Microthermogravimetric studies of the kinetics and thermodynamics of heterogeneous reactions in sulfur-containing atmospheres

A thermobalance for studies of the high-temperature sulfidation of metals and alloys is described. This apparatus permits the determination of mass changes in the sulfidized sample as functions of temperature and sulfur vapour pressure with an accuracy of 10^–6 g. Besides heterogeneous kinetics measurements, it has been shown that the chemical diffusion coefficients and deviations from stoichiometry of metal sulfides can also be studied as functions of temperature and sulfur pressure by means of equilibration measurements.

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Zusammenfassung Es wird eine Thermowaage zur Untersuchung der Hochtemperatur-Sulfidierung von Metallen und Legierungen beschrieben. Diese Apparatur ermöglicht die Bestimmung von MassenverÄnderungen der sulfidierten Proben in AbhÄngigkeit von der Temperatur und dem Schwefeldampfdruck mit einer Genauigkeit von 10^–6 g. Es wird gezeigt, da\ damit, sofern Gleichgewichtsmessungen ausgeführt werden, auch die chemischen Diffusionskoeffizienten und Abweichungen der Metallsulfide von der Stöchiometrie in AbhÄngigkeit von der Temperatur und dem Schwefeldampfdruck untersucht werden können.

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A versatile new catalyst for the enantioselective isomerization of allylic alcohols to aldehydes: scope and mechanistic studies.

A new planar-chiral bidentate phosphaferrocene ligand (2) has been synthesized and structurally characterized. The derived rhodium complex, [Rh(cod)(2)]BF(4), serves as an effective catalyst for asymmetric isomerizations of allylic alcohols to aldehydes, furnishing improved yields, scope, and enantioselectivities relative to previously reported methods. The catalyst is air-stable and can be recovered at the end of the reaction. Mechanistic studies establish that the isomerization proceeds via an intramolecular 1,3-hydrogen migration and that the catalyst differentiates between the enantiotopic C1 hydrogens.     

Biosorption of Cr(VI) ions from aqueous solutions: kinetics, equilibrium, thermodynamics and desorption studies

Cr(VI) is a major water pollutant from industrial effluent whose concentration is to be reduced within the permissible limit. Present study reports a systematic evaluation of six different natural adsorbents for the removal of Cr(VI) from aqueous solutions in batch process. The adsorption kinetic data were best described by pseudo-second order model. The values of mass transfer coefficient for Cr(VI) adsorption indicated that the velocity of the adsorbate transport from the bulk to the solid phase was quite fast. The effective diffusivity of Cr(VI) removal for all the adsorbents were of the order of 10(-10) m(2)/s which suggested chemisorption of the process. The adsorption process was jointly controlled by film diffusion and intraparticle diffusion. Maximum monolayer adsorption capacities onto the natural adsorbents used were comparable to the other natural adsorbents used by other researchers. The thermodynamic studies and sorption energy calculation using Dubinin-Radushkevich isotherm model indicated that the adsorption processes were endothermic and chemical in nature. FT-IR studies were carried out to understand the type of functional groups responsible for Cr(VI) binding process. Desorption study was carried out with different concentration of NaOH solutions. Application study was carried out using electroplating industrial wastewater.     

Adsorption of toluene and acetone vapors on microwave-prepared activated carbon from agricultural residues: isotherms,kinetics, and thermodynamics studies

Research on Chemical Intermediates - Activated carbons produced with agricultural residues have attracted substantial attention in recent years. However, much work remains to be done. The current...     

Addition of hetero allenyl copper reagents to aldehydes: scope and behavior

Cyanocuprates derived from propargylic amines or ethers react with aldehydes to give regioselectively the corresponding anti-homopropargylic alcohols with a high level of diastereoselectivity. Such selectivity could be obtained independently of the nature of the heteroatom (amine or ethers) or the acetylenic substituents. Excellent selectivities can be reached regardless of the aldehydes used, remarkably also with vinylic or acetylenic ones. A reactivity scale for the cuprates bearing different acetylenic substituents was established to be: SiMe3 >Ph> Et. The rate of the addition reaction to aldehydes was also found to be slowed down in the presence of HMPA underlining the crucial role of the Li counterion. DFT calculations have shown that the relationship between the rate and the acetylenic substituent is not connected to a possible metallotropic equilibrium but to the stability of the reactive allenic species compared to the less-stable propargylic isomer.     

