Behavior of H2O molecules in the channels of natrolite and scolecite: A Raman and IR spectroscopic investigation of hydrous microporous silicates

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, Na 16 [Al 16 Si 24 O 80 ]·16H 2 O, and scolecite, Ca 8 [Al 16 Si 24 O 80 ]·24H 2 O to determine the behavior of H 2 O molecules in the channels. Both IR and Raman spectra show intense O-H stretching and H 2 O bending modes derived from the hydrogen-bonded H 2 O 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 H 2 O 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 H 2 O 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 H 2 O translational vibration, T(H 2 O), which is also observed directly in single-crystal Raman spectra. In addition, two H 2 O 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 H 2 O vibrations. They permit the wavenumber of several T(H 2 O) 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 H 2 O bending modes decreases with an increase of the H-O-H angle of the H 2 O molecule in natrolite and scolecite. The dehydration behavior of H 2 O 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 H 2 O-molecule behavior in microporous silicates and energetics and the behavior of hydrogen bonding.