Anomalous Volume Changes in the Siliceous Zeolite Theta‑1 TON due to Hydrogen Insertion under High-Pressure, High-Temperature Conditions

High-pressure X-ray diffraction and
Raman spectroscopy in a diamond
anvil cell were used to study the insertion of the chemical hydrogen
storage material, ammonia borane, in the one-dimensional pores of
the zeolite theta-1 TON. Heating of this material up to 300 °C
under pressures up to 5 GPa resulted in the release of a significant
amount of hydrogen due to the conversion of ammonia borane confined
in the channels of TON and outside the zeolite to polyaminoborane
and then polyiminoborane chains. The filling of TON with hydrogen
resulted in a much greater increase in unit cell volume than that
corresponding to thermal expansion of normal compact inorganic solids.
This process at high temperature is accompanied by a phase transition
from the collapsed high-pressure Pbn21 form to the more symmetric Cmc21 phase
with expanded pores. This material has a high capacity for hydrogen
adsorption under high-temperature, high-pressure conditions.