Examples Of Heterogeneous Catalysis Research Articles

Heterogeneous Copper-Catalyzed Cross-Coupling for Sustainable Synthesis of Chiral Allenes: Application to the Synthesis of Allenic Natural Products.

Classical Crabbé type SN 2' substitutions of propargylic substrates has served as one of the standard methods for the synthesis of allenes. However, the stereospecific version of this transformation often requires either stoichiometric amounts of organocopper reagents or special functional groups on the substrates, and the chirality transfer efficiency is also capricious. Herein, we report a sustainable methodology for the synthesis of diverse 1,3-di and tri-substituted allenes by using a simple and cheap cellulose supported heterogeneous nanocopper catalyst (MCC-Amp-Cu(I/II)). This approach represents the first example of heterogeneous catalysis for the synthesis of chiral allenes. High yields and excellent enantiospecificity (up to 97 % yield, 99 % ee) were achieved for a wide range of di- and tri-substituted allenes bearing various functional groups. It is worth noting that the applied heterogeneous catalyst could be recycled at least 5 times without any reduced reactivity. To demonstrate the synthetic utility of the developed protocol, we have applied it to the total synthesis of several chiral allenic natural products.

Heterogeneous Copper‐Catalyzed Cross‐Coupling for Sustainable Synthesis of Chiral Allenes: Application to the Synthesis of Allenic Natural Products

AbstractClassical Crabbé type SN2' substitutions of propargylic substrates has served as one of the standard methods for the synthesis of allenes. However, the stereospecific version of this transformation often requires either stoichiometric amounts of organocopper reagents or special functional groups on the substrates, and the chirality transfer efficiency is also capricious. Herein, we report a sustainable methodology for the synthesis of diverse 1,3‐di and tri‐substituted allenes by using a simple and cheap cellulose supported heterogeneous nanocopper catalyst (MCC‐Amp‐Cu(I/II)). This approach represents the first example of heterogeneous catalysis for the synthesis of chiral allenes. High yields and excellent enantiospecificity (up to 97 % yield, 99 % ee) were achieved for a wide range of di‐ and tri‐substituted allenes bearing various functional groups. It is worth noting that the applied heterogeneous catalyst could be recycled at least 5 times without any reduced reactivity. To demonstrate the synthetic utility of the developed protocol, we have applied it to the total synthesis of several chiral allenic natural products.

Kinetic Isolation between Turnovers on Au18 Nanoclusters: Formic Acid Decomposition One Molecule at a Time

Formic acid (HCOOH or FA) is a clean, safe, and renewable hydrogen storage material. Although Au catalysts decompose vapor-phase FA with high activity and selectivity toward hydrogen, the active site and reaction mechanism remain unclear. Here, we show that the subnanometric Au18 cluster (0.8 nm in diameter) is likely the active species for FA decomposition. We performed coverage self-consistent, density functional theory-based kinetic Monte Carlo simulations of FA decomposition on gas-phase Au18 clusters, predicting 100% selectivity toward hydrogen and turnover frequencies close to experimentally determined values. The active site is made up of a triangular ensemble of three atoms each possessing a coordination number of 5. Although there are two active site ensembles on the Au18 cluster, their occupations are strongly correlated because of strong, stabilizing interactions between pairs of open-shell adsorbates mediated by the superatomic nature of the cluster. Because the occupation of the active sites blocks the dissociation of additional HCOOH molecules, there is kinetic isolation between turnovers: only one HCOOH molecule can dissociate on the cluster at a time. This explains the extraordinary, experimentally observed selectivity of Au catalysts toward HD during decomposition of HCOOD and DCOOH. Our work offers nanoscale insights into the reaction mechanisms of FA decomposition over Au. This represents a rare example of heterogeneous catalysis by a cluster that catalyzes reactants one molecule at a time. Our work on Au18 thus sheds light on how the unique electronic properties of subnanometric clusters can be used to design quasi-molecular catalysts with high activity and selectivity.

