Pure Reactants Research Articles

After 118 years, the isolation of two common radical anion reductants as simple, stable solids

Two common radical anion reductants, potassium benzophenone ketyl (K(Ph(2)CO)) and potassium naphthalenide (K(2)(C(10)H(8))(2)(THF)), have been isolated and characterized for the first time in solvent-free form or with low solvent content, allowing their use as pure solid reactants in preparative redox chemistry in accurate stoichiometric amounts.

Diesel fuel jet lift-off stabilization in the presence of laser-induced plasma ignition

The mechanisms affecting lift-off stabilization at diesel conditions were investigated by laser-igniting a diesel fuel jet upstream of its natural lift-off position. Single-nozzle fuel sprays penetrating into an optically accessible constant-volume chamber were ignited using laser-induced plasma formation both prior to natural autoignition or after a quasi-steady lift-off length was established. Fuel sprays ignited readily, with reaction kernels growing in connected regions. After laser-ignition, the lift-off persists upstream of the natural lift-off position for a substantial period of time indicating that upstream ignition has a strong influence on lift-off stabilization. While not discounting the role of flame propagation downstream of the ignition event, these results show that upstream ignition sites can start a chain of events that effectively controls lift-off. Lift-off eventually returns to its natural position, but only after injection times that are too long for practical engines. The time of return to the natural position depends upon the relative distance of the laser-ignition site to the natural lift-off length. A theory for fuel jet lift-off stabilization based on flame propagation into pure fuel-ambient reactant streams fails to predict the long upstream stabilization away from the natural lift-off length because turbulent velocities are higher in upstream regions of the fuel jet. Likewise, upstream lift-off stabilization by autoignition of pure reactants (no mixing with combustion products) fails because of cooler temperatures and shorter residence times. A potential mechanism explaining the transient lift-off response to laser-ignition is offered based on turbulent mixing with high-temperature combustion products found at the jet edges.

Monitoring structural transformations in crystals. 12. Course of an intramolecular [4 + 4] photocycloaddition in a crystal

Variations in crystal and molecular structures, brought about by the intramolecular [4 + 4] photocycloaddition of bi(anthracene-9,10-dimethylene), were monitored using X-ray diffraction. The cell volume increased by 0.8% until the reaction was ca 40% complete, and afterwards decreased by 1.6% during the remainder of the photoreaction. The changes of the a and b lattice parameters were correlated with the changes of the molecular shape and packing. The distance between the directly reacting C atoms varied in a manner not observed for other photochemical reactions in crystals. It was constant until ca 20% photoreaction progress, then decreased, and later stabilized from ca 40% photoreaction progress. This phenomenon was explained by interplay between stress resulting from the presence of product molecules and the rigidity of reactant molecules. Changes of the orientation of molecules during the photoreaction were smaller than in the case of other monitored photochemical reactions in crystals owing to similarities in the shape and packing of reactant and product molecules. Weak C-H...pi hydrogen bonds exist among reactant molecules in the pure reactant and partly reacted crystals.

Jasminaldehyde Synthesis Catalyzed by Al(Ga)PON Oxynitrides: Influence of the Reaction Conditions

Nitridation of amorphous aluminium phosphate (AlPO4) and mixed aluminium gallium phosphate (Al0.5Ga0.5PO4) under ammonia flow allows preparing “AlPON” and “AlGaPON” bifunctional acid-base heterogeneous catalysts. Their acid-base bi-functional character allows their use as selective catalysts for the production of jasminaldehyde through the cross-condensation of heptanal and benzaldehyde, followed by the dehydration of the aldol intermediate product. Three parameters enabling to maximize the jasminaldehyde yields in a batch reactor operated at 125°C were investigated: the catalyst composition (Al/Ga and O/N ratio), the reactant concentration and the amount of catalyst in the reactor. Maximum jasminaldehyde selectivities were obtained for intermediate nitrogen contents (7-10 wt.%). Maximum reaction rates were obtained using pure reactants and a catalysts weight equal to 10% of the total reagent weight.

