Stoichiometric Reaction Conditions Research Articles

A 1,2-Aryl Migration Reaction in Visible-Light-Mediated Synthesis of Quinoxaline Derivatives: Mechanistic Studies.

The synthesis of quinoxaline derivatives holds critical importance in various fields ranging from pharmaceuticals to material science. In this study, we introduce a practical light-mediated method for the efficient synthesis of quinoxaline derivatives. This approach enabled the sequential two-step, one-pot synthesis of 1,2-dihydro-2,2-diaryl-substituted quinoxalines from quinones, alkynes, and diamines. By adjusting the stoichiometric ratios and reaction conditions, the method was shifted to yield 2,3-diaryl-substituted quinoxalines exclusively, demonstrating remarkable versatility and efficiency. This switch in reaction outcomes was revealed to involve an oxidative 1,2-aryl migration through a combination of thorough experimental and computational mechanistic studies.

Electrochemical Promotion of Bimetallic Palladium‐Cobalt Nano‐Catalysts for Complete Methane Oxidation

AbstractThe catalytic activity and electrochemical promotion of catalysis (EPOC) of the bimetallic Pd−Co nanoparticles (10 nm) deposited on yttria‐stabilized zirconia were investigated for complete methane oxidation. The reaction was conducted under open‐circuit and under polarization in a temperature range of 320–400 °C in reducing, stoichiometric, and oxidizing conditions. The catalytic activity of the catalyst nanoparticles increased to 40 % upon positive polarization in all gaseous compositions. A comparison of three reaction conditions revealed that the highest reaction rate increase (enhancement ratio, ρ=1.4) occurs under reducing conditions. The reaction rate increased upon anodic polarization, resulting in non‐Faradaic electrochemical modification of catalytic activity (Λ≫1) in reducing and oxidizing conditions and Faradaic or electrochemical enhancement (Λ<1) in stoichiometric reaction condition. This work demonstrates that the formation of different Pd and Co oxide phases can be accurately controlled by electrochemical stimuli and, in reducing conditions, result in pseudo‐capacitor behaviour.

Operando characterisation of alumina-supported bimetallic Pd–Pt catalysts during methane oxidation in dry and wet conditions

Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) was used to study the chemical states of a range of alumina-supported monometallic Pd and bimetallic Pd–Pt nanocatalysts, under methane oxidation conditions. It has been suggested before that for optimal complete methane oxidation, palladium needs to be in an oxidised state. These experiments, combining NAP-XPS with a broad range of characterisation techniques, demonstrate a clear link between Pt presence, Pd oxidation, and catalyst activity under stoichiometric reaction conditions. Under oxygen-rich conditions this behaviour is less clear, as all of the palladium tends to be oxidised, but there are still benefits to the addition of Pt in place of Pd for complete oxidation of methane.

Reduction of the Diazo Functionality of α-Diazocarbonyl Compounds into a Methylene Group by NH3BH3 or NaBH4 Catalyzed by Au Nanoparticles.

Supported Au nanoparticles on TiO2 (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group [C=(N2) → CH2] by NH3BH3 or NaBH4 in methanol as solvent. The Au-catalyzed reduction that occurs within a few minutes at room temperature formally requires one hydride equivalent (B-H) and one proton that originates from the protic solvent. This pathway is in contrast to the Pt/CeO2-catalyzed reaction of α-diazocarbonyl compounds with NH3BH3 in methanol, which leads to the corresponding hydrazones instead. Under our stoichiometric Au-catalyzed reaction conditions, the ketone-type carbonyls remain intact, which is in contrast to the uncatalyzed conditions where they are selectively reduced by the boron hydride reagent. It is proposed that the transformation occurs via the formation of chemisorbed carbenes on Au nanoparticles, having proximally activated the boron hydride reagent. This protocol is the first general example of catalytic transfer hydrogenation of the carbene-like α -ketodiazo functionality.

Mixed RuxIr1–xO2 Oxide Catalyst with Well-Defined and Varying Composition Applied to CO Oxidation

The Pechini method allows for compositional and structural control of mixed ruthenium–iridium powder samples. Extensive characterization reveals that the calcination step leads to agglomerates with a metallic core that is encapsulated by the mixed RuxIr1–xO2 oxides. In the composition range of 21 up to 74 mol % ruthenium, the metal core reveals a miscibility gap with an Ir-rich fcc structure and a Ru-enriched hcp structure. Quite in contrast, the mixed oxide particles form a well-defined RuxIr1–xO2 solid solution throughout the entire composition range. Catalytic activity tests of the mixed RuxIr1–xO2 oxide catalysts are conducted with the prototypical CO oxidation reaction under oxidizing and under both stoichiometric reaction conditions; note that the metallic core is buried and does therefore not contribute to the activity. The least active catalyst is pure IrO2. At moderate reaction temperatures above 100 °C, the most active oxidation catalyst is identified with the Ir0.125Ru0.875O2 mixed oxide. All mixed RuxIr1–xO2 oxide catalysts are bulk-stable under the reaction conditions considered. However, upon CO oxidation reaction, the Ir4+ concentration at the mixed oxide surface is significantly enhanced with respect to its bulk composition. This information may be important for other catalytic oxidation reactions as well, such as the anodic oxygen evolution reaction in the electrocatalytic water splitting.

