Gas Separation through Bilayer Silica, the Thinnest Possible Silica Membrane

Membrane-based gas
separation processes can address key challenges
in energy and environment, but for many applications the permeance
and selectivity of bulk membranes is insufficient for economical use.
Theory and experiment indicate that permeance and selectivity can
be increased by using two-dimensional materials with subnanometer
pores as membranes. Motivated by experiments showing selective permeation
of H2/CO mixtures through amorphous silica bilayers, here
we perform a theoretical study of gas separation through silica bilayers.
Using density functional theory calculations, we obtain geometries
of crystalline free-standing silica bilayers (comprised of six-membered
rings), as well as the seven-, eight-, and nine-membered rings that
are observed in glassy silica bilayers, which arise due to Stone–Wales
defects and vacancies. We then compute the potential energy barriers
for gas passage through these various pore types for He, Ne, Ar, Kr,
H2, N2, CO, and CO2 gases, and use
the data to assess their capability for selective gas separation.
Our calculations indicate that crystalline bilayer silica, which is
less than a nanometer thick, can be a high-selectivity and high-permeance
membrane material for 3He/4He, He/natural gas,
and H2/CO separations.