Seeded growth, silylation, and organic/water separation properties of MCM-48 membranes

A seeded growth method is reported for synthesis of MCM-48 membranes on α-alumina supports, thereby extending the seeded growth technique used for zeolite membranes to mesoporous silica membrane synthesis. The surface properties of the MCM-48 membranes are then modified by silylation with hexamethyldisilazane. The physicochemical properties of these membranes are characterized by multiple techniques. X-ray diffraction demonstrates that the structure of the MCM-48 membranes is maintained after surfactant removal and silylation. In comparison to MCM-48 membranes previously synthesized by the in situ growth technique, much less silica infiltration into the alumina support is observed by energy dispersive X-ray spectroscopy. The pore structure of the MCM-48 membranes is assessed by direct nitrogen physisorption measurements on membrane samples, demonstrating that a large accessible pore volume is available for molecular permeation and pore modification to tailor selectivity. The gas permeation properties of the calcined and silylated MCM-48 membranes are consistent with a Knudsen-like mechanism, albeit with a substantial influence of gas-solid interactions in the mesopores. The silylated MCM-48 membranes are evaluated for pervaporative separation of ethanol (EtOH), methyl ethyl ketone (MEK), and ethyl acetate (EA) from their dilute aqueous solutions (5 wt%). The membranes showed pervaporative separation factors in the range of 5–35 and organic fluxes in the range of 0.03–0.22 kg-m−2h−1 at 303 K. The intrinsic selectivities (water/organic permeability ratios) of the silylated membranes are 6 (water/EtOH), 10 (water/MEK), and 26 (water/EA) respectively, in comparison to selectivities of 66, 450, and 3900 for the unmodified membranes, at 303 K. The selective separation of organic/water mixtures with MCM-48 membranes is attributed to both the organophilic nature of the surface and the effective pore size of the silylated mesopores.