Organocatalytic asymmetric direct phosphonylation of alpha,beta-unsaturated aldehydes: mechanism, scope, and application in synthesis

The first direct organocatalytic enantioselective phosphonylation of alpha,beta-unsaturated aldehydes with phosphite, in combination with a Br?nsted acid and a nucleophile, is presented. Mechanistic investigations have revealed that the first step in the catalytic process, after the formation of the iminium intermediate, is the addition of phosphite to the beta-carbon atom, leading to the phosphonium ion-enamine intermediate. The rate-determining step for the reaction is the transformation of P(III) to P(V), which occurs via a nucleophilic SN2-type dealkylation, and a screening of various nucleophiles shows that soft nucleophiles in combination with a Br?nsted acid improve the reaction rate and enantioselectivity. The reaction conditions developed show that the use of 2-[bis(3,5-bistrifluoromethylphenyl)trimethylsilanoxymethyl]pyrrolidine as the catalyst and tri-iso-propyl phosphite as the phosphonylation reagent, in the presence of stoichiometric amount of benzoic acid and sodium iodide, gave the beta-phosphonylation of aromatic and aliphatic alpha,beta-unsaturated aldehydes in good yields and enantioselectivities. The products formed by this new reaction have been used for the synthesis of a number of biologically important compounds, such as optically active hydroxyl phosphonate esters, phosphonic acids, and especially glutamic acid and fosmidomycin precursors, of which the two latter are showing important properties for the treatment of central nervous system diseases and as anti-malarial compounds, respectively. DFT calculations have been applied to explain the approach of the phosphite to the reactive carbon atom in the iminium intermediate in order to account for the observed absolute enantioselectivity in the reaction.     

The kinetics of dimerization of chlorotrifluoroethene

Rate constants determined for the thermal dimerization of chlorotrifluoroethene to dichlorohexafluorocyclobutane at 404–672 K have been correlated with previously published results. For the temperature range of 404–800 K, The dependence of the preexponential factor on temperature corresponds to a value of 17 J/K·mol for the difference in heat capacity (ΔC_p^±) between the activation complex and the reactant.     

Removal of nickel ions by adsorption on nano-bentonite: Equilibrium,kinetics, and thermodynamics

Nano-bentonite was used as an adsorbent to remove nickel ions from aqueous solutions. Experimental investigation was carried out to identify the effect of pH, contact time, initial concentration, and adsorbent dose of Ni(II). Equilibrium data were described by and fitted to Langmuir, Freundlich, and Dubinin–Radushkevich models. Results showed that the optimum conditions for the removal of the Ni(II) are initial concentration 100 mg/L, adsorbent dose 0.5 g, and pH 6. Surface morphology and functionality of nano-bentonite were characterized by SEM and FTIR. The kinetics data were more accurately described by pseudo-second-order model. The intra-particle diffusion model gave multi-linear curves, so more than one step controlled the adsorption process. Nano-bentonite removed nickel with maximum adsorption capacity of 39.06 mg/g (30°C, pH) and thermodynamic data indicated that adsorption reaction is spontaneous and of an endothermic nature.     

Tautomerism of pyrimidyl-2-methane derivatives: medium effects,thermodynamics, kinetics and mechanism

The Investigation of tautomeric equilibria among pyrimidyl-2-methanes has been undertaken. Pyrimidyl-pyrimidyledene equilibrium has been found in the case of 5-substituted 2-pyrimidyl-cyanoacetic esters. Unsymmetrically substituted pyrimidyl-2-methanes form two types (1-NH and 3-NH) of ylidene tautomers, which differ in ring proton spin coupling constants. The effect of substituents in the pyrimidine ring and of the solvent on tautomeric equilibrium are discussed. A drastic effect is produced by acids (CF₃ COOH) which shift the equilibrium toward the ylidene tautomer of higher basicity. The thermodynamic and kinetic characteristics and the mechanism of the tautomeric equilibrium were examined.     

