Highly permeable and oriented AlPO-18 membranes prepared using directly synthesized nanosheets for CO₂/CH₄ separation

Oriented AlPO-18 membranes with super high CO₂ permeance were prepared for the first time by single secondary hydrothermal synthesis using directly synthesized nanosheet seeds. The membranes were grown on the inner surface of tubular asymmetric alumina supports. The morphology of membrane crystals and the membrane quality were highly affected by synthesis temperature and the amount of the structure directing agent in the membrane gel. The optimized AlPO-18 membranes had a ∼3 μm (110)-oriented top AlPO-18 layer upon a ∼1 μm (002)-oriented one. Both channels in the (110) and (002) directions of AlPO-18 zeolite provide fast transport channels, and thus the (110) & (002)-oriented AlPO-18 membranes showed the highest separation index [=CO₂ permeance × (selectivity - 1) × permeate pressure] for CO₂/CH₄ separation among those reported in the literature. Single-gas permeances for H₂, CO₂, N₂ and CH₄ with different temperature and pressure drop were measured. The effects of temperature and pressure drop on the separation performance of the oriented membrane were also studied. The best membrane showed CO₂ permeance as high as 3.6 × 10^-6 mol (m² s Pa)^-1 (∼10500 GPU) with a CO₂/CH₄ selectivity of 91.5 for an equimolar CO₂/CH₄ mixture at 0.2 MPa pressure drop and room temperature.