Acceleration Of Reaction Research Articles

Reaction and phase equilibria of esterification of isoamyl alcohol and acetic acid at 760 mm Hg

The thermodynamic behavior of esterification reaction equilibrium and vapor–liquid equilibrium (VLE) of acetic acid and isoamyl alcohol mixture and its products, isoamyl acetate and water were investigated in this study. The experiments of chemical and phase equilibria were conducted in an Othmer type equilibrium cell. Since this esterification reaction proceeds very slowly, thus, a commercial Y type zeolite, NaY catalyst, was added to accelerate reaction rate such that the reaction equilibrium will be reached sooner and before phase equilibrium is considered. Using experimental data and phase and reaction equilibrium equations, the reaction equilibrium constants were calculated at different temperatures. The experimental results showed that the reaction equilibrium constant was very slightly dependent on temperature. The experimental data were then correlated by the Wilson, NRTL, and UNIQUAC models with the consideration of association effect of acetic acid in vapor phase. The concept of transformation composition by Barbosa and Doherty [D. Barbosa, M.F. Doherty, 1987, Proc. R. Soc. London Ser. A 413, 443–459.] was applied to construct three-dimensional figures showing experimental data and the calculated composition surfaces. It is observed from figures that the reactive azeotropy described by Barbosa and Doherty does not exist for this quaternary mixture. This study provides the required thermodynamic information for the development and design of this esterification process.

Preparation and characterisation of solvent-stabilised nanoparticulate platinum and palladium and their catalytic behaviour towards the enantioselective hydrogenation of ethyl pyruvate

Solvent-stabilised Pt and Pd nanoparticles, of size range 2.3–2.8 nm and 2.7–3.8 nm, respectively, have been prepared by metal vapour synthesis routes, characterised by transmission electron microscopy (TEM), and their behaviour as catalysts for the enantioselective hydrogenation of ethyl pyruvate (EP) investigated; comparisons have been effected with the performance of standard supported Pt and Pd catalysts. Cinchona alkaloid-modified Pt nanoparticles display parallel behaviour to that exhibited by their conventional supported counterparts both in terms of the sense of the enantioselectivity in the ethyl lactate product and in the acceleration in reaction rate relative to the unmodified system. With Pd, however, significant differences are noted. Here, the sense of the enantioselectivity relative to that reported previously over conventional supported catalysts is reversed, i.e., an ( R)- vs. ( S)-enantiomer switch occurs, and a rate acceleration rather than retardation is noted on cinchona alkaloid modification. The Pt particle size distribution shows a higher degree of monodispersity after use in catalysis, although the average particle size remains essentially unchanged, whereas the behaviour of the Pd nanoparticles shows evidence of concentration dependence, lower concentrations showing Pt-like behaviour but more highly concentrated preparations showing evidence of significant aggregation during catalysis. With Pt catalysts, the presence of water as a component of the ketonic solvent system is shown to result in a significant acceleration in overall reaction rate with both conventional supported catalysts and their solvent-stabilised counterparts. In sharp contrast, totally aqueous-based colloidal platinum preparations, obtained by conventional salt reduction, display very low reaction rates and enantioselectivities.

Microwave Activation in Organic Synthesis: Natural Indian Clay, EPICR EPZGR and EPZ10R as Novel Heterogenous Catalysts for Rapid Synthesis of Nitriles from Aldoximes in Absence of Solvent

The Conversion of aldoximes into nitriles was carried out in the absence of solvent under microwave irradiation using environmentally-friendly catalysts like natural kaolinitic clay, EPICR EPZGR and EPZ10R. Acceleration of reaction rate, simple work-up and formation of clean Products are salient features of this method.

