Alumina-supported AlPO-18 membranes for CO₂/CH₄ separation

Selective AlPO-18 membranes were reproducibly synthesized on the outside surface of macroporous tubular α-alumina supports in a single hydrothermal synthesis step. The AlPO-18 crystals and membranes were characterized by SEM, XRD, adsorption and permeation measurements. Single-gas permeances of CO₂, N₂, CH₄ and i-C₄H₁₀ decreased with increased kinetic diameter, but CO₂ permeated faster than the smaller H₂, because CO₂ adsorbs more strongly. Eleven AlPO-18 membranes had an average CO₂ permeance of (1.8±0.5)×10^-7mol/(m²sPa) and average CO₂/CH₄ separation selectivities of 101±16 at room temperature and a pressure drop of 0.1MPa for the equimolar CO₂/CH₄ mixture. The membrane was also selective (selectivity=31) at 2MPa feed pressure and had a CO₂ permeance of 0.47×10^-7mol/(m²sPa) [CO₂ flux of 15kg/(m²h)]. The CO₂/CH₄ selectivities decreased as temperature and pressure drop increased. The membranes stability was investigated for different calcination times, calcination temperature ramps, and following exposures to water vapor and liquid.