Optimization for catalytic performances of Hβ zeolite in the acylation of 2-methylfuran by surface modification and solvents effect

The liquid phase acylation of 2-methylfuran with acetic anhydride over modified Hβ zeolite was first conducted in a continuous flow reactor. The deactivation of Hβ zeolites was attributed to strong adsorption of reactants or products and was verified by GC–MS and ¹³C MAS NMR. Deactivated zeolites can be regenerated to their original state by calcination. The acidic properties was adjusted by surface modification on Hβ, the maximum yield of 89.5 mol% and selectivity of 100 % were obtained over tartaric acid modified by Hβ. The deposition of tetraethoxysilane to silica on Hβ contributed to enhancing the catalytic stability. Combined with the results of NH₃-TPD and Py-FTIR, the amount of Broensted acids played a major role on catalytic activity. A close relationship between the catalytic stability and the ratio of the amount of strong to weak acids at 1:1 was highlighted here. The solvents' effect on the catalytic performances was examined, and 1,2-dichloroethane with moderate polarity exerted a positive effect on catalytic stability.     

Friedel–Crafts alkylation of 2-naphthols with enamides and enethers induced by catalytic amount of triarylaminium salt

A novel and efficient regioselective Friedel–Crafts alkylation reaction of 2-naphthols with electron-rich enamides or enethers initiated by catalytic amounts of stable radical cation triarylaminium salt has been developed. The processes occur under mild conditions.     

Conformation and rotational mobility ofэSiOĊH2 radicals grafted onto the silica surface

The conformation ofэSiOC·H₂ radicals was determined by comparison of the ESR data and results of quantum-chemical calculations. Based on the experimental data, the characteristic times τ_c of rotational mobility ofэSiOC·H₂ radicals grafted onto a silica surface were estimated over the temperature interval from 77 (τ_c = 15.8 · 10^-8 s) to 295 K (τ_c 1.3 · 10^-8 s). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2092–2095, November, 1999.     

Role of O?H bonding in catalytic activity of Nb2O5 during the course of dehydrogenation of MgH2

The hydrogen desorption reaction of MgH₂, MgH₂ → Mg + H₂, is accelerated by the addition of metal oxide catalysts (e.g., Nb₂O₅). From our theoretical calculation of electronic structure, it was predicted that the catalytic activities of metal oxides are related closely to the O? H interaction operating at the interface between oxide catalyst and MgH₂. In this study, the O? H vibration on the Nb₂O₅‐catalyzed MgH₂ was investigated experimentally using FTIR spectroscopy. The broad absorption band due to the O? H stretching mode was observed in the region of 2,800–3,600 cm^?1 in the FTIR spectra of the specimens when hydrogen desorption reaction was in progress. The absorbance of the band decreased monotonously with decreasing hydrogen content in the specimen during the course of dehydrogenation of MgH₂. This experimental result was in agreement with our prediction for the existence of O? H interaction in the hydrogen desorption process. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Quantum instanton calculation of rate constants for the C2H6 + H → C2H5 + H2 reaction: anharmonicity and kinetic isotope effects

Thermal rate constants and kinetic isotope effects for the title reaction are calculated by using the quantum instanton approximation within the full dimensional Cartesian coordinates. The obtained results are in good agreement with experimental measurements at high temperatures. The detailed investigation reveals that the anharmonicity of the hindered internal rotation motion does not influence the rate too much compared to its harmonic oscillator approximation. However, the motion of the nonreactive methyl group in C(2)H(6) significantly enhances the rates compared to its rigid case, which makes conventional reduced-dimensionality calculations a challenge. In addition, the temperature dependence of kinetic isotope effects is also revealed.     

Recent progress in catalytic oxygenation of aromatic C–H groups with the environmentally benign oxidants H2O2 and O2

Developing efficient approaches for the selective oxygenation of aromatic C–H groups has been a challenging goal, interesting from both fundamental and practical perspectives. Designing catalyst systems for the direct production of phenol from benzene holds significant practical promise; on the contrary, novel strategies for the late-stage selective introduction of oxygen into aromatic rings of complex organic molecules could be useful for the synthesis of fine chemicals, including biologically active compounds. Another aspect of the overall picture is toughening environmental constraints, resulting in a steady increase in the number of catalyst systems relying on environmentally benign oxidants such as H₂O₂ and O₂. Overall, today, the realm of selective catalytic aromatic C–H oxidations is developing rapidly. This review summarizes the progress made in homo- and heterogeneous catalyst systems of aromatic C–H oxygenations with hydrogen peroxide and molecular oxygen, achieved in the last decade.     

