Preparation of zeolite T membranes by a two-step temperature process for CO₂ separation

A thin and well-intergrown zeolite T membrane with high CO₂ permeability and selectivity was rapidly prepared on seeded tubular supports by the two-step varying-temperature hydrothermal synthesis process (two-step method). The influence of synthesis parameters such as synthesis temperature and crystallization time during the two-step durations on crystals growth and separation performance was investigated systematically. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and gas permeation measurements were used to characterize the as-synthesized membranes. Compared with the conventional hydrothermal synthesis at a constant temperature (one-step method), the two-step method is more effective to synthesize a thin and continuous zeolite membrane layer in short time, which fully covered the support surface. Membrane synthesis temperature during the two-step durations could seriously influence the crystal growth and CO₂ separation properties. It was found that a lower temperature favored nucleation, while a higher crystallization temperature promoted crystallization during the two-step method, thus improving zeolite T crystal growth and membrane separation properties. The membranes prepared by the two-step method under the optimized conditions exhibited good reproducibility, high CO₂ permeance, and relatively high permselectivity for the separation of CO₂ from CH₄ or N₂ gas mixtures. For example, CO₂ permeance and CO ₂/CH₄ selectivity of the membranes reached 6.2 × 10^-8 mol m^-2 s^-1 Pa^-1 and 80 for the equimolar CO₂/CH₄ binary gaseous mixtures at 35 C, respectively.