| Popis: |
Infrared spectra in the framework absorption region reflect the various degrees of offretite and erionite character of several zeolites belonging to the offretite-erionite family. I.r. spectra of materials rich in offretite contained an absorption band in the 600–610 cm −1 region that is absent in erionite. Spectra of materials rich in erionite contained bands (at 410–425 cm −1 , 550–555 cm −1 , and 665–685 cm −1 ) that are absent in offretite. Thus, i.r. spectroscopy is diagnostic in detecting the presence of these two structures. In the 29 Si MAS n.m.r. spectra of these zeolites, the dispersion of the chemical shift due to crystallographic site inequivalence is approximately equal to the shift due to the different number of aluminium atoms in the first coordination sphere (∼ 5 ppm). The Si/Al ratios calculated from solid-state n.m.r. have been found to agree only semiquantitatively with those obtained from chemical analyses. This result indicates that the assumption of aluminium a toms randomly distributed over two nonequivalent sites present in a 2:1 ratio in offretite is not valid. The effects of calcination temperature on dealumination is documented by n.m.r. spectra of both 29 Si and 27 Al. |