Photoinduced Proton-Transfer Reactions for Mild O-H Functionalization of Unreactive Alcohols.

Hexafluoroisopropanol is typically considered as an unreactive solvent and not as a reagent in organic synthesis. Herein, we report on a mild and efficient photochemical reaction of aryl diazoacetates with hexafluoroisopropanol that enables, under stoichiometric reaction conditions, the synthesis of fluorinated ethers in excellent yield. Mechanistic studies indicate there is a preorganization of hexafluoroisopropanol and the diazoalkane acts as an unreactive hydrogen‐bonding complex. Only after photoexcitation does this complex undergo a protonation‐substitution reaction to the reaction product. Investigations on the applicability of this photochemical transformation show that a broad variety of acidic alcohols can be subjected to this transformation and thus demonstrate the feasibility of this concept for O‐H functionalization reactions (54 examples, up to 98 % yield).

Stoichiometric Photochemical Carbene Transfer by Bamford-Stevens Reaction.

The photolysis of diazoalkanes is a timely strategy to conduct carbene‐transfer reactions under mild and metal‐free reaction conditions, and has developed as an important alternative to conventional metal‐catalyzed carbene‐transfer reactions. One of the major limitations lies within the rapidly occurring side reaction of the carbene intermediate with remaining diazoalkane molecules that result in the use of an excess of the reaction partner and thus impacts on the reaction efficiency. Herein, we describe a protocol that takes advantage of the in situ generation of donor–acceptor diazoalkanes by Bamford–Stevens reaction. Following this strategy, the concentration of the diazoalkane reaction partner can be minimized to reduce unwanted side reactions and to now conduct photochemical carbene transfer reactions under stoichiometric reaction conditions. We have explored this approach in the C−H and N−H functionalization and cyclopropanation reaction of N‐heterocycles and could demonstrate the applicability of this method in 51 examples.

Clash of Chemists: A Gamified Blog To Master the Concept of Limiting Reagent Stoichiometry

In a first-year university course, students experienced a new learning activity (“Clash of Chemists”) prompting them to create and share personal analogies explaining the difference between stoichiometric and nonstoichiometric reaction conditions, also known in the literature as “limiting reagent stoichiometry”. To support students’ commitment to this unusual assignment, the instructional design drew on a blog enriched with game mechanics (tournament, video rewards, and leaderboard), as found in popular mini-games. The paper reports on the activity’s outputs and on participants’ perceptions of its usability, usefulness, and generated satisfaction. Overall students’ reception of this mini-game was positive. A significant difference between players and nonplayers’ end-of-term exam results was highlighted.

Nucleophilic reactivity of copper(ii)-alkylperoxo complexes.

Copper(ii)-alkylperoxo adducts, [Cu(CHDAP)(OOR)]+ (CHDAP = N,N'-dicyclohexyl-2,11-diaza[3,3](2,6)pyridinophane; R = C(CH3)2Ph and tBu), were prepared and characterized using various physicochemical methods. These are the first synthetic Cu(ii)-alkylperoxo complexes that can perform aldehyde deformylation (i.e., nucleophilic reactivity) under the stoichiometric reaction conditions, which was confirmed by kinetic studies.

Photocatalytic Asymmetric Epoxidation of Terminal Olefins Using Water as an Oxygen Source in the Presence of a Mononuclear Non-Heme Chiral Manganese Complex.

Photocatalytic enantioselective epoxidation of terminal olefins using a mononuclear non-heme chiral manganese catalyst, [(R,R-BQCN)MnII]2+, and water as an oxygen source yields epoxides with relatively high enantioselectivities (e.g., up to 60% enantiomeric excess). A synthetic mononuclear non-heme chiral Mn(IV)-oxo complex, [(R,R-BQCN)MnIV(O)]2+, affords similar enantioselectivities in the epoxidation of terminal olefins under stoichiometric reaction conditions. Mechanistic details of each individual step of the photoinduced catalysis, including formation of the Mn(IV)-oxo intermediate, are discussed on the basis of combined results of laser flash photolysis and other spectroscopic methods.

