The catalytic nanodiode: detecting continuous electron flow at oxide-metal interfaces generated by a gas-phase exothermic reaction.

Continuous flow of ballistic charge carriers is generated by an exothermic chemical reaction and detected using the catalytic metal-semiconductor Schottky diode. We obtained a hot electron current for several hours using two types of catalytic nanodiodes, Pt/TiO2 or Pt/GaN, during carbon monoxide oxidation at pressures of 100 Torr of O2 and 40 Torr of CO at 413-573 K. This result reveals that the chemical energy of an exothermic catalytic reaction is directly converted into hot electrons flux in the catalytic nanodiode. By heating the nanodiodes in He, we could measure the thermoelectric current which is in the opposite direction to the flow of the hot electron current. The chemicurrent is well correlated with the turnover rate of CO oxidation, which is separately measured with gas chromatography. The influence of the flow of hot charge carriers on the chemistry at the oxide-metal interface, and the turnover rate in the chemical reaction are discussed.     

A flow microreactor for laboratory high pressure vapor phase catalytic reactions

A high pressure laboratory flow microreactor was designed and built to study the kinetics of hydroprocessing reactions. New features of this system are: the high pressure saturator-condenser (up to 7·10⁶ Pa), the small size of the reactor (1 cm³), the automatic on-line analysis device. The operation of the unit is illustrated by conversion data for the hydrogenation of biphenyl.

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Measurement of gas phase reactions using automated headspace gaschromatography

The kinetics of the gas phase-conversion of 0,0-dimethylmaleic ester into 0,0-dimethyl fumaric ester, catalyzed by piperidine, were measured in the gas phase at 95°C. Headspace-sample flasks served as the reaction vessels for the gaseous components and an automated headspace gaschromatograph as the sampling and analyzing system. The method was applied to investigate the thermal decomposition ofDFP and sarin to propene in the range of 148 to 189°C.

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Messung der Kinetik von Gasphasenreaktionen unter Verwendung der automatischen Dampfraumanalyse

Zusammenfassung Die Piperidin-katalysierte Isomerisierung von 0,0-Dimethylmaleinsäureester zum Fumarsäureester wurde in der Gasphase bei 95° durchgeführt. Als Reaktionsgefäße dienten Probenflaschen, wie sie für die automatische Dampfraumanalyse (HSGC) zur Verwendung kommen. Mit Hilfe der HSGC-Analyse wurde die Kinetik der Reaktion untersucht. Dieses Verfahren wurde sodann angewandt, um die thermolytische Zersetzung vonDFP und Sarin zu Propen zwischen 148 und 189°C zu studieren.

     

Physicochemical quantities in catalytic reactions measured simultaneously by gas chromatography

Summary A small volume of reactant, 1-butene, is injected onto a catalytic bed and is allowed to diffuse away from it together with the product butane, along a narrow empty chromatographic tube; the latter is connected perpendicularly to the middle point of another similar tube through which hydrogen flows as reactant and carrier gas, transferring both 1-butene and butane to the detector through an analytical column. By using the reversed-flow GC technique, extra peaks are obtained in the chromatographic trace, sampling the concentration of both the readtant and product at the junction of the two tubes as a function of time. These concentrations are the result of the diffusion of the substances along the narrow empty tube, modified by the adsorption-desorption rates and the rate of the catalytic reaction. From the extra peaks of the reactant and product, a number of physicochemical quantities pertaining to the catalytic reaction can be calculated simultaneously, using appropriate mathematical analysis. These include adsorption rate constants, reaction rate constants, desorption rate constants, partition coefficients, and the overall mass transfer coefficients of the reactant across the gas-solid boundary of the catalytic bed. Presented at the 17th International Symposium on Chromatography, September 25–30, 1988, Vienna, Austria.     

Fragmentation of organic anions induced by exothermic addition reactions

The addition reactions of a series of carbanions with CO₂, COS and CS₂ have been studied in a flowing afterglow apparatus. Carbon dioxide simply forms the adduct, but the more highly exothermic additions to COS and CS₂ lead both to addition and addition followed by fragmentation. A number of novel fragmentation pathways have been observed for additions to diazo anions and to anions derived by proton abstraction from allenes. In addition to these reactions, highly basic anions are observed to undergo sulfur atom transfer reactions with CS₂.     

Measuring H-bonding in supramolecular complexes by gas phase ion-molecule reactions

H/D and guest-exchange ion-molecule reactions have been used as a new tool to elucidate the operation of multiple hydrogen bonding in gas-phase complexes formed between phosphonate cavitands and ethyl-substituted ammonium ions.     

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.     

