Complex Catalytic Reactions Research Articles

A Complex Catalytic Reaction Caught in the Act: Intermediates and Products Sampling Online by Liquid μ-Beam Mass Spectrometry and Theoretical Modeling.

Liquid-beam IR-laser desorption mass spectrometry has been used to monitor the reactants, intermediates, and products of a complex organic signature reaction in real time on multiple timescales directly from the liquid phase. The reaction was chosen because it has advantages in medicinal chemistry applications, and the three-component, modular construction provides a means to generate molecular diversity rapidly. Under Lewis acid catalysis, a vinylogous Mannich reaction was monitored as it generated a δ-amino-α-silyloxy-α,β-unsaturated ester, which upon hydrolysis to the corresponding α-keto ester spontaneously reacted in a [3+2] cycloannulation to the final pyrrolo[2,1-b]benzoxazole. The kinetic data were compared with predictions of quantum chemical calculations to elucidate and verify or exclude reaction pathways and mechanisms for a possible rational optimization of the reaction. The simplicity and rapid response of this approach make it a very powerful technique for online characterization of chemical reactions on timescales spanning several orders of magnitude. This enables full control over chemical reactions, thereby maximizing the product yield. This combined experimental and theoretical approach opens up a new route for the study of novel chemistry in liquid-phase reactions.

Kinetic investigation of cross-coupling reaction steps by advanced competing reaction methods

Investigating the mechanism of a complex catalytic reaction is usually quite complicated because many processes, such as active catalyst formation and deactivation, are linked to the main catalytic cycle. Using artificial multi-route reactions with similar competing substrates is a unique way to elucidate the fine details of a reaction mechanism. Applying such multi-route reaction-based methods to the cross-coupling of alkynes with aryl halides (Cu-free Sonogashira reaction) and alkenes with aromatic carboxylic anhydrides (modified Heck reaction) catalysed by ligand-free Pd allowed us to elucidate the fast, reversible, and rate-determining steps involved in the relevant catalytic cycles.

ChemInform Abstract: Solvent Effects in Catalysis: Implementation for Modelling of Kinetics

AbstractReview: 52 refs.

ChemInform Abstract: Computational Catalysis Using the Artificial Force Induced Reaction Method

AbstractReview: 33 refs.

Computational Catalysis Using the Artificial Force Induced Reaction Method.

The artificial force induced reaction (AFIR) method in the global reaction route mapping (GRRM) strategy is an automatic approach to explore all important reaction paths of complex reactions. Most traditional methods in computational catalysis require guess reaction paths. On the other hand, the AFIR approach locates local minima (LMs) and transition states (TSs) of reaction paths without a guess, and therefore finds unanticipated as well as anticipated reaction paths. The AFIR method has been applied for multicomponent organic reactions, such as the aldol reaction, Passerini reaction, Biginelli reaction, and phase-transfer catalysis. In the presence of several reactants, many equilibrium structures are possible, leading to a number of reaction pathways. The AFIR method in the GRRM strategy determines all of the important equilibrium structures and subsequent reaction paths systematically. As the AFIR search is fully automatic, exhaustive trial-and-error and guess-and-check processes by the user can be eliminated. At the same time, the AFIR search is systematic, and therefore a more accurate and comprehensive description of the reaction mechanism can be determined. The AFIR method has been used for the study of full catalytic cycles and reaction steps in transition metal catalysis, such as cobalt-catalyzed hydroformylation and iron-catalyzed carbon-carbon bond formation reactions in aqueous media. Some AFIR applications have targeted the selectivity-determining step of transition-metal-catalyzed asymmetric reactions, including stereoselective water-tolerant lanthanide Lewis acid-catalyzed Mukaiyama aldol reactions. In terms of establishing the selectivity of a reaction, systematic sampling of the transition states is critical. In this direction, AFIR is very useful for performing a systematic and automatic determination of TSs. In the presence of a comprehensive description of the transition states, the selectivity of the reaction can be calculated more accurately. For relatively large molecular systems, the computational cost of AFIR searches can be reduced by using the ONIOM(QM:QM) or ONIOM(QM:MM) methods. In common practice, density functional theory (DFT) with a relatively small basis set is used for the high-level calculation, while a semiempirical approach or a force field description is used for the low-level calculation. After approximate LMs and TSs are determined, standard computational methods (e.g., DFT with a large basis set) are used for the full molecular system to determine the true LMs and TSs and to rationalize the reaction mechanism and selectivity of the catalytic reaction. The examples in this Account evidence that the AFIR method is a powerful approach for accurate prediction of the reaction mechanisms and selectivities of complex catalytic reactions. Therefore, the AFIR approach in the GRRM strategy is very useful for computational catalysis.

