Effect of H₂O on the Pressure-Induced Amorphization of AlPO₄-54·xH₂O

Abstract

Microporous AlPO 4 -54•xH 2 O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H 2 O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H 2 O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO 4 -54•xH 2 O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure, two water molecules enter in the coordination sphere of IV Al, thereby increasing the coordination number from 4 to 6, which destabilizes the structure. The present results show that the insertion of and/or reaction with guest species can be used to strongly modify the stability of microporous materials with respect to PIA.

Effect of H2O on the Pressure-Induced Amorphization of Hydrated AlPO4-17

“…At this pressure range, part of the sample becomes amorphous and local depressurization occurs leading to an apparent increase of the cell parameters. Such effect has been observed on other structural studies performed on zeolites under high pressures [13,20,21] in which local depressurization effects were reported due to important volume changes during the amorphization with the external pressure not being transmitting to the remaining crystalline material.…”

“…AlPO 4 -54• x H 2 O possesses a 1-D pore system along the c direction, in which H 2 O molecules form a disordered hydrogen-bonded network [11]. While its dehydrated phase undergoes to pressure-induced phase transition starting at 0.8 GPa, its hydrated phase shows PIA beginning at 2.0 GPa [12,13]. In H 2 O, the material undergoes superhydration effects and a decrease in the onset of amorphization from 2.0 in non-penetrating PTM to 0.9 GPa in penetrating H 2 O PTM is observed [12,13].…”

“…While its dehydrated phase undergoes to pressure-induced phase transition starting at 0.8 GPa, its hydrated phase shows PIA beginning at 2.0 GPa [12,13]. In H 2 O, the material undergoes superhydration effects and a decrease in the onset of amorphization from 2.0 in non-penetrating PTM to 0.9 GPa in penetrating H 2 O PTM is observed [12,13]. The insertion of H 2 O in the pores hinders pore collapse at lower pressures.…”

Pressure-Induced Amorphization of Small Pore Zeolites—the Role of Cation-H2O Topology and Anti-glass Formation

“…Furthermore, the P1 phases of the NAT II materials Rb-NAT and Cs-NAT have higher bulk compressibilities, both transform to two denser phases, and subsequently amorphize at higher pressures than NAT I ones. We suggest that the closer proximity of water to the aluminosilicate framework in NAT I materials appears to promote coordination with Al 3+ which has been proposed as one mechanism for PIA in large pore materials7. The persistence of long-range order in Cs- and Rb-NAT after pressure release observed in X-ray powder data and HAADF-STEM images suggests an anti-glass structure.…”

“…In AlPO 4 -54-xH 2 O water coordination to the Al 3+ framework has been suggested to initiate PIA7. We present systematic studies of PIA in small pore zeolites with natrolite (NAT) frameworks.…”