Engineering selectivity in heterogeneous catalysis: Ag nanowires as selective ethylene epoxidation catalysts

Controlling selectivity in heterogeneous catalysis is critical for the design of environmentally friendly catalytic processes that minimize the production of undesired byproducts and operate with high energy efficiency. We show that the Ag nanowire catalysts exhibit higher selectivity in the ethylene epoxidation reaction than conventional spherical particle catalysts. The higher selectivity of the nanowire catalysts was attributed to a higher concentration of the Ag(100) surface facets in the nanowire catalysts compared to the particle catalysts. Density functional theory calculations show that the transformation of the surface oxametallacycle intermediate to form the selective product, EO, is more favorable on the Ag(100) than on Ag(111). The studies show that recent advances in the controlled synthesis of uniform nanostructures with well-defined surface facets might provide an important platform for the design of highly selective heterogeneous catalysts.     

Synergetic promotion of Ba, Cs and Cl to silver supported catalyst for butadiene epoxidation

Summary Ag/-Al₂O₃ catalysts promoted by Ba, Cs and Cl were prepared and evaluated for butadiene epoxidation. The results indicated that Cs could enhance butadiene conversion, Cl was favorable for improving the selectivity to vinyloxirane (VO), and Ba probably plays an important role for catalyst stability during the long time on stream operation.     

On the mechanism of propylene epoxidation catalyzed by molybdenum naphthenate

The mechanism of the catalytic epoxidation of propylene with -phenylethyl hydroperoxide has been investigated. The epoxidation step is a molecular interaction between propylene and a complex formed from hydroperoxide and catalyst, while part of side products is formed in free radical reactions. This mechanism is valid for the kinetics of both epoxidation and catalytic decomposition of hydroperoxide.

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On the performance of Au(111) for ethylene epoxidation: a density functional study

This work presents a periodic density functional study of the epoxidation mechanism of ethylene on Au(111). It is found that, once atomic oxygen is adsorbed on the surface, partial oxidation to ethylene oxide becomes possible. Calculated transition state theory rate constants for the elementary steps involved in the reaction predict that the selectivity of Au(111) toward epoxide is of approximately 40% in good agreement with recent experimental findings for styrene epoxidation on Au(111).     

Control of ethylene epoxidation selectivity by surface oxametallacycles

Surface science experiments, DFT calculations, and kinetic isotope effect data are utilized to understand the elementary steps that govern the selectivity of silver catalysts for the partial oxidation of ethylene to produce ethylene oxide. It is proposed that selective and unselective pathways proceed via a common intermediate, the surface oxametallacycle. The structures of the transition states leading from this intermediate to selective and unselective products are calculated. From the calculated Gibbs free energies of activation for competing pathways, it is possible to predict selectivity to ethylene oxide as well as the magnitude of the kinetic isotope effect. The proposed mechanism is qualitatively and quantitatively in accord with experimental results.     

Theoretical investigation on reaction pathways for ethylene epoxidation on Ti-decorated graphene

The activities of atomic Ti-decorated graphene (Ti/dG) for ethylene epoxidation and competitive paths for acetaldehyde (AA) formation are investigated by means of density functional theory together with the D3 dispersion correction (UM06-L-D3). Two reaction mechanisms for ethylene epoxidation, namely concerted and stepwise mechanisms, were considered. The computational results reveal that the electron transfer from graphene can effectively enhance the catalytic activity of Ti atom. Without graphene support, atomic Ti becomes an inert metal for this reaction. Strong adsorption and significant activation of the reactant O₂ molecule were observed on the Ti-decorated graphene material. Over the O₂-adsorbed Ti/dG, the direct attack of the olefin on an peroxo oxygen center is preferred. The activation for this step is 10.9 kcal mol^?1. After the reaction, an ethylene oxide is formed with one atomic oxygen on top of Ti. Consequently, a gaseous ethylene reacts with the remaining O atom of TiO moiety for the formation of the second ethylene oxide molecule. The formation of ethylene oxide over the TiO/dG involves a two-step process which is the formations of oxametallacycle intermediate and EO, respectively. The calculated barriers for these two steps are 9.9 and 18.9 kcal mol^?1, respectively. Furthermore, the Ti/dG showed a lower activation barrier toward EO formation than that of AA. Therefore, our theoretical study suggests that atomic Ti-decorated graphene could possess catalytic activity for ethylene epoxidation comparable to that of potential catalysts.     