Crosslinking of imprinted proteases to maintain a tailor-made substrate selectivity in aqueous solutions

A covalent method to keep imprinted properties of proteins stable in aqueous as well as in organic environment is described. To stabilize the ligand induced acceptance for d-configured substrates by α-chymotrypsin or subtilisin Carlsberg, each protein was first vinylated by acylation with itaconic anhydride. Then, the tailoring of the derivatized proteins by precipitation in the presence of N-acetyl-d-tryptophan from an aqueous medium with 1-propanol, and the subsequent crosslinking of the enzyme preparations with ethylene glycol dimethacrylate in cyclohexane was carried out. The crosslinked imprinted proteins (CLIPs) obtained catalyzed the hydrolysis of N-acetyl-d-tryptophan ethyl ester in phosphate buffer and the corresponding back reaction in cyclohexane, respectively. The repeated use of CLIP-α-chymotrypsin in d-ester hydrolysis was demonstrated. Furthermore, this particular CLIP-α-chymotrypsin showed no loss in activity when it subsequently was used in the synthesis of N-acetyl-d-tryptophan ethyl ester in cyclohexane again. In the case of d-ester hydrolysis the reaction rate acceleration (kenz/knonenz) was in the same order of magnitude of about 104 −105 mM−1 for the two CLIP-proteases. The results suggest that enzymes tailored by imprinting technique do not lose their induced “new” property in the presence of water when they are prepared according to the described vinylation/crosslinking method (CLIP technique).

Acceleration of chemical reaction by AOT micelles under supercritical conditions

The alkaline fading of crystal violet (CV) is accelerated by using AOT micelles formed in supercritical ethane as a continuous phase. The rate of fading can be promoted by over two orders of magnitude by controlling operating pressure and molar ratio of water to AOT( W). The structure of AOT molecules, the type of interior water molecules, and the E T (30) value indicative of micropolarity of a dye-organized cybotactic region in the micelle core seem to account for the rate enhancement with an increase in the W. It is further suggested that a decrease in the size of the micelle core due to significant changes in the intermicellar and AOT-ethane interactions with increasing pressure, which causes an increase in the micropolarity inside the core, is of importance for the promotion effect by pressure.

Enzymatic synthesis of silicon-containing dipeptides with 3-trimethylsilylalanine

In this study, the enzymatic synthesis of silicon-containing dipeptides with a silicon-containing amino acid, 3-trimethylsilylalanine (TMS-Ala), was attempted in ethyl acetate, and the effects of TMS-Ala on thermolysin-catalyzed dipeptide synthesis are also discussed. Benzyloxycarbonyl(Z)-TMS-Ala was recognized by thermolysin as a better substrate than Z-Leu, and various silicon-containing dipeptides, Z-(TMS-Ala)-Xaa-OMe (Xaa = Leu, Ile, Phe, etc.), could be obtained. The acceleration of the reaction rate in the synthesis of Z-(TMS-Ala)-Leu-OMe compared with Z-Leu-Leu-OMe synthesis was explained by the higher hydrophobicity of the side-chain of TMS-Ala containing a trimethylsilyl group. On the other hand, TMS-Ala-OMe was not accepted as the amino component because of the bulkiness of the trimethylsilyl group. The enantioselectivity of thermolysin was very high. Z-d-TMS-Ala was not a substrate, while Z-l-TMS-Ala served as a good substrate.

The Decomposition Kinetics of Monocrotophos in Aqueous Solutions by the Hydrogen Peroxide–Ozone Process

Decomposition of monocrotophos in an aqueous solution by the hydrogen peroxide–ozone (H2O2–O3) process was studied under various solution pHs and H2O2–O3) molar ratios. Experimental results indicated that monocrotophos could be almost completely (>95%) decomposed by the H2O2–O3) process within 20 minutes. Formation of carbon dioxide was observed, followed by the formation of nitrate and phosphate. A simplified kinetic model, consisting of three two‐step consecutive reactions based on elemental mass balances, was found to accurately describe the temporal behaviors of reacting species during the decomposition of monocrotophos in aqueous solution by the H2O2–O3) process. In most experiments, acidic conditions favored the disappearance of monocrotophos, possibly because of breakage of the carbon–carbon double bond to form various organic intermediates, but mineralization of organic intermediates by free radical hydroxyl (OH·) attack was more favored in alkaline conditions. The decomposition of monocrotophos by the H2O2–O3) process was found to be significantly affected by the mode of H2O2 addition. For the batch mode of H2O2 addition, increased H2O2 dose reduced the decomposition rate, possibly because large amounts of H2O2 added initially served as a scavenger for OH· free radicals. For the continuous mode of H2O2 addition, increased H2O2 dose accelerated reaction rates of monocrotophos until an optimum H2O2–O3) molar ratio was reached, which was found to vary for different solution pHs.