Electron probe microanalysis of chemical gradients in gas pressure sintered silicon nitride

During gas pressure sintering of silicon nitride (Si₃N₄) — which normally contains oxide additives such as SiO₂, Al₂O₃ and Y₂O₃ — in a resistance heated graphite furnace, a reduction of the Si₃N₄ sample takes place. At high temperatures (>1800°C) this effect is accompanied by decomposition reactions of Si₃N₄. Both lead to chemical gradients in larger components which influence the strength of the sintered article. Electron probe microanalysis (EPMA) has been carried out in order to study the influence of the crucible material [graphite (C), boron nitride (BN)] and the quantity of filling on the gradient formation.     

Efficient removal of thallium from flow gas using manganite‐based catalysts by gas–solid‐phase catalytic oxidation at low temperature

Thallium (Tl) is a highly toxic heavy metal, for which its removal from aqueous environments has gained increasing attention. However, removal of Tl from the gas phase is yet to be addressed, especially at low temperature. Herein, we synthesized and characterized a manganite‐based active carbon (Mn–AC) catalyst via an impregnation method process. The Mn–Ac catalyst was then used for elemental Tl removal from simulated flow gas in a laboratory‐scale fixed‐bed reactor. Tl(I) was adsorbed by the AC and oxidized by active Mn, resulting in Tl(III). In addition, experiments were performed at varying temperatures and different Mn dosages to evaluate the efficiency of Tl removal in various conditions. The Mn–AC catalysts were shown to oxidize Tl(I) to Tl(III) on their surface with Tl removal efficiency decreasing with increasing temperature, reaching a removal rate of 90% over 8 h at 25°C. Further, increasing Mn content led to a decrease in surface area and a convolution of the AC morphology. Mn(10%)–AC was shown to have the highest gas‐phase Tl removal efficiency (≥90%, over 8 h).     

Properties of combustion waves in a model with competitive exothermic reactions

We study the properties of travelling combustion waves in a diffusional thermal model with a two-step competitive exothermic-exothermic reaction mechanism. This investigation considers the system in one spatial dimension under adiabatic conditions. Based on the notion of the crossover temperature, the model is examined analytically using the activation energy asymptotic method to predict travelling combustion wave behaviour in the limit of large activation energies. The model is then studied numerically using a shooting-relaxation method over a wide range of parameter values, such as those describing the ratios of enthalpies, pre-exponential factors and activation energies. It is demonstrated that the flame speed as a function of these parameters is a single-valued monotonic function and there are two flame regimes identified—each region representing parameter values when one reaction dominates the other.     

Carbonylative Heck reactions using CO generated ex situ in a two-chamber system

A carbonylative Heck reaction of aryl iodides and styrene derivatives employing a two-chamber system using a stable, crystalline, and nontransition metal based carbon monoxide source is reported. By applying near-stoichiometric amounts of the carbon monoxide precursor, an effective exploitation of the hazardous CO gas is obtained affording chalcone derivatives in good yields. Application to isotope labeling, incorporating (13)CO, was further established.     

The use of high pressure DTA,heat flow and adiabatic calorimetry to study exothermic reactions

Cellulose, a commonly found carbohydrate, exhibits exothermic decomposition, when heated above 180C. These reactions have been detected and quantified by a range of thermal analysis and calorimetric techniques giving complementary information. The presented results are of interest for process safety.

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Zusammenfassung Cellulose, ein weit verbreitetes Kohlenhydrat, zeigt beim ErwÄrmen auf Temperaturen oberhalb 180C exotherme Zersetzung. Die damit verbundenen Reaktionen wurden mittels einer Reihe komplementÄrer thermoanalytischer und kalorimertrischer Methoden erfasst und quantifiziert. Die vorgestellten Resultate sind für die AbschÄtzung von Prozessrisiken bedeutungsvoll.

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The authors gratefully acknowledge many discussions with C. Foetisch (Linor, Orbe, CH), Prof. R. C. Reid (Lexington, Massachusetts, USA) and I. Horman (Nestlé Research Centre, Vers-chez-les-Blanc, CH).     

Ab initio kinetics of gas phase decomposition reactions

The thermal and kinetic aspects of gas phase decomposition reactions can be extremely complex due to a large number of parameters, a variety of possible intermediates, and an overlap in thermal decomposition traces. The experimental determination of the activation energies is particularly difficult when several possible reaction pathways coexist in the thermal decomposition. Ab initio calculations intended to provide an interpretation of the experiment are often of little help if they produce only the activation barriers and ignore the kinetics of the decomposition process. To overcome this ambiguity, a theoretical study of a complete picture of gas phase thermo-decomposition, including reaction energies, activation barriers, and reaction rates, is illustrated with the example of the β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) molecule by means of quantum-chemical calculations. We study three types of major decomposition reactions characteristic of nitramines: the HONO elimination, the NONO rearrangement, and the N-NO(2) homolysis. The reaction rates were determined using the conventional transition state theory for the HONO and NONO decompositions and the variational transition state theory for the N-NO(2) homolysis. Our calculations show that the HMX decomposition process is more complex than it was previously believed to be and is defined by a combination of reactions at any given temperature. At all temperatures, the direct N-NO(2) homolysis prevails with the activation barrier at 38.1 kcal/mol. The nitro-nitrite isomerization and the HONO elimination, with the activation barriers at 46.3 and 39.4 kcal/mol, respectively, are slow reactions at all temperatures. The obtained conclusions provide a consistent interpretation for the reported experimental data.     