Why copper is intrinsically more selective than silver in alkene epoxidation: ethylene oxidation on Cu(111) versus Ag(111)

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically important. Although large-scale ethylene epoxidation is universally carried out with Ag catalysts, recent laboratory studies on single crystal surfaces show that Cu is intrinsically much more selective than Ag in the epoxidation of a variety of terminal alkenes. The reasons for this striking difference between Ag and Cu have been investigated by means of density functional theory. It is found that the fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation (the latter being the first step on the route to combustion). On Cu, epoxide formation is less activated than aldehyde formation; the opposite is true on Ag. This behavior is associated with a late transition state to epoxidation on Cu (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on Ag.     

The mechanism of polyleucine catalysed asymmetric epoxidation

Catalysis in the Julia-Colonna epoxidation of alpha,beta-unsaturated ketones is due to binding of the hydroperoxide enolate intermediate by the three N-terminal amidic N-H groups of alpha-helical poly-leucine; the N-terminal pair forms an oxy-anion hole, whilst the third aids displacement of hydroxide.     

On the mechanism of arylation of ethylene by palladium(II)

The phenylation of ethylene with benzene under the effect of palladium acetate at [C₂H₄] [Pd(OAc)₂] has been found to give styrene and trans-stilbene via parallel rather than consecutive reactions. The data obtained indicate that styrene and stilbene are formed through the same intermediate. The reaction mechanism is discussed.

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, [C₂H₄][Pd(OAc)₂] - . , . .

     

On the reaction between ethylene and cyclopentene,a radical mechanism

The bimolecular reaction is shown to proceed via a simple, nonchain, radical mechanism: with the net reaction the same as (1). Rate constants are estimated for each step and for each possible competing reaction and shown to yield minor or negligible side reactions in agreement with the observations of Lalonde and Back. Estimated and observed rate constants (1) and (1′) are in excellent agreement with the assumption that k'_-1 is a typical radical disproportionation with zero activation energy. From the reported data a best value for k′₁ is where θ = 2.303RT kcal/mol.     

New insights into ethene epoxidation on two oxidized Ag[111] surfaces

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime.     

Cs+ -selective membrane electrodes based on ethylene glycol-functionalized polymeric microspheres

A new strategy for improving the robustness of membrane-based ion-selective electrodes (ISEs) is introduced based on the incorporation of microsphere-immobilized ionophores into plasticized polymer membranes. As a model system, a Cs⁺-selective electrode was developed by doping ethylene glycol-functionalized cross-linked polystyrene microspheres (P-EG) into a plasticized poly(vinyl chloride) (PVC) matrix containing sodium tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate (TFPB) as the ion exchanger. Electrodes were evaluated with respect to Cs⁺ in terms of sensitivity, selectivity, and dynamic response. ISEs containing P-EG and TFPB that were plasticized with 2-nitrophenyl octyl ether (NPOE) yielded a linear range from 10⁻¹ to 10⁻⁵ M Cs⁺, a slope of 55.4 mV/decade, and a lower detection limit (log a_Cs) of −5.3. In addition, these membranes also demonstrated superior selectivity over Li⁺, Na⁺, and alkaline earth metal ion interferents when compared to analogous membranes plasticized with bis(2-ethylhexyl) sebacate (DOS) or membranes containing a lipophilic, mobile ethylene glycol derivative (ethylene glycol monooctadecyl ether (U-EG)) as ionophore.     

Mechanism, selectivity promotion, and new ultraselective pathways in Ag-catalyzed heterogeneous epoxidation

The selective oxidation of styrene on clean and modified Ag(100) surfaces has been studied by synchrotron fast XPS and temperature-programmed reaction spectroscopy. By following the time dependence of surface species, it is unequivocally demonstrated that the necessary and sufficient conditions for epoxide formation are oxygen adatoms and pi-adsorbed alkene molecules. Increased oxygen coverage and coadsorbed Cs have pronounced and opposite effects on epoxidation selectivity, consistent with the view that the valence charge density on O(a) is pivotal in determining this property. Submonolayer quantities of Cs nitrate generated in situ open a new, low-temperature ultraselective, epoxidation pathway thought to involve direct oxygen transfer from the oxyanion to the alkene.     

