Heterogeneous Alkene Epoxidation Research Articles

Synthesis and Catalytic Application of Robust Grafted Tripodal (Phenoxy) Ti-POSS Complexes in Crosslinked Hyperbranched Poly(siloxysilane) Matrix

We have studied synthesis, activity, epoxide selectivity, H2O2 efficiency, and recyclability of new robust heterogeneous alkene epoxidation catalysts of grafted tripodal (phenoxy) Ti-polyhedral oligomeric silsesquioxane (Ti-POSS) complexes in hyperbranched poly(siloxysilane) matrix. Crosslinked hyperbranched (CHB) poly(siloxysilane)-grafted [Ti(OC6H4CN-p){(HSiMe2(CH2)3)-(i-C4H9)6Si7O12}] (5) and CHB-poly(siloxysilane)-grafted [Ti(OC6H5){(HSiMe2(CH2)3)(i-C4H9)6Si7O12}] (6) have demonstrated high activity, epoxide selectivity (≥99%) and H2O2 efficiency (≥98%) in cyclohexene and 1-octene epoxidation with aqueous H2O2 higher than titanosilicate catalysts such as TS-1 and Ti-β, due to high Lewis acidity of the Ti-center and to the use of CHB-poly(siloxysilane) matrix. The catalysts are readily recyclable with retained activity.

Diamine-Decorated Graphene Oxide with Immobilized Gold Nanoparticles of Small Size for Alkenes Epoxidation with H2O2

Diamine-decorated graphene oxide (NH2-GO) is successfully synthesized by covalently intercalated ethanediamines into the flat planes and edge of the GO sheet. The diamines could be act as buffer layer, thereby avoiding the stacking of graphene sheets, and offer abundant metal-chelating sites to subsequently obtain well-dispersed metal nanoparticles of small size. After the in situ reduction and deposition of AuNPs, it was found that the Au particle size of Au0.9%@NH2-GO estimated to be around 2–5 nm. The as-prepared GO-supported small AuNPs exhibited good catalytic performance for the heterogeneous epoxidation of alkenes with H2O2 under mild conditions. Notably, they presented good expansibility for either bulky or less bulky alkenes in this reaction, owing to the facilitated efficiency of mass transfer of substrates and products resulted from the supporting effect of intercalated diamines and high surface area of GO. Moreover, these heterogeneous catalysts could be readily recovered and recycled for five times with the remained activity and selectivity. Gold nanoparticles of small size immobilized on the diamine-decorated graphene oxide has been developed, which employed as an efficient, universal and reusable catalyst for the epoxidation of alkenes using H2O2 as an oxidant.

New perspective to catalytic epoxidation of olefins by Keplerate containing Keggin polyoxometalates

Different Keggin encapsulated in Keplerate polyoxometalates (Mo72Fe30, PMo12 ⊂ Mo72Fe30, SiMo12 ⊂ Mo72Fe30 and BW12 ⊂ Mo72Fe30) have been synthesized and their catalytic efficiency in the epoxidation of olefins with hydrogen peroxide investigated. Results were confirmed that Keggin encapsulated in Keplerates could show higher catalytic activity than parent ones. These POM catalysts lead to heterogeneous epoxidation of alkenes by hydrogen peroxide with green features of convenient recovery, steady reuse, high conversion and selectivity, and simple preparation.

Amphiphilic Mesoporous Poly(Ionic Liquid) Immobilized Heteropolyanions Towards the Efficient Heterogeneous Epoxidation of Alkenes with Stoichiometric Hydrogen Peroxide

AbstractNew mesoporous poly(ionic liquid)s (MPILs) that feature a pure polycationic framework and tunable hydrophobicity were synthesized through the free‐radical polymerization of alkyl‐functionalized ionic liquids (ILs) and bisvinylimidazolium salt. Phosphotungstic (PW) anions were immobilized firmly on these MPILs with a high dispersion and density. The obtained catalyst was effective in the epoxidation of cis‐cyclooctene with stoichiometric hydrogen peroxide (H2O2) to give a high yield (>98 %) and a remarkable H2O2 efficiency (>98 %). The maximum turnover number and turnover frequency was 2375 and 685 h−1, respectively. The catalyst can be recovered easily by filtration and reused with stable activity. Other alkenes can be converted with high yields. The extraordinary performance was attributed to the pure ionic porous framework with an enhanced hydrophobicity that not only provides strong cation–anion interactions towards the stable immobilized PW anions with a high dispersion but also enables the strong affinity of the catalyst with both the substrate and oxidant.

