MCM-22/silica selective flake nanocomposite membranes for hydrogen separations

(Chemical Equation Presented) MCM-22/silica composite films were fabricated using layer-by-layer deposition toward a nanoscale realization of the selective flake concept first proposed by Cussler in (J. Membr. Sci. 1990, 52 (3), 275-288). According to this concept, considerable zeolitic transport selectivity can be harvested if plate-like zeolite particles were oriented flat within an appropriate thin film matrix. c-Out-of-plane oriented MCM-22 flakes were chosen because of the expected H₂-selective (over other permanent gases) molecular sieving action through their c-axis-transport-limiting aperture defined by six SiO₄ tetrahedra. To fill the gaps between MCM-22 particles the evaporation induced self-assembled (EISA) mesoporous silica, introduced by Brinker et al. in (Nature 1997, 389 (6649), 364-368), was selected as a tunable matrix, through which Knudsen diffusion would be the dominant transport mechanism for permanent gases. The repetition of appropriate deposition cycles (i.e., particle deposition and subsequent silica coating) led to the gradual increase of separation performance achieving H₂/N ₂ ideal selectivity as high as 120.