An oriented, siliceous deca-dodecasil 3R (DDR) zeolite film for effective carbon capture: Insight into its hydrophobic effect

An all-silica deca-dodecasil 3R (Si-DDR) zeolite with a pore size of 0.36 × 0.44 nm² is highly desirable for membrane-based separation of CO₂ (0.33 nm) from N₂ (0.364 nm), which is critical in the post-combustion carbon capture process, via molecular recognition of their slight size difference. For the first time, we acquired h0h-oriented, hydrophobic DDR zeolite films through the epitaxial growth of a DDR seed layer with a structure directing agent of methyltropinium iodide. The degree of the out-of-plane orientation and inter-growth was increased with the secondary growth time, while reducing the defects that provide non-selective pathways. The resulting DDR membrane showed a CO₂/N₂ separation factor (SF) as high as 11.9 at 50 °C (a representative flue-gas temperature) under dry conditions. More desirably, it could achieve a much enhanced CO₂/N₂ SF of up to 15.9 at 50 °C in the presence of H₂O vapor (3^rd largest component in the flue-gas). The transport of the larger N₂ molecule, plausibly its entering the pore mouth of DDR zeolites, was more inhibited by H₂O molecules adsorbed on the membrane surface; it appears that this surface resistance was due to the hydrophobicity of the highly siliceous DDR membrane and beneficial for improving CO₂/N₂ SFs under wet conditions.