(742e) Molecular Modeling of Silica Based Membranes

Previous
studies have shown that the permeation through these silica-based membranes is
based on a mechanism that involves the hopping of gas species between
solubility sites. A molecular modeling approach, namely density functional
theory, is used to calculate activation energies of permeation for various
gases (He, H₂, Ne, CO, CO₂, CH₄) through
silica-based membranes. Permeation through these membranes is modeled as
passage of the permeating species through 6-membered siloxane
rings, which act as critical openings between the solubility sites. Within DFT,
the Becke3lyp with accurate basis sets (6-311G (2d, p), LANL2DZ) is applied to
optimize these ring structures and the permeating species. The activation
energies are estimated as the interaction energy of the permeating species
approaching and passing through H₁₂Si₅O₆X cyclosiloxane based rings. These siloxane
rings contain aluminum, boron, silicon, titanium, yttrium and zirconium, which
can be introduced into the structure to potentially improve the permeation
properties of the silica membranes. The optimized siloxane
based rings have a buckled conformation due to the asymmetry created by the
inorganic oxides. The calculated activation energies of the buckled silica-yttria and silica-zirconia rings
(20–40 kJ mol^-1) are found to be similar to those reported for
planar silica rings (10–30 kJ mol^-1). The calculated
activation energies for the buckled silica-alumina, silica-boria,
silica-titania rings (60–100 kJ mol^-1)
are found to be higher than those reported for planar silica rings and suggest
a much denser structure for these membranes.