On defects in highly a-oriented MFI membranes

Previously, uniformly a-oriented MFI films were synthesized via the secondary growth of an a-oriented seed layer, which was obtained by chemically depositing ∼2 μm thin-along-the-a-axis particles on a silica-coated α-Al₂O₃ porous disc (Choi et al., Angew. Chem. Int. Ed. 45 (2006) 1154-1158). Despite the uniformity of the a-orientation throughout the film, we found that the MFI membranes are not applicable for p-/o-xylene separation since they have significant crack and grain boundary defects. Accordingly, a-oriented MFI films were fabricated via alternative methods, motivated by the expected high separation performance that could be comparable to p-/o-xylene separations (100-400 separation factor) of a b-oriented MFI membrane. First, smaller 500 nm thin-along-the-a-axis MFI particles (small seeds) were used instead of 2 μm thin-along-the-a-axis particles (large seeds) for seed deposition. Second, a seed layer was formed by a sonication-assisted physical deposition of the large seeds. Finally, an a-oriented seed layer comprising of the large seeds was epitaxially-grown to be the a-oriented MFI film by conducting two consecutive hydrothermal growth processes. The separation performance of the resultant MFI membranes was investigated via n-/i-butane and p-/o-xylene permeation tests. Maximum separation factors (SF) of ∼4 and ∼17 were achieved for p-/o-xylene and n-/i-butane isomers, respectively. These values are below the expected performance, indicating the presence of non-zeolitic pathways in a-oriented films.