Solvent effects in catalysis: implementation for modelling of kinetics

A mathematical framework is developed for analysis of solvent dependent reaction rates and selectivity in the case of complex catalytic reactions by incorporating solvent permittivity into the rate expressions.

Visualizing reacting single atoms in chemical reactions: Advancing the frontiers of materials research

Abstract

ChemInform Abstract: Differential Selectivity Measurements and Competitive Reaction Methods as Effective Means for Mechanistic Studies of Complex Catalytic Reactions

AbstractReview: 105 refs.

Differential selectivity measurements and competitive reaction methods as effective means for mechanistic studies of complex catalytic reactions

In most kinetic investigations of catalytic reactions, catalytic activity is a traditionally measured kinetic parameter. However, differential selectivity measurements have many advantages for mechanistic studies. This Perspective gives an overview of the basic principles and practices of mechanistic studies based on differential selectivity measurements, including the use of artificially created competitive reactions.

Nearly 50 years in the making: defining the catalytic mechanism of the multifunctional enzyme, pyruvate carboxylase

Numerous steady-state kinetic studies have examined the complex catalytic reaction mechanism of the multifunctional enzyme, pyruvate carboxylase (PC). Through initial velocity, product inhibition, isotopic exchange and alternate substrate experiments, early investigators established that PC catalyzes the MgATP-dependent carboxylation of pyruvate by HCO3 (-) through a nonclassical sequential Bi Bi Uni Uni reaction mechanism. This review surveys previous steady-state kinetic investigations of PC and evaluates the proposed hypotheses concerning the overall catalytic mechanism, nonlinear kinetics and active site coupling in the context of recent structural and mutagenic analyses of this multifunctional enzyme. The determination several PC holoenzyme structures have aided in corroborating the proposed molecular mechanisms by which catalysis occurs and established the inextricable link between the dynamic protein motions and complex kinetic mechanisms associated with PC activity. Unexpectedly, the conclusions drawn from these early steady-state kinetic investigations have consistently proven to be in fundamental agreement with our current understanding of PC catalysis, which is a testament to the overarching sophistication of the methods pioneered by Michaelis and Menten and further developed by Northrop, Cleland and others.

Identification and analysis of synthesis routes in complex catalytic reaction networks for biomass upgrading

In this paper, we present a method to construct and query the spectrum (>15,000) of potential biomass-derived compounds that can be synthesized using heterogeneous catalysis. Specifically, it involves the construction of an exhaustive network of reactions using RING, a rule-based network generator that we have developed, to identify potentially synthesizable compounds from biomass. Subsequently, we: (a) employ quantitative structure–property relationships to query compounds with desired properties, (b) seek different synthesis routes to these compounds, and (c) evaluate these routes in terms of stoichiometric, energetics, and physical parameters such as atom efficiency, enthalpy and free energy change of reactions, and aqueous-organic partition coefficients (LogP) of intermediates. Thermochemistry and LogP are both estimated using group additivity methods, and all property estimations are performed on-the-fly during network generation. We present this method in context of identifying and evaluating heterogeneous catalytic routes from biomass to fatty alcohols that are potential constituents of nonionic surfactants. We also show that group additivity-based estimates of enthalpy of reaction are accurate to within 10kJ/mol and LogP values reliably predict the relative hydrophobicity of compounds. This method is generic, flexible, reliable and fast in terms of the scope of chemistry that can be considered, properties that can be included, predictive accuracy, and speed of execution; it can, therefore, be used to rapidly screen a large spectrum of compounds and synthesis routes in biomass conversion.