The relationship between SnII fraction and visible light activated photocatalytic activity of SnOx·SiO2 glass studied by Mössbauer spectroscopy

The relationship between local structure and visible-light photocatalytic ability of tin silicate glass prepared by sol–gel method was investigated. ¹¹⁹Sn Mössbauer spectrum of SnO_x·SiO₂ glass prepared from SnCl₂ showed a small peak of Sn^II component besides the major amount of Sn^IV. The smallest bandgap energy of 2.5 ± 0.5 eV was estimated from Tauc plot, and the largest first order rate constant (k) of (13.8 ± 0.1) 10⁻³ min⁻¹ was recorded from the methylene blue degradation test under visible-light irradiation. It is concluded that Sn^II shows remarkable photocatalytic ability when it is incorporated into silica glass matrix.

     

Complexes of type C6H7(+)·L (L = N2 and CO2) studied by explicitly correlated coupled cluster theory

Complexes of the benzenium ion (C(6)H(7)(+)) with N(2) or CO(2) have been studied by explicitly correlated coupled cluster theory at the CCSD(T)-F12x (x = a, b) level [T. B. Adler et al., J. Chem. Phys. 127, 221106 (2007)] and the double-hybrid density functional B2PLYP-D [T. Schwabe and S. Grimme, Phys. Chem. Chem. Phys. 9, 3397 (2007)]. Improved harmonic vibrational wavenumbers for C(6)H(7)(+) have been obtained by CCSD(T?)-F12a calculations with the VTZ-F12 basis set. Combining them with previous B2PLYP-D anharmonic contributions we arrive at anharmonic wavenumbers which are in excellent agreement with recent experimental data from p-H(2) matrix isolation IR spectroscopy [M. Bahou et al., J. Chem. Phys. 136, 154304 (2012)]. The energetically most favourable conformer of C(6)H(7)(+)·N(2) shows a π-bonded structure similar to C(6)H(7)(+)·Rg (Rg = Ne, Ar) [P. Botschwina and R. Oswald, J. Phys. Chem. A 115, 13664 (2011)] with D(e) ≈ 870 cm(-1). For C(6)H(7)(+)·CO(2), a slightly lower energy is calculated for a conformer with the CO(2) ligand lying in the ring-plane of the C(6)H(7)(+) moiety (D(e) ≈ 1508 cm(-1)). It may be discriminated from other conformers through a strong band predicted at 1218 cm(-1), red-shifted by 21 cm(-1) from the corresponding band of free C(6)H(7)(+).     

Effect of small amounts of Al on the surface silanol structure and their correlation to the improved catalytic performances of WOx/SiO2–Al2O3 in the propene self-metathesis

Surface silanol structures, acid properties, and tungsten dispersion of the sol-gel-derived 7W/SiO₂–xAl₂O₃ (x = 0.2–23 wt%) were investigated by means of ²⁹Si, ²⁷Al, and ¹H MAS NMR, NH₃-TPD, in-situ NH₃-IR spectroscopy, XRD, and Raman spectroscopy. The surface silanol structure changed upon Al and tungsten loadings; however, loading of 1 wt% Al₂O₃ appeared to be the threshold for preserving the Si(OH)Al with isolated bridge after impregnation of 7 wt% W. The 7W/SiO₂–1Al₂O₃ (1 wt% Al₂O₃) was also found to exhibit the lowest ratio of Bronsted to Lewis acid with the highest amount of Lewis acid sites and the best catalyst performances in propene self-metathesis at 550 °C in terms of both propene conversion and ethylene/butene selectivity. Despite its low tungsten dispersion, the metathesis activity was correlated well with the higher amount of tungsten carbene species, which were formed on the catalysts containing higher isolated bridge silanol and the presence of higher Lewis acid sites.     