Microwave‐Hydrothermal Synthesis of Nanophase Ferrites

This paper reports the synthesis of technologically important ferrites such as ZnFe2O4, NiFe2O4, MnFe2O4, and CoFe2O4 by using novel microwave‐hydrothermal processing. Nanophase ferrites with high surface areas, in the range of 72‐247m2/g, have been synthesized in a matter of a few minutes at temperatures as low as 164°C. The rapid synthesis of nanophase ferrites via an acceleration of reaction rates under microwave‐hydrothermal conditions is expected to lead to energy savings.

The design of protein-based catalysts using semisynthetic methods

The combination of site-directed mutagenesis and chemical modification has resulted in the preparation of protein conjugates with new and useful properties. Proteins modified with metal-chelating groups are proving useful for mapping tertiary and quaternary interactions using the technique of affinity cleavage. The attachment of cofactors, including pyridoxal and pyridoxamine, has resulted in the preparation of semisynthetic transaminases that display enzyme-like properties, including enantioselectivity, substrate specificity and reaction-rate acceleration.

Phase and reaction equilibria of acetic acid–1-pentanol–water– n-amyl acetate system at 760 mm Hg

The esterification reaction of acetic acid and alcohol is one of the processes applying reactive distillation technology. It is known that the thermodynamic properties are essential to chemical process design. In this study, the thermodynamic behaviors of vapor–liquid equilibrium (VLE) and reaction equilibrium of acetic acid, 1-pentanol, n-amyl acetate, and water mixture were determined experimentally. Since the present esterification reaction is very slow, thus, a sulfuric-aluminum oxide catalyst prepared in our laboratory was used to accelerate reaction rate such that the chemical reaction equilibrium will be reached ahead of phase equilibrium. Then this complicated problem was simplified and readily to be solved. The reaction equilibrium constants at different temperatures were calculated with experimental VLE data and the reaction and phase equilibrium equations. We found that the reaction equilibrium constant was slightly dependent on temperature. The experimental VLE data were correlated by the models of UNIQUAC and NRTL with the consideration of vapor molecular association of acetic acid. Figures showing the experimental data and the calculated composition surfaces were plotted against temperature. From these figures, we observed that reactive azeotropy as described by Barbosa and Doherty [D. Barbosa, M.F. Doherty, 1987. Proc. R. Soc. London Ser. A, 413, 443–459.] does not exist for this quaternary mixture.

Noncatalytic gas-solid reactions: modelling of simultaneous reaction and formation of surface with a nonisothermal crackling core model

This work presents a novel numerical gas-solid reaction model which is based on the mass and the heat transport in the pellet. The initial nonporous pellet is transformed into a porous pellet upon the reaction. Due to this transformation, the increased surface of the pellet accompanied by an accelerated reaction rate leads to a periodic ignition-extinguishing behaviour of the system. These oscillations are discovered observing hydrochlorination of metallurgical-grade silicon (Si + 3HCl → SiHCl3 + H2), a process of importance in industry, and could be explained by the novel model.

