Nests of three silanol groups are located on the internal pore surface of calcined zeolite SSZ-70. 2D 1 H double/triple-quantum single-quantum correlation NMR experiments enable a rigorous identification of these silanol triad nests. They reveal a close proximity to the structure directing agent (SDA), that is, N,N'-diisobutyl imidazolium cations, in the as-synthesized material, in which the defects are negatively charged (silanol dyad plus one charged SiO- siloxy group) for charge balance. It is inferred that ring strain prevents the condensation of silanol groups upon calcination and removal of the SDA to avoid energetically unfavorable three-rings. In contrast, tetrad nests, created by boron extraction from B-SSZ-70 at various other locations, are not stable and silanol condensation occurs. Infrared spectroscopic investigations of adsorbed pyridine indicate an enhanced acidity of the silanol triads, suggesting important implications in catalysis.
Keywords: defects; heterogeneous catalysis; silanols; solid-state NMR spectroscopy; zeolites.
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.