Snapshots of Enzymatic Baeyer-Villiger Catalysis: OXYGEN ACTIVATION AND INTERMEDIATE STABILIZATION

Baeyer-Villiger monooxygenases catalyze the oxidation of carbonylic substrates to ester or lactone products using NADPH as electron donor and molecular oxygen as oxidative reactant. Using protein engineering, kinetics, microspectrophotometry, crystallography, and intermediate analogs, we have captured several snapshots along the catalytic cycle which highlight key features in enzyme catalysis. After acting as electron donor, the enzyme-bound NADP(H) forms an H-bond with the flavin cofactor. This interaction is critical for stabilizing the oxygen-activating flavin-peroxide intermediate that results from the reaction of the reduced cofactor with oxygen. An essential active-site arginine acts as anchoring element for proper binding of the ketone substrate. Its positively charged guanidinium group can enhance the propensity of the substrate to undergo a nucleophilic attack by the flavin-peroxide intermediate. Furthermore, the arginine side chain, together with the NADP(+) ribose group, forms the niche that hosts the negatively charged Criegee intermediate that is generated upon reaction of the substrate with the flavin-peroxide. The fascinating ability of Baeyer-Villiger monooxygenases to catalyze a complex multistep catalytic reaction originates from concerted action of this Arg-NADP(H) pair and the flavin subsequently to promote flavin reduction, oxygen activation, tetrahedral intermediate formation, and product synthesis and release. The emerging picture is that these enzymes are mainly oxygen-activating and "Criegee-stabilizing" catalysts that act on any chemically suitable substrate that can diffuse into the active site, emphasizing their potential value as toolboxes for biocatalytic applications.

Dry reforming of methane over Pt/PrCeZrO catalyst: Kinetic and mechanistic features by transient studies and their modeling

The effect of pretreatment and Pt content on the catalytic properties as well as mechanistic features of DR were investigated for structured catalysts comprised of Pt supported on CeO 2–ZrO 2 oxide doped by Pr. Progressive reduction of cationic Pt species by the reaction feed lowers the activity in CH 4 dry reforming while accelerating the reverse water gas shift reaction catalyzed only by Pt 0, which then decreases progressively the H 2/CO ratio in the effluent. This process is counteracted by the mobility of surface oxygen supplying oxygen atoms to reduced Pt centers thus ensuring their reoxidation and generating in parallel surface oxygen vacancies for the dissociation of CO 2. A mathematical model and software were developed for numerically studying the transients of the complex catalytic reactions. The processing of experimental data was fulfilled taking into account the importance of cationic forms of Pt, reactivity of carbonate complexes coordinated to these cations and oxygen surface/bulk diffusion. A quantitative evaluation of the density of catalyst's active sites and their coverage by reactive species was accomplished and the rates both of the lattice oxygen diffusion and main stages of the catalytic reaction were estimated.

ChemInform Abstract: Contributions to the Coordination Chemistry of Technetium. Synthesis and Structural Clarification of New Complex Types and Their Testing in Selected Complex Catalytic Reactions

ChemInform Abstract: Contributions to the Coordination Chemistry of Technetium. Synthesis and Structural Clarification of New Complex Types and Their Testing in Selected Complex Catalytic Reactions

The steady-state kinetics of parallel reaction networks

The conventional kinetic analysis of an overall reaction (OR) is limited to a single sequential pathway of molecular steps at a time, based either on the general quasi-steady state (QSS) approach of Bodenstein, or on the much simpler but limited Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach based on assuming a single rate-determining step (RDS), the remaining being quasi-equilibrated (QE). We recently described a new algebraic methodology for deriving the QSS rate expression for a reaction sequence, which allowed interpretation of the final result in an Ohm's law form, i.e., OR rate=OR motive force/OR resistance of an equivalent electric circuit, where the consecutive mechanistic steps represent resistors in series. Here, we propose a similar Ohm's law form of QSS rate for a reaction system involving parallel pathways, whose equivalent electrical circuit derives directly from the reaction route (RR) Graph of its mechanism, as proposed earlier by us. The results are exact for a reaction network with mechanistic steps linear in intermediates concentrations, while they are approximate, albeit accurate, for non-linear step kinetics. We further show how the LHHW methodology, combined with the concept of intermediate reaction might be utilized to obtain the step resistances involved. For illustration, we utilize the relatively simple examples of: (1) the gas-phase hydrogen–bromine non-catalytic reaction (non-linear kinetics), and (2) zeolite catalyzed N 2O decomposition reaction (linear kinetics). However, the approach is useful for more complex non-catalytic, catalytic and enzymatic reactions networks as well.

