Refining siliceous zeolite framework structures with 29Si 2D J-resolved NMR spectroscopy

Abstract

A modified shifted-echo PIETA pulse sequence is developed to acquire natural abundance ²⁹Si 2D J-resolved spectra in crystalline silicates. The sequence is applied to the highly siliceous zeolites Sigma-2 and ZSM-12. The 2D J-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 ²⁹Si chemical shifts and geminal ²J_Si–O–Si 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 J-couplings.