Investigations on the intrinsic and template-dependent transformations of AlPO4-11 molecular sieve under high pressure

Abstract

In this study, aluminophosphate molecular sieve AlPO4-11, with one-dimensional elliptical channels, was investigated under high pressure in non-penetrating silicone oil by X-ray powder diffraction, infrared spectroscopy and theoretical simulation, focusing on its intrinsic and template-modified mechanical behaviors. Two distinct compressible regimes were observed for both the calcined and as-synthesized AlPO4-11 frameworks before pressure-induced amorphization (PIA). The channel cross-section in the calcined AlPO4-11 framework showed an obvious increase in the ellipticity during this process. The framework structure began to lose crystallinity at approximately 2–3 GPa and underwent significant amorphization at 6.6 GPa. PIA was mainly induced via the destruction of the constructed species along the periodic crystallographic planes which are parallel to the channel axis. The bulk moduli showed that this empty framework was rather soft in these regimes. For the as-synthesized AlPO4-11 framework, the mechanical stability of the channels was markedly improved by the template molecules. A different elliptic deformation of the cross-section with a reduced contractility in the minor axis direction was presented. Discontinuous volume evolution occurs at 3–4 GPa and significant amorphization was postponed to 13.7 GPa. The as-synthesized AlPO4-11 was stiffer than calcined AlPO4-11 and showed a soft to hard transformation due to the strengthened host-guest interaction upon compression. Moreover, infrared spectra indirectly confirmed the structural collapses by detecting the vibrational characteristics of the embedded template molecules. The present results provided a comparative and basic knowledge of the intrinsic and extrinsic stability, compressibility, channel deformation and collapse of the AlPO4-11 framework under high pressure. This study emphasized the importance of guest molecules in modifying the mechanical behaviors of zeolites and resisting framework collapse.