Acceleration of Chemical Reaction in Reactive Extrusion Accompained by Devolatilization

Abstract This work concerns the reaction acceleration in bubble enhanced devolatilization (DV) occurring in reactive extrusion (REX), to provide further evidence that overall reactivity can be enhanced by boiling. Using monoesterification of a styrenemaleic anhydride copolymer with an alcohol as a model reaction, the rate increase was determined by comparing the conversions of the samples collected at different screw locations with those predicted from the combined kinetic equation for the bubble free homogeneous system and the RTD information obtained from both the RTD measurements and theoretical calculations. Significant reaction acceleration was observed. The reaction enhancement was found to be more significant than found previously by batch operation. Also, the reaction enhancement beyond the DV zone showed a trend consistent with the previous observations in batch after DV has been halted. Further, we show that the apparent reaction rate increase during continuous REX/DV is caused by local mass transfer effects, in agreement with the previous work in batch. The good agreement between the theoretical predictions and the experimental results, observed here, lends some extended generality to the mass transfer model for reaction acceleration during REX/DV.

ChemInform Abstract: A New Powerful and Practical BLA Catalyst for Highly Enantioselective Diels‐Alder Reaction: An Extreme Acceleration of Reaction Rate by Broensted Acid.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Microwave‐Hydrothermal Processing of BiFeO3 and CsAl2PO6

Microwave‐hydrothermal (M‐H) processing was compared with conventional‐hydrothermal (C‐H) processing in the crystallization of BiFeO3 and CsAl2PO6 phases. The presence of the microwave field led to accelerated kinetics of the crystallization of both these phases as detected by powder X‐ray diffraction. The acceleration of reaction rates under microwave field is expected to lead to energy savings during ceramic processing.

超臨界流体の基礎と応用 超臨界二酸化炭素中での有機合成

The possibilities of the practical application of the supercritical carbon dioxide for the organic syntheses have been discussed based upon the recent experimental results obtained at author's laboratory, focusing on the "unusual phenomena", i. e., the acceleration of reaction rates, the shift of equiliblium constants, or the selectivity of reaction products which were used to be induced near the critical points (Tc & Pc).

A New Powerful and Practical BLA Catalyst for Highly Enantioselective Diels−Alder Reaction: An Extreme Acceleration of Reaction Rate by Brønsted Acid

A New Powerful and Practical BLA Catalyst for Highly Enantioselective Diels−Alder Reaction: An Extreme Acceleration of Reaction Rate by Brønsted Acid

Sonolytic Hydrolysis of p-Nitrophenyl Acetate: The Role of Supercritical Water

Ultrasonic irradiation is shown to accelerate the rate of hydrolysis of p-nitrophenyl acetate (PNPA) in aqueous solution by 2 orders of magnitude over the pH range of 3-8. In the presence of ultrasound, the observed first-order rate constant for the hydrolysis of PNPA is found to be independent of pH and ionic strength with k_(obs) = 7.5 x 10^(-4) s^(-1) with Kr as the cavitating gas, and k_(obs) = 4.6 x 10^(-4) s^(-1) with He as the cavitating gas. The apparent activation parameters for sonolytic catalysis are ΔH^≠ (sonified) = 211 Kj/mol, ΔS^≠(sonified) = -47/Jl(mol K), and ΔG^≠ (sonified) = 248 kJ/mol. Under ambient conditions and in the absence of ultrasound, k_(obs) is a strong function of pH where k_(obs) = k_(H_2O)[H_2O] + k_(OH)[OH-] with k_(H_2O) = 6.0 x 10^(-7) s^(-1) and k_(OH)- = 11.8 M^(-1) s^(-1) at 25 °C. The corresponding activation parameters are ΔH^≠ = 71.5 kJ/mol, ΔS^≠ = -107 J/(mol K), and ΔG^≠ = 155 kJ/mol. During cavitational bubble collapse, high temperatures and pressures exceeding the critical values of water (T > T_c = 647 K and P > P_c = 221 bar) occur in the vapor phase of the cavitating bubbles and at the interfaces between the hot vapors and the cooler bulk aqueous phase. The formation of transient supercritical water (SCW) appears to be an important factor in the acceleration of chemical reactions in the presence of ultrasound. The apparent activation entropy, ΔS^≠, is decreased substantially during the sonolytic catalysis of PNPA hydrolysis, while ΔG^≠ and ΔH^≠ are increased. The decrease ΔS^≠ is attributed to differential solvation effects due to the existence of supercritical water (e.g., lower ρ and є) while the increases in ΔG^≠ and ΔH^≠ are attributed to changes in the heat capacity of the water due to the formation of a transient supercritical state. A dynamic heat-transfer model for the formation, lifetime, and spatial extent of transient supercritical water at cavitating bubble interfaces is presented.

