Molecular Structure and Confining Environment of Sn Sites in Single-Site Chabazite Zeolites

Chabazite (CHA) molecular sieves, which are industrial catalysts for the selective reduction of nitrogen oxides and the conversion of methanol into olefins, are also ideal materials in catalysis research because their crystalline frameworks contain one unique tetrahedral-site. The presence of a single lattice site allows for more accurate descriptions of experimental data using theoretical models, and consequently for more precise structure-function relationships of active sites incorporated into framework positions. A direct hydrothermal synthesis route to prepare pure-silica chabazite molecular sieves substituted with framework Sn atoms (Sn-CHA) is developed, which is required to predominantly incorporate Sn within the crystalline lattice. Quantitative titra-tion with Lewis bases (NH3, CD3CN, pyridine) demonstrates that framework Sn atoms behave as Lewis acid sites, which catalyze intermolecular propionaldehyde reduction and ethanol oxidation, as well as glucose-fructose isomerization. Aqueous-phase glucose isomerization turnover rates on Sn-CHA are four orders-of-magnitude lower than on Sn-Beta zeolites, but similar to those on amorphous Sn-silicates. Further analysis of Sn-CHA by dynamic nuclear polarization enhanced solid-state nuclear magnetic reso-nance (DNP NMR) spectroscopy enables measurement of 119Sn NMR chemical shift anisotropy (CSA) of Sn sites. Comparison of experimentally determined CSA parameters to those computed on cluster models using density functional theory supports the pres-ence of closed sites (Sn-(OSi)4) and defect sites ((HO)-Sn-(OSi)3) adjacent to a framework Si vacancy), which respectively be-come hydrated hydrolyzed-open sites and defect sites when Sn-CHA is exposed to ambient conditions or aqueous solution. Kinetic and spectroscopic data show that large substrates (e.g., glucose) are converted only on Sn sites located within disordered mesopo-rous voids of Sn-CHA, which are selectively detected and quantified in IR and 15N and 119Sn DNP NMR spectra using pyridine titrants. This integrated experimental and theoretical approach allows precise description of the primary coordination and secondary confining environments of Sn active sites isolated in crystalline silica frameworks, and clearly establishes the role of confinement within microporous voids for aqueous-phase glucose isomerization catalysis

5,6,7,8-Tetra­hydro­quinoline 1-oxide hemihydrate

In the title compound, C9H11NO·0.5H2O, the asymmetric unit contains two similar mol­ecules of 5,6,7,8-tetra­hydro­quinoline 1-oxide and one water mol­ecule. The water mol­ecule links the two O atoms of both independent N-oxides into dimers via O—H⋯O hydrogen bonds, forming a three-dimensional network along [101], which is additionally stabilized by weak C—H⋯O inter­molecular inter­actions. In each mol­ecule, the saturated six-membered rings exist in a conformation inter­mediate between a half-chair and sofa

NMR Signatures of the Active Sites in Sn-beta Zeolite

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mossbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-beta zeolite. This approach unambiguously shows the presence of framework Sn-IV-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework)

Hybrid polarizing solids for pure hyperpolarized liquids through dissolution dynamic nuclear polarization

Hyperpolarization of substrates for magnetic resonance spectroscopy (MRS) and imaging (MRI) by dissolution dynamic nuclear polarization (D-DNP) usually involves saturating the ESR transitions of polarizing agents (PAs; e.g., persistent radicals embedded in frozen glassy matrices). This approach has shown enormous potential to achieve greatly enhanced nuclear spin polarization, but the presence of PAs and/or glassing agents in the sample after dissolution can raise concerns for in vivo MRI applications, such as perturbing molecular interactions, and may induce the erosion of hyperpolarization in spectroscopy and MRI. We show that D-DNP can be performed efficiently with hybrid polarizing solids (HYPSOs) with 2,2,6,6-tetramethyl-piperidine-1-oxyl radicals incorporated in a mesostructured silica material and homogeneously distributed along its pore channels. The powder is wetted with a solution containing molecules of interest (for example, metabolites for MRS or MRI) to fill the pore channels (incipient wetness impregnation), and DNP is performed at low temperatures in a very efficient manner. This approach allows high polarization without the need for glass-forming agents and is applicable to a broad range of substrates, including peptides and metabolites. During dissolution, HYPSO is physically retained by simple filtration in the cryostat of the DNP polarizer, and a pure hyperpolarized solution is collected within a few seconds. The resulting solution contains the pure substrate, is free from any paramagnetic or other pollutants, and is ready for in vivo infusion

Catalytic N-oxidation of tertiary amines on RuO2NPs anchored graphene nanoplatelets

Ultrafine ruthenium oxide nanoparticles (RuO2NPs) with an average diameter of 1.3 nm were anchored on graphene nanoplatelets (GNPs) using a Ru(acac)3 precursor by a very simple dry synthesis method. The resultant material (GNPs–RuO2NPs) was used as a heterogeneous catalyst for the N-oxidation of tertiary amines for the first time. The transmission electron microscopy (TEM) images of the GNPs–RuO2NPs showed the excellent attachment of RuO2NPs on GNPs. The loading of Ru in GNPs–RuO2NPs was 2.68 wt%, as confirmed by scanning electron microscope-energy dispersive spectroscopy (SEM-EDS). The X-ray photoelectron spectrum (XPS) and the X-ray diffraction pattern (XRD) of GNPs–RuO2NPs revealed that the chemical state of Ru on GNPs was +4. After the optimization of reaction conditions for N-oxidation of triethylamine, the scope of the reaction was extended to various aliphatic, alicyclic and aromatic tertiary amines. The GNPs–RuO2NPs showed excellent catalytic activity in terms of yields even at a very low amount of Ru catalyst (0.13 mol%). The GNPs–RuO2NPs was heterogeneous in nature, chemically as well as physically, very stable and could be reused up to 5 times.ArticleCATALYSIS SCIENCE & TECHNOLOGY. 4(7):2099-2106 (2014)journal articl

Creating a regular array of metal-complexing molecules on an insulator surface at room temperature

Controlling self-assembled nanostructures on bulk insulators at room temperature is crucial towards the fabrication of future molecular devices, e.g., in the field of nanoelectronics, catalysis and sensor applications. However, at temperatures realistic for operation anchoring individual molecules on electrically insulating support surfaces remains a big challenge. Here, we present the formation of an ordered array of single anchored molecules, dimolybdenum tetraacetate, on the (10.4) plane of calcite (CaCO3). Based on our combined study of atomic force microscopy measurements and density functional theory calculations, we show that the molecules neither diffuse nor rotate at room temperature. The strong anchoring is explained by electrostatic interaction of an ideally size-matched molecule. Especially at high coverage, a hard-sphere repulsion of the molecules and the confinement at the calcite surface drives the molecules to form locally ordered arrays, which is conceptually different from attractive linkers as used in metal-organic frameworks. Our work demonstrates that tailoring the molecule-surface interaction opens up the possibility for anchoring individual metal complexing molecules into ordered arrays