Structural Evolution of CO2-Filled Pure Silica LTA Zeolite under High-Pressure High-Temperature Conditions

Abstract

The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied at high pressures and temperatures using synchrotron based x ray powder diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them accommodated in the large alpha cages and 1 in the beta cages, giving a SiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stable under pressure up to 20 GPa with a slight pressure dependent rhombohedral distortion, indicating that pressure induced amorphization is prevented by the insertion of guest species in this open framework. The ambient-temperature lattice compressibility has been determined. In situ high pressure resistive heating experiments up to 750 K allow us to estimate the thermal expansivity at 5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal expansion of the empty zeolite structure. No evidence of any chemical reaction was observed. The possibility of synthesizing a silicon carbonate at high temperatures and higher pressures is discussed in terms of the evolution of C-O and Si-O distances between molecular and framework atoms.