Characterization of micropores in zeolites by 3He NMR

3He is used as an NMR probe to characterize micropores. We have traced NMR spectra of 3He 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 3He 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 3He relaxation, although paramagnetic impurities also contribute to it. 3He NMR can probe the smaller pores than 129Xe NMR can, because He atom (0.28 nm) is smaller than Xe atom (0.44 nm). The total experiment time of 3He NMR is much shorter than that of 129Xe NMR, because 3He has much higher sensitivity than 129Xe.