Compressibility, Phase Transition, and Argon Insertion in the Siliceous Zeolite Mobil-Twelve at High Pressure

The siliceous zeolite Socony Mobil-twelve
(ZSM-12) or MTW (Mobil-TWelve)
with a one-dimensional pore system was studied at high pressure by
synchrotron X-ray powder diffraction in nonpenetrating DAPHNE7474
oil and in a penetrating argon pressure medium. A phase transition
from the space group C2/c to P2/n is observed in the nonpenetrating
medium at close to 1.5 GPa with a 4% volume decrease and a strong
increase in compressibility in the ac plane corresponding
to partial collapse of the pores. Strong decreases in diffracted intensity
are observed with further compression, and the diffraction pattern
contains broad features characteristic of an amorphous material above
10 GPa. Distinct behavior is observed when this material is pressurized
in argon. Argon fills the pores with 9 ± 1 Ar atoms per unit
cell. In this medium-pore zeolite, the quantity of inserted argon
was not found to vary with pressure. Argon-filled MTW is 35% less
compressible than the corresponding empty form.