Thermal treatment effects on the Ru/CeO2 catalysts performance for partial hydrogenation of benzene

The aim of this work is to study the effect of the preparation conditions of Ru/CeO₂ catalyst (calcination temperature and/or reduction) over the performance in the partial hydrogenation of benzene reaction in the presence of TiCl₃. The catalysts were prepared through chlorinated precursors by incipient wetness impregnation method. The reaction occurred in three-phase reactional medium in presence of water at 373 K and 5.0 MPa. Temperature programmed reduction (TPR) profiles of calcinated catalysts indicate the presence of oxidated ruthenium. X-ray photoelectron spectroscopy (XPS) analysis confirms this supposition, showing that the ruthenium appears in the form of RuO₂ for the sample calcinated at 673 K, while for the reduced solid at 773 K, the Ru appears in the metallic state. However, the calcination step followed or not by reduction, strongly hinders the catalytic performance. In its turn, the direct reduction leads to a more active Ru/CeO₂ catalysts, as well as higher cyclohexene yields throughout all the reaction.     

Hydroxylation of Aromatics by H2O2 Catalyzed by Mononuclear Non-heme Iron Complexes: Role of Triazole Hemilability in Substrate-Induced Bifurcation of the H2O2 Activation Mechanism

Rieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O₂ and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe^V(O)(OH) moiety. Molecular models allow the generation of a Fe^III(OOH) species with H₂O₂, to yield a Fe^V(O)(OH) species with tetradentate ligands, or {Fe^IV(O); OH^.} pairs with pentadentate ones. We have designed a new pentadentate ligand, mtL₄², bearing a labile triazole, to generate an “in-between” situation. Two iron complexes, [(mtL₄²)FeCl](PF₆) and [(mtL₄²)Fe(OTf)₂]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H₂O₂. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe^II state, but decoordinates in the Fe^III(OOH). The resulting [(mtL₄²)Fe^III(OOH)(MeCN)]²⁺ then lies on a bifurcated decay pathway (end-on homolytic vs. side-on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it is proposed to follow a side-on pathway leading to a putative (N₄)Fe^V(O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N₅)Fe^IV(O); OH^.} reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity.     

Recommended sublimation pressure and enthalpy of benzene

Recommended vapor pressures of solid benzene (CAS Registry Number: 71-43-2) which are consistent with thermodynamically related crystalline and ideal-gas heat capacities as well as with properties of the liquid phase at the triple point temperature (vapor pressure, enthalpy of vaporization) were established. The recommended data were developed by a multi-property simultaneous correlation of vapor pressures and related thermal data. Vapor pressures measured in this work using the static method in the temperature range from 233 K to 260 K, covering pressure range from 99 Pa to 1230 Pa, were included in the simultaneous correlation. The enthalpy of sublimation was established with uncertainty significantly lower than the previously recommended values.     

Cover Feature: Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity? (Chem. Eur. J. 63/2019)

The biodegradation of compounds with C−F bonds is challenging due to the fact that these bonds are stronger than the C−H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N-bridged diiron-phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H₂O₂ effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C₆F₆ activation by an iron(IV)-oxo active species of the N-bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate-determining electrophilic C−O addition reaction followed by a 1,2-fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C−F bond is the aliphatic C−F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C−H hydroxylation by iron(IV)-oxo intermediates such as cytochrome P450 Compound I.     

Second-Coordination Sphere Effects on Selectivity and Specificity of Heme and Nonheme Iron Enzymes

Mononuclear iron-containing enzymes are highly versatile oxidants that often react stereospecifically and/or regioselectively with substrates. Combined experimental and computational studies on heme monooxygenases, nonheme iron dioxygenases and halogenases have revealed the intricate details of the second-coordination sphere, which determine this specificity and selectivity. These second-coordination sphere effects originate from the positioning of the substrate and oxidant, which involve the binding of the co-factors and substrate into the active site of the protein. In addition, some enzymes affect the selectivity and reactivity through charge-stabilization from nearby bound cations/anions, an induced electric field or through the positioning of salt bridges and hydrogen-bonding interactions to first-coordination sphere iron ligands and/or the substrate. Examples of all of these second-coordination sphere effects in iron-containing enzymes and how these influence structure and reactivity are given.     

Impact of surface hydroxylation in MgO-/SnO-nanocluster modified TiO2 anatase (101) composites on visible light absorption,charge separation and reducibility

Anatase TiO₂ surfaces, whether oxidised or hydroxylated, can be modified by nanoclusters of SnO and MgO to give a red shift in light absorption, enhanced charge separation and high reducibility.     

Theoretical study of structures and properties of cyclobutadiene, cyclopentadiene and benzene and their nitrogen isoelectronic equivalents

B3LYP/aug-cc-pvDZ level of theory is applied to study the geometric structures, electronic topologies, heats of formation, hyperconjugations and steric repulsions of 27 kinds of compounds obtained by successive replacement of CH groups with nitrogen atoms in cyclobutadiene, cyclopentadiene and benzene. The results reveal that the total energy linearly decreases along with the replacement of CH groups by nitrogen atoms for the three systems. To estimate the potential of high nitrogen content high energy materials (HNC–HEMs), heats of formation are calculated by G3 method. With the increase of the number of nitrogen atoms in ring, heats of formation increase substantially. The four-membered ring system is found to have the greatest heat of formations, followed by the six-membered ring system, and then by the five-membered ring system. Especially, hexazine and tetraazacyclobutadiene have great heats of formation relative to the other compounds, which implies that they should be applicable as HNC–HEMs. In addition, our studies indicate that the relationship between the total energy or heats of formation of isomers and the position of nitrogen atoms is (ortho) meta < (ortho) para < ortho. NBO analysis shows that it is hyperconjugation, not steric repulsion that plays a key role in the relative stability of isomers.     

苯与丙烯在MCM-22型分子筛内吸附行为的Monte Carlo研究

采用巨正则统计系综Monte Carlo模拟方法研究了不同温度、不同吸附方式下纯硅MCM-22型分子筛ITQ-1上苯与丙烯分子的吸附行为. 分子筛内吸附质粒子云分布模拟结果显示, 苯和丙烯主要吸附在超笼和十元环孔道内, 其中丙烯分子几乎充满了孔道内部大部分区域, 在链接超笼之间的十元环窗口也充满了丙烯分子, 而苯分子在超笼内和十元环孔道内的吸附却较为分散、均匀. 丙烯与分子筛之间相互作用能高于苯与分子筛之间的相互作用能, 使苯分子吸附相对丙烯分子更为稳定. 温度变化对分子筛上丙烯吸附远大于对苯吸附的影响, 100 kPa时温度由298 K升高至443 K导致丙烯分子吸附量迅速减少, 而对苯分子却没有显著的影响. ITQ-1分子筛上存在苯和丙烯分子的竞争吸附, 使两者吸附相互作用能最可几分布朝着折中方向移动. 苯与丙烯在分子筛内吸附等温线的模拟结果表明, 在温度较高、压力较低时, 丙烯的吸附量小于苯的吸附量.