Strong Swelling and Symmetrization in Siliceous Zeolites due to Hydrogen Insertion at High Pressure

Hydrogen and helium saturate the 1D pore systems of the high-silica (Si/Al>30) zeolites Theta-One (TON), and Mobile-Twelve (MTW) at high pressure based on x-ray diffraction, Raman spectroscopy and Monte Carlo simulations. In TON, a strong 22 % volume increase occurs above 5 GPa with a transition from the collapsed P2₁ to a symmetrical, swelled Cmc2₁ form linked to an increase in H₂ content from 12 H₂/unit cell in the pores to 35 H₂/unit cell in the pores and in the framework of the material. No transition and continuous collapse of TON is observed in helium indicating that the mechanism of H₂ insertion is distinct from other fluids. The insertion of hydrogen in the larger pores of MTW results in a strong 11 % volume increase at 4.3 GPa with partial symmetrization followed by a second volume increase of 4.5 % at 7.5 GPa, corresponding to increases in hydrogen content from 43 to 67 and then to 93 H₂/unit cell. Flexible 1D siliceous zeolites have a very high H₂ capacity (1.5 and 1.7 H₂/SiO₂ unit for TON and MTW, respectively) due to H₂ insertion in the pores and the framework, in contrast to other atoms and molecules, thereby providing a mechanism for strong swelling.