Dehydration of Methanol to Dimethyl Ether by Catalytic Distillation

The kinetics of liquid catalytic dehydration of methanol over an ion exchange resin (Amberlyst 35) has been determined for the temperature range 343 to 403 K using a batch reactor. The experimental data are described well by an Eley‐Rideal type kinetic expression, for which the surface reaction is the rate‐determining step. A catalytic distillation process for methanol dehydration to dimethyl ether (DME) has been modeled using the experimentally determined kinetic data. The results were incorporated into the rate‐controlled reaction mode for RadFrac, a part of the commercial simulation program Aspen Plus. It was shown that synthesis of high purity DME can be achieved using a single catalytic distillation column. Thus there is significant potential for reduction of overall capital cost for a plant for methanol dehydration to DME when compared to conventional production facilities that involve separate reaction and distillation processes.     

Analysis of potentiostatic transients under charge-transfer and diffusion control on the basis of a characteristic differential equation—I. Theoretical treatment and simulated curves

It is shown that the analysis of single-pulse potentiostatic transients for a metal/metal-ion electrode with one charge-transfer step coupled with semi-infinite linear diffusion becomes possible over the entire time range, if a differential equation obtained from the i/t relation is rearranged for linear plotting. At the same time, the method proposed should facilitate the study of electrode reactions with two charge-transfer steps under both charge-transfer and linear-diffusion control and that of a model involving generalized higher-order charge-transfer processes coupled with semi-infinite linear diffusion.Experimental side effects such as charging of the double layer, convection and spherical diffusion on small, unshielded electrodes can fairly be distinguished from the models considered here. The reason for the high degree of sensitivity in the plots lies in the fact that the transformation of the data points includes the derivative di/dt.     

A Kinetic Study on the Catalytic Reductive Methylation of Acetylhydrazine

Catalytic reductive methylation of acetylhydrazine (AH) to dimethyl acetylhydrazine (DMAH) is a key step in the production of unsymmetrical dimethyl hydrazine (UDMH). The results of the present study show that the limiting step in this process is the hydrogenation of AH to mono methyl acetylhydrazine (MMAH). The kinetics of the hydrogenation of acetylhydrazone (AOH) using an activated carbon catalyst impregnated with 5 % palladium were studied in a stirred slurry reactor at 40–65 °C and 5–25 bar hydrogen pressure. Analysis of the data from this investigation indicates that the rate of hydrogenation is consistent with an Eley‐Rideal reaction mechanism. The activation energy for the hydrogenation was determined as 44 kJ/mol.     

Kinetics and mechanism of liquid-phase alkylation of 3-methylthiophene with 2-methyl-2-butene over a solid phosphoric acid

Drastic regulations concerning sulfur content in fuels require the development of new processes in the refineries. In the case of gasoline, AOTS process based on the alkylation of thiophene by butenes was proposed. In the present work, we attempted to describe the mechanism of such a reaction with model molecules. Liquid-phase alkylation of 3-methylthiophene with 2-methyl-2-butene was performed on supported phosphoric acid. It is shown that this reaction is mainly selective to monoalkylation products. The kinetics is following an Eley–Rideal law and the reaction intermediate seems to be an ester of phosphoric acid or polyphosphate.     

Light-induced electron spin polarization in the ground state of water-soluble copper porphyrins

Light-induced spin-polarized transient EPR spectra are reported for several water-soluble copper porphyrins. The spectra are assigned to the doublet ground state, with emissive spin polarization resulting from photoexcitation and subsequent electronic relaxation. In contrast to other systems for which polarization of a doublet ground state has been observed, the exchange interactions in the copper porphyrins are strong and the geometry is fixed. It is proposed that intersystem crossing from the photoexcited trip-doublet to the trip-quartet state can lead to net polarization of the spin system and that this polarization is maintained during electronic decay, possibly via charge-transfer and exciplex states. The intensity of the observed spin polarization is essentially independent of the molecular orientation in the external field, but is strongly dependent on the nature of the charged peripheral groups. Possible reasons for this behavior are discussed.     

