Characterization of micropores in zeolites by 3He NMR

He-3 is used as an NMR probe to characterize micropores. We have traced NMR spectra of He-3 introduced in the micropores of various zeolites: KA, NaA, CaA, NaX, NaY, HY, Na and H type mordenites and Na and H type ZSM-5. The peak position depends on the kind and the content of cations as well as the framework. The mechanisms determining the position are discussed, which are interaction of cations and bulk magnetic susceptibility. The former causes a positive shift, while the latter produces a negative shift for diamagnetic substances. We have also studied spin-lattice relaxation of He-3 in micropores. Zeolites with higher cation contents show shorter relaxation times than those with lower cation contents. Nuclear spins on the cations, Na+ and K+, might contribute to He-3 relaxation, although paramagnetic impurities also contribute to it. He-3 NMR can probe the smaller pores than Xe-129 NMR can, because He atom (0.28 nm) is smaller than Xe atom (0.44 nm). The total experiment time of He-3 NMR is much shorter than that of Xe-129 NMR, because He-3 has much higher sensitivity than Xe-129. (c) 2006 Elsevier Inc. All rights reserved.