Cycles Across an Equilibrium: A Kinetic Investigation of the Reverse and Forward WGS Reaction over a 2% Pt/CeO 2 Catalyst (Experimental Data and Qualitative Interpretation)

The forward and reverse water–gas-shift (WGS)-reactions over 2%Pt/CeO 2 catalyst are studied under steady-state conditions using a procedure which entails adjusting feed gas concentrations to cycle back and forth across the equilibrium line. The kinetic data were collected on “both sides” of the equilibrium under varying feed gas compositions (CO and CO 2 concentrations) and temperatures. The obtained data are interpreted using the concept of the apparent “kinetic resistance” developed recently based on the original non-linear theory [see Lazman, M.Z., Yablonsky, G.S., 2008. Overall reaction rate equation of single-route complex catalytic reaction in terms of hypergeometric series. Advances in Chemical Engineering 34, 47–102].

Biomimetic Chemoselectivity Control of Aldol Reactions

The synthesis and utilization of polymeric catalyst 1 bearing proline moieties as active sites is reported. This catalyst aggregate was applied successfully in aldol reactions of aldehydes and ketones in aqueous media. In a self-­aldol process of butanal, mainly reversibly formed β-hydroxyaldehyde 2 was obtained, whereas the formation of aldol condensation product 3 could be suppressed. These results were transferred to more complex dynamic catalytic cross-aldol reactions, giving rise to α,β-unsaturated ketones 4, favored over formation of self-condensation product 5. The likewise expected ketones 6 were not observed.

Estimating kinetic parameters of complex catalytic reactions using a curve resolution based method

A Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) based algorithm is used to extract kinetic parameters from on−line FT−NIR data of a series of Heck reactions between iodobenzene and n-butyl acrylate (NBA), measured at different temperatures with different catalysts. Four catalysts are prepared in situ from a Pd(OAc) 2 (precursor), and a ligand (triphenyl-phosphine and triphenyl-phosphite) in different ligand/Pd ratios. The entire dataset for all reactions is then used to calculate the kinetic parameters directly from the spectra. The algorithm is first validated using simulated data and then applied to the experimental data. The results are compared with those from a calibration model. Coupled with a kinetic model and some a priori information over the system, the algorithm yields good results within a range of feasible starting values of the model parameters.

Simulation of the kinetics of complex heterogeneous catalytic reactions under diffusion limitations

A method is suggested for determining kinetic parameters of arbitrary complex heterogeneous catalytic reactions under conditions such that the reaction rate is significantly affected by heat and mass transfer to and in the catalyst pellets. Linear relationships are established between the concentrations of the key and dependent reactants (diffusion stoichiometry relationships) and between temperature and the concentrations of the key components. A solution procedure is suggested for the problems involved in the determination of kinetic parameters for reactions proceeding under diffusion limitations. The procedure is illustrated by analysis of data earlier reported for steam methane reforming over a nickel catalyst.

Reaction Calorimetry as a Tool for Understanding Reaction Mechanisms: Application to Pd-Catalyzed Reactions

Reaction calorimetry can be employed for kinetic studies of complex catalytic reactions. In the calorimeter, the enthalpy balance around the reactor is continuously monitored and gives a profile of reaction heat vs time, which can be easily expressed in terms of reaction rate, conversion, and concentration of reactants/products. The data can be further analyzed using a methodology called reaction progress kinetic analysis, where the rate is expressed in terms of the concentration of substrates and a graphical expression is used to analyze the order in each substrate and in the catalyst. When studying a catalytic cycle, we can obtain useful information on catalyst activation/deactivation, substrate or product activation/deactivation, and the rate-limiting step. Thus, a detailed mechanistic picture can be obtained. In the present work we show an example of the role of reaction calorimetry in both fingerprinting and detailed study of some Pd-catalyzed aminations of aryl halides. These reactions are considere...