Special Issue: Frontiers in Catalysis Research—The Catalysis Research Center of Technische Universität München

Special Issue: Frontiers in Catalysis Research—The Catalysis Research Center of Technische Universität München

A Model for the Interfacial Kinetics of Phospholipase D Activity on Long-Chain Lipids

The membrane-active enzyme phospholipase D (PLD) catalyzes the hydrolysis of the phosphodiester bond in phospholipids and plays a critical role in cell signaling. This catalytic reaction proceeds on lipid-water interfaces and is an example of heterogeneous catalysis in biology. Recently we showed that planar lipid bilayers, a previously unexplored model membrane for these kinetic studies, can be used for monitoring interfacial catalytic reactions under well-defined experimental conditions with chemical and electrical access to both sides of the lipid membrane. Employing an assay that relies on the conductance of the pore-forming peptide gramicidin A to monitor PLD activity, the work presented here reveals the kinetics of hydrolysis of long-chain phosphatidylcholine lipids in situ. We have developed an extension of a basic kinetic model for interfacial catalysis that includes product activation and substrate depletion. This model describes the kinetic behavior very well and reveals two kinetic parameters, the specificity constant and the interfacial quality constant. This approach results in a simple and general model to account for product accumulation in interfacial enzyme kinetics.

Heterogeneous Catalysis: On Bathroom Mirrors and Boiling Stones

Though heterogeneous nucleation of liquid droplets on a smooth surface (such as a bathroom mirror) is a classical topic in nucleation theory, it is not well-known that this topic is actually a pedagogical example of heterogeneous catalysis: the one and only effect of the surface is to lower the activation Gibbs energy of droplet formation. In contrast to, for example, catalysis involving molecular reactions, the lowering of the activation barrier can be calculated with basic thermodynamics. This is an instructive exercise for a physical chemistry course that also explains to students why boiling stones have to be added to distillation vessels.

Catalytic Oxidation of Sulfur Dioxide by Activated Carbon: A Physical Chemistry Experiment

The experiment described in this paper is concerned with the use of activated carbon as a catalyst for oxidation of SO2 to sulfuric acid. The experiment is a good example of heterogeneous catalysis and environmental clean-up. It is simple to set up and uses a conductometer (conductivity meter) to follow the reaction. We describe the experiment in detail, from the preparation of sulfur dioxide to its oxidation by oxygen, the reaction catalyzed by activated carbon. Typical results and suggestions for performing a larger study of the kinetics of the reaction are presented.

Langmuir‐hinshelwood model of soil phosphorus kinetics1

A mathematical model is needed to relate the dynamics of soil phosphorus (P) chemistry in a batch reactor to the soil/solution ratio and to initial P in the reactor. The Langmuir‐Hinshelwood model appears to describe this system quite well. According to this model reversible adsorption of P from solution to the colloidal surface (Langmuir component) is followed by an irreversible reaction of the surface species (Hinshelwood component). The system is an example of heterogeneous catalysis. Adsorption follows 2nd order kinetics related to solution P concentration and concentration of available surface sites for adsorption. Desorption and reaction are assumed to follow 1st order kinetics. The system is described by three simultaneous equations, two of which are 1st order nonlinear ordinary differential equations. Numerical integration is by the Euler finite difference method. Data from the literature are used to calibrate the model and to demonstrate its characteristics for a sandy soil. Rapid drop in solution P concentration is followed by a gradual decline. The first step in calibration is to assume quasi‐equilibrium in the early phase (analogous to that in enzyme kinetics), which leads to values for the total concentration of adsorption sites, So, and the Langmuir coefficient, potassium (K). The second step then estimates the adsorption coefficient, ka, and the reaction coefficient, kr. Full simulation of the transient process over time requires great care in size of time step, t, to maintain stability and accuracy in the procedure. Analysis shows a linear correlation between total surface sites, So, and soil/solution ratio, M, as expected.

Heterogeneous and homogeneous hydrogen kinetics in plasma chemistry

A review of gas phase and surface kinetics of hydrogen radicals in plasma chemistry is presented. It is shown that the presence of rovibrationally excited hydrogen molecules strongly influences the mechanisms of the chemical reactions. Both homo- and heterogeneous collisional processes are analysed in that respect. As will be shown, ionizing plasmas are dominated by gas phase rovibrational excitation phenomena. By contrast the kinetics of recombining plasmas are strongly influenced by heterogeneous elementary processes, where they can be distinguished in a natural way. Examples of heterogeneous catalysis in plasmas, i.e. particle excitation phenomena at surfaces, are discussed.

Use of isotopic transient kinetics in heterogeneous catalysis

The main features of isotopic transient kinetics -principle, implementation, performances and limits- are reviewed through various examples of heterogeneous catalysis (hydrogenation, oxidation). Powerful “in situ” method for surface investigation, this technique is shown to provide unique information on mechanistic pathways.