Mechanism Of Enolization Research Articles

Kinetic study of Fe & Co perovskite catalyst in Fischer–Tropsch synthesis

The investigation of the reaction's kinetics is one of the most crucial aspects of the design of a commercial process. The current research investigates the kinetics of Fischer–Tropsch synthesis using a perovskite catalyst. The LaFe0.7 Co0.3 O3 perovskite catalyst was prepared via the thermal sol–gel technique and characterized using BET, XRD, SEM, and H2-TPR techniques. According to operating conditions (e.g. H2/CO: 1–2, pressure: 10–20 barg, temperature: 240–300 °C, and GHSV: 3000 1/h), Fischer–Tropsch reaction kinetics (CO conversion) were carried out in a fixed-bed reactor. Using the framework of Langmuir–Hinshelwood–Hougen–Watson (LHHW) theories, 18 kinetic expressions for CO conversion were derived, and all were fitted to experimental data one by one to determine the optimum condition. The correlation was derived from experimental data and well-fitted using LHHW form (according to the enol mechanism, carbon monoxide and dissociated hydrogen atoms are adsorbed and reacted on the surface of the catalyst) −rCO = kpbCOPCO(bH2PH2)0.5/(1 + bCOPCO + (bH2PH2)0.5)2. Finally, the activation energy of the optimum kinetic model was determined with respect to the Arrhenius equation under various operating conditions. The activation energy of perovskite catalyst is about 106.25 kJ/mol at temperatures 240–300 °C, pressures 10–20 barg, and H2/CO ratios 1–2, which is lower than other types of catalyst. Therefore, the catalyst was activated at a high temperature and demonstrated stable performance without any temperature runaway and coking issues.

Insights to the reaction kinetics of Fischer-Tropsch synthesis using an integral system over Cu-Mo promoted Fe catalyst

This work focused on the design and kinetics study of the Cu-Mo promoted Fe catalyst supported on biomass-derived carbon material with engineered surface chemistry and textural properties for Fischer-Tropsch synthesis (FTS). The kinetics tests were carried out at different operational conditions including temperature in the range of 280–310 °C, pressures: 20.6–41.3 bar, and gas hourly space velocity (GHSV): 1500–3000 h−1. The reaction rate was evaluated based on the enol mechanism using Eley–Rideal theory. To study the reaction rates along the catalytic bed, the reactor was assumed as an integral system. The production rates of the hydrocarbons were estimated by introducing a mathematical expression based on the lumped reaction network. The kinetics parameters, profiles of the partial pressures for the reactants and products were optimized using genetic algorithm. The proposed reaction mechanism proceeded with associative adsorption of CO, followed by hydrogenation to an oxygenated intermediate (CHO*); and the activation energy for CO consumption rate model was obtained as 51.90 kJ. mol−1. The provided one-dimensional model can lead to better understanding of the reaction mechanism of FTS as a sustainable energy resource with a novel catalyst design.

Computational fluid dynamics simulation of Fischer-Tropsch synthesis coupled with a novel reaction kinetics using trimetallic catalyst over a biomass-based support

A mathematical modeling based on the computational fluid dynamics (CFD) was performed in this work. The reactive flow behavior in a tubular reactor was modeled for Fischer-Tropsch synthesis (FTS) using a novel trimetallic catalyst supported on a low-cost and renewable carbonaceous material. The fixed bed reactor was simulated as a porous media using a commercial software package (COMSOL Multiphysics). In this study, major attention has been paid to the species transport and reaction kinetics, as a fundamental stage in CFD simulation of FTS. The reaction kinetics was evaluated using Eley-Rideal theory assuming the molecular adsorption of CO based on the enol mechanism. Furthermore, the products distribution was accounted in the kinetics study by defining a mathematical expression and the kinetics parameters were optimized using genetic algorithm (GA). The activation energy and heat of H2 adsorption were estimated as 62.63 and −29.52 kJ/mol, respectively. The role of operational conditions was critically evaluated on the partial pressure profiles of the species, and the CFD results were validated with the experimental data. While increasing total pressure improved the partial pressure of C5+ hydrocarbons to 1.2 bar, increasing temperature to 310 °C limited the chain growth. The employed modeling and novel catalyst design can provide promising overview of the FTS reaction mechanism and products selectivity in fixed bed reactors.