Revisiting aromatic thiols effects on radical photopolymerization

Influence of thiols on kinetics of acrylate radical photopolymerization in the presence and absence of photoinitiators was studied. The kinetics was monitored by infrared absorption and photo-differential scanning calorimetry. It was found that aromatic thiols at concentrations, <0.5% (∼0.05 M), can accelerate radical photopolymerization. Initiation of radical photopolymerization by some aromatic thiols in the absence of conventional photoinitiators was observed. As well as expected inhibition of photopolymerization at higher concentration of aromatic thiols due to chain transfer was detected. A ground state charge-transfer complex formation between thiols and benzoin based photoinitiators was detected.Mechanism of thiols participation in photopolymerization involving reduction of oxygen inhibition and dissociation of charge-transfer complexes is discussed.     

Esterification of Pentanoic Acid with 1-Propanol by Sulfonated Cation Exchange Resin: Experimental and Kinetic Studies

Liquid-phase esterification reaction of 1-propanol with pentanoic acid to give propyl pentanoate over Amberlyst 15 has been studied. The reaction carried out in a batch reactor was found to be independent of mass transfer effects in the temperature range of 323.15–363.15 K. The reaction is highly temperature sensitive. Increase in 1-propanol concentration has an enhancing effect on the conversion, whereas water acts as an inhibitor. As alcohol–acid mixtures behave nonideally, kinetic analysis has been performed using UNIFAC liquid-phase activities of pentanoic acid, 1-propanol, propyl ester, and water. Observed kinetics is best represented by Eley–Rideal mechanism. Surface reaction was observed to be a rate-limiting step.     

A DFT Study of Ethylene Hydrogenation Reaction Mechanisms on Ni13 Nanocluster

Metal nanoclusters can potentially exhibit high catalytic activity and selectivity due to not only a high number of exposed surface atoms but also active sites with different coordination numbers as compared to bulk catalyst surface. In this study ethylene hydrogenation was used as a model reaction in an effort to elaborate reactivity of small Ni nanoclusters and the ethylene hydrogenation reaction mechanism. Two mechanistic pathways; representing Eley–Rideal and Horiuti–Polanyi type of mechanism for ethylene hydrogenation reaction are studied. It has been concluded that Horiuti–Polanyi type is more favorable when compared to Eley–Rideal type.     

Ru(II) and Zn(II) complexes of multicomponent ligands incorporating triazine-based tridentate ligands

Ru(II) and Zn(II) complexes of multicomponent ligands have been synthesised and characterised incorporating triazine-based coordinating motifs with pendant phenanthryl and phenyl–phenanthryl groups. At room temperature the ligands emit from intra-ligand charge-transfer (ILCT) states, the energy of which may be lowered significantly by metal–ion coordination (e.g. Zn(II)). The ILCT state is efficiently quenched in the Ru(II) complexes by energy transfer to a low-lying metal–ligand charge-transfer ³MLCT state.     

The mechanism of oxidation of chromium to chromate

The oxidation of chromium to chromate proceeds via six consecutive charge-transfer reactions. The intermediates cover the chromium surface completely with an adsorbed monolayer. In a seventh step chromate is irreversibly desorbed.     

Water effect on the kinetics of 1-pentanol dehydration to di-n-pentyl ether (DNPE) on amberlyst 70

Experiments performed at 433 K and 453 K with 1-pentanol/water and 1-pentanol/DNPE (di-n-pentyl ether) clearly states the inhibiting effect of water released in the liquid-phase dehydration of 1-pentanol to DNPE on ion-exchange resin. The inhibiting effect of water was described by an empirical correction factor in the Eley–Rideal rate model deduced in a previous work. Best results were obtained with a factor including a Freundlich-like adsorption isotherm expression. The activation energy of dehydration reaction lowers slightly on splitting off the water effect, and it is 114.0 ± 0.1 kJ/mol.     

Brownian dynamics simulations of the effects of hydrodynamic interactions on the polymer viscoelastic behavior

Brownian dynamics simulation is used to investigate the viscoelastic response of the Gaussian chain to a step shear deformation in both the linear and non-linear regions under the influence of hydrodynamic interactions. Both the preaveraged Oseen tensor and the Rotne-Prager tensor are used in the study. In the former case, the simulation results are shown to be in agreement with the expected results calculated using the eigenvalues of the normal modes of motion obtained numerically. It is shown that an initial state with zero second normal-stress difference is generated by the step shear deformation. The subsequent rise of the second normal-stress difference as revealed by the simulation is shown mainly arising from the coupling of the recoil of the stretched bond in the direction of deformation and the anisotropy in the hydrodynamic interaction created by the step deformation.     