Non-isothermal two-membrane reactors for reversible gas phase reactions

The performance of a non-isothermal two-membrane reactor for reversible chemical reactions in gas phase has been analyzed by numerical simulation. The analyzed reactions were of the form: aA = bB + cC. Two membranes, that are permeable to all the components of the reaction mixture, are supposed to be the most permeable to one of the two reaction products, satisfying the condition of reverse products permselectivities. The reactant is taken to be the slowest permeating component. A negative temperature influence on the permeabilities of components has been assumed. Co-current plug flow pattern has been accepted. It has been shown that it is possible to enhance reactant conversion above that of a conventional reactor for both endothermic and exothermic reversible reactions, including adiabatic and non-adiabatic case. By using a two-membrane reactor, considerable lowering of feed temperatures is enabled for an endothermic reaction. For endothermic reactions, there is the optimum feed temperature, whereas for exothermic reactions, the higher the temperature, the lower is the attained conversion. In reactor design, the optimal external heat exchange for both endothermic and exothermic reactions can be determinated.     

Elimination reactions of ions in the gas phase

The loss of water from the molecular ion of 2-adamantanol was investigated using specifically labelled deuterium derivatives, and, in particular that stereospecifically labelled in position 4. Water is lost predominantly in a stereospecific 1, 3 fashion by two clearly distinguishable mechanisms. Determination of metastable ion characteristics proved to be essential for drawing this distinction.     

Michael reactions carried out using a bench-top flow system

The Michael reaction between methyl 1-oxoindan-2-carboxylate and methyl vinyl ketone was achieved successfully by pumping solutions of the reactants in toluene through a fluid bed of Amberlyst A21 at 50 degrees C. The use of a fluid bed reactor is attractive as it allows gel-type beads, i.e. the type of bead used in most studies of polymer-supported (PS) organic reactions, to be used satisfactorily in a flow system. When polymer-supported cinchonidine was used in place of Amberlyst A21, the Michael product was obtained in high yield with an enantiomeric excess (ee) of 51%. This % ee is comparable to that achieved when the reaction was catalysed by cinchonidine itself.     

Cycloaddition reactions of 2,3-didehydrothiophene generated by flow vacuum thermolysis of thiophene-2,3-dicarboxylic anhydride

Flow vacuum thermolysis (FVT) of thiophene-2,3-dicarboxylic anhydride ($\text{2}$) in the presence of 2,3-dimethylbutadiene ($\text{6}$) gives, in addition to 5,6-dimethylthianaphthene ($\text{9}$). small quantities of a dihydrodimethylthianaphthene ($\text{12}$) and another dimethylthianaphthene ($\text{13}$) which is probably also formed by dehydrogenation of $\text{12}$ with chloranil. The partial structures of these minor products are consistent with their being formed by a [2+2]-cycloaddition between $\text{6}$ and an intermediate aryne, 2,3-didehydrothiophene ($\text{1}$), followed by a rearrangement of the resulting adduct $\text{11}$ and dehydrogenation. FVT of $\text{2}$ in the presence of 2,5- ($\text{17b}$) or 3,4-dimethylthiophene ($\text{17c}$) also gave a mixture of the dimethylthianaphthenes ($\text{18}$$\text{22}$, $\text{23}$) which can be rationalized as arising by a [4+2]- and two [2+2]-cycloadditions of the aryne $\text{1}$ to the thiophenes $\text{17}$ with subsequent desulfurization. The lack of equilibration of the products $\text{18}$, $\text{22}$ and $\text{23}$, was demonstrated and their origin as a function of the structure and reactivity of the aryne $\text{1}$ discussed.     

Direct catalytic aldol-type reactions using RCH2CN

[reaction: see text] A copper fluoride-catalyzed cyanomethylation that can be applied to a wide range of ketones and aldehydes was developed using TMSCH(2)CN as a nucleophile. The reaction was extended to a conceptually more advanced copper alkoxide-catalyzed direct addition of alkylnitriles to aldehydes, which can act as a surrogate direct catalytic aldol reaction of esters. These reactions can be applied to the first catalytic enantioselective cyanomethylation of ketones and direct catalytic enantioselective cyanomethylation of aldehydes.