Some Thoughts on the Mechanism of Ester Interchange Reactions Involving Acylglycerols

AbstractEster interchange reactions involving acylglycerides are industrially important because the interesterification process provides trans‐isomer free hardstocks and the transesterification process produces biodiesel. This Commentary proposes to extend the enolate mechanism that has been suggested for the interesterification process to the transesterification process. It also highlights the gaps in people's knowledge and discusses the implication of these gaps.Practical Applications: Insight into the mechanism of the interesterification reaction provides means to eliminate the yield loss suffered by the current industrial processes. Use of an alkali metal glycerolate will avoid the formation of fatty acid methyl esters and their subsequent loss. Using the same catalyst during the transesterification will reduce the variable cost of this process since this catalyst can be easily produced on site from by‐product glycerol.

Mono-endo-6-N,N-diethylcarbamoyl and bis-endo,endo-6,12-N,N-diethylcarbamoyl derivatives of Tröger’s base. Synthesis and exo-endo isomerization study

An efficient synthetic route to the mono-endo-6- and bis-endo,endo-6,12-N,N-diethylcarbamoyl derivatives of Troger’s base (TB), endo-7 and endo-8, is reported. Studies of reaction time, proton source, and additive allowed establishment of optimized conditions for the conversion of exo-7 into the corresponding isomer endo-7. With a longer reaction time, the exo,exo-6,12 bis-carbamoyl derivative exo-8 was converted into the corresponding endo,endo-bis-carbamoyl product endo-8. Single crystal X-ray crystallographic analysis confirmed the structural and stereochemical assignments made on the basis of 1H NMR, mechanistic, and calculational studies. Deuterium quench experiments using LDA, CD3ONa/CD3OD and DCl/CD3OD conditions of both exo-7 and exo-8 afforded exo-7d1 and exo-8d2, respectively (> 95% deuterium incorporation), supporting an enolate mechanism for the isomerization. In contrast, when repeating the experiment with DCl/CD3OD, no deuterium was incorporated, suggesting the traditional ring-opening mechanism involving an iminium ion. (Less)

Evidence for an enolate mechanism in the asymmetric Michael reaction of α,β-unsaturated aldehydes and ketones via a hybrid system of two secondary amine catalysts

The key nucleophile was found to be neither an enamine nor an enol, but an enolate in the direct Michael reaction of α,β-unsaturated aldehydes and non-activated ketones catalyzed by two amine catalysts namely diphenylprolinol silyl ether and pyrrolidine. This is a rare example of an enolate from a ketone serving as a key intermediate in the asymmetric organocatalytic reaction involving secondary amine catalysts because the ketone enolates are generally generated using a strong base, and the enamine is a common nucleophile in this type of reaction.

Highly active hybrid mesoporous silica-supported base organocatalysts for C C bond formation

Abstract New base hybrid catalysts, based on silyl-derivatives of molecules carrying amino, diamino, pyrrolidine, pyrazolium and imidazolium functionalities have been successfully achieved through post synthetic grafting onto M41S-type support. Different characterization techniques were implemented to study the characteristics of the materials, such as elemental analysis, solid state MAS NMR and FTIR spectroscopies, X-ray diffraction (XRD), thermogravimetric and differential thermal analyses (TGA-DTA) and textural properties through N2 physisorption analysis. The catalytic activity and recyclability of these compounds as base catalysts was demonstrated for C C bond forming reactions such as Knoevenagel condensations and Michael additions rationalizing the differences observed as function of the reaction mechanisms. An enamine mechanism was proposed for Knoevenagel condensations and an enolate mechanism for Michael additions.

Mechanistic Insights into One-Step Catalytic Conversion of Ethanol to Butadiene over Bifunctional Zn–Y/Beta Zeolite

Bifunctional Zn–Y/Beta catalyst was applied in the reaction mechanism study of the ethanol to butadiene conversion to clarify the roles of Zn and Y functional sites in each individual reaction step. According to the results of several complementary methods, i.e., ethanol temperature-programmed desorption (TPD), temperature-programmed surface reaction (TPSR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the reaction network consisting of several key steps, i.e., ethanol dehydrogenation, acetaldehyde aldol condensation, and crotonaldehyde reduction, was elucidated. An enolization mechanism was verified to involve in the coupling step. During this reaction, the Lewis acidic Zn and Y species in [Si]Beta zeolite were both active in the ethanol dehydrogenation, aldol condensation, and Meerwein–Ponndorf–Verley reduction. In this cycle, Zn species exhibited the higher dehydrogenation activity but lower coupling activity than that of Y species. Through the combination of the tw...