ChemInform Abstract: Heterogeneous Epoxidation of Alkenes with H2O2 Catalyzed by a Recyclable Organic—Inorganic Polyoxometalate‐Based Framework Catalyst.

Four organic–inorganic polyoxometalate-based frameworks (HSiW-MOF, PMo-MOF, HPMo-MOF and PW-MOF) were synthesised and characterized by elemental analysis, UV-vis and FT-IR spectroscopy, cyclic voltammetry and XRD analysis. Organic–inorganic polyoxometalate-based frameworks could act as effective and reusable heterogeneous catalysts for selective epoxidation of alkenes with aqueous hydrogen peroxide in acetonitrile. In the presence of PW-MOF, various kinds of alkenes could selectively be converted into the corresponding epoxides with high conversion and selectivity. PW-MOF was stable to leaching, behaved as true heterogeneous catalysts, easily recovered by filtration, and reused four times with maintenance of the catalytic performance.

Heterogeneous epoxidation of alkenes with H2O2 catalyzed by a recyclable organic–inorganic polyoxometalate-based framework catalyst

PW-MOF as an efficient, recyclable and stable catalyst for the selective oxidation of alkenes with hydrogen peroxide.

Synthesis and Catalytic Applications of Chemically Grafted SiH-Functionalized Tripodal Ti-POSS Complexes in Crosslinked Hyperbranched Poly(siloxysilane)

This study investigated the synthesis, activity, epoxide selectivity, H2O2 efficiency, and recyclability of new heterogeneous alkene epoxidation catalysts prepared by chemical grafting of new SiH-functionalized tripodal Ti–polyhedral oligomeric silsesquioxane (Ti-POSS) complexes in hyperbranched poly(siloxysilane) via hydrosilation. Crosslinked hyperbranched poly(siloxysilane)-grafted [{(p-HSiMe2(CH2)2C6H4)(c-C6H11)6Si7O12}Ti(NMe2)] (11) and crosslinked hyperbranched poly(siloxysilane)-grafted [{(HSiMe2(CH2)3)(i-C4H9)6Si7O12}- Ti(NMe2)] (12) displayed high activity, epoxide selectivity (≥98 %), and H2O2 efficiency (≥97 %) in cyclohexene and 1-octene epoxidation with aqueous H2O2. Moreover, these catalysts were highly recyclable with retained activity and durability and proved to be truly heterogeneous. Using chemical grafting for the synthesis of 11 and 12 enhanced their recyclability and durability with retained activity. The high H2O2 efficiency can be attributed to the uniformly non-polar environment provided about Ti in 11 and 12 by the polymer; this results in low water concentrations and higher [alkene] : [H2O2] ratios at the Ti active site than in the rest of the solution. These effects enhance the epoxide selectivity and minimize leaching of titanium.

ChemInform Abstract: Efficient Heterogeneous Epoxidation of Alkenes by a Supported Tungsten Oxide Catalyst.

AbstractA heterogenous catalyst comprising of tungsten and zinc oxides (3.5 wt% W and 0.8 wt% Zn) on SnO2 support is found to efficiently promote the epoxidation of alkenes and the oxidation of amines, silanes, and sulfide by hydrogen peroxide.

Efficient Heterogeneous Epoxidation of Alkenes by a Supported Tungsten Oxide Catalyst

Optimierte Oxidation: Eine Kombination von Wolfram- und Zinkoxid auf einem SnO2-Träger (W–Zn/SnO2) dient als Heterogenkatalysator in der selektiven Oxidation von Alkenen, Aminen, Silanen, Sulfiden und anderen Substraten mit wässrigem H2O2 (siehe Schema). Die Produkte werden in hohen Ausbeuten erhalten, und der Katalysator kann ohne größere Leistungseinbuße mehrmals wiederverwendet werden.

