A novel layered aluminophosphate with corner-sharing AlO₆ chains

The synthesis, Raman spectroscopy and crystal structure of a novel layered aluminophosphate is described. The new phase was derived by the sol-gel method starting from a modified low hydrothermal ALPO4-34 zeolite synthesis procedure[1].The structure was solved by direct methods using single-crystal X-ray diffraction. The synthesized layered material, with composition [AlPO₃(OH)F(H₂O)]-(H₉C₄ON), crystallizes in the monoclinic space group P2₁/a with a = 9.2282(5) Å, b = 6.9152(4) Å, c = 14.4615(9) Å, β = 101.57(1)⁰. Layered aluminophosphates with AlO₆ polyhedra have been previously described [2], although in these compounds Al octahedral share edges. The novel compound has corner sharing AlO₄F(H₂O) chains along [010], where fluorine is at the shared apex, four oxygen atoms are shared with PO₄ tetrahedra and the fifth oxygen is a H₂O group. This kind of aluminophosphate chains is found in nature in tancoite [3]. Chains are linked along [100] through corner sharing with a PO₄ group of the adjacent chain plus hydrogen bonding of the H₂O group. Layers are stacked along c* through hydrogen bonding with a double layer of morpholine (H₉C₄ON) molecules. The chemical stability field of the novel materialis strongly dependent from the fluorine/aluminum ratio of the starting gel. At lower fluorine concentrations only ALPO4-34 and/or AlPO₄ (berlinite) are stable depending on the morpholine content. Crystals growth morphology depends on the supersaturation conditions of the starting gel: at low concentrations crystals are well developed hexagonal like plate shaped and are very thin. At higher concentrations they show a more elongated morphology. A treatment with H₂CO₃ leads to a complete morpholine removal, as shown by in situ Raman spectroscopy. Powder X-ray diffraction reveals that after morpholine extraction, the material diffract still coherently in two dimensions while a strong broadening is shown for basal planes.

HP-induced penetration of guest molecules in high-Si mordenite

A high-Si mordenite (HS-MOR, SiO2/Al2O3 ~ 200, s.g. Cmcm) was investigated by in-situ synchrotron XRPD under HP using silicone oil (s.o.) as non-penetrating pressure transmitting medium (PTM), and the following penetrating PTM: (16:3:1) methanol: ethanol:water (m.e.w), (3:1) water:ethanol (w:e), ethylene glycol (e.g.) and resorcinol (res). The experiments were performed in DAC at SNBL1 (ESRF, Grenoble). The evolution of the structural features was followed by full profile Rietveld refinements. In the Pamb-1.2 GPa range, the volume contraction of HS-MOR compressed in s.o. (Tab. 1) is the highest found among the HS zeolites studied up to now in the same P range [1-2]. Above this P value, a rapid and irreversible loss of long range order is observed in the diffraction patterns. These findings suggest a gradual P-induced amorphization. The main results of the experiments with penetrating PTM are the following (Tab. 1 and Fig. 1): i) no complete X-ray amorphization and phase transitions achieved up to the highest investigated P; ii) penetration of additional guest species into the channels, even at very low P; iii) lower cell-volume reduction with respect to that found in s.o. in the same P range; iv) partial reversibility of the P-induced effects upon decompression. The lower compressibility of HS-MOR in penetrating PTM with respect to s.o. is due to the entrapping of additional guest molecules, which contributes to sustaining the mordenite framework and stiffening the material.

HP-induced supra-molecular organization of guest molecules in FER-type zeolites

The response to pressure of a synthetic all-silica ferrierite (Si-FER) and of a natural ferrierite from Monastir (Sardinia, Italy) (Mon-FER, Na0.56 K1.19 Mg2.02 Ca0.52 Sr0.14)(Al6.89Si29.04)O72 ·17.86 H2O) is here investigated combining HP synchrotron XRPD experiments and molecular dynamics simulations. The experiments were carried out by using penetrating (methanol:ethanol:water 16:3:1, m.e.w.; ethanol:water 1:3, e.w.) and non-penetrating (silicone oil, s.o.) pressure transmitting media (PTM). In Si-FER compressed in e.w., both water (w.) and ethanol molecules (e.) enter the pore system even at 0.2 GPa. The structural refinement of the data collected at 0.8 GPa reveals 8 w. and 4 e. molecules in the 10- and 6-membered ring channels, in tight agreement with the results of MD simulations. In Si-FER compressed at 0.2 GPa in m.e.w., only water molecules penetrate the 10-membered ring channels (15 per u.c.), organized in chains running along the channel axis. The interactions among the guest species and the framework oxygen atoms are very weak, due to the hydrophobicity of the framework. Upon decompression, the intruded extra-molecules are not completely released, so giving rise to new materials with different extra-framework contents. The results obtained for Si-FER compressed in m.e.w. and s.o. were compared to those obtained for Mon-FER, demonstrating that the zeolite composition and the PTM strongly influence the overall elastic parameters of the investigated samples. Specifically, Mon-FER shows a much higher rigidy than Si-FER in both media, due to the stiffening effect of the numerous extraframework species present in the natural sample. The higher rigidity of Si-FER in m.e.w. with respect to s.o. can be explained by the penetration, in the former case, of the PTM molecules, which contribute to stiffen the framework.