ChemInform Abstract: Iodine(III)‐Mediated para‐Selective Direct Imidation of Anilides.

AbstractThe imidation of acetanilides proceeds exclusively para‐selectively under iodine(III)‐mediated stoichiometric or catalyzed reaction conditions with good tolerance to various functional groups at both ortho or meta position.

Comparison of precious metal doped and impregnated perovskite oxides for TWC application

Four high-surface-area (50–60m2/g) perovskite-based powder catalysts, Pd-doped LaFeO3, Pd-impregnated LaFeO3, Rh-doped CaTiO3, and Rh-impregnated CaTiO3, were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14h at 800°C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts.

Ageing induced improvement of methane oxidation activity of Pd/YFeO3

The ageing characteristics of flame-made 2 wt% Pd supported on YFeO3 were analysed in comparison with a Pd/Al2O3–CeO2–ZrO2 three-way catalyst (TWC) with respect to structural changes and catalytic performance for methane oxidation under stoichiometric reaction conditions. Thermal treatment under lean conditions (air, 900 °C) resulted in slight decrease in the methane oxidation activity of the TWC. In marked contrast, YFeO3-supported Pd catalysts exhibit an increase in activity after such treatment. Activity enhancement is even higher when the treatment was performed under stoichiometric conditions (air–fuel equivalence ratio, λ = 1, 900 °C). To explain this observation, in-depth characterization (BET, STEM, OSCC, XAS, and CO chemisorption) of fresh and aged catalysts was performed. Both thermal and stoichiometric ageing cause a severe sintering of the support particles and the phase transformation from hexagonal to orthorhombic YFeO3. Despite the absence of a mixed Pd–YFeO3 phase, the growth of Pd particles appears to be limited under the λ = 1 atmosphere. In contrast to thermally aged catalysts where large PdO particles are formed, well-defined metallic Pd nanoparticles of 10–20 nm are present after stoichiometric ageing along with higher methane oxidation activity. Although it is tempting to conclude that metallic Pd is active for methane oxidation under the given conditions, reversible and periodic partial oxidation of the large metallic particles is observed in modulation excitation high energy X-ray diffraction (HXRD) experiments designed to simulate the oscillating redox conditions experienced during operation. These results indicate that large Pd particles exhibit improved methane oxidation activity but equally confirm that activity under stoichiometric conditions is the result of a delicate equilibrium dictated by the bulk-Pd/surface-PdO pair.

Methane oxidation hysteresis over Pt/Al2O3

With growing, accessible natural gas reserves, there is renewed interest in natural gas engines for the wider automotive market. A potential issue, if legislated, is controlling (reducing) the un-combusted CH4 emissions. The key challenge in catalytic CH4 oxidation is the high temperature required, relative to other hydrocarbon species. In this context, a monolith-supported Pt/Al2O3 catalyst was evaluated for CH4 combustion under fuel lean and fuel rich mixtures using temperature programmed reaction (TPRxn) and step-change temperature and oxygen level experiments. The experiments included performance evaluation during both ignition (increasing temperature) and extinction (decreasing temperature after ignition). Conversion hysteresis was observed, with the conversions during extinction higher than those during ignition under fuel lean or stoichiometric combustion reaction conditions. Results obtained demonstrate that this hysteresis effect can be used to achieve high CH4 oxidation conversions at temperatures lower than that required for ignition, admittedly first through using high temperatures to obtain ignition, then lowering the temperature to take advantage of the hysteresis. Results also suggest that changing O2 levels can lead to similar benefits. With the assumption that lean conditions and lower exhaust temperatures are associated with improved fuel economy, while higher exhaust temperatures and fuel rich conditions lead to higher CH4 oxidation rates over the catalyst, the findings presented clearly demonstrate the potential to achieve both via a cyclic operating approach, with the frequency on the order of 10s of minutes. Data obtained when cycling between temperatures above and below the ignition point and between excess O2 and stoichiometric O2 levels proved this hypothesis.

Iron-Catalyzed Chlorination of Silanes

A simple and highly efficient iron-catalyzed method for the chlorination of silanes has been developed. By use of 0.5–2% of the Fe(III)-based catalyst FeCl3 or Fe(acac)3 in the presence of 1–1.5 equiv of acetyl chloride as the chlorine donor, a large number of silanes, alkoxysilanes, and silanols were converted to the corresponding chlorosilanes in 50–93% yields. In contrast to earlier reported methods often suffering from expensive catalysts or use of stoichiometric metal salts, hazardous reagents, and reaction conditions, the presently described methodology allows benign reaction conditions and simple workup while using only catalytic amounts of a readily available and economically viable iron catalyst.

