Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

N-Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine-chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal-based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non-noble metal-based heterogeneous catalysts to synthesize N-heterocycles from alcohols and N-nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid-base properties on these reactions.

Preparation of zeolitic bismuth vanadomolybdate using a ball-shaped giant polyoxometalate for olefin epoxidation

Bismuth vanadomolybdate-based zeolitic octahedral metal oxide was synthesized from {Mo72V30} and used as a heterogeneous catalyst for olefin epoxidation.

Vanadium-Enhanced Intramolecular Redox Property of a Transition-Metal Oxide Molecular Wire

Transition-metal oxide molecular wires are inorganic 1D polymers with elemental diversity. The properties of the materials are tuned by tuning the chemical compositions. The phosphovanadomolybdate molecular wire is synthesized, which is an isostructural material of the phosphomolybdate molecular wire. V is randomly located in the crystal to form {[(HP^IIIO₃)(Mo^VI₅O₁₅)(V^VO₃)]^3-}_n, which is incorporated into the material after the formation of the phosphomolybdate molecular wire. The heat-triggered redox reaction via the intramolecular electron-transfer and oxygen-transfer procedure is promoted after V substitution. Oxygen transfers from {V^VO₆} to {HP^IIIO₃}, and an electron transfers from {HP^IIIO₃} to {V^VO₆} with oxidation of the triangle {HP^IIIO₃} to the corner-sharing tetrahedral {P^V₂O₇} and reduction of the octahedral {V^VO₆} to the pyramidal {V^IVO₅}. The material shows catalytic activity for the aerobic oxidation of alcohol to aldehyde, and good activity with high selectivity is obtained.

Fabrication of Keggin-type Polyoxometalate Membranes at the Gas-Liquid Interface

Keggin-type polyoxometalates are effective catalytic materials for acid-catalyzed reactions and oxidation reactions. In this paper, we describe the fabrication of a porous membrane of Keggin-type polyoxometalates at the gas-liquid interface and the structural characterization of the membrane via aberration-corrected scanning transmission electron microscopy. The membrane is formed by the introduction of n-octylamine vapor into aqueous solution of Keggin-type silicotungstic acid (H₄SiW₁₂O₄₀) and cesium chloride. The membrane obtained has the cubic structure of cesium salts of silicotungstic acid (Cs_xH_4-xSiW₁₂O₄₀), exposing the (110) crystal plane preferentially. The single-unit cell layer of the upper surface of the membrane is densely packed with Keggin anions, while the lower layers are rich in vacancies at both the anion and cation sites. This structure results from the fast formation of the single-unit cell layer at the surface of the solution and slow stacking of Keggin anions under the layer.

Tin oxide-coated transition metal oxide molecular wires for biomass conversion

Herein, acid catalysts were prepared by coating Sn oxide on molecular wires for the production of levulinic acid from cellulose.

Control of coordinatively unsaturated Zr sites in ZrO2 for efficient C-H bond activation

Due to the complexity of heterogeneous catalysts, identification of active sites and the ways for their experimental design are not inherently straightforward but important for tailored catalyst preparation. The present study reveals the active sites for efficient C-H bond activation in C₁-C₄ alkanes over ZrO₂ free of any metals or metal oxides usually catalysing this reaction. Quantum chemical calculations suggest that two Zr cations located at an oxygen vacancy are responsible for the homolytic C-H bond dissociation. This pathway differs from that reported for other metal oxides used for alkane activation, where metal cation and neighbouring lattice oxygen form the active site. The concentration of anion vacancies in ZrO₂ can be controlled through adjusting the crystallite size. Accordingly designed ZrO₂ shows industrially relevant activity and durability in non-oxidative propane dehydrogenation and performs superior to state-of-the-art catalysts possessing Pt, CrO_x, GaO_x or VO_x species.

Keggin-type polyoxometalate nanosheets: synthesis and characterization via scanning transmission electron microscopy

Polyoxometalate nanosheets were synthesized and their structure was elucidated at the atomic and molecular levels.

Synthesis of crystalline molybdenum oxides based on a 1D molecular structure and their ion-exchange properties

Crystalline molybdotellurate based on a 1D molecular structure exhibits interesting ion-exchange properties with both inorganic and organic cations.