The Structure of Water in Crystalline Aluminophosphates: Isolated Water and Intermolecular Clusters Probed by Raman Spectroscopy, NMR and Structural Modeling

Abstract

Raman spectroscopy, thermo-gravimetry and NMR together with EEM-Monte Carlo calculations are used to elucidate the properties of water in Metavariscite, AlPO_4−H3, AlPO_4-8 and VPI-5. The framework density of the aluminophosphates decreases along this sequence, the pore size increases, and the water is less confined. The Raman vibrations of water in different aluminum phosphates reflects the polarization by Al and the number, and the type of hydrogen bonds in which they participate. Metavariscite and AlPO_4−H3 have well-defined symmetric νOH doublets in the region between 3000 and 3450 cm^(-1), corresponding to structural water molecules. AlPO_4−H3 has additional free water with ν_(OH) at 3505 and 3563 cm^(-1). In AlPO_4-8 and VPI-5 the majority of the water is zeolitic, and the corresponding feature is a broad band around 3200−3400 cm^(-1). The differences between Al-coordinated and zeolitic water is also evidenced by EEM-Monte Carlo calculations. For the octahedral Al, ^(27)Al NMR is used to probe the different types involved. Mapping between principle hardness from electron population normal modes and force constants from the vibrational normal modes relates the influence of the environment of the water on the position and shape of the Raman bands. Perturbations imposed on water by the lattice Al (ν_(OH) < 3350 cm^(-1)) and H bonding by O atoms alone (ν_(OH) > 3400 cm^(-1)), change the symmetric OH-stretching polarization and decrease the principle hardness. The ν_(OH) vibrational frequency and the principal hardness splits as a result of different types of hydrogen bonding. When the number of interacting water molecules increases (Metavariscite → AlPO_4-H3 → VPI-5) a broadening of the Raman bands and the hardness distribution of the OH polarization is seen. This EEM-Monte Carlo approach is used for the semiquantitative interpretation of Raman data.