Behavior of H (sub 2) O 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 (super -1) at 4< or =T< or =700 K) and powder IR spectra (1500 to 4000 cm (super -1) at 50<T<300K) were measured for two microporous zeolites natrolite, Na (sub 16) [Al (sub 16) Si (sub 24) O (sub 80) ] . 16H (sub 2) O, and scolecite, Ca (sub 8) [Al (sub 16) Si (sub 24) O (sub 80) ] . 24H (sub 2) O to determine the behavior of H (sub 2) O molecules in the channels. Both IR and Raman spectra show intense O-H stretching and H (sub 2) O bending modes derived from the hydrogen-bonded H (sub 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 (sub 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 (sub 2) O molecules of natrolite and scolecite are located between 3200 and 3700 cm (super -1) and bending vibrations occur around 1650 cm (super -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 (sub 2) O translational vibration, T(H (sub 2) O), which is also observed directly in single-crystal Raman spectra. In addition, two H (sub 2) O librational modes are located at about 440 and 500 cm (super -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 (sub 2) O vibrations. They permit the wavenumber of several T(H (sub 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 (sub 2) O bending modes decreases with an increase of the H-O-H angle of the H (sub 2) O molecule in natrolite and scolecite. The dehydration behavior of H (sub 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 (sub 2) O-molecule behavior in microporous silicates and energetics and the behavior of hydrogen bonding.