Synthesis of high-silica Sigma-1 zeolite membranes on hollow fibers for gas separation

DDR-type zeolite, owning small eight-membered-ring (8MR) pores, is a good membrane material for CO₂/CH₄ and H₂/CH₄ separation. All-silica DDR (DD3R) zeolite membranes have been studied for many years but it is difficult to avoid the growth of impurity crystals and hazardous ethylenediamine is used as a mineralizer. Al-containing Sigma-1 zeolite which is isostructural with DD3R, is synthesized using sodium hydroxide as the mineralizer. In this study, high-silica Sigma-1 zeolite membranes (Si/Al = ∼60) were synthesized for the first time and employed in H₂/CH₄ and CO₂/CH₄ separation. Effects of template concentration and H₂O/SiO₂ ratio in precursor solution on membrane quality were investigated extensively. When the precursor solution has molar composition of 9 ADA: 60 SiO₂: 1 NaAlO₂: 3 NaOH: 6000 H₂O, a high-quality Sigma-1 zeolite membrane (3.6 μm thick) was acquired. The membrane exhibited the highest H₂/CH₄ and CO₂/CH₄ separation selectivities (117 and 304), and the corresponding H₂ and CO₂ permeances were 1.60 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 3.51 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹, respectively. Additionally, the membrane showed good separation stability under both dry and humid conditions.