Nanoporous layered silicate AMH-3/cellulose acetate nanocomposite membranes for gas separations

• The layered silicate AMH-3 was delaminated into flakes using a high-shear mixer.
• Composite membranes were made with cellulose acetate and 2–6 wt% AMH-3 flakes.
• SAXS and HRTEM showed a high degree of exfoliation of AMH-3 in the membranes.
• Large increase in CO2 permeance and maintenance of CO2/CH4 selectivity were seen.
• This non-conventional behavior indicates complex transport paths in the membrane.

Nanoporous layered silicate/polymer composite membranes are of interest because they can exploit the high aspect ratio of exfoliated selective flakes/layers to enhance molecular sieving and create a highly tortuous transport path for the slower molecules. In this work, we combine membrane synthesis, detailed microstructural characterization, and mixed gas permeation measurements to demonstrate that nanoporous flake/polymer membranes allows significant improvement in gas permeability while maintaining selectivity. We begin with the primary-amine-intercalated porous layered silicate SAMH-3 and show that it can be exfoliated using a high shear rate generated by a high-speed mixer. The exfoliated SAMH-3 flakes were used to form SAMH-3/cellulose acetate (CA) membranes. Their microstructure was analyzed by small angle X-ray scattering (SAXS), revealing a high degree of exfoliation of AMH-3 layers in the CA membrane with a small number of layers (4–8) in the exfoliated flakes. TEM analysis visualized the thickness of the flakes as 15–30 nm, and is consistent with the SAXS analysis. The CO2/CH4 gas separation performance of the CA membrane was significantly increased by incorporating only 2–6 wt% of SAMH-3 flakes. There was a large increase in CO2 permeability with maintenance of selectivity. This cannot be explained by conventional models of transport in flake-containing membranes, and indicates complex transport paths in the membrane. It is also in contrast to the much higher loadings of isotropic particles required for similar enhancements. The present approach may allow avoidance of particle aggregation and poor interfacial adhesion associated with larger quantities of inorganic fillers.

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