Active Surfaces for CO Oxidation on Palladium in the Hyperactive State

Hyperactivity was previously observed for CO oxidation over palladium, rhodium, and platinum surfaces under oxygen-rich conditions, characterized by reaction rates 2-3 orders higher than those observed under stoichiometric reaction conditions [Chen et al. Surf. Sci. 2007, 601, 5326]. In the present study, the formation of large amounts of CO(2) and the depletion of CO at the hyperactive state on both Pd(100) and polycrystalline Pd foil were evidenced by the infrared intensities of the gas phase CO(2) and CO, respectively. The active surfaces at the hyperactive state for palladium were characterized using infrared reflection absorption spectroscopy (IRAS, 450-4000 cm(-1)) under the realistic catalytic reaction condition. Palladium oxide on a Pd(100) surface was reduced eventually by CO at 450 K, and also under CO oxidation conditions at 450 K. In situ IRAS combined with isotopic (18)O(2) revealed that the active surfaces for CO oxidation on Pd(100) and Pd foil are not a palladium oxide at the hyperactive state and under oxygen-rich reaction conditions. The results demonstrate that a chemisorbed oxygen-rich surface of Pd is the active surface corresponding to the hyperactivity for CO oxidation on Pd. In the hyperactive region, the CO(2) formation rate is limited by the mass transfer of CO to the surface.

Precipitation of calcium carbonate particles by gas–liquid reaction: Morphology and size distribution of particles in Couette-Taylor and stirred tank reactors

The morphology and size of CaCO 3 precipitated by CO 2–Ca(OH) 2 reaction in stirred tank and Couette-Taylor reactors were experimentally investigated. The Taylor vortex in CT reactor encouraged more homogeneous mixing conditions, resulting in the production of smaller particles with a uniform shape throughout the reactor. However, in the stirred tank reactor, the local non-homogeneity of the mixing intensity led to the simultaneous production of cube-like and spindle-like particles at a high reactant concentration. The agglomeration of CaCO 3 resulted in a bimodal size distribution. However, the morphology and size of a single particle were predominantly changed by the excess species in the solution. The largest mean size and cube-like particles were observed under stoichiometric reaction conditions. As the excess species concentration increased, the morphology was transformed to a spindle-like shape and the mean size decreased due to selective adsorption of the excess species on the crystal faces.

Characterization and Catalytic Behavior of Nano Pt/MgO Catalysts

Pt/MgO catalysts were prepared by using wet impregnation method. X-ray diffraction, transmission electron microcopy (TEM), electron spectroscopy for chemical analysis, and temperature programmed surface reaction (TPSR) were used to characterize the calcined and used catalysts. Partial Oxidation of Methane (POM) to synthesis gas was carried out in a fixed bed micro-reactor to test the catalytic activity. TEM images revealed that Pt particles have the size of less than 10 nm, the size of the support MgO is about 50200 nm. At 800C, the Pt/MgO catalyst exhibited a very high activity and stability for POM reaction. Both the conversion of methane and selectivities to CO and H2 remained almost constant up to 120 h. The dispersed Pt existed in the metallic state. The state and the dispersion of active Pt and the structure of support MgO were very stable, together with the high ability to resist carbon deposition, made the catalysts show a very high stability for POM under stoichiometric reaction conditions.

Highly active surfaces for CO oxidation on Rh, Pd, and Pt

Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O 2/CO ratio above 0.5. The catalytic surfaces, formed at discrete O 2/CO ratios >0.5, exhibit rates 2–3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O 2/CO ratios required to achieve these so-called “hyperactive” states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O 2/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation.

Method for Determination of Association and Dissociation Rate Constants of Reversible Bimolecular Reactions by Isothermal Titration Calorimeters

The rate law equation for reversible bimolecular reactions, which are describable by association and dissociation rate constants (k1 and k-1), is not solvable to a plain formula under stoichiometric reaction conditions. Therefore, it is a general technique to observe such reactions under pseudo first-order conditions, which make the reactions a single-exponential process, and enable us to determine k1 and k-1 without any complicated iterative computations needed to analyze the same reactions under stoichiometric reaction conditions. However, the accelerated reaction rates under pseudo first-order conditions are not always favorable to the physicochemical tools employing a slow or medium response time, such as thermal analysis instruments. In this study, we have developed a simple non-iterative analytical method to determine k1 and k-1 of reversible bimolecular reactions under stoichiometric conditions on the basis of experimental data of isothermal titration calorimetry (ITC), which is generally used to determine thermodynamic parameters rather than kinetic constants. Our method is principally based on the general principle of chemical bindings caused along with the titration processes, that is, the chemical relaxation kinetics, which had been hitherto considered in the analysis on the ITC data.