Crystalline Molecular Assemblies of Complexes Showing Eightfold Coordinated Niobium(IV) Dodecahedral Geometry in the Pyridine-Dicarboxylic Acid System.

The reactivity of 2,3-pyridine-dicarboxylic (known as quinolinic or H2qui) acid and 2,5-pyridine-dicarboxylic (known as isocinchomeronic or H2icc) acid has been investigated as a complexing agent toward the niobium(IV) tetrachloride precursor (NbCl4·2THF) in different organic solvent mixtures. It resulted in the isolation of four crystalline assemblies of mononuclear coordination complexes 1-4 [Nb(HL)4·solvent], where HL is the monoprotonated quinolinate (Hqui) ligand (complexes 1-3) or the monoprotonated isocinchomeronate ligand (complex 4). For each complex, the discrete niobium(IV) center is eightfold coordinated to four oxygen atoms from the deprotonated carboxylate arm and four nitrogen atoms from the pyridine part of the dicarboxyl ligand with a dodecahedral environment [NbO4N4]. The remaining carboxyl arm (either in 3 or in 5 position) remained under its protonated form, leading to neutral [Nb(HL)4] moieties for compounds 1, 2, and 4, or the anionic [Nb(qui)(Hqui)3]- moiety for compound 3. The complexes are observed in various molecular arrangements, involving intercalated solvent molecules such as acetonitrile in compound 1 ([Nb(Hqui)4·0.8(CH3CN)], obtained at room temperature), a mixture of acetonitrile and pyridine in compound 2 ([Nb(Hqui)4·0.7CH3CN·2PYR], obtained via the solvothermal reaction at 80 °C), a mixture of pyridine and triethylamine, in addition with water and chloride species, in compound 3 ([Nb(qui)(Hqui)3·Cl·HPYR·HTEA·1.5H2O], obtained via solvothermal reaction at 80 °C), and N,N-dimethylformamide in compound 4 ([Nb(Hicc)4·6DMF], obtained at room temperature). The d1 configuration expected for the niobium(IV) centers has been analyzed by magnetic measurements, as well as by EPR and XPS. An anti-ferromagnetism transition has been observed at very low temperatures for complexes 1 (3.6 K) and 4 (3.3 K), for which the shortest Nb···Nb interatomic lengths occur.

Mixed-Valence Iron Dumortierite Fe13.52.22+(As5+O4- x)8(OH)6 and Its Intricate Topotactic Exsolution at Mild Temperatures.

The mixed-valent iron arsenate hydroxide Fe13.52.22+(AsO4- x)8(OH)6, x = 0.25, was prepared using the reaction of iron metal with arsenate in aqueous solution and autogenous pressure. Its crystal structure reveals a dumortierite-like framework with mixed-valent Fe2+/Fe3+ in double chains creating channel walls. Remarkably, hexagonal channels consist of chains of face-sharing Fe2+O6 octahedra, 3/4th occupied, whereas AsO4 tetrahedra occupy triangular ones with a single " up" orientation according to the polar P63 mc symmetry. We have analyzed the transformation of this phase upon heating, in which several chemical processes interact, including dehydroxylation, arsenate to arsenite reduction, and oxidative exsolution of a significant part of iron (ca. 15%) found at the surface as hematite and amorphous Fe-rich surficial layer. It leaves a strongly disordered composite structure between several Fe3+-based subunits, in which ∼80% of them is ordered in a complex supercell. Because of the high degree of disorder, the crystal chemistry of the individual subunits and their plausible imbrication were considered to unravel the most plausible ideal 3D model.

SBA-15 supported Ni-Cu catalysts for hydrodeoxygenation of m-cresol to toluene.

Amidst concerns over fossil fuel dependency and environmental sustainability, the utilization of biomass-derived aromatic compounds emerges as a viable solution across diverse industries. In this scheme, the conversion of biomass involves pyrolysis, followed by a hydrodeoxygenation (HDO) step to reduce the oxygen content of pyrolysis oils and stabilize the end products including aromatics. In this study, we explored the properties of size controlled NiCu bimetallic catalysts supported on ordered mesoporous silica (SBA-15) for the catalytic gas-phase HDO of m-cresol, a lignin model compound. We compared their performances with monometallic Ni and Cu catalysts. The prepared catalysts contained varying Ni to Cu ratios and featured an average particle size of approximately 2 nm. The catalytic tests revealed that the introduction of Cu alongside Ni enhanced the selectivity for the direct deoxygenation (DDO) pathway, yielding toluene as the primary product. Optimal performance was observed with a catalyst composition comprising 5 wt.% Ni and 5 wr.% Cu, achieving 85 % selectivity to toluene. Further increasing the Cu content improved turnover frequency (TOF) values, but reduced DDO selectivity. These findings underscore the importance of catalyst design in facilitating biomass-derived compound transformations and offer insights into optimizing catalyst composition for more selective HDO reactions.

