BEHAVIOR OF H2O MOLECULES IN THE CHANNELS OF NATROLITE AND SCOLECITE: A RAMAN AND IR SPECTROSCOPIC INVESTIGATION OF HYDROUS MICROPOROUS SILICATES

Abstract

Single-crystal polarized Raman spectra (80 to 4000 cm-1 at 4 ≤ T ≤ 700 K) and powder IR spectra (1500 to 4000 cm-1 at 50 < T < 300 K) were measured for two microporous zeolites natrolite, Na16[Al16Si24 O80]·16H2O, and scolecite, Ca8[Al16Si24 O80]·24H2O to determine the behavior of H2O molecules in the channels. Both IR and Raman spectra show intense O-H stretching and H2O bending modes derived from the hydrogen-bonded H2O molecule(s) in the channels. Using published crystal structural data for natrolite and scolecite, and a consideration of Raman mode intensities that are sensitive to the H2O orientation in the framework channels, the internal stretching and bending modes could be assigned. The Raman spectra also show lower energy lattice modes and, in addition, second-order scattering in the wavenumber range where O-H stretching vibrations occur. The stretching vibrations of H2O molecules of natrolite and scolecite are located between 3200 and 3700 cm-1 and bending vibrations occur around 1650 cm-1. In the case of natrolite, two intense O-H stretching modes can be observed and also several weaker combination modes. The latter was used to derive a low energy external H2O translational vibration, T(H2O), which is also observed directly in single-crystal Raman spectra. In addition, two H2O librational modes are located at about 440 and 500 cm-1. For scolecite, six O-H stretching modes are observed in the Raman spectra recorded at 4 K, but only five are found at room temperature in the IR or Raman. The single-crystal Raman spectra also show several second-order combination modes consisting of external and internal H2O vibrations. They permit the wavenumber of several T(H2O) modes at low wavenumbers to be determined. These combination bands are analyzed based on their temperature behavior between 0 and 300 K. It is shown that the wavenumber of the H2O bending modes decreases with an increase of the H-O-H angle of the H2O molecule in natrolite and scolecite. The dehydration behavior of H2O in natrolite and scolecite was investigated by Raman measurements of the intensities of the O-H stretching modes at temperatures from 300 K to 570 K and 720 K, respectively. IR and Raman spectra, obtained over a large temperature range, permit one to obtain a better understanding of inner surface H2O-molecule behavior in microporous silicates and energetics and the behavior of hydrogen bonding.