Char gasification in mixtures of CO 2 and H 2O: Competition and inhibition

Our understanding of char gasification reaction kinetics at high pressures is generally limited to analyses of data generated using pure reactants, or reactants diluted with inert gases. This paper presents data generated for the reaction of coal chars with mixtures of CO 2 and H 2O at high pressures, to determine how existing pure-gas rate data can be applied to more realistic gasification systems. The data presented here show that the rate of reaction in a mixture of CO 2 and H 2O is not the sum of the two pure-gas reaction rates; rather, it is a complex combination of the two that appears to be dependant on the blocking of reaction sites by the relatively slow C–CO 2 reaction. Analysis of the results using a Langmuir–Hinshelwood (LH) style of rate equation with the assumption that both reactions compete for the same active surface, produces a model that explains the experimental data. This model allows reaction rates in mixtures of CO 2 and H 2O to be estimated based on existing, measured pure-gas reactivity data or from ‘first principles’ using knowledge of the rate constants for the LH equation.

MnCl 2 modified H 4SiW 12O 40/SiO 2 catalysts for catalytic oxidation of dimethy ether to dimethoxymethane

The catalytic oxidation of dimethy ether (DME) to dimethoxymethane (DMM) was carried out over MnCl 2 modified H 4SiW 12O 40/SiO 2 catalysts prepared by impregnation method in a continuous flow type fixed-bed reactor with ratio of n DME / n O 2 = 1 : 1 at 593 K and 0.1 MPa. The effect of calcination temperatures, different impregnation sequences, reaction time and different feeding modes over MnCl 2(5 wt%)–H 4SiW 12O 40/SiO 2 catalyst were all investigated. The results show that the catalyst, which was prepared by first H 4SiW 12O 40 then MnCl 2 impregnated in SiO 2 and calcined at 673 K demonstrates the optimum activity with 8.6% of DME conversion and 39.1% of DMM selectivity. The pure DME reactants form higher selectivity of DMM than pure CH 3OH reactants. The BET surface area, total pore volume, X-ray diffraction (XRD), NH 3-TPD, TEM, FT-IR, XPS and ICP-AES were used to characterize the structure and performance of the catalysts deeply. XPS shows that Mn 4+ species of the fresh catalyst play important role and may be reduced to Mn 2+ or Mn 3+ species in the synthesis of DMM from DME oxidation reaction. Besides, the amount of superficial Mn declines by 15.5% after the reaction. ICP-AES indicates that the total amount of Mn after the reaction decreases by only 4.9% than that of the fresh sample. XRD patterns of the catalysts calcined at different temperatures show that the Keggin structure of the catalysts are destroyed when the temperature exceeds 673 K, Besides, NH 3-TPD profiles also indicate that the acidity of the catalyst becomes weaker with the calcination temperature increased. Compared to the fresh catalyst, the acidity of the catalyst after the reaction is also weakened. It is found that Mn modified H 4SiW 12O 40/SiO 2 improves the catalyst oxidative activity and moderates the acidity of the catalyst. The mechanism of this reaction may be that DMM can be synthesized via methoxy groups formed by DME dissociation under the cooperation of the acid sites and redox sites of Mn modified H 4SiW 12O 40/SiO 2 catalysts.

Isothermal activation of Mo2O52+–ZSM-5 precursors during methane reactions: effects of reaction products on structural evolution and catalytic properties

