Multiple-Quantum ¹H MAS NMR Studies of Defect Sites in As-Made All-Silica ZSM-12 Zeolite

Abstract

Multiple-quantum 1H MAS NMR spectroscopy has been used to study defect sites in siliceous as-made ZSM-12 zeolite synthesized with perdeuterated benzyltrimethylammonium cations. The results of two-dimensional double-quantum and triple-quantum 1H MAS NMR experiments show that three silanol (SiOH) protons of the defect site engage in hydrogen bonding with the charge-compensating siloxy (SiO-) group. Simulations of the double-quantum sideband pattern were used to determine an average dipolar coupling between the protons of 4.0 (±0.5) kHz (an equilateral triangular geometry is assumed). This value corresponds to a maximum distance of 3.1 (±0.1) Å, if the protons are rigid. We believe that these results have important implications concerning the nucleation mechanism of high-silica zeolites, as they provide new information on the structure of charge-compensating units in the zeolite colloidal precursors reported by others.