Molecular-sieve thin films are attracting research interest because of emerging applications as membranes, [1][2][3][4][5][6] chemical sensors, [7][8][9] low-k dielectrics, [10] and hosts for supramolecular organization of guest molecules and/or stabilization of guest nanostructures. [11][12][13][14] Siliceous ZSM-5 (Si 98 O 196 -ZSM-5; structure type MFI) films, from now on referred to as MFI films, are studied widely because they serve as a model system and hold promise for practical applications. [15][16][17][18][19] For example, MFI membranes are ideally suited for separating xylene isomers, since the pore size of the MFI framework should allow p-xylene (kinetic diameter ca. 5.8 ) to permeate relatively fast, while the bulkier o-and m-xylene (kinetic diameters ca. 6.8 ) should permeate at a much lower rate. The separation of xylene isomers is important in the petrochemical industry, since they are widely used as industrial solvents and precursors. Separations based on MFI membranes could be a better alternative to current technology that is based on energy-intensive operations such as fractional crystallization, adsorption in a simulated moving bed, and distillation. Among other microstructural characteristics, the preferred orientation correlates strongly with film performance, and therefore controlling the preferred orientation is important. [20,21] Although continuous MFI films with certain types of preferred orientation have been reported, [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] a method that allows systematic manipulation of molecular-sieve pore orientation in supported, thin, continuous films has not been demonstrated. Syntheses of MFI membranes described recently can be broadly classified into two categories: in situ and secondary (or seeded) growth. In the in situ technique, the support surface is directly contacted with an alkaline solution containing the zeolite precursors and subjected to hydrothermal conditions. Under appropriate conditions, zeolite crystals nucleate on the support and grow to form a

c-Oriented columnar MFI films made by secondary growth of randomly oriented seed monolayers, deposited using sonication-assisted covalent attachment, exhibit n-hexane/2,2-dimethylbutane separation factor of up to 10 4 , n-/i-butane separation factor of up to 50, and p-/o-xylene separation factor of up to 2. A MFI film from a-oriented seed layer shows lower separation factors for the linear vs. branched isomers but higher separation factor for p-/o-xylene.

Die Aggregation von Zeolithkristallen auf einem porösen Träger mit anschließendem Sekundärwachstum führt zu kontinuierlichen, dünnen, einheitlich orientierten MFI‐Zeolithfilmen mit Zickzack‐Kanälen von 5.1×5.5 Å senkrecht zum Träger (siehe SEM‐Querschnittbild). Zusammen mit den zuvor beschriebenen b‐orientierten Analoga bilden diese a‐orientierten Filme den ersten Satz dünner Molekularsiebmembranen mit jeweils einheitlichen, aber drastisch unterschiedlichen Orientierungen.

A layer-by-layer deposition method is presented for the fabrication of compact c-oriented-MCM-22/silica films on aluminum alloys and porous α-alumina discs. The film fabrication procedure combines deposition of platelike MCM-22 crystals on substrates by covalent attachment under reflux and/or by sonication-assisted covalent attachment (using the methods introduced by Yoon and co-workers and recently reviewed [Acc. Chem. Res. 2007, 40 (1), 29−40]) with evaporation-induced-self-assembly (EISA) of surfactant-templated silica. The composite c-oriented MCM-22/silica films exhibited corrosion resistance barrier properties comparable to commercial chromate conversion coatings. Moreover, they exhibited hydrogen ideal selectivities (e.g., H2/N2 ∼ 7) above those expected by Knudsen diffusion indicating molecular sieving potential.