No abstract

Substoichiometric tungsten oxide single-crystal nanosheets are successfully prepared via the exfoliation of layered tungstic acid and subsequent introduction of oxygen vacancies. The combination of different strategies, i.e., 2D-structure construction, the introduction of surface oxygen vacancies, and the creation of localized surface plasmon resonance can promote the light-harvesting performance of tungsten oxide through accumulative and synergistic effects.

Nanocrystalline Sn-Beta zeolites have been successfully prepared via an improved two-step postsynthesis strategy, which consists of creating vacant T sites with associated silanol groups by dealumination of parent H-Beta and subsequent dry impregnation of the resulting Si-Beta with organometallic dimethyltin dichloride. Characterization results from UV−vis, XPS, Raman, and 119 Sn solid-state MAS NMR reveal that most Sn species have been successfully incorporated into the framework of Beta zeolite through the postsynthesis process and exist as isolated tetrahedral Sn(IV) in open arrangement. The creation of strong Lewis acid sites upon Sn incorporation is confirmed by FTIR spectroscopy with pyridine adsorption. The Sn-Beta Lewis acid catalysts are applied in the ring-opening hydration of epoxides to the corresponding 1,2-diols under near ambient and solvent-free conditions, and remarkable activity can be obtained. The impacts of Lewis acidity, preparation parameters, and reaction conditions on the catalytic performance of Sn-Beta zeolites are discussed in detail.

Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethylbenzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via 1 H MAS NMR and 13 C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethylcyclopentenyl, and polymethylcyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream.

Shewanella thrives in redox-stratified environments where accumulation of H 2 O 2 becomes inevitable because of the chemical oxidation of reduced metals, sulfur species, or organic molecules. As a research model, the representative species Shewanella oneidensis has been extensively studied for its response to various stresses. However, little progress has been made toward an understanding of the physiological and genetic responses of this bacterium to oxidative stress, which is critically relevant to its application as a dissimilatory metal-reducing bacterium. In this study, we systematically investigated the mechanism underlying the response to H 2 O 2 at cellular, genomic, and molecular levels. Using transcriptional profiling, we found that S. oneidensis is hypersensitive to H 2 O 2 in comparison with Escherichia coli, and well-conserved defense genes such as ahpCF, katB, katG, and dps appear to form the first line of defense, whereas iron-sulfur-protecting proteins may not play a significant role. Subsequent identification and characterization of an analogue of the E. coli oxyR gene revealed that S. oneidensis OxyR is the master regulator that mediates the bacterial response to H 2 O 2 -induced oxidative stress by directly repressing or activating the defense genes. The sensitivity of S. oneidensis to H 2 O 2 is likely attributable to the lack of an inducible manganese import mechanism during stress. To cope with stress, major strategies that S. oneidensis adopts include rapid removal of the oxidant and restriction of intracellular iron concentrations, both of which are achieved predominantly by derepression of the katB and dps genes.

SAPO-34 materials with comparable Brønsted acid site density but different crystal sizes were applied as methanol-to-olefin (MTO) catalysts to elucidate the effect of the crystal size on their deactivation behaviors. 13 C HPDEC MAS NMR, FTIR, and UV/vis spectroscopy were employed to monitor the formation and nature of organic deposits, and the densities of accessible Brønsted acid sites and active hydrocarbon-pool species were studied as a function of timeon-stream (TOS) by 1 H MAS NMR spectroscopy. The abovementioned spectroscopic methods gave a very complex picture of the deactivation mechanism consisting of a number of different steps. The most important of these steps is the formation of alkyl aromatics with large alkyl chains improving at first the olefin selectivity, but hindering the reactant diffusion after longer TOS. The hindered reactant diffusion leads to a surplus of retarded olefinic reaction products in the SAPO-34 pores accompanied by their oligomerization and the formation of polycyclic aromatics. Finally, these polycyclic aromatics are responsible for a total blocking of the SAPO-34 pores, making all catalytically active sites inside the pores nonaccessible for further reactants.

The selective hydrogenation of alkynes to alkenes is an important type of organic transformation with large-scale industrial applications. This transformation requires efficient catalysts with precise selectivity control, and palladium-based metallic catalysts are currently employed. Here we show that four-coordinated cationic nickel­(II) confined in zeolite can efficiently catalyze the selective hydrogenation of acetylene to ethylene, a key process for trace acetylene removal prior to the polymerization process. Under optimized conditions, 100% acetylene conversion and an ethylene selectivity up to 97% are simultaneously achieved. This catalyst system also exhibits good stability and recyclability for potential applications. Spectroscopy investigations and density functional theory calculations reveal the heterolytic dissociation of hydrogen molecules and the importance of hydride and protons in the selective hydrogenation of acetylene to ethylene. This work provides an efficient strategy toward active and selective zeolite catalysts by utilizing the local electrostatic field within the zeolite confined space for small-molecule activation and by linking heterogeneous and homogeneous catalysis.

Size-controlled and coated magnetite nanoparticles with glucose and gluconic acid have been successfully synthesized via a simple and facile hydrothermal reduction route using a single iron precursor, FeCl 3 , and a combination of the inherent chemical reduction capability of sucrose decomposition products and their inorganic coordinating ability. The particle size can be easily controlled in the range of 4-16 nm. Results obtained with and without the addition of sucrose indicate that sucrose is required for the formation of nanoscale and coated magnetite instead of the much larger hematite. Mass spectrometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetry analysis were used to investigate the formation mechanism of the coated nanomagnetite from the single Fe(III) precursor in sucrose. Sucrose acts as a bifunctional agent: (i) it decomposes into reducing species, causing partial reduction of the Fe 3+ ions to Fe 2+ ions as required for the formation of Fe 3 O 4 and (ii) acts as the source of a capping agent to adjust the surface properties and enable the formation of nanoscale particles. The saturation magnetization of the asobtained magnetite is measured and greatly related to the particle size.