Kinetics of Reactions Between Silica and Carbon and the Formation of Silicon Carbide

ABSTRACTThe reactions taking place between coarse grained mixtures of silica (cristobalite) and carbon (graphite) at 1558°C in pure CO as well as mixtures of CO and Ar, have been subjected to a thermogravimetric investigation. A model is developed in the regime where the formation of SiC does not take place. The primary steps are assumed to be: Reaction betweefn SiO2 and CO to give SiO(g) and CO2 followed by the reaction between CO2 and C to give CO. The model predicts the prevailing partial pressure of SiO throughout the charge, and the correlation between observations and model strongly supports the above given reaction mechanism.Enhanced and accelerating reaction rates were observed when the formation of β-SiC took place. It is suggested that this is due to the continuous formation of stable SiC-nucleus on the Csurface and the subsequent shortening of the diffusion path for SiO.

Evidence against the reactive rotamer explanation of the gem-dialkyl effect

Acceleration of intramolecular reaction rates with increasing substitution has been known for a long time. The most common explanation used for this phenomenon is the 'Reactive Rotamer Effect' which states that “The ring closure reaction proceeds at a greater rate on geminal (or alkyl) substitution because of the resultant decrease in unprofitable rotamer distribution”. 1 This research presents evidence against this explanation and suggests an alternate explanation for the cause of the gem-dialkyl effect.

Field ion microscopic studies of the CO oxidation on platinum: Field ion imaging and titration reactions

Elementary steps of the CO oxidation—which are important for understanding the oscillatory behavior of this catalytic reaction—are investigated simultaneously on different Pt-single crystal surfaces by field ion microscopy. Due to preferential ionization probabilities of oxygen as imaging gas on those surface sites, which are adsorbed with oxygen, these sites can be imaged in a lateral resolution on the atomic scale. In the titration reaction a COad-precovered field emitter surface reacts with gaseous oxygen adsorbed from the gas phase or, vice versa, the Oad-precovered surface with carbon monoxide adsorbed from the gas phase. The competition of the manifold of single crystal planes exposed to the titration reaction at the field emitter tip is studied. The surface specificity can be documented in the specific reaction delay times of the different planes and in the propagation rates of the reaction-diffusion wave fronts measured on these individual planes during the titration reaction with a time resolution of 40 ms. At 300 K the COad-precovered surfaces display the {011} regions, precisely the {331} planes as the most active, followed by {012}, {122}, {001}, and finally by {111}. Reaction wave fronts move with a velocity of 8 Å/s at {012}, with ≊0.8 Å/s at {111}, and have a very fast ‘‘switch-on’’ reaction at the (001) plane with 500 Å/s. At higher temperature, T=350 K, an acceleration of reaction rates is combined with shorter delay times. The titration reaction of a precovered Oad surface with COgas at T=373 K shows the formation of CO islands starting in the {011} regions with a quickly moving reaction front into the other surface areas without showing particular delay times for different surface symmetries. The two reverse titration reactions have a largely different character. The titration of COad with oxygen adsorbed from the gas phase consists of three different steps, (i) the induction times, (ii) the highly surface specific reaction, and (iii) different rates of wave front propagation. The reaction of COgas with a precovered Oad layer on the other hand starts with nucleating islands around the {011} planes from where the whole emitter surface is populated with COad without pronounced surface specifity.