The dynamics of carburization of Mo-oxo precursors exchanged onto H-ZSM-5 strongly influence initial induction periods and steady-state rates during catalytic pyrolysis of CH4 to alkenes and arenes at 900-1000 K. The effects of co-reactants and of activating conditions were examined by on-line time-resolved mass spectrometric analysis of effluent streams using rigorous analyses to account for equilibrium effects on measured rates. Ethene co-reactants and the larger hydrocarbons to which it converts on acid sites in H-ZSM-5 led to much faster carburization of exchanged (Mo2O5)(5+) dimers and to shorter induction periods than with pure CH4 reactants, but steady-state pyrolysis rates were unchanged, indicating that CH4 and C2H4 form similar MoCx clusters during carburization of exchanged Mo-oxo precursors. H2 treatment at 973 K before CH4 reactions led to reduction of Mo(6+) species to Mo(4+), which carburize faster than (Mo2O5)(5+) precursors during initial contact with CH4. This H2 pretreatment or the use of CH4-H2 reactant mixtures did not influence steady-state pyrolysis rates, once contributions from reverse reactions were taken into account. With pure CH4 streams, (Mo2O5)(5+)-ZSM-5 converts to active MoCx clusters within zeolite channels via autocatalytic processes, in which higher hydrocarbons, initially formed during initial conversion of MoOx to MoCx structures, lead to faster carburization of downstream catalyst sections. Concurrently, H2O and CO2 formed during this incipient carburization of exchanged (Mo2O5)(5+) and unexchanged MoO3 present in trace amounts inhibit and even prevent carburization and lengthen activation periods. Activation protocols with C2H4 were also successful in the activation of more refractory high-valent metal-oxo species, such as WOx and VOx, exchanged onto H-ZSM-5. The formation of active carbide structures occurred in less than 300 s, instead of 4 ks and 16 ks for VOx and WOx samples, respectively, in pure CH4 reactants. These activation protocols led to VCx-ZSM5 catalysts about three times more active than those activated in pure CH4 reactants.

Long-term operation of AFC electrodes with CO2 containing gases

Low temperature fuel cells represent an important component in pollution-free energy conversion for mobile application. Alkaline fuel cells are an interesting alternative for polymer electrolyte membrane fuel cells (PEFC), because they do not need expensive membranes and catalysts. It is commonly accepted, that carbon dioxide intolerance is the most pronounced disadvantage of air breathing alkaline fuel cells. Thus, alkaline technology is commonly not considered for mobile fuel cell application. In this study, the long-term influence of carbon dioxide containing reaction gases on electrodes manufactured by a rolling process were investigated. To investigate the mechanisms of carbon dioxide poisoning, gas diffusion electrodes (GDEs) which were designed for alkaline fuel cells had been operated in a half cell configuration up to several thousand of hours with a constant load of 150 mA/cm2. The reactants were contaminated with 5% CO2. The experiments were performed with both electrodes, anode and cathode. Control measurements were performed by operating electrodes with pure reactants. During these long-term experiments, the V–I curves were recorded daily. Recently also the chemical surface compositions of the gas diffusion electrodes were investigated by X-ray-induced photoelectron spectroscopy (XPS) after long-term operation. In addition to the characterization of the electrode, the carbonate concentration in the electrolyte was determined after different operation times. The electrochemical performance of the electrodes, anodes and cathodes, decreases during the operation time in both operation modes, operated with pure gases as well as with CO2 containing gases. The decreases in both modes are comparable; this means the CO2 neither enhances the degradation process nor induces a new, detrimental degradation process. The XPS characterization of used cathodes operated with pure and with CO2 containing oxygen yielded no significant difference. A slightly higher carbon concentration was observed on the surface of the anodes which was operated with CO2 containing hydrogen than on surfaces of anodes which were operated with pure hydrogen. The carbonate concentration in the electrolyte increases during fuel cell operation with CO2 containing gases. The long-term behavior of the investigated alkaline fuel cell (AFC) electrodes supplied with CO2 containing gases gives no evidence that CO2 affects significantly the degradation process.

Mechanisms for the Formation of Isoprostane Endoperoxides from Arachidonic Acid: “DIOXETANE” INTERMEDIATE VERSUSऔ-FRAGMENTATION OF PEROXYL RADICALS

