SiO^-•••HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites

Abstract

The high-silica zeolites NON, DDR, MTW, AFI, and MFI synthesized with organic quaternary ammonium cations show in the as-synthesized form a line at 10.2 ± 0.2 ppm in the ^1H MAS NMR spectra that does not originate from the organic structure directing agents (SDA's). This signal is assigned to SiO^-•••HOSi hydrogen bonds between defect sites with an O•••O distance of 2.7 Å. The presence of Q^3 sites is confirmed by ^(29)Si MAS NMR. The intensity of the line at 10.2 ppm in the ^1H MAS NMR spectra decreases when the positive charge of the quaternary ammonium cations is balanced by negative framework charge by the incorporation of aluminum or boron into the zeolite structure or when the amount of defects is reduced by using fluoride ions as mineralizing agents in the synthesis. Diffuse reflectance IR spectroscopy of high-silica MFI reveals a broad band centered at 3200 cm^(-1) for the O-H stretching vibration of the SiO^-•••HOSi hydrogen bonds and the O•••O distance compares well to that obtained from ^1H MAS NMR data. Heating the sample induces protonation of the siloxy groups of the hydrogen bonds 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 ^1H NMR lines are found at 6.5 and 4.5-5.5 ppm and are assigned to silanol groups forming weaker hydrogen bonds and water molecules associated with alkali-metal cations, respectively. A model of the defect sites in as-synthesized high-silica zeolites is proposed.

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