Hydrogen bonded silanols in the 10 Å phase: Evidence from NMR spectroscopy

Using ²⁹Si, ¹H, and ²H magic-angle spinning (MAS) and ²⁹Si{¹H} heteronuclear correlation (Het-Cor) nuclear magnetic resonance (NMR) spectroscopy the tetrahedral sheets of the 10 Å phase are shown to contain Q²-type Si bonded to silanol groups that donate hydrogen bonds to interlayer H₂O. ²⁹Si NMR spectra of 10 Å phase samples synthesized from oxide and from crystalline talc starting materials contain a peak near -87 ppm for Q² Si, in addition to the main peak at -98 ppm for the talc-like Q³ of the tetrahedral sheet. The ¹H MAS NMR spectra of the 10 Å, phase contain two distinct peaks, at chemical shifts of +7.8 and +3.2 ppm, in addition to a narrow peak near +0.9 ppm from the talc-like hydroxyl groups. ²⁹Si {¹H} HetCor data indicate that the +7.8 ppm ¹H resonance corresponds to silanol groups and that at +3.2 ppm arises from interlayer H₂O. Comparison of the observed data with correlations of ¹H NMR chemical shift and ²H quadrupolar coupling indicates that the silanol groups donate moderate hydrogen bonds to interlayer H₂O, d(O···O) ≈ 2.8 Å, whereas most interlayer H₂O donate only very weak or no hydrogen bonds at ambient conditions. Our results suggest that formation of the 10 Å phase involves formation of vacancies, which allow favorable hydrogen bond interaction between interlayer H₂O and the normally hydrophobic talc-like 2:1 layers.