The isoprostanes are a class of autoxidation products generated from arachidonic acid (or its esters) by a free radical initiated process. The potent biological activity of these compounds has been attracting intense research interest since they were detected in humans as well as animal models in the early 1990s. The measurement of these compounds has been regarded as one of the most useful non-invasive biomarkers for oxidative stress status. Two mechanisms for the formation of these compounds have been proposed. In the first mechanism, a peroxyl radical undergoes successive 5-exo cyclizations analogous to the enzymatic mechanism proposed for prostaglandin biosynthesis. The second mechanism starts with a 4-exo cyclization of a peroxyl radical leading to an intermediate dioxetane, a mechanism that has also been proposed for prostaglandin biosynthesis as well as for the formation of 4-hydroxy nonenal (HNE). Autoxidation of cholesteryl-15-HpETE under free radical conditions provides Type IV isoprostanes. The "dioxetane" mechanism for isoprostane generation from 15-HpETE requires that optically pure products are formed from an optically pure reactant, whereas an alternate mechanism for the process involving beta-fragmentation of the 15-peroxyl would give racemic isoprostane products. We have carried out a test of the mechanism based upon these stereochemical requirements. The results of analysis of the product mixture derived from autoxidation of optically pure Ch-15-HpETE by atmospheric pressure chemical ionization-mass spectrometry coupled with chiral high performance liquid chromatography indicate that the major isoprostane diastereomers are formed as a racemic mixture. These experimental results are consistent with a mechanism for isoprostane formation involving beta-fragmentation of the 15-peroxyl radical followed by re-addition of oxygen to form the 11-HPETE peroxyl, and they exclude a mechanism proceeding through the formation of a dioxetane intermediate.

Liquid phase isomerisation of dichlorobenzenes over H-zeolites

Liquid phase isomerisation of dichlorobenzenes (dCBs), i.e. ortho-dichlorobenzene (o-dCB) and/or para-dichlorobenzene (p-dCB) to meta-dichlorobenzene (m-dCB) catalysed by solid acid catalysts could provide an alternative to the traditional process based on AlCl3-type catalysts. The reaction was studied over a series of H-zeolites in a batch autoclave reactor at a temperature of 340°C and a pressure of 40bar, with pure reactants, i.e. without solvent. Under these conditions, H-ZSM-5 and mordenite exhibited the highest activity, while β-zeolite, faujasite and ferrierite showed only low activity. These results are directly related to the pore architecture of the different zeolites, which governs deactivation by coke formation. The nature of the coke formed in the two most active zeolites H-ZSM-5 and mordenite has been analysed by different techniques.

Low-melting glasses based on natural aluminosilicates from Karelia

The effect of the chemico-mineralogical composition of aluminosilicate rocks on the properties and structure of low-melting technical glasses is studied. Comparative studies of glasses based on traditional glass materials such as pegmatite and nepheline syenite and glasses based on nontraditional materials such as halleflinta and pure chemical reactants are performed. It has been established that the use of various natural aluminosilicate in technical glasses does not significantly disturb the glass-forming conditions but makes it necessary to adjust the ratio of the main components and additives.

Isotopic Tracer Studies of Thiophene Desulfurization Reactions Using Hydrogen from Alkanes on H-ZSM5 and Co/H-ZSM5

Reaction pathways for the desulfurization of thiophene using alkanes as hydrogen sources were probed by measuring the chemical and isotopic composition of products formed from 13C-labeled C3H8 and unlabeled C4H4S mixtures on H-ZSM5 and Co/H-ZSM5. Aliphatic hydrocarbons formed only from propane carbon atoms while aromatic molecules contained carbon atoms from both propane and thiophene. Hydrogen-deficient thiophene and thiophene-derived species react with surface hydrogen and alkenes formed from propane and desorb as unreactive aromatics, in steps that lead to the irreversible exit of alkenes from oligomerization-cracking cycles. These steps remove kinetic bottlenecks in thiophene desulfurization resulting from the irreversible formation of strongly adsorbed unsaturated species, which cannot desorb without reactions with hydrogen or hydrogen-rich surface species. This kinetic coupling between alkane and thiophene reactions leads to the observed concurrent increase in the rates of both propane aromatization and thiophene desulfurization compared with those achieved with each pure reactant. The scavenging of unsaturated intermediates formed via thiophene decomposition using hydrogen or hydrogen-rich intermediates formed from propane decreases the rate of bimolecular Diels–Alder reactions, which lead to larger organosulfur compounds and to low H2S selectivity. As a result, H2S selectivities are higher and deactivation rates are lower when propane is present as a co-reactant during reactions of thiophene.