The kinetics of photosensitized thymine dimerization

Since thymine dimerization is the main photochemical lesion occuring in uv irradiated DNA, an understanding of the mechanism of dimerization is biologically significant. Both photosensitized and direct dimerization are important in DNA, but because photosensitized thymine dimerization has been less thoroughly investigated, this has been the major topic of study in this laboratory. By comparing experimental results with those obtained by computer simulation, attempts have been made to deduce mechanisms for photosensitization by acetone, acetophenone, and benzophenone. Photolysis of photosensitized solutions was performed using a xenon lamp and quantitative detection of dimer was achieved using h.p.l.c. techniques. A program designed to solve differential rate equations was used for the computer simulation of reaction mechanisms. Based on the results obtained it has been confirmed that acetone photosensitization over the entire range of thymine concentration considered (10^?4 mol dm^?3 to 10^?2 mol dm^?3) proceeds via diffusion controlled triplet transfer from the photosensitizer to thymine, followed by bimolecular collision of ground and excited state thymine monomers. For acetophenone and benzophenone photosensitization this method applied at low thymine concentrations, but at higher concentrations (<10^?3 mol dm^?3) predicted far lower yields than those observed experimentally. The effect of thymine base stacking was then considered, but it was found that this did not significantly increase the dimer yield. A mechanism involving association of thymine and photosensitizer molecules was therefore proposed. This mechanism was found to give reasonably good agreement between experimental and computed data. On the basis of present data the authors regard this as the most likely mechanism for thymine dimerization and work is in progress to confirm this proposal.     

Theoretical studies on the thermodynamics and kinetics of one-pot synthesis of aromatic PCP and PNP pincer ligands

In the present study, the kinetics and thermodynamics of the successful one-pot synthesis of arene-based PCP/PNP pincer ligands by less nucleophilic secondary halophosphines (mono/bis(phosphinomethyl)benzene/pyridine) via S_N2 reactions, which have been recently developed by Shih and Ozerov (Organometallics 34:4591, 35), have been studied. By utilizing different substituents on the target molecules, the impact of such modifications has been investigated both in gas phase and in solution. Performed calculations showed clear dependence of the activation energy and reaction rates on the presence of lithium in the structure. Additionally, the nature of the leaving group was observed to influence the favorability of the reaction. Similarly, the type of the halogen on both substrate and nucleophile had impact on the reaction rates, with the use of iodide leading to the observation of the fastest reaction rates. The solvent used was also noted to exert considerable influence on the thermodynamics and kinetics of the reactions. However, the R group in the nucleophile (XPR₂) showed no significant inductive effect on the activation energy, but a slight steric effect could be observed.     

Comparative pyrolysis upcycling of polystyrene waste: thermodynamics, kinetics, and product evolution profile

Pyrolysis is one important way to treat polystyrene waste and upcycle it into useful materials. A comparative pyrolysis study of virgin polystyrene (VPS) and two types of commonly used polystyrene products, expanded polystyrene (EPS) and polystyrene container (CPS) was carried out. Various values were found in the thermodynamic study and kinetic study of VPS, EPS, and CPS pyrolysis, suggesting distinct thermal degradation characteristics of these materials. The energy barrier order of the pyrolysis processes was EPS, CPS, VPS, showing activation energy of 230, 219, and 145 kJ mol^?1, respectively. The order of amount of heat absorbed was EPS, CPS, VPS, with enthalpy of 224, 213, and 139 kJ mol^?1, respectively. The reaction favorability order was EPS, CPS, and VPS with Gibbs free energy of 118, 132, and 210 kJ mol^?1, respectively. Thermogravimetric analysis indicated the use of high heating rate would increase the reaction rate and shorten the reaction time. Product evolution profiles showed that VPS and CPS pyrolysis produced mainly aromatics, while EPS pyrolysis produced aromatics at the initial phase of the reaction and aliphatic hydrocarbon at the latter phase. The diverse pyrolysis behaviors of VPS, EPS, and CPS demonstrated that an examination on different polystyrene materials was desired to optimize the pyrolysis conditions and product distribution, and thus benefit the process of valuable materials recovery.     

