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

Abstract

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 ( 29 Si NMR, 1 H NMR, 31 P NMR, and IR) combined with X-ray diffraction and HRTEM. In addition, a DFT-based theoretical modeling of a large Si 154 O 354 H 92 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.

Chemical Shift Anisotropy: A Promising Parameter To Distinguish the 29Si NMR Peaks in Zeolites

“…11 The development of such methodologies in the last decades has contributed to the assignment of the experimental spectra, giving rise to the palette of techniques referred to as 'NMR crystallography.' 12,13 While the 29 Si NMR isotropic chemical shift is one of the most important NMR parameters for zeolites, 14 the partial overlap of signals corresponding to different coordination states is a common problem hindering precise assignments, 5,15 and chemical shift anisotropy was used to distinguish different Q 4 (0Al) 29 Si environments in siliceous zeolites previously. 16,17 In this work, we show that chemical shift anisotropy (CSA) provides a promising parameter to distinguish 29 Si NMR signals of Q 4 species, being either Q 4 (0Al) or Q 4 (1Al) (Brønsted acid sites) and silanols in zeolites.…”

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Using a combination of (i) symmetry-based recoupling in 29 Si NMR, (ii) spin dynamics-based simulations, and (iii) DFT-based theoretical calculations, we show how the 29 Si NMR peaks of (�Si−O−Si�) (Q 4 (0Al)), Brønsted acid sites (� Si−OH−Al�) (Q 4 (1Al)), and silanols (�Si−OH) (Q 3 ) can be characterized in nanosized ZSM-5 zeolites. Significant differences in the chemical shift anisotropy are calculated theoretically and observed experimentally for silicon nuclei close to aluminum (Q 4 (1Al)) compared to those which are not, i.e., Q 4 (0Al) and Q 3 , allowing one to clearly assign the 29 Si NMR peaks.

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Coke Formation over Zeolite Catalysts in Light Alkanes Aromatization and Anti-Carbon-Deposition Strategies and Perspectives: A Review

“…Providing a fluorine environment for zeolite synthesis can also effectively inhibit the formation of silanol defects . In addition, isomorphic substitution can change an orthorhombic system (Si-OH and Si vacancies) into a monoclinic system (Si-O-Si) by changing the crystal system of the zeolite to reduce defects. , …”

“…154 In addition, isomorphic substitution can change an orthorhombic system (Si-OH and Si vacancies) into a monoclinic system (Si-O-Si) by changing the crystal system of the zeolite to reduce defects. 120,155 4.4.2. Healing.…”

Selective Incorporation of Iron Sites into MFI Zeolite Framework by One-Pot Synthesis

“…Besides, a weak peak at 3630 cm −1 can be observed over these spectra, which can be attributed to the Brønsted acid hydroxyl groups bridging between Si and Fe framework atoms in the form Fe(OH)Si bonds. 61,62 Noticeably, the intensities of these two peaks (∼3724 and 3510 cm −1 ) gradually decrease with increasing Fe contents, suggesting that the isolated silanol groups and silanol nests decrease with increasing Fe contents. In contrast, the peak at 3630 cm −1 increases with Fe contents, indicating that the Brønsted acid bridging hydroxyl groups increase with the Fe contents.…”