Ion beam techniques for functionalization of polymer surfaces

Polymer surface modification often requires a proper control of reactants and surface activation. Ion beam techniques offer, in a rather unique manner, such processing controls. Firstly, unusual chemical species in an ion form can be generated in and extracted from a plasma, thus a large variety of reactants can be studied. The extracted ions can also be readily purified with a mass filter, such that a pure reactant can be delivered to a polymer surface. In addition, the ion fluence can be easily controlled for the prevention of an excessive reactant exposure, an exposure which can lead to undesirable multiple functionalization products. Furthermore, possible only with an ion beam approach is the control of a reactant's kinetic energy to a high enough value for promoting the chemisorption of the reactant, but a low enough value to prevent impact dissociation of the reactant. In an alternative methodology, a surface modification procedure is partitioned into a step for generating an appropriate amount of surface active sites and a subsequent step for supplying a reactant to those sites. The surface activation step can be efficiently performed by surface impact with inert gas atoms with energy of 10–20 eV. The reaction sequence involves electron impact ionization of an inert gas, extraction and acceleration of the ions, ion-neutral collision cascade in gas phase for generating energetic neutrals, and surface activation with the energetic neutral bombardment. Such a procedure can be implemented in a simple and inexpensive reactor for activating a large sheet of polymer in less than a second. For the completion of the surface functionalization process, a well-defined radical reactant can be generated by dissociative adsorption on hot filaments, or by photolysis, and then transported via a gas flow in vacuum to an activated polymer.

Radiolysis of binary systems containing germanium and carbon hydrides

Radiolysis of the mixtures GeH 4/C 2H 6 and GeH 4/C 3H 8 have been studied. The decomposition of the parent molecules leads to solid and volatile products. The decomposition of GeH 4, according to the composition of the mixtures, is 1.7–2.5 times larger than in pure GeH 4. Also and hydrocarbons decompose 1.6–3.4 times more than in systems containing the pure reactants. The formation of the solid, a polymer containing Ge, C and H, has been attributed to a polymerization process initiated by the germylene-like species, while the gaseous products seem to derive from germyl-like species. The radical mechanism seems to prevail on the ionic.

Measuring the Burning Rate of Premixed Turbulent Flames in Stagnation Flows

Abstract A method is proposed to measure the burning rate of planar premixed turbulent flames in stagnation flows. The method is based on the balance of conditional mass fluxes in a control volume that extends from pure reactants to the stagnation plane. The principle is that the difference between the mass of reactants flowing into and out of the control volume is the mass that has reacted within the control volume. Similarly, if only reactants flow into the control volume then the mass flow rate of products out of the control volume constitutes the burning rate in that region of the flame. The measurement techniques necessary to use this approach are all available. Experimental measurements were conducted on the stagnation plate geometry. The burning rate characterized by a leading edge velocity overestimated the value determined from the conditioned mass fluxes by 41% The extension of this approach to other burner geometries and more general flame shapes is discussed and suggestions for its use in rod stabilized flames are made. In this case the shape and orientation of the control volume are set by the maximum gradient in space of the progress variable. For curved flame zones and more general flows the choice of control volume remains ambiguous.

4,6,4′-Trimethylangelicin induces interstrand cross-links in mammalian cell DNA

4,6,4′-Trimethylangelicin, a well-known effective photosensitizer described as a pure monofunctional reactant with DNA, can induce interstrand cross-links in mammalian cell DNA in vivo (about 15% relative to 8-methoxypsoralen), as observed using alkaline elution and Chinese hamster ovary cells. Experiments performed using the two-step irradiation method and HeLa cells support these data. In contrast with 4,6,4′-trimethylangelicin, 4′-methylangelicin and 4,4′-dimethylangelicin do not form interstrand cross-links. These results are consistent with those recently reported by Chen et al. (X. Chen, J. Kagan, F. Dall'Acqua, D. Averbeck and E. Bisagni, J. Photochem. Photobiol. B: Biol., 22 (1994) 51–57) using pBR322 and M13 DNA. The cross-linking ability of 4,6,4′-trimethylangelicin does not seem to be related to a particular feature of these DNAs but to the compound itself.