Quantitative regio-selective Diels–Alder reaction of an unsymmetrical 1,4-dithiin and anthracene through heterogeneous solid state conversion

The solid state, Diels–Alder reaction of an unsymmetrical 1,4-dithiin and anthracene produced an adduct regioselectively via a charge-transfer complex crystal prearranged to undergo a transformation among a constitutional isomeric mixture generated in the reaction solution. The heterogeneous solid state conversion allowed the reaction to proceed in almost quantitative yield.     

Étude de l'hydrogénation sélective du cyclopentadiène sur l'aérogel Cu/Al2O3 fluidisé

This work was conducted to evaluate the use of Cu/Al₂O₃ aerogel for the selective hydrogenation of the cyclopentadiene. Aerogel particles belonging to the C group of Geldart's classification, fluidize via a discrete clustering process above a minimum superficial velocity of 0.04 m/s. A kinetic study of the cyclopentadiène hydrogenation was performed on an integral reactor. The kinetic data suggest that the rate model can be represented by an equation corresponding to the Rideal—Eley mechanism. The reaction between gaseous hydrogen and the cyclopentadiène adsorbed on the catalyst surface is, in this mechanism, the controlling step. Because of the low activity of the catalyst, several reactor models (homogeneous and heterogeneous) were able to describe the experimental conversion rates observed in a small pilot fluidized bed reactor. However, the single phase plug flow model, which conforms more to the pseudo-homogeneous behaviour of the bed, was the most appropriate based on statistical discrimination.     

Molecular rotors: what lies behind the high sensitivity of the thioflavin-T fluorescent marker

Thioflavin-T (ThT) can bind to amyloid fibrils and is frequently used as a fluo-rescent marker for in vitro biomedical assays of the potency of inhibitors for amyloid-related diseases, such as Alzheimer's disease, Parkinson's disease, and amyloidosis. Upon binding to amyloid fibrils, the steady-state (time-integrated) emission intensity of ThT increases by orders of magnitude. The simplicity of this type of measurement has made ThT a common fluorescent marker in biomedical research over the last 50 years. As a result of the remarkable development in ultrafast spectroscopy measure-ments, researchers have made substantial progress in understanding the photophysical nature of ThT. Both ab initio quantum-mechanical calculations and experimental evidence have shown that the electronically excited-state surface potential of ThT is composed of two regimes: a locally excited (LE) state and a charge-transfer (CT) state. The electronic wave function of the excited state changes from the initial LE state to the CT state as a result of the rotation around a single C-C bond in the middle of the molecule, which connects the benzothiazole moiety to the dimethylanilino ring. This twisted-internal-CT (TICT) is responsible for the molecular rotor behavior of ThT. This Account discusses several factors that can influence the LE-TICT dynamics of the excited state. Solvent, temperature, and hydrostatic pressure play roles in this process. In the context of biomedical assays, the binding to amyloid fibrils inhibits the internal rotation of the molecular segments and as a result, the electron cannot cross into the nonradiative "dark" CT state. The LE state has high oscillator strength that enables radiative excited-state relaxation to the ground state. This process makes the ThT molecule light up in the presence of amyloid fibrils. In the literature, researchers have suggested several models to explain nonradiative processes. We discuss the advantages and disadvantages of the various nonradiative models while focusing on the model that was initially proposed by Glasbeek and co-workers for auramine-O to be the best suited for ThT. We further discuss the computational fitting of the model for the nonradiative process of ThT.     

Cinetique du couple redox ferro-ferricyanure a des electrodes d'oxyde et de graphite. Effets de la nature de l'electrode et du cation alcalin

At an electrode made of graphite or of the mixed oxide La_0.8Sr_0.2CoO₃, it is shown with the rotating disk method that the limited current of oxidation of ferrocyanide is governed by the diffusion of a participating species only with K ⁺ and Cs ⁺. Conversely, with Na ⁺ and Li ⁺, it is governed by the diffusion and by a heterogenous reaction which takes place before the charge-transfer. This step is apparently catalysed by the surface of the oxide. By scanning potential cyclic voltametry, it is shown that, with Na ⁺ and Li ⁺, the oxidation reaction is semifast, though it is an order of magnitude faster with K ⁺. By conductibility measurements, we have verified that the number of ion pairs formed is larger with K ⁺ than with Na ⁺ and Li ⁺. A mechanism in term of “contact” and “non contact” ion pairs is proposed.     