Influence of Dab2 and Pro3 configuration of [Leu]-enkephalins on the interactions with β-cyclodextrin studied by fluorescence spectroscopy, microcalorimetry and 1H NMR spectroscopy

Natural enkephalins and their analogues are very important as potential therapeutic agents (analgetics). Herein we describe the influence of Dab and Pro chirality of cyclic [Leu]enkephalins (X¹-c[Dab²-Pro³-βNal(2)⁴-Leu⁵], where X = Tyr or Phe) on the binding constant with β-cyclodextrin and spatial and mutual orientation of guest and host molecules. The formation of complexes is enthalpy driven for all cyclic [Leu]enkephalins studied as well as for Nal and AcNalNH₂. Moreover, change of Dab residue configuration has a greater influence on changes of the binding constant of cyclic enkephalin with β-CD than change of Pro chirality has. Also, the replacement of Tyr¹ residue by Phe¹ substantially changes the peptide chain conformation. An analysis of 2D NMR spectra reveals that, apart from inclusion complex formed by penetration of cyclodextrin cavity from wider and narrow rims by Nal, Tyr or Phe or Leu residue, a side and/or bottom association complexes are formed.     

Multicomponent QM study on the reaction of HOSO + NO2 with H2O: Nuclear quantum effect on structure and reaction energy profile

The hydrogen transfer reaction in the reaction of HOSO + NO₂ with and without H₂O have been investigated using multicomponent quantum-mechanics method, which can directly take nuclear quantum effect (NQE) of light nuclei into account. For the case of the reaction without H₂O, our calculation reveals that the reaction leading to trans-HONO is preferred. For the reaction with H₂O, water-non-mediated and water-mediated (hydrogen-relay) hydrogen transfer mechanism are investigated. The NQE of hydrogen nucleus lowers the relative energy of the stationary point structures and reduces the activation barrier of the reactions. The largest stabilization is found in the transition state structure of the hydrogen-relay type reaction. H/D isotope effects for the reactions are also analyzed. In particular, H/D isotope effect on the activation barrier is analyzed in detail with the aid of the active strain model.     

Fluorinated hydrotreatment catalysts: Promotion of the catalytic activity of unsupported MoS2 by doping with F? ions

Influence of fluorination on the HDS activity of unsupported MoS₂ phase has been studied and correlated with the influence of fluoride on the CoMo and NiMo catalysts supported on -Al₂O₃.

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MoS₂ . CoMo NiMo, -Al₂O₃.

     

Improve the catalytic activity of α-Fe2O3 particles in decomposition of ammonium perchlorate by coating amorphous carbon on their surface

Sphere- and pod-like α-Fe₂O₃ particles have been selectively synthesized using NH₃·H₂O and NaOH solution to adjust the pH value of the designed synthetic system, respectively. The sphere-like α-Fe₂O₃ particles with diameter about 25 nm on average were encapsulated into carbon shells to fabricate a novel core-shell composite (α-Fe₂O₃@C) through the coating experiments. The catalytic performance of the products on the thermal decomposition of ammonium perchlorate (AP) was investigated by thermal gravimetric analyzer (TG) and differential thermal analysis (DTA). The thermal decomposition temperatures of AP in the presence of pod-like α-Fe₂O₃, sphere-like α-Fe₂O₃ and α-Fe₂O₃@C are reduced by 72, 81 and 109 °C, respectively, which show that α-Fe₂O₃@C core-shell composites have higher catalytic activity than that of α-Fe₂O₃.     

Convenient and simple synthesis of 2-aminothiazoles by the reaction of α-halo ketone carbonyls with ammonium thiocyanate in the presence of N-methylimidazole

Substituted 2-aminothiazole derivatives were obtained as a result of N-methylimidazole catalyzed cyclization of α-halo ketone carbonyls with ammonium thiocyanate in water-alcoholic media. The generality of the method has been demonstrated by screening a series of aromatic/heteroaromatic/aliphatic α-halo ketones, α-halo β-diketones, and α-halo β-ketoesters. The developed method is simple, mild, and general route for the preparation of diversely functionalized 2-aminothiazoles in good to moderate yields from readily available starting materials.     

