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.
Bibliographical noteFunding Information:
This work was supported by NSF ( CBET-0855863 ) and also by the Human Resources Development Program of KETEP Grant (No. 20114010203050 ) funded by the Korea Government Ministry of Knowledge Economy . Portions of this work were conducted at the University of Minnesota Characterization Facility, which receives partial support from the National Science Foundation through the NNIN program.
Copyright 2013 Elsevier B.V., All rights reserved.
- Butane isomers
- Film orientation
- MFI (silicalite-1) membranes
- Secondary (seeded) growth
- Xylene isomers