Cumene Hydroperoxide Hydrogenation on a Pd/Al2O3 Catalyst in a Trickle Bed Reactor – Kinetics of Hydrogenation and Deactivation

Liquid‐phase hydrogenation using a Pd/Al₂O₃ catalyst provides a potential technique for the reduction of cumene hydroperoxide (CHP) to α‐cumyl alcohol (CA). In this paper, CHP hydrogenation was carried out in a cocurrent downflow trickle‐bed reactor over a wide range of reaction conditions to study the reaction and deactivation kinetics. The proposed intrinsic rate expression for CHP hydrogenation is based on an Eley‐Rideal mechanism that accounts for an irreversible surface reaction between the absorbed CHP with nonabsorbed hydrogen molecules. During CHP hydrogenation, an exponential decay in activity of the Pd/Al₂O₃ catalyst and the presence of residual activity were observed. A kinetic deactivation model with residual activity was developed. Based on reaction and deactivation kinetics, catalyst deactivation was attributed to oxidation of the catalyst surface by CHP. The presence of residual activity was due to the partial reduction of oxidized catalyst surface by hydrogen.     

Untersuchungen zur thioladdition an polybutadiene

The free-radical addition of 1-octanediol (RSH) on polybutadienes with different molecular weights and configuration of the C? C double bonds occurs selectively to the 1,2-vinyl and 1,4-cis double bond, while the 1,4-trans double bond remains unchanged. The reactivity order 1,2- > cis > trans is confirmed by the model experiments with the isomers of pentene. The addition is favoured by the formation of an olefin/RSH charge-transfer complex as preinitiation step.     

Photochemie arylsubstituierter Δ2-1,3,4-Oxadiazoline. II. Zum Einfluß der Substituenten und des Lösungsmittels auf die Absorptions- und Fluoreszenzeigenschaften arylsubstituierter Δ2-1,3,4-Oxadiazoline

Photochemistry of Arylsubstituted Δ²-1,3,4-Oxadiazolines. II. The Influence of the Substituents and the Solvent on the Absorption and Fluorescence Properties of Arylsubstituted Δ²-1,3,4-Oxadiazolines The electronic absorption and the fluorescence spectra of 16 arylsubstituted Δ²-1,3,4-oxadiazolines were measured in benzene at room temperature. The push-pull effect of the substituents supports an increasing intramolecular charge-transfer in the electronically excited singlet state, which increases its dipole moment relative to the ground state. The significant linear correlation of the Hammett σ_p-values with the energies of the fluorescence and absorption maxima of the Δ²-oxadiazolines, p-substituted in the phenylring at the C 2-atom, demonstrate these intramolecular charge transfer. The titel compounds demonstrate with increasing solvent polarity a bathochromic shift of the absorption maxima. For some oxadiazolines the dipole moment for the excited singlet state proved to be about 30 Cm as Stoke's shift dipendency from the solvent indicate.     

Die kinetik der zinkelektrode in wässrigen malonatlösungen

The kinetics of dissolution and deposition of zinc in sodium malonate solutions containing NaClO₄ as the base electrolyte was investigated. The steady state polarization curves and the frequency dependence of the faradaic impedance indicate a consecutive charge-transfer reaction with adsorbed monovalent zinc as the intermediate. The dependence of the polarization curves and of the faradaic impedances on the concentrations of malonate and zinc salt are explained by the following mechanism: During the anodic transfer reaction of zinc yielding monovalent zinc a complex with one malonate ion is formed. The subsequent oxidation to divalent zinc can follow two parallel paths of comparable rates. The charge-transfer reaction may directly yield a zinc complex with two malonate ions, or, a complex with one malonate ion may participate in the charge-transfer, the complex with two malonate ions being the product of a homogeneous reaction in solution.