Questions begging for answers (Part III) – Interesterification

The interesterification mechanism that assumed the diacylglycerolate anion to be the only catalytically active intermediate has been unchallenged for some forty years. Then, I listed a number of observations that could not be explained by the glycerolate mechanism and proposed a new one in which the enolate anion is the main catalytic intermediate. After independent support had been provided, this enolate mechanism was published as a ’hypothesis paper' [1].This new mechanism raises further questions but it also shows that it should be possible to reduce the yield loss, which at the moment is the largest cost element of the process, to a possibly negative value. That would make the process far more attractive and could encourage the development of interesterified hardstock to replace partially hydrogenated fats.

(16) O/(18) O Exchange of Aldehydes and Ketones caused by H2 (18) O in the Mechanistic Investigation of Organocatalyzed Michael, Mannich, and Aldol Reactions.

Organocatalyzed Michael, Mannich, and aldol reactions of aldehydes or ketones, as nucleophiles, have triggered several discussions regarding their reaction mechanism. H2 (18) O has been utilized to determine if the reaction proceeds through an enamine or enol mechanism by monitoring the ratio of (18) O incorporated into the final product. In this communication, we describe the risk of H2 (18) O as an evaluation tool for this mechanistic investigation. We have demonstrated that exchange of (16) O/(18) O occurs in the aldehyde or ketone starting material, caused by the presence of H2 (18) O and amine catalysts, before the Michael, Mannich, and aldol reactions proceed. Because the newly generated (18) O starting aldehydes or ketones and (16) O water affect the incorporation ratio of (18) O in the final product, the use of H2 (18) O would not be appropriate to distinguish the mechanism of these organocatalyzed reactions.

Diastereomer Interconversion via Enolization: A Case Study.

The three-component reaction of indole, isobutyraldehyde, and methyl acetoacetate affords methyl 2-(acetyl)-3-(1H-indol-3-yl)-4-methylpentanoate as a single diastereomer. To investigate the origin of the observed diastereoselectivity, the thermodynamics and kinetics of interconversion of diastereomers 1 and 2 in solution were studied by a combination of (1)H nuclear magnetic resonance (NMR) spectroscopy, high-performance liquid chromatography (HPLC), mass spectrometry, and deuteration experiments. The results indicate that interconversion is both acid- and base-catalyzed, and that the alpha carbon is the only stereolabile center in the molecule. The evidence points to an enolization mechanism for the interconversion process. The selective precipitation of 1 in the presence of the equilibrium 1⇆2 eventually results in the exclusive formation of 1 (crystallization-induced asymmetric transformation).

Enolization as an Alternative Proton Delivery Pathway in Human Aromatase (P450 19A1)

Human aromatase catalyzes the last step of estrogen biosynthesis, the aromatization of ring A of androstenedione (ASD) and testosterone leading to estrone and estradiol. The enolization of the substrate molecule has been suggested to play an essential role in this process. In this work using quantum mechanical and hybrid QM/MM calculations, the reaction mechanism of enolization was investigated. It is shown that the energetically unfavorable enolization of andostenedione occurs in a coupled process with the energetically favorable protonation of the ferrous superoxo complex (traditionally called ferric peroxo complex) via a low barrier of about 5 kcal/mol. This mechanism implies an alternative way for protonation of the ferrous superoxo complex to form compound 0, which occurs via the Asp309-water-ASD proton delivery pathway instead of the Asp-water-Thr pathway suggested for other P450 enzymes. It is also shown that Thr310, which is known experimentally to be important for catalysis, plays a key role in the conversion of compound 0 to compound I.