Efficient Heterogeneous Epoxidation of Alkenes by a Supported Tungsten Oxide Catalyst

The tungsten and zinc oxides supported on SnO2 (W-Zn/SnO2) could act as effective and reusable solid catalysts for selective oxidn. with aq. H2O2. Various kinds of org. substrates such as olefins, amines, silanes, and sulfides could be converted into the corresponding epoxides, N-oxides, silanols, and sulfoxides in high to excellent yields. The present system was applicable to larger-scale epoxidn. of cyclooctene with one equiv. of H2O2 (20 mmol scale) with respect to cyclooctene, and the corresponding epoxide could be isolated in 88% yield. In this case, the turnover no. reached up to 650 and the turnover frequency was 108 h-1, and these values were larger than those of the heterogeneous tungsten catalysts. The obsd. catalysis for the present epoxidn. was truly heterogeneous and the recovered catalyst could be reused several times without an appreciable loss of its high catalytic performance. The kinetic, mechanistic, and spectroscopic studies show that polytungstates with dioxo groups play an important role in the present epoxidn. [on SciFinder(R)]

Polyoxometalate-based amino-functionalized ionic solid catalysts lead to highly efficient heterogeneous epoxidation of alkenes with H2O2

Polyoxometalate(POM)-based ionic hybrid nanospheres prepared by protonating and anion-exchanging amino-attached ionic liquid cations with Keggin POM-anions have proven to be highly efficient heterogeneous catalysts for the epoxidation of alkenes with H2O2, presenting advantages of convenient recovery, steady reuse, simple preparation, and flexible composition.

Tripodal titanium silsesquioxane complexes immobilized in polydimethylsiloxane (PDMS) membrane: Selective catalysts for epoxidation of cyclohexene and 1-octene with aqueous hydrogen peroxide

We investigated the activity, epoxide selectivity, H 2O 2 efficiency, and recyclability of heterogeneous alkene epoxidation catalysts prepared by encapsulation of tripodal Ti silsesquioxane complexes in polydimethylsiloxane (PDMS) membrane. We found that [Ti(NMe 2){( c-C 6H 11) 7Si 7O 12}]/PDMS ( 3) and [Ti(NMe 2){( i-C 4H 9) 7Si 7O 12}]/PDMS ( 4) displayed high activity, epoxide selectivity (⩾97%), and H 2O 2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H 2O 2. Moreover, these catalysts were highly recyclable. The high H 2O 2 efficiency can be attributed to the uniformly non-polar environment provided about the Ti silsesquioxane complex in 3 and 4 by the PDMS membrane, which presumably results in low local water concentrations and higher [alkene]:[H 2O 2] ratios at the Ti center than in the bulk reaction medium; both of these effects favor the epoxide selectivity and minimal leaching of titanium. Our study of the effect of solvent on catalyst activity revealed that acetonitrile is to be preferred as solvent, since methanol inhibits the epoxidation at long reaction times.

Polystyrene-supported triphenylarsines: useful ligands in palladium-catalyzed aryl halide homocoupling reactions and a catalyst for alkene epoxidation using hydrogen peroxide

The utility of both soluble (non-cross-linked) and insoluble (cross-linked) polystyrene-supported triphenylarsine reagents were examined. These reagents were prepared by standard radical polymerization methodology and used in palladium-catalyzed homocoupling reactions of aryl halides. The insoluble reagent was also used as a catalyst precursor in heterogeneous alkene epoxidation reactions in which aqueous hydrogen peroxide was the stoichiometric oxidant. For the aryl halide homocoupling reactions, both reagents worked well and afforded similar results. Unhindered aryl iodides afforded the best yields in the shortest reaction times compared to aryl bromides. The epoxidation reactions of unfunctionalized alkenes were not very efficient. This was probably due to the hydrophobicity of the polystyrene matrix, which did not swell in the reaction medium. Thus, since a microporous, gel-type polystyrene matrix was used, the majority of the arsine groups were inaccessible to the reaction components and therefore incapable of participating in catalysis.

Heterogeneous Alkene Epoxidation: Past, Present and Future

AbstractFor Abstract see ChemInform Abstract in Full Text.