Boosting Gas-Phase TiO2 Photocatalysis with Weak Electric Field Strengths of Volt/Centimeter.

Among semiconductor nanomaterials, titanium dioxide is at the forefront of heterogeneous photocatalysis, but its catalytic activity greatly suffers from the loss of photoexcited charge carriers through deleterious recombination processes. Here, we investigate the impact of an external electric field (EEF) applied to conventional P25 TiO2 nanopowder with or without Au nanoparticles (NPs) to circumvent this issue. The study of two redox reactions in the gas phase, water splitting and toluene degradation, reveals an enhancement of the photocatalytic activity with rather modest electric fields of a few volt/centimeters only. Such an improvement arises from the electric-field-induced quenching of the green emission in anatase, allowing the photoexcited charge carriers to be transferred to the adsorbed reactants instead of pointless radiative recombinations. Applying an EEF across a trap-rich metal oxide material, such as TiO2, which, when impregnated with Au NPs, leads, respectively, to 12- and 6-fold enhancements in the production of hydrogen and the oxidation of toluene for an electric field of 8 V/cm, without any electrolysis, is a simple and elegant strategy to meet higher photocatalytic efficiencies.

Liquid metals for boosting stability of zeolite catalysts in the conversion of methanol to hydrocarbons.

Methanol-to-hydrocarbons (MTH) process has been considered one of the most practical approaches for producing value-added products from methanol. However, the commonly used zeolite catalysts suffer from rapid deactivation due to coke deposition and require regular regeneration treatments. We demonstrate that low-melting-point metals, such as Ga, can effectively promote more stable methanol conversion in the MTH process by slowing coke deposition and facilitating the desorption of carbonaceous species from the zeolite. The ZSM-5 zeolite physically mixed with liquid gallium exhibited an enhanced lifetime in the MTH reaction, which increased by a factor of up to ~14 as compared to the parent ZSM-5. These results suggest an alternative route to the design and preparation of deactivation-resistant zeolite catalysts.

An unusual O2-/F- distribution in the new pyrochlore oxyfluorides: Na2B2O5F2 (B = Nb, Ta).

Two new oxyfluorides with a pyrochlore-type structure, Na2Nb2O5F2 and Na2Ta2O5F2, have been obtained, for which the XRD crystallographic study coupled with 19F solid state NMR reveals an unusual O/F distribution. Both materials are n-type semiconductors exhibiting photoconductive properties.

A high dimensional oxysulfide built from large iron-based clusters with partial charge-ordering.

Herein we report the original Ba10Fe7.75Zn5.25S18Si3O12 oxysulfide which crystallizes in a new structural type. Contrary to the usual oxychalcogenides, it crystallizes with a non-centrosymmetric 3D spatial network structure built from large magnetic clusters consisting of twelve (Fe2+/3+/Zn)S3O tetrahedra decorating a central Fe2+S6 octahedron and exhibiting a spin glass state.

Direct conversion of uranium dioxide UO2 to uranium tetrafluoride UF4 using the fluorinated ionic liquid [Bmim][PF6].

The industrial fluorination of UO2 to UF4 is based on a complex process involving the manipulation of a large amount of HF, a very toxic and corrosive gas. We present here a safer way to accomplish this reaction utilizing ionic liquid [Bmim][PF6] as a unique reaction medium and fluoride source.

The hidden story in BaNiO3 to BaNiO2 transformation: adaptive structural series and NiO exsolution.

BaNiO3 crystal to BaNiO2 crystal transformation is reported. Contrary to an intuitive topochemical reduction, a two step reaction was observed. In the first step, NiO exsolution occurs and intermediate Ba1+xNiO3 phases were obtained and isolated. A composite approach was used to solve the novel structure for x ∼ 1/6 with charge ordered Ni2+ and Ni4+. We argue that this NiO exsolution is responsible for the increased oxygen enhanced reactivity recently reported. Upon re-oxidation, oxygen-deficient mixed valent BaNi3/4+O3-x are obtained such that the full redox cycle is irreversible and goes through a diversity of structural and nickel valence adaptative oxides.