ZEOLITE THIN-FILMS WITH TUNABLE MOLECULAR-SIEVE FUNCTION

We report on tunable zeolite-based thin film nanostructures whose mole cular size selectivity can be controlled between 3 and 5 Angstrom by f ast ion exchange. The composite films are assembled from molecularly a ttached zeolite CaA crystals and a silica binding phase on the gold el ectrodes of piezoelectric quartz crystal microbalances (QCM). Intrazeo litic cations can be reversibly exchanged with other cations from aque ous solution, such as Na+, K+, and Rb+. The mass changes resulting fro m cation exchange in the film can be measured ex situ due to the high stability of the film and the nanogram sensitivity of the piezoelectri c crystal. The surface area and microporosity of the films were studie d in situ on the piezoelectric crystals by measuring sorption isotherm s. Distinctive differences in the nitrogen sorption between the CaA fi lms and the Na, K, and Rb forms are due to blocking of the zeolite eig ht-ring windows. The effective pore sizes of the films were evaluated by measuring isotherms and sorption rates of vapors with different mol ecular sizes and shapes. Step by step exclusion of progressively small er molecules from the interior porosity of the film is observed as a f unction of the cation size and distribution in the zeolitic framework.