Unraveling the Effect of Silanol Defects on the Insertion of Single-Site Mo in the MFI Zeolite Framework

The preparation of defect-free MFI crystals containing single-site framework Mo through a hydrothermal postsynthesis treatment is reported. The insertion of single Mo sites in the MFI zeolite samples with different crystal sizes of 100, 200, and 2000 nm presenting a diverse concentration of silanol groups is revealed. The nature of the silanols and their role in the incorporation of Mo into the zeolite structure are elucidated through an extensive spectroscopic characterization (²⁹Si NMR, ¹H NMR, ³¹P NMR, and IR) combined with X-ray diffraction and HRTEM. In addition, a DFT-based theoretical modeling of a large Si₁₅₄O₃₅₄H₉₂ nanoparticle containing 600 atoms is carried out to understand the expansion of the unit cell volume measured by X-ray diffraction. An accurate quantification of the silanols in the MFI crystals with different particle sizes and the insertion of Mo in the zeolitic framework is reported for the first time. The results confirmed that the non-H-bonded silanols seem to be the gateway for the insertion of single Mo atoms in the zeolite structure. Such materials with single metal sites present high crystallinity and perfect structure, thus providing great stability in catalytic applications.