4 ) 567 cm-1).26 An IR study of phosphatoantimonates has shown that the various Sb-O stretching modes occur between 880 and 470 cm-1.27 Therefore the absorptions at 1173 and 966 cm-1 in Figure 5a can be attributed to the antisymmetric and symmetric stretching modes of the PO4 group, respectively. The domain of 880-400 cm-1 can be assigned to various Sb-O stretching modes and the different bending vibrations of the P-O and Sb-O bonds. An IR study of the sample after heating showed that at 250 °C the absorption strength at 1173 cm-1 was lowered, and a shoulder appeared at 1217 cm-1. This suggested that the PO4 group has a terminal P-O bond which was weakened by hydrogen bonding involving water of crystallization. Upon heating, the crystallization water was lost and the hydrogen bond was destroyed; the antisymmetric stretching absorption partially shifted from 1173 to 1217 cm-1 because the terminal P-O bonds were reformed. The IR study also suggested that the PO4 groups do not share all vertexes with the SbO6 octahedra. This result is in agreement with that of 31P NMR. The product was characterized by DTA and TGA (Figure 6). There were two endothermal peaks at 185 and 550 °C corresponding to water losses of 4.7 and 2.7 wt %, respectively. The first thermal effect resulted from the dehydration of crystallization water, and this (25) Bleam, W. F.; Pfeffer, P. E.; Frye, J. S. Phys. Chem. Miner. 1989, 16, 455. (26) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1978. (27) Husson, E.; Genet, F.; Lachgar, A.; Piffard, Y. J. Solid State Chem. 1988, 75, 305.
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Figure 6. DTA-TGA curves of the K8Sb8P2O29‚8H2O.
process was completely reversible when the sample was exposed to moisture. This was proved by XRD and adsorption experiments; when the first thermal effect occurred, an XRD study of the residue showed that the structure was maintained. The second thermal effect was assigned to the dehydration of OH groups. After the second thermal effect, XPS results showed that the valence state of Sb was the same as before, but the XRD study indicated that the structure of the sample had changed. So the formula should be K8Sb8P2O26(OH)6‚5H2O according to the above results. The adsorption experiment (prior to adsorption, the sample used was pretreated at 200 °C under vacuum for 2 h) showed that the sample did not absorb cyclohexane, and the absorbed amount of water is 4.2 wt % at 25 °C. This result was in agreement with that obtained from TGA and indicated the reversibility of the first dehydration process.
Conclusions In summary, a new potassium phosphatoantimonate, K8Sb8P2O29‚8H2O was synthesized from the K2OSb2O5-P2O5-H2O system. The Sb/P molar ratio and pH value in the initial reaction mixture, along with the reaction temperature, dominated the crystallization of the product. The K8Sb8P2O29‚8H2O has a layered structure which can be intercalated by organic amine ions, and the potassium ions can be exchanged. The as-synthesized compound provides a possible host material for some guest molecules. The hydrothermal method can be used to access not only the phosphatoantimonates obtained by solid-state reaction but also new phases which usually can not be obtained via the solidstate route, because the temperature needed for the solid-state reaction is much higher than the transformation temperature of the new phase. The successful synthesis of this new phase suggests the possibility of obtaining other new antimonate phases, such as phosphatoantimonates, arsenoantimonates, silicoantimonates, and germanatoantimonates. Acknowledgment. This work was supported by the Chinese NSF through the National Outstanding Youth Science Foundation (S.F.) and partially supported by the Chinese State Science and Technology Commission. CM950182+
