SIO-CENTER-DOT-CENTER-DOT-CENTER-DOT-HOSI HYDROGEN-BONDS IN AS-SYNTHESIZED HIGH-SILICA ZEOLITES

The high-silica zeolites NON, DDR, MTW, AFI, and Mm synthesized with o rganic quaternary ammonium cations show in the as-synthesized form a l ine at 10.2 +/- 0.2 ppm in the H-1 MAS NMR spectra that does not origi nate from the organic structure directing agents (SDA's). This signal is assigned to SiO-... HOSi hydrogen bonds between defect sites with a n 0 ... 0 distance of 2.7 Angstrom. The presence of Q(3) sites is conf irmed by Si-29 MAS NMR. The intensity of the line at 10.2 ppm in the H -1 MAS NMR spectra decreases when the positive charge of the quaternar y ammonium cations is balanced by negative framework charge by the inc orporation of aluminum or boron into the zeolite structure or when the amount of defects is reduced by using fluoride ions as mineralizing a gents in the synthesis. Diffuse reflectance IR spectroscopy of high-si lica MFI reveals a broad band centered at 3200 cm(-1) for the O-H stre tching vibration of the SiO-... HOSi hydrogen bonds and the 0 ... 0 di stance compares well to that obtained from]H-1 MAS NMR data. Heating t he sample induces protonation of the siloxy groups of the hydrogen bon ds due to the decomposition of the organic tetrapropylammonium cations followed by the disappearance of the SiO-... HOSi hydrogen bonds. For MFI prepared with deuterated tetrapropylammonium cations, it is found that approximately one to two hydrogen-bonded protons in these defect sites exist per quaternary ammonium cation. Other H-1 NMR lines are f ound at 6.5 and 4.5-5.5 ppm and are assigned to silanol. groups formin g weaker hydrogen bonds and water molecules associated with alkali-met al cations, respectively. A model of the defect sites in as-synthesize d high-silica zeolites is proposed.