Selective Epoxidation Research Articles

Core–shell structured Fe3O4@SiO2 supported cobalt(ii) or copper(ii) acetylacetonate complexes: magnetically recoverable nanocatalysts for aerobic epoxidation of styrene

Organic–inorganic hybrid heterogeneous nanocatalysts were successfully fabricated via covalent anchoring of cobalt(II) or copper(II) acetylacetonate complexes ([Co(acac)2] or [Cu(acac)2]) onto core–shell structured Fe3O4@SiO2 previously functionalized with 3-aminopropyltriethoxysilane (APTES). Surface functionalized nanomaterials were analyzed by a series of characterization techniques such as SEM, TEM, XRD, FT-IR, XPS, ICP-AES and VSM. The catalytic performance of the prepared nanocatalysts was evaluated in the epoxidation of styrene using eco-friendly air as the oxygen source. Both of the nanocomposites Fe3O4@SiO2–NH2–Co and Fe3O4@SiO2–NH2–Cu presented excellent styrene conversion (90.8% vs. 86.7%) and good epoxide selectivity (63.7% vs. 51.4%), much higher than the corresponding homogeneous counterparts. In addition, the magnetically recoverable nanocatalysts (MRNCs) can be conveniently separated and recovered from the reaction system by applying an external magnetic field and reused for four cycles with insignificant loss of catalytic activity. These results demonstrate that the heterogeneous nanocatalysts possess potential applications for green and sustainable development.

Eco-friendly solvents and amphiphilic catalytic polyoxometalate nanoparticles: a winning combination for olefin epoxidation

Eighteen eco-friendly solvents were examined to carry out the epoxidation of olefins with the amphiphilic catalytic dodecyltrimethylammonium polyoxometalate nanoparticles [C12]3[PW12O40] in comparison with [H]3[PW12O40] and [Na]3[PW12O40]. Surprisingly, the screening of solvents with cyclooctene has revealed that the [C12]3[PW12O40] catalyst is much more active with initial turn-over frequencies, TOF0, increasing up to a factor of 10. Moreover, the reaction occurs at competitive rates in four relevant solvents, i.e. cyclopentyl methyl ether, 2-methyl tetrahydrofuran, methyl acetate and glycerol triacetate, for which TOF0 values are higher than 260 h−1. The recyclability of the systems is demonstrated and the scope of substrates has been successfully extended to cyclohexene, 1-octene, limonene, 3-carene, α-pinene, β-pinene and neryl acetate with good epoxide selectivity. The catalytic performances in the “green” solvent are assigned to the formation of stable [C12]3[PW12O40] nanoparticle dispersions which have been characterized by transmission electron microscopy and dynamic and multiple light scattering experiments. Finally, the Kamlet–Taft parameters were measured in order to correlate the physicochemical properties of the solvents and the catalytic activity.

Preparation of γ-Fe2O3@C@MoO3 core/shell nanocomposites as magnetically recyclable catalysts for efficient and selective epoxidation of olefins

Magnetic γ-Fe2O3@C@MoO3 core-shell structures were fabricated via a general approach and characterized by several techniques such as XRD, XPS, ICP-AES, Raman etc. The catalytic performance of them has been demonstrated in heterogeneously catalyzed oxidation of cyclooctene with tert-butyl hydroperoxide (tBuOOH) as an oxidant, and high conversion (100%) and selectivity (over 99%) were achieved with relatively low catalyst dosage under mild conditions. It should be emphasized that the hybrid catalyst could be easily separated by an external magnet and reused up to 9 times with no significant loss of activity and selectivity. The reaction mechanism over MoO3 is also discussed on the basis of the FTIR and XPS measurements. It is suggested that the high catalytic activity is related to the formation of the transition state, a Mo-alkyl peroxide and olefin complex, which is beneficial for an efficient oxygen atom transfer by converting the intermolecular process directly into an intramolecular process.

