Separation of organic/water mixtures with silylated MCM-48 silica membranes

Abstract

MCM-48 membranes were synthesized on an alumina support. The MCM-48 membranes were silylated with trimethylchlorosilane (TMCS) to enhance the hydrothermal stability and hydrophobicity. X-ray diffraction, N 2 adsorption, and permporometry were carried out with the MCM-48 and the silylated MCM-48 (sily-MCM-48) membranes and powders. The XRD patterns indicated that the structure of the MCM-48 membrane was retained after calcination and silylation. The sily-MCM-48 membranes were used for the separation of ethanol (EtOH), methylethyl ketone (MEK), and ethyl acetate (EA) in an aqueous solution. At 303 K, the separation factor and the flux were higher in the order: EA > MEK > EtOH, a finding which was related to the hydrophobicity of organic molecules. The separation factors of MEK/water increased with a decrease in the feed concentration from 85 (20 wt.%) to 318 (1 wt.%). The separation factors of MEK/water (5:95) and EA/water (5:95) were, respectively, 400 and 1000 at 283 K, and 45 and 100 at 323 K. The adsorbed organic molecules must have blocked the pores of the sily-MCM-48 membrane, especially at low temperatures. The pore blocking effect due to the selective adsorption of the organic molecules caused the obstruction of the diffusion of water molecules in the pores. From the results of the permporometry, we found that the average Kelvin diameter of the sily-MCM-48 membranes was 1.9 nm, which is consistent with the intrinsic pore size of sily-MCM-48. We believe that the highly selective separation of organic/water mixtures is attributable not only to the hydrophobic nature of the pore surface but also to the uniform mesopores of sily-MCM-48. Up to 723 K, the sily-MCM-48 membrane was thermally stable; furthermore, repeated calcination and silylation were effective in regenerating it.