MCM-41 and SBA-15 mesoporous silica materials with different pore sizes (3.08 nm for small pore size MCM-41 ( P 1 ), 5.89 nm for medium pore size SBA-15 ( P 2 ), and 7.81 nm for large pore size SBA-15 ( P 3 )) were synthesized by the hydrothermal method and then functionalized with 3-aminopropyltrietoxysilane by postsynthesis treatments. Next, polysulfone-mesoporous silica mixed matrix membranes (MMMs) were prepared by the solution casting method. The obtained materials and MMMs were characterized by various techniques including X-ray diffraction, scanning electron microscopy, and N 2 adsorption-desorption, and Brunauer-Emmett-Teller method to examine the crystallinity, morphology, and particle size, pore volume, specific surface area, and pore size distribution, respectively. Finally, the gas permeation rates of prepared MMMs were measured in 8 bar and 25 °C and the effect of pore size of modified and unmodified mesoporous silica on the gas separation performance of these MMMs were investigated. The experimental results indicate that the carbon dioxide (CO 2 ) and methane (CH 4 ) permeability and CO 2 /CH 4 selectivity were increased with an enhancement in the particle pore size.

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