AlPO4-5 zeolite at high pressure: Crystal–fluid interaction and elastic behavior

The high-pressure behavior of the aluminophosphate zeolite AlPO4-5 (AFI-topology) has been investigated by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell, using the “non-penetrating” silicone oil and the “penetrating” 16:3:1 methanol:ethanol:water (m.e.w.) mixture as P-transmitting fluids. The crystals used in this study showed diffraction patterns typical of an incommensurate modulated structure, with modulation vector q ≈ 0.37c*. Compression in silicone oil showed that AlPO4-5 is one of the softest zeolites reported so far, being its refined bulk modulus (KV0 = 1/βV) at ambient conditions only 13.2(11) GPa (βV = 0.076(6) GPa−1), with an elastic anisotropy Ka:Kc = 1.6. The high-pressure experiment performed using m.e.w. showed a significantly lower compressibility of AlPO4-5, if compared to the behavior in silicone oil, suggesting the occurrence of a P-induced intrusion of the fluid molecules into the zeolitic cavities. An indirect evidence for the molecules intrusion is provided by the comparative analysis of the structure deformation mechanisms at the atomic scale. When zeolite AlPO4-5 is compressed in m.e.w., a lower degree of distortion of the secondary building units is observed, with respect to the compression in silicone oil. Overall, this study suggests zeolite AlPO4-5 as a promising microporous material for the pressure-mediated intrusion and hyper-confinement of molecules into the structural micropores.