Mechanistic double ASF product distribution study of Fischer–Tropsch synthesis on precipitated iron catalyst

Product distributions of iron based catalyst represented by double ASF distributions characterized by two growth probabilities. Based on enol mechanism and double ASF distributions the complete set of elementary reactions is given. By using mechanistic kinetic studies of Fischer–Tropsch reaction, the chain growth probabilities (α1 and α2) for double ASF distributions are formulated. Fe–Cu–La catalyst was prepared by co-precipitation procedure, and steady-state FTS reaction rates were measured in a continuous spinning basket reactor. The experimental results for low carbon monoxide conversions have been satisfactorily fit with the double ASF model. But for higher carbon monoxide conversions deviate substantially because the FTS reaction highly depends on the hydrogen formed by the WGS as the carbon monoxide conversion increases.

Organocatalytic Asymmetric Conjugate Addition of Aldehydes to Nitroolefins: Identification of Catalytic Intermediates and the Stereoselectivity‐Determining Step by ESI‐MS

Looking back: The asymmetric organocatalytic 1,4-addition of aldehydes to nitroolefins was studied by ESI-MS. Analysis of the back reaction starting from quasienantiomeric mass-labeled 1,4-adducts (see scheme) provided conclusive evidence for an enamine rather than an enol mechanism, and allowed identification of the enantioselectivity-determining step.

ChemInform Abstract: An Uncatalyzed Aldol Reaction of Thiazolidinediones.

Thiazolidinediones have been used as aldol donors with aromatic aldehydes “on water” and in DMSO without using any catalyst to give the corresponding β-hydroxy carbonyl compounds in high yield and purity. The p-cyano and p-nitro substituted aldol products undergo syn/anti isomerization via an enolization mechanism providing an “on water” diastereoselectivity switch. Water molecules play a significant role in stabilizing the syn aldol products of pyridine and thiazole containing aldehydes.

Highly Enantioselective Three-Component Direct Mannich Reactions of Unfunctionalized Ketones Catalyzed by Bifunctional Organocatalysts

A highly stereoselective three-component direct Mannich reaction between aromatic aldehydes, p-toluenesulfonamide, and unfunctionalized ketones was achieved through an enolate mechanism for the first time with a bifunctional quinidine thiourea catalyst. The corresponding N-tosylated β-aminoketones were obtained in high yields and excellent diastereo- and enantioselectivities (up to >99:1 dr and >99% ee).

An uncatalyzed aldol reaction of thiazolidinediones

Thiazolidinediones have been used as aldol donors with aromatic aldehydes “on water” and in DMSO without using any catalyst to give the corresponding β-hydroxy carbonyl compounds in high yield and purity. The p-cyano and p-nitro substituted aldol products undergo syn/anti isomerization via an enolization mechanism providing an “on water” diastereoselectivity switch. Water molecules play a significant role in stabilizing the syn aldol products of pyridine and thiazole containing aldehydes.

Diastereoselectivity of the α,α-diphenylprolinol trimethylsilyl ether-catalyzed two-component domino reactions: toward cyclohexanes with six stereogenic centers

Theoretical calculations were employed to investigate the diastereoselectivity of the α,α-diphenylprolinol trimethylsilyl ether-catalyzed two-component cascade reactions between (i) pentane-1,5-dial and nitrostyrene, and (ii) pentane-1,5-dial and an α,β-unsaturated carbonyl compound. In both cases, the enol mechanism was able to account for the enantioselectivity of not only the major diastereomer formed, but also that of the minor diastereomers. Insights gained from our mechanistic investigations have enabled us to develop a scheme for the synthesis of cyclohexanes with six stereogenic centers, via two-component one-pot catalytic domino reactions.

ChemInform Abstract: Quinidine Thiourea‐Catalyzed Aldol Reaction of Unactivated Ketones: Highly Enantioselective Synthesis of 3‐Alkyl‐3‐hydroxyindolin‐2‐ones.

AbstractThe asymmetric aldol reaction of unactivated ketones and activated carbonyl compounds is realized with a quinidine‐derived thiourea catalyst, and involves an enolate mechanism instead of the widely used enamine mechanism.

Quinidine Thiourea Catalyzed Aldol Reaction of Unactivated Ketones

A highly efficient aldol reaction of unactivated ketones and activated carbonyl compounds using a quinidine thiourea catalyst 1 is presented. Zhao and co-workers obtained the ­desired products in good yields and high enantio­selectivities. The authors propose an enolate mechanism and they also illustrate the synthetic utility of their reaction process for the preparation of 3-alkyl-3-hydroxyindolin-2-ones, which are lead compounds for the synthesis of bioactive molecules.