Direct Evidence for Single Molybdenum Atoms Incorporated in the Framework of MFI Zeolite Nanocrystals

Abstract

Direct evidence of the successful incorporation of atomically dispersed molybdenum (Mo) atoms into the framework of nanosized MFI zeolite is demonstrated for the first time. Homogeneous distribution of Mo with a size of 0.05 nm is observed by scanning transmission electron microscopy high-angle annular dark-field imaging (STEM-HAADF). 31P magic-angle spinning nuclear magnetic resonance (MAS NMR) and Fourier-transform infrared (FT-IR) spectroscopy, using trimethylphosphine oxide (TMPO) and deuterated acetonitrile as probe molecules, reveal a homogeneous distribution of Mo in the framework of MFI nanozeolite, and the presence of Lewis acidity. 31P MAS NMR using TMPO shows probe molecules interacting with isolated Mo atoms in the framework, and physisorbed probe molecules in the zeolite channels. Moreover, 2D 31P–31P MAS radio frequency-driven recoupling NMR indicates the presence of one type of Mo species in different crystallographic positions in the MFI framework. The substitution of framework Si by Mo significantly reduces the silanol defect content, making the resulting zeolite highly hydrophobic. In addition, the insertion of Mo into the MFI structure induces a symmetry lowering, from orthorhombic (Pnma), typical of high silica MFI, to monoclinic (P21/n), as well as an expansion of unit cell volume. The novel material opens many opportunities of catalysts design for application in mature and emerging fields.

Titanosilicate zeolite precursors for highly efficient oxidation reactions

“…As summarized in Table S2, † the weight loss of ATZ-TMAda and ATZ-PhDC samples (30.2 and 17.2 wt%, respectively) is higher than that of the rest of the samples, demonstrating the strong structure-directing ability of the corresponding OSDAs and the highly open structure of the obtained samples. The 29 Si MAS NMR spectra of ATZ samples show similar silicon microenvironments, and display resonance bands centered at À110 $ À112 ppm, ascribed to the tetrahedrally coordinated silicon. Notably, the resonance bands are also wider than those of the crystallized TS-1 zeolite, ascribed to the incompletely formed zeolite frameworks (Fig.…”

“…with isolated active sites in the zeolite framework exhibit unique catalytic performances in numerous important processes. [27][28][29][30][31][32] Titanosilicate zeolites, as a typical family of heteroatom-containing zeolites, have been widely used in oxidation reactions with H 2 O 2 as the oxidant under mild conditions, such as alkene epoxidation, phenol hydroxylation, cyclohexanone ammoximation, and deep oxidative desulfurization. [33][34][35][36] The catalytic activity of titanosilicate zeolites strongly correlates with the lattice Ti contents and the Ti coordination states.…”

Novel Strategy for the Synthesis of Ultra‐Stable Single‐Site Mo‐ZSM‐5 Zeolite Nanocrystals

“…%o fM oa ccording to standard procedures reported in the MDAl iterature. [12] In contrast to the direct synthesis approach reported earlier by our group, [13] the current method allows stabilization of single metal species in already synthesized aluminium containing MFI type (ZSM-5) zeolites (Scheme S1). This new procedure can be applied to any calcined pure silica and aluminosilicate zeolites including commercially available materials.…”

Zeolite (In)Stability under Aqueous or Steaming Conditions

“…[70] Impregnation of zeolites with phosphorus improves the hydrothermal stability without loss of acidity due to the stabilization of framework aluminum with extra-framework protonated orthophosphoric acid. [71] Incorporation of tungsten [72] and molybdenum [73] atoms into zeolite framework allowed the preparation of silanol-free nanosized zeolites, thus, allowing to adjust hydrophilicity/hydrophobicity of these catalysts. Increased stability of zeolites on steam can be also achieved by covering the zeolite particle surface with a thin layer of amorphous silica [74] or, preferably, with an ordered all-silica zeolite with the same topology.…”