High-Pressure Insertion of Dense H₂ into a Model Zeolite

Abstract

Our combined high-pressure synchrotron x-ray diffraction and Monte Carlo modelling studies show super-filling of the zeolite, and computational results suggest an occupancy by a maximum of nearly two inserted H 2 molecules per framework unit, which is about twice more than observed in gas hydrates. Super-filling prevents amorphization of the host material up to at least 60 GPa, which is a record pressure for zeolites and also for any group IV element being in full 4-fold coordination, except for carbon. We find that the inserted H 2 forms an exotic topologically constrained glassylike form, otherwise unattainable in pure hydrogen. Raman spectroscopy on confined H 2 shows that the micro-porosity of the zeolite is retained over the entire investigated pressure range (up to 80 GPa) and that inter-molecular interactions share common aspects with bulk hydrogen while they are also affected by the zeolite framework.

Monoclinic–Orthorhombic Phase Transition with a Positive Volume Change in the Siliceous Zeolite Mobil-Five Induced by the High-Pressure Insertion of Dense Fluid Helium

“…Helium is different from the other guest species mentioned above as it has been clearly shown to penetrate the frameworks of various silica forms such as silica glass − and cristobalite and thus can fill various rings and cages in the framework in addition to the 5.5 Å diameter pores. The situation is similar for H 2 and in the orthorhombic form of MFI at high pressure, hydrogen molecules not only occupy the pores but also the cages in the framework built up of 5- and 6-membered rings of SiO 4 tetrahedra . In the case of H 2 , however, measurements were not performed on the monoclinic form at low pressure, and the monoclinic–orthorhombic phase transition was not investigated .…”

“…The situation is similar for H 2 and in the orthorhombic form of MFI at high pressure, hydrogen molecules not only occupy the pores but also the cages in the framework built up of 5- and 6-membered rings of SiO 4 tetrahedra . In the case of H 2 , however, measurements were not performed on the monoclinic form at low pressure, and the monoclinic–orthorhombic phase transition was not investigated . In another siliceous zeolite TON, insertion of H 2 at high pressure and high temperature was found to induce an anomalous volume increase and a phase transition to a higher symmetry structure …”

“…Relative volume ( V / V 0 ) of MFI in He as a function of pressure on compression (solid squares) and on decompression (empty squares). Circles, dashed, and solid lines represent the behavior of MFI in H 2 , silicone oil, and Ar and CO 2 , respectively. Error bars are smaller than the symbol size.…”

“…Such a behavior does not occur on compression in argon but can occur with slow kinetics above a threshold pressure in the case of the intermediate-sized neon atom . In the case of porous, pure silica zeolites, larger atoms and molecules readily enter the pores at high pressure. − In the case of the archetypal all-silica zeolite, silicalite-1 with the monoclinic Mobil-Five (MFI) structure and a three-dimensional pore system, , argon or simple molecules such as water, nitrogen, and carbon dioxide can completely fill the pore system but do not enter the framework. ,, The exception is hydrogen, which has very recently been found to enter the secondary building units of the framework. The structure of MFI , is formed by linear 5.3 Å × 5.6 Å channels along the b direction and 5.1 Å × 5.5 Å sinusoidal pores in the xz plane (Figure ).…”

“…High silica, MFI undergoes a ferroelastic phase transition from the monoclinic P 2 1 / n to the orthorhombic Pnma form at 349 K. A similar phase transition is also observed at high pressure in the vicinity of 1 GPa . When siliceous MFI is compressed in penetrating medium, such as H 2 O or CO 2 , this transition can occur at even lower pressures. , Filling with guest species was also found to stabilize the orthorhombic phase with respect to pressure-induced amorphization at very high pressures. ,, In H 2 -filled MFI, the crystalline phase is still observed at 60 GPa.…”