Synthesis of thin DD3R zeolite membranes on hollow fibers using gradient-centrifuged seeds for CO₂/CH₄ separation

All-silica DD3R zeolite has been recognized as a promising CO₂-selective membrane material owing to its appropriate pore size (0.36 nm × 0.44 nm), strong hydrophobicity, excellent thermal and chemical stabilities. In order to reduce membrane cost, it is meaningful to synthesize thin DD3R zeolite membranes that possess high gas permeance. In this work, high-performance DD3R zeolite membranes were synthesized on TiO₂/α-Al₂O₃ composite hollow fibers by secondary growth method. Sigma-1 zeolite seeds were ball-milled and then fractionated into different-sized seeds by gradient centrifugation. Smaller seed particles were collected after centrifuged at higher rotation speed rate. A very thin and dense DD3R zeolite membrane (thickness: 1.2 µm) was synthesized by using the smallest seeds (ZS4) with average particle size of 163 nm, which were obtained from the supernatant of the third centrifugation at 11,000 rpm. The as-synthesized membranes were employed in separation of equimolar CO₂/CH₄ mixture. Compared with the original ball-milled seeds (ZS1)-induced membrane, the seed ZS4-induced membrane exhibited an improved CO₂ permeance of 1.2 × 10⁻⁷ mol m⁻² Pa⁻¹ s⁻¹ by 62%. All the resultant membranes performed good CO₂/CH₄ separation selectivities between 262 and 364.