Inorganic Electrides Formed by Alkali Metal Addition to Pure Silica Zeolites

Up to 4 Cs or Rb atoms per 32 Si atoms can be incorporated into the channels of the all-silica (SiO2) zeolites ITQ-4 and beta with effective oxidation states of zero. The optical properties and 29Si MAS NMR spectra suggest partial or complete ionization within the channels to yield M+ ions and relatively free electrons. This view is supported by a published structural model (Petkov, V.; Billinge, S. I.; Vogt, T.; Ichimura, A. S.; Dye, J. L. Phys. Rev. Lett. 2002, 89, 075502) based on pair distribution functions for Cs in ITQ-4. As with solutions of alkali metals in ammonia, however, the trapped electrons in this inorganic electride interact with the cations, and spin-pairing between adjacent electrons also occurs. The crystal morphology is unaffected by the inclusion of alkali metals. Structural distortions that reduce or eliminate long-range order are completely reversed upon oxidation and removal of the metal. At higher temperatures, Na and K can also be incorporated, but the optical spectra indicate that either substantial ionization does not occur or the released electrons react reversibly with the silica to form strongly reducing species.