Characterization of Microporosity in Ordered Mesoporous Material MAS-7 by 129Xe NMR Spectroscopy

Abstract

A new kind of ordered mesoporous aluminosilicate material MAS-7, containing micropores inside its pore walls, has been investigated by 129Xe NMR spectroscopy as a function of xenon pressure over a wide range of temperatures. 129Xe NMR results reveal that two different independent micropore systems are present in the mesoporous walls of MAS-7. Two signals are observed in the 129Xe NMR spectra, which can be attributed to two kinds of fast exchange of xenon atoms in the two different types of micropores (micropores I and II) with those in the mesopores and in the interparticle space of MAS-7. Coadsorption experiments of xenon and guest molecules with different sizes provide an estimation of the sizes of micropores in MAS-7. Micropore I has a diameter of ca. 0.74 nm, which is most likely formed by the primary structure units of zeolite beta. Micropore II possesses a relatively small pore diameter of 0.44−0.49 nm, which may be generated from partial occlusion of triblock polymer (template used in the synthesis of MAS-7) chains into the mesoporous walls of MAS-7. The details of xenon adsorption and motion are also thoroughly studied by variable-temperature (VT) 129Xe NMR experiments. The VT 129Xe NMR experiments suggest that even bulk (solid or liquid) xenon atoms probably formed on the mesoporous wall still undergo fast exchange with xenon in the other adsorption states (such as xenon in the micropores) at 124 K, indicating there is a good communication among the various types of pores in MAS-7.