High-resolution solid-state magic-angle spinning nuclear magnetic resonance studies on the thermal decomposition of the layered antimony hydrogen phosphate, HSb(PO4)2·2H2O
Introduction
Recent renewed interest in the structure, properties and chemistry of antimony hydrogen phosphates (the so-called phosphatoantimonic acids) has arisen owing to their ability to act as excellent cation exchangers 1, 2. Interest also arises from their increased use as acidic catalyst supports within the petrochemicals industry. The antimony hydrogen phosphate studied in the present paper, HSb(PO4)2·2H2O, is the most common member of a series of acidic antimony hydrogen phosphates whose ideal composition may be described by the following formula:where: n=an odd integer; i.e. 1, 3, 5... and x is a variable depending upon the degree of hydration of the phosphate.
The members of this series with n equal to 1 or 3 have been found to possess a two-dimensional lattice structure [3]. In the case of the title antimonic acid (referred to hereafter as H1SbP2·2H2O or antimony hydrogen phosphate), these layers are arranged in a series of infinite [SbP2O8−]n sheets composed of SbO6 edge-sharing octahedra and –PO4 tetrahedra. Each octahedron is connected to six tetrahedra and each tetrahedron is connected to three SbO6 octahedra 4, 5. This structure, which is thought to be similar to that of α-zirconium hydrogen phosphate [6], is shown schematically in Fig. 1b.
The present study details an investigation into the structure of the phosphate groups in the antimonic acid, H1SbP2·2H2O, using both single pulse (SP) and cross-polarisation (CP) 31P high-resolution solid-state Magic-angle Spinning Nuclear Magnetic Resonance (MAS NMR) techniques. The nature of the decomposition of the antimonic acid was also studied by 1H and 31P NMR techniques.
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Materials
The synthesis of the Sb2O5(s) precursor required Sb2O3(s) and 30% mass/volume H2O2(aq) both of which were GPR grade and supplied by BDH Ltd. The synthesis of the crystalline K1SbP2 required the following: NH4H2PO4(s), KH2PO4(s) and 16 M HNO3 which were also supplied (as GPR reagents) by BDH Ltd.
Preparation of Sb2O5·xH2O
The preparation of hydrated antimony pentoxide, Sb2O5·xH2O, was achieved by heating Sb2O3 (100 g, 0.34 mol) in 30% mass/volume H2O2 solution (0.5 dm−3) for a period of 24 h at 80°C. Caution should be
Scanning electron microscopy
Fig. 2a shows the scanning electron micrograph obtained for the parent KSb(PO4)2 salt. The individual crystallites are easily identifiable. This SEM clearly shows that the potassium salt of the antimonic acid forms via the so-called `deck of cards' mechanism 2, 9. In this mechanism [2]the lamellae are stacked at random as in the act of shuffling (or dropping) a deck of playing cards. A schematic diagram of the KSb(PO4)2 is shown in Fig. 1a. Fig. 2b shows the micrograph of the proton exchanged
Conclusions
The synthesised H1SbP2·2H2O material was prepared according to the established method. SEM analysis suggested that the precursor potassium salt precipitated out via the so-called `deck of cards' mechanism of random lamellae orientation. Studies using both 31P and 1H MAS NMR indicated that the synthesised phosphate consisted of Q3-type phosphate tetrahedra which were protonated in all cases via interaction of the water of crystallisation. Simultaneous TG and DSC thermogravimetry revealed a
Acknowledgements
The authors would like to thank Laporte Research and Development for financial support for S.C. and A.E.A. Technology Ltd. (Harwell) for financial support for W.J.L. We should also like to thank Dr D.C. Apperley of the EPSRC Solid-state NMR Service, University of Durham Industrial Research Laboratories, for the Solid-state NMR spectra and for his suggestions regarding their interpretation.
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