Influence of Pretreatment on the Interaction of Oxygen with Silver and the Catalytic Activity of Ag／SiO2 Catalysts for CO Selective Oxidation in H2

The interactions of oxygen with pre~reduced silver catalysts as well as their catalytic propertiesfor CO selective oxidation in H2 after oxygen pre-treatment are studied in this paper. It is found that the pretreatment exerts a strong influence on the activity and selectivity of the silver catalyst. A drop in activity and selectivity is observed after treating a pre-reduced catalyst with oxygen at low temperatures,whereas a converse result is obtained after an oxidizing treatment at high temperatures (T≥350℃). O2-TPD results show that surface oxygen species adsorbs on silver surface after the oxygen treatment at low temperatures. However, penetration of oxygen into the silver is enhanced by a high temperature treatment, meanwhile the surface oxygen species disappear. No other silver species except metallic silver are observed on all the catalysts by XRD, and the size of silver particle is not changed after the treatment with oxygen at low temperatures. The surface oxygen species formed by oxygen treatment can also be removed by hydrogen reduction. The strongly-adsorbed surface oxygen species prohibit the adsorption and diffusion of oxygen species in reaction gas on the surface of silver catalyst, causing the decrease in CO oxidation activity, in other words, it is important to obtain a clean silver surface for increasing the catalyst activity in CO removal from H2-rich feed gas. The differences in activity and selectivity due to the oxygen pretreatment at different temperatures axe discussed in terms of the changes in the surface/subsurface oxygen species of the silver particles.     

A study on the structure and catalytic performance of ZnxCu1−xAl2O4 catalysts synthesized by the solution combustion method for the esterification reaction

A Zn_xCu_1−xAl₂O₄ catalyst was prepared via the microwave-assisted solution combustion method (MSC). This method presents a fast procedure for industrial scale catalyst preparation. The physicochemical properties of the fabricated catalyst were characterized using XRD, FTIR, BET, SEM and TEM analyses. The catalytic performance through the esterification reaction was examined under the following conditions: reaction temperature = 180 °C, catalyst concentration = 3% (w/w), molar ratio of oleic acid to methanol = 9 and reaction time = 6 h. XRD results showed that loading both zinc and copper oxides on alumina at a ratio of amounts that were nearly the same resulted in decreased crystalline size and well-dispersed copper-alumina and zinc-alumina crystals. Moreover, the mean pore diameter of the sample was increased by simultaneous loading of zinc and copper oxides on alumina that enhanced permeation of the reactants within pores and increased the interaction of the reactant with the catalyst active sites. The catalyst showed minimum tendency towards adsorbing moisture from air, which was attributed to it having less atoms on the surface through which binding with H₂O molecules takes place. The highest level of activity in the esterification reaction (96.9%) was obtained at the optimum ratio of the Zn:Cu molar ratio, identified to be 2:3. The sample particles ranged from 10 to 30 nm in size, without agglomeration.     

Interaction of acetone, hydroxyacetone, acetaldehyde and benzaldehyde with the surface of water ice and HNO3·3H2O ice

Oxygenated volatile organic compounds (OVOCs) influence the oxidative properties of the atmosphere, and their transport from the ground may occur by scavenging by the HNO(3)-rich supercooled water droplets found in polluted convective air masses. With infrared spectroscopy, we have studied the interactions of four typical atmospheric OVOCs (acetone, hydroxyacetone, acetaldehyde and benzaldehyde) with model surfaces of water ice and of trihydrated nitric acid (NAT) ice. We show that these molecules weakly adsorb on water ice and NAT by hydrogen bonding. No chemical reaction occurs between the molecules and the NAT substrate, the OVOCs remaining intact when in contact with hydrated HNO(3) in atmospheric ice clouds.     

The gas-phase and solution mechanism of the isotope exchange reaction OH- + D2 = OD- + HD: Beam study of the solvated-ion reaction OH-. H2O + D2 in the collision energy range 0–2 eV

In a tandem mass spectrometer we have measured the excitation functions (reaction cross section as a function of collision energy) for the following solvated-ion reactant pairs: OH^-.(H₂O) + H₂; OD^-.(D₂O) + D₂; and OH^-.(H₂O) + D₂—in the collision energy range 0–2 eV. Product channels include H₃O^--type production, collision-induced dissociation of reactants and products (OH^- and H^- types) and isotopic mixing. These solvated-ion reactions are used to correlate the reactivity of the isotope exchange reaction OH^- + D₂→OD^- + HD occuring in the gas phase and solution, identifying a proton-transfer mechanism occuring within an H₃O^- intermediate.