STRUCTURAL INVESTIGATIONS OF SAPO-37 MOLECULAR-SIEVE BY COHERENCE-TRANSFER AND DIPOLAR-DEPHASING SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE EXPERIMENTS

In this work, dipolar-based coherence-transfer and dipolar-dephasing solid-state nuclear magnetic resonance (NMR) experiments were performed to investigate the structure of the silicoaluminophosphate molecular sieve SAPO-37. The Al-27,Si-29 transferred-echo double-resonance (TEDOR) experiment directly proves, for the first time, that silicon substitutes for phosphorus atoms in the framework via the SM2 mechanism. The Al-27 (P-31) and Al-27 (Si-29) dipolar-dephasing difference experiments and the two-dimensional Al-27-->P-31 TEDOR experiment indicate the presence of three types of aluminum environments: tetrahedral Al surrounded by symmetrical [4P] or [4Si] environments which give rise to a sharp resonance, tetrahedral Al surrounded by asymmetrical [3P, Si], [2P, 2Si] or [P, 3Si] environments which give rise to a broad resonance, and also extra-framework amorphous octahedral Al.