Theoretical models for the combustion of alloyable materials

The purpose of this work is to extend a theoretical model of layered (laminar) media for SHS combustion presented in an earlier article [1] to explore possible mechanisms for after-burning in SHS (i.e., gasless) combustion. As before, our particular interest is how the microscopic geometry of the solid reactants is reflected in the combustion wave and in the reaction product. The model is constructed from alternating lamina of two pure reactants that interdiffuse exothermically to form a product. Here, the laminar model is extended to contain layers of differing thicknesses. Using asymptotic theory, it was found that under certain conditions, the combustion wave can become “detached,” and an initial thin flame propagates through the media, leaving a slower, thicker flame following behind (i.e., afterburning). Thin laminae are consumed in the initial flame and are thick in the secondary. The thin flame has a width determined by the inverse of the activation energy of diffusion, as found previously. The width of the afterburning zone, however, is determined by the absolute time of diffusion for the thicker laminae. Naturally, when the laminae are all the same thickness, there is only one thin flame. The condition for the appearance of afterburning is found to be contingent on the square of the ratio of smallestto-largest thicknesses being considerably less than unity.

Measurement of various terms in the turbulent kinetic energy balance within a flame and comparison with theory

Direct measurements have been made of four of the six terms in the exact form of the turbulent kinetic energy (TKE) equation within a premixed turbulent flame, in order to understand how the combustion interacts with the turbulence. Results are compared to the predictions of the Bray-Moss-Libby (B-M-L) model. The mean reaction ( w ) also was estimated from the data, using a local control volume concept. The measurements were made possible by the simultaneous use of laser velocimetry and Rayleigh scattering. Increasing the heat release was found to increase the “flame generated” turbulence as well as the “apparent” turbulence and the turbulent flame speed. Part of this flame-generated turbulence is due to vorticity created by the curved flamelets, as evidenced by the unexpected existence of low speed products. The flame also is found to exert an upstream influence that causes an increase in the turbulence of the pure reactants before these reactants ever reach a flamefront. The measured diffusion term in the TKE balance indicates a countergradient diffusion of TKE toward the center of the flame where TKE is maximum; however, this is not true diffusion since the TKE balance includes apparent turbulence. The B-M-L model correctly predicts quantities which are dominated by intermittency due to flame motion such as the diffusion term in the TKE, and fluxes ϱu″c″ and ϱu″ 2c″ which change sign within the flame. This is because the model successfully accounts for intermittency by separating the physics of reactants and products. The model does not successfully predict even the trends associated with the more subtle “true turbulence” associated with products ( u′ p). It is felt that additional work is needed to remove “apparent turbulence” from the TKE balance, and eventually to replace one closure relation in the model to account for vorticity produced by curved flamelets.

An X-ray powder diffraction study of the chloride-bromide systems of trivalent gadolinium, terbium, and ytterbium

The lanthanoid mixed halide systems, MCl 3 MBr 3, for M = Gd, Tb, and Yb, have been prepared by mixing and fusing the pure reactants and have been examined by X-ray powder diffraction procedures. For M = Gd, UCl 3, ( P6 3 m )-, PuBr 3 ( Cmcm)- and AlCl 3 ( C2 m )- type solid solution regions were found. For M = Tb, PuBr 3- and AlCl 3-type solid solution regions were found. For M = Yb, AlCl 3 − and FeCl 3-(R 3)- type solid solution areas were observed. Pure TbBr 3 appears to be isostructural with GdBr 3 rather than with DyBr 3. These systems are discussed and compared to related systems.

CONCENTRATION FORCING IN AMMONIA SYNTHESIS: PLUG-FLOW EXPERIMENTS AT HIGH TEMPERATURE AND PRESSURE

Square-wave concentration forcing experiments have been performed for the ammonia synthesis reaction in an isothermal, fixed-bed, plug-flow reactor at a temperature and pressure approaching industrial conditions (703 K, 4.14Mpa). Using the optimal steady-state (OSS) production rate as a base for comparison, we have found that cycling between pure reactants (N2, H2) results in decreased time-averaged production rates as predicted by Wilson and Rinker (1982), while mixture cycling under appropriate conditions results in modest (but not insignificant) improvements over the OSS rate. We also have observed that the catalyst activity changes significantly under a variety of dynamic operating conditions.