Refining siliceous zeolite framework structures with Si 2D -resolved NMR spectroscopy.

A modified shifted-echo PIETA pulse sequence is developed to acquire natural abundance Si 2D -resolved spectra in crystalline silicates. The sequence is applied to the highly siliceous zeolites Sigma-2 and ZSM-12. The 2D -resolved spectra are used to develop a silicate framework structure refinement strategy based on Si-O, O-O, and Si-Si distance restraints and analytical relationships between local structure and Si chemical shifts and geminal couplings. The refinement of the Sigma-2 structure showed that the Si-O and O-O distances were in excellent agreement with the single-crystal X-ray diffraction (SCXRD) data. The refinement of the ZSM-12 structure, initially determined from synchrotron powder XRD data, highlighted significant improvements in Si-O and O-O distances, and better agreement between calculated and experimental chemical shifts and -couplings.