Heterogeneous alkene epoxidation: past, present and future

Recent progress in the metal-catalysed heterogeneous epoxidation of alkenes is reviewed with respect to key mechanistic issues, drawing on results obtained with both single crystals and practical dispersed catalysts. New data are also presented and future prospects examined. It is shown that on silver surfaces oxygen adatoms are responsible for all the chemistry—both combustion and selective oxidation. In particular, the minimum necessary and sufficient conditions for epoxidation to occur are π-adsorbed alkene and O a. Studies with co-adsorbed F, Cl, Br, and I demonstrate that promotion of epoxidation selectivity correlates with halogen electronegativity—supporting the view that this is an electronic effect involving control of the valence charge density on O a. The observations that (i) selectivity markedly increases with increasing O a coverage and (ii) alkalis strongly decrease selectivity are in excellent accord with this view. In the presence of alkali, addition of NO x to the reaction gas leads to formation of absorbed nitrate species that open an ultra-selective epoxidation channel that involves transfer of oxygen from submonolayer quantities of adsorbed alkali nitrate to alkene, probably by a direct mechanism. Our comprehension of silver surface chemistry suggests that copper should also be an epoxidation catalyst—an expectation borne out in practice. In fact, Cu is more selective than Ag. When allylic hydrogen atoms are present (propene, allylbenzene and its isomers), Cu surpasses Ag by an even greater margin: copper continues to catalyse selective oxidation effectively, whereas silver simply burns these alkenes. The proximity of the C C bond to the Cu surface is critically important in determining the degree to which epoxidation occurs.

Manganese(III) Porphyrin Supported on Polystyrene as a Heterogeneous Alkene Epoxidation and Alkane Hydroxylation Catalyst.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Bonding and reactivity of styrene on Cu(110): heterogeneous alkene epoxidation without the use of silver

Styrene adsorption on oxygenated Cu(110) has been studied by temperature programmed reaction, fast XPS and NEXAFS. When the molecule is adsorbed on Cu metal sites in the presence of co-adsorbed oxygen, it undergoes efficient epoxidation to styrene epoxide. This is the first demonstration of alkene epoxidation on the surface of a clean metal, other than silver. However, styrene adsorbed on the ‘oxidic’ p(2×1)-O surface is catalytically inert. The NEXAFS data indicate that the reactant molecule is oriented approximately parallel to the surface plane on both the clean and oxidic p(2×1)-O surfaces, so that the very different reactivity is unlikely to be due to geometrical effects. The implications of these findings for the mechanism of heterogeneous alkene epoxidation are discussed.

Molecular Structural and Morphological Characterization of Polymer-Supported Mo(VI) Alkene Epoxidation Catalysts

Heterogeneous alkene epoxidation catalysts prepared by immobilization of species derived from MoO2(acac)2on two polymers, a polystyreneN-hydroxypropyl aminomethylpyridine resin and a polybenzimidazole resin, are known to behave differently. The two catalyst species have now been characterized extensively in terms of both the molecular structure of the Mo(VI) centers and the resin morphologies, and significant differences have been demonstrated in both aspects. Molecular structural characterization has involved elemental analysis and FTIR, electronic, and X-ray photoelectron spectroscopies, together with EXAFS analysis and associated modeling. Morphological studies have employed high-resolution transmission electron microscopy with Mo mapping, N2sorption, and Hg intrusion porosimetry and solvent imbibition measurements. The differences observed account for the observed variation in catalytic behavior, and these structural and morphological differences are discussed in detail.

Molecular mechanism of heterogeneous alkene epoxidation: A model study with styrene on Ag(111)

On atomically clean Ag(111) oxygen adatoms selectively oxidise styrene (phenylethene) to the epoxide with high efficiency, providing strong and direct confirmation of the mechanism advanced earlier for ethene epoxidation on single crystal Ag and on Ag catalysts. The effects of coadsorbed promoter species (K and Cl) are in good accord with the view that the principal mode of action of such species involves an electronic interaction which modifies the ability of adsorbed atomic oxygen to act as an electrophilic reagent towards the adsorbed alkene.

Molecular mechanism of heterogeneous alkene epoxidation: A model study with styrene on Ag(111)

On atomically clean Ag(111) oxygen adatoms selectively oxidise styrene (phenylethene) to the epoxide with high efficiency, providing strong and direct confirmation of the mechanism advanced earlier for ethene epoxidation on single crystal Ag and on Ag catalysts. The effects of coadsorbed promoter species (K and Cl) are in good accord with the view that the principal mode of action of such species involves an electronic interaction which modifies the ability of adsorbed atomic oxygen to act as an electrophilic reagent towards the adsorbed alkene.