Estimating the thermodynamics and kinetics of chlorinated hydrocarbon degradation

Many different degradation reactions of chlorinated hydrocarbons are possible in natural groundwaters. In order to identify which degradation reactions are important, a large number of possible reaction pathways must be sorted out. Recent advances in ab initio electronic structure methods have the potential to help identify relevant environmental degradation reactions by characterizing the thermodynamic properties of all relevant contaminant species and intermediates for which experimental data are usually not available, as well as provide activation energies for relevant pathways. In this paper, strategies based on ab initio electronic structure methods for estimating thermochemical and kinetic properties of reactions with chlorinated hydrocarbons are presented. Particular emphasis is placed on strategies that are computationally fast and can be used for large organochlorine compounds such as 4,4′-DDT     

Alpha,beta-unsaturated delta-lactones from copper-catalyzed asymmetric vinylogous Mukaiyama reactions of aldehydes: scope and mechanistic insights

A direct regio-, diastereo-, and enantiocontrolled access to alpha,beta-unsaturated delta-lactones is described, based on the reaction of a silyl dienolate and an aldehyde in the presence of 10 % of Carreira's catalyst. The scope and limitations of this reaction, as well as mechanistic insights concerning the reactivity of an allyl copper species, are discussed.     

Iridium-catalyzed dehydrogenation of substituted amine boranes: kinetics, thermodynamics, and implications for hydrogen storage

Dehydrogenation of amine boranes is catalyzed efficiently by the iridium pincer complex (kappa (3)-1,3-(OP ( t )Bu 2) 2C 6H 3)Ir(H) 2 ( 1). With CH 3NH 2BH 3 (MeAB) and with AB/MeAB mixtures (AB = NH 3BH 3), the rapid release of 1 equiv of H 2 is observed to yield soluble oligomeric products at rates similar to those previously reported for the dehydrogenation of AB catalyzed by 1. Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry. The experimental heats of reaction are compared to results from computational studies.     

Receptor-assisted combinatorial chemistry: thermodynamics and kinetics in drug discovery

Current drug discovery using combinatorial chemistry involves synthesis followed by screening, but emerging methods involve receptor-assistance to combine these steps. Adding stoichiometric amounts of receptor during library synthesis alters the kinetics or thermodynamics of the synthesis in a way that identifies the best-binding library members. Three main methods have emerged thus far in receptor-assisted combinatorial chemistry: dynamic combinatorial libraries, receptor-accelerated synthesis, and a new method, pseudo-dynamic libraries. Pseudo-dynamic libraries apply both thermodynamics and kinetics to amplify library members to easily observable levels, and attain selectivity heretofore unseen in receptor-assisted systems.     

Aluminum hydride for solid-state hydrogen storage:Structure,synthesis, thermodynamics,kinetics, and regeneration

Aluminum hydride(AlH3) is a binary metal hydride that contains more than 10.1 wt% of hydrogen and possesses a high volumetric hydrogen density of 148 kg H2 m^-3.Pristine AlH3 can readily release hydrogen at a moderate temperature below 200℃.Such high hydrogen density and low desorption temperature make AlH3 one of most promising hydrogen storage media for mobile application.This review covers the research activity on the structures,synthesis,decomposition thermodynamics and kinetics,regeneration and application validation of AlH3 over the past decades.Finally,the future research directions of AlH3 as a hydrogen storage material will be revealed.     

Temperature dependence of the thermodynamics and kinetics of micellar solutions

We predict theoretically the thermodynamics and relaxation kinetics of solutions of cylindrical branched micelles. Using a recently developed theory in combination with the experimental data, we explain the unusual, inverted temperature dependence of the phase separation observed in wormlike micelles and dilute microemulsions. We extend the model to treat the temperature dependence of the relaxation kinetics and explain the observations.