Theoretical study of the mechanism of alkane hydroxylation and ethylene epoxidation reactions catalyzed by diiron bis-oxo complexes. The effect of substrate molecules

The hybrid density functional method B3LYP was used to study the mechanism of the hydrocarbon (methane, ethane, methyl fluoride, and ethylene) oxidation reaction catalyzed by the complexes cis-(H(2)O)(NH(2))Fe(mu-O)(2)(eta(2)-HCOO)(2)Fe(NH(2))(H(2)O), I, and cis-(HCOO)(Imd)Fe(mu-O)(2)(eta(2)-HCOO)(2)Fe(Imd)(HCOO) (Imd = Imidazole), I_m, the "small" and "medium" model of compound Q of the methane monooxygenase (MMO). The improvement of the model from "small" to "medium" did not change the qualitative conclusions but significantly changed the calculated energetics. As in the case of methane oxidation reported by the authors previously, the reaction of all the substrates studied here is shown to start by coordination of the substrate molecule to the bridging oxygen atom, O(1) of I, an Fe(IV)-Fe(IV) complex, followed by the H-atom abstraction at the transition state III leading to the bound hydroxy alkyl intermediate IV of Fe(III)-Fe(IV) core. IV undergoes a very exothermic coupling of alkyl and hydroxy groups to give the alcohol complex VI of Fe(III)-Fe(III) core, from which alcohol dissociates. The H(b)-atom abstraction (or C-H bond activation) barrier at transition state III is found to be a few kcal/mol lower for C(2)H(6) and CH(3)F than for CH(4). The calculated trend in the H(b)-abstraction barrier, CH(4) (21.8 kcal/mol) > CH(3)F (18.8 kcal/mol) > or = C(2)H(6) (18.5 kcal/mol), is consistent with the C-H(b) bond strength in these substrates. Thus, the weaker the C-H(b) bond, the lower is the H(b)-abstraction barrier. It was shown that the replacement of a H-atom in a methane molecule with a more electronegative group tends to make the H(b)-abstraction transition state less "reactant-like". In contrast, the replacement of the H-atom in CH(4) with a less electronegative group makes the H(b)-abstraction transition state more "reactant-like". The epoxidation of ethylene by complex I is found to proceed without barrier and is a highly exothermic process. Thus, in the reaction of ethylene with complex I the only product is expected to be ethylene oxide, which is consistent with the experiment.     

A possible mechanism for enantioselectivity in the chiral epoxidation of olefins with

The origin of enantioselectivity in the Jacobsen-Katsuki reaction has been investigated by applying density functional calculations in combination with molecular mechanics methodologies. The calculations suggest that a high enantiomeric excess is connected to three specific features: 1) a chiral diimine bridge, which induces folding of the salen ligand(H2salen = bis(salicylidene)ethylenediamine), and hence the formation of a chiral pocket; 2) bulky groups at the 3,3'-positions of the salen ligand, which cause a preferential approach from the side of the aromatic rings; and 3) pi conjugation of the olefinic double bond, which confers regioselectivity and, consequently, enantioselectivity. In combination with experimental studies, the model also provides a rationale for the decrease in ee values when one of these components is missing.     

Organocatalysis of asymmetric epoxidation mediated by iminium salts: comments on the mechanism

Non-aqueous conditions developed for catalytic asymmetric epoxidation mediated by iminium salt organocatalysts have allowed NMR spectroscopy to be carried out on the reaction mixtures for the first time.     

Ethene epoxidation selectivity inhibited by twisted oxametallacycle: a DFT study on Ag surface-oxide

Competitive ethene oxidation pathways are presented for a p(4 x 4) surface-oxide phase on Ag(111) obtained from density functional theory (DFT) calculations. Both parallel routes are found to proceed from a common oxametallacycle intermediate (OMME) in agreement with previous mechanistic studies on low coverage O adatom phase, although acetaldehyde (AcH) is favored by almost 2 kcal/mol. An even more striking difference with pure metal surface appears with the oxide regeneration pathways, which are found non-rate controlling. Furthermore, a kinetic model is developed on the basis of these DFT calculations and yields 96% selectivity in favor of AcH for a simulation in realistic catalytic conditions (600 K and respective partial pressures of 1 atm for ethene and oxygen reactants). As a key finding, this low ethene epoxide selectivity is proposed to be directly linked to the conformational barrier of the pivotal intermediate. In fact, the elasticity of the ultrathin oxide adlayer enables a twisted OMME structure as a true minimum, which agrees well with orbital prerequisite of the concerted H migration toward AcH. On the contrary, the desired selective ring closure forming ethene epoxide (EO) requires conformational inversion although the eclipsed form lies 2 kcal/mol above.