An in-situ x-ray diffraction and infrared spectroscopic study of the dehydration of AlPO4-54
Graphical abstract
Introduction
AlPO4-54•xH2O, also known as VPI-5, with the VFI structure type [1] is the aluminophosphate with the largest pores [2]. This material has a one dimensional pore system with 12.7 Å pores lying along the c direction. The hexagonal structure of this material, space group P63 with a = 18.9752 Å and c = 8.1044 Å has been refined using powder [3] and single crystal x-ray diffraction [[4], [5], [6]]. It consists of a framework of alternating Al-centered polyhedra and P-centered tetrahedra. The Al atoms are in different environments; one third are in octahedral coordination due to the presence of two water molecules in the coordination sphere, the remaining two thirds of the Al atoms are in tetrahedral coordination. In addition to the structural water, the pores also contain confined adsorbed water, which presents some degree of order [3]. This is in fact a model system for the confinement of water at the nanoscale [4].
AlPO4-54. xH2O can be dehydrated with great care conserving the framework structure and removing the structural and adsorbed water [[7], [8], [9], [10], [11], [12]]. This results in the Al atoms all being tetrahedrally coordinated. The dehydration process involves the use of vacuum and/or heating and if it is not optimized, transformation to a smaller-pore aluminophosphate AlPO4-8 is observed [[12], [13], [14]]. The dehydrated form of this material is of interest as the removal of water can allow for the insertion of other guests.
Using neutron powder diffraction, the structure of dehydrated VFI was first refined using a P63/mcm model with a = 18.549 Å and c = 8.404 Å as the Al and P atoms could not be distinguished [7]. Subsequent x-ray powder diffraction studies indicated that the symmetry was in fact lower with a P63cm space group due to alternating AlO4 and PO4 tetrahedra in the structure [8]. NMR studies confirm that all the Al atoms are in tetrahedral coordination; however, 31P NMR spectra are not in agreement with such high hexagonal symmetry [9].
In addition, unit cell parameters differ depending on the vacuum pressure, temperature and duration of the treatment, for example after 8 h at 1.3 mPa the unit cell parameters were found to be a = 18.524 Å and c = 8.332 Å [10] as compared to a = 18.544 Å and c = 8.3847 Å after 48 h at 0.1 Pa [11], with no further evolution under high vacuum nor with heating. Very long dehydration with increasing vacuum and temperature over the period of one week produced a hexagonal material with a = 18.5457 Å and c = 8.3992 Å [15].
It was later shown that the structure and cell parameters also depend on the age of the sample and its detailed history [9]. In the latter study, dehydrated VFI was found to adopt two monoclinic Cm structures with simple and doubled c lattice parameters. No deviation from a hexagonal unit cell metric was observed. The 31P NMR spectra of both materials are the same and are not consistent with these monoclinic crystal structures. It was proposed that these monoclinic structures are commensurate or incommensurate modulated structures.
In the present study, in order to determine the conditions required for dehydration of VFI without the need for heating for optimized filling with other guest species, an in situ study under vacuum by infrared spectroscopy, powder and synchrotron single crystal x-ray diffraction was performed.
Section snippets
Synthesis of AlPO4-54.xH2O
Polycrystalline samples and large single crystals, with linear dimensions on the order of 30 × 1x1 μm3 and 200 × 25 × 25 μm3 (needle-like), respectively, were synthesized by an optimized sol-gel procedure followed by hydrothermal treatment. The starting materials for the synthesis of the polycrystalline material were nanometric alumina and phosphoric acid. An alternative method using polyphosphoric acid was used for the synthesis of the large single crystals [16]. The details of the synthesis
In situ infrared spectroscopy under vacuum
The infrared spectrum of AlPO4-54•xH2O was measured as a function of time and vacuum pressure, Fig. 1. As the OH stretching region contains band from water and also hydroxyl defects in the framework, attention was primarily focused on the bending vibration of H2O at 1632 cm−1. This band loses 88.3% of its intensity after 2 h in primary vacuum (5 Pa) and 94.2% of its intensity after 2 h in secondary vacuum (0.05 Pa). These results provide quantitative information on the residual water content of
Conclusion
The present infrared and x-ray diffraction results on powder samples are consistent with the dehydration of AlPO4-54•xH2O occurring via a distinct partially dehydrated intermediate phase AlPO4-54•1/2H2O corresponding to a form with empty pores, but with some remaining structural H2O in the Al-centered polyhedra. At ambient temperature, the vacuum pressure appears to be a key parameter in the dehydration process. Adsorbed pore water is removed readily under primary vacuum followed by gradual
CRediT authorship contribution statement
Marco Fabbiani: Methodology, Investigation, Formal analysis, Writing - review & editing. Michelangelo Polisi: Methodology, Investigation, Formal analysis, Writing - review & editing. Bernard Fraisse: Methodology, Investigation, Writing - review & editing. Rossella Arletti: Writing - review & editing, Supervision. Mario Santoro: Methodology, Investigation, Writing - review & editing. Frederico Alabarse: Methodology, Investigation, Writing - review & editing. Julien Haines: Methodology,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We acknowledge the PRIN project ZAPPING, number 2015HK93L7, granted by the Italian Ministry of Education, Universities and Research, MIUR. The synchrotron single crystal X-ray diffraction experiments were performed at the Xpress beamline from Elettra Sincrotrone Trieste (proposal number: 20195123).
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