Tetradentate N2O2 type Nickel(II) Schiff base complexes derived from meso-1,2-diphenyle-1,2-ethylenediamine: synthesis, characterization, crystal structures, electrochemistry, and catalytic studies

A series of Ni(II) complexes of salen type Schiff base ligands were synthesized and characterized. The ligands were synthesized from the condensation of meso-1,2-diphenyle-1,2-ethylenediamine with salicylaldehyde (H2L1), 5-bromosalicylaldehyde (H2L2), 5-bromo-3-nitrosalicylaldehyde (H2L3), and 2′-hydroxyacetophenone (H2L4). The complexes were characterized by means of 1HNMR, IR, and UV–Vis spectroscopy and elemental analysis. Crystal structures of NiL3 and NiL4 were also determined by x-ray crystallography. Electrochemistry of the complexes was studied by means of cyclic voltammetry. Catalytic performance of the complexes was studied in the oxidation of cyclooctene using tert-butylhydroperoxide (TBHP) as oxidant. Various factors including solvent type, reaction temperature, time, catalyst amount, and substrate to oxidant ratio were optimized. Solvent free oxidation of cyclooctene with these catalysts was also studied. Increased catalytic activity and higher epoxide selectivity was achieved in solvent-free conditions. NiL4 with more electron donating substituents on the ligand was the most efficient oxidation catalyst.

A comparative DFT study on the mechanism of olefin epoxidation catalyzed by substituted binuclear peroxotungstates ([SeO4WO(O2)2MO(O2)2]n− (M = TiIV, VV, TaV, MoVI, WVI, TcVII, and ReVII))

The selective epoxidation of olefins catalyzed by substituted binuclear peroxotungstates ([SeO4WO(O2)2MO(O2)2]n− (M = TiIV, VV, TaV, MoVI, WVI, TcVII, and ReVII)) are investigated at the density functional theory level. The computational results reveal that the activation barrier corresponding to the oxygen transfer to the ethylene step decreases with M = V > Ti > Ta > Mo > W > Tc > Re. The Re and Tc substituted species can effectively improve the catalytic activity with lower Gibbs free energy barriers of 22.53 and 25.82 kcal/mol relative to the others under normal conditions. This suggests that Re and Tc center peroxo complexes would improve the catalytic performance. The higher activity of the substituted species is directly attributed to the lower energy of the σ*(OO) orbital. The reaction barriers in epoxidation process are rationalized by analyzing the atomic charge, the OO bond length, and the interaction between the substituted metal and the peroxo group of the precursor complexes. © 2013 Wiley Periodicals, Inc.

Solvent-free biodiesel epoxidation

Biodiesel emerged again recently as an alternative for fossil fuels. Besides energy, biodiesel can be used as raw material to synthesize high value products such as epoxides. Indeed, epoxides are versatile intermediates in organic synthesis for numerous reactions due to the high reactivity of the oxirane ring. Actually common epoxidation by peracids in organic solvent can last several hours according to the double bonds number. The solvent confers selectivity to the reaction, preventing di-hydroxylation. Alternatively solvent-free reactions can be done in shorter times, but hydroxylation is less controlled. Here, we set up the synthesis of epoxides from methyl and ethyl esters of waste cooking oil, without solvent or metal catalyst. We tested molar ratio of hydrogen peroxide and formic acid, double bond number, reaction time, and temperature. Results show that the highest epoxide yields and selectivity, with high conversion of the double bonds, were obtained for temperature reactions between 50 and 65 °C, reaction times from 2 to 3 h, and molar ratio of 20/2/1. For those conditions, the double bonds conversion is equal or near 100 %, with epoxide yield and selectivity between 85 and 93 %. Organic solvent suppression, besides being environmentally friendly, also saves reaction time and subsequent purification steps.

Cooperative Tandem Catalysis by an Organometallic Complex and a Metalloenzyme

AbstractAlthough chemical and enzymatic catalysts have been combined, reactions in which an organometallic catalyst and a metalloenzyme work cooperatively to create products, which cannot be generated with either catalyst alone or in comparable yields by sequential reactions of the two catalysts, have not been reported. Such reactions are challenging to achieve, in part because the milieu in which these catalysts operate are typically different. Herein, two classes of catalysts are demonstrated to react cooperatively in the same system. Combination of a metathesis catalyst and a P450 enzyme lead to a dynamic equilibration of alkenes and a selective epoxidation of the cross‐metathesis products. These results show the potential of combining the two classes of catalysts for synthetic transformations.

Cooperative Tandem Catalysis by an Organometallic Complex and a Metalloenzyme

Although chemical and enzymatic catalysts have been combined, reactions in which an organometallic catalyst and a metalloenzyme work cooperatively to create products, which cannot be generated with either catalyst alone or in comparable yields by sequential reactions of the two catalysts, have not been reported. Such reactions are challenging to achieve, in part because the milieu in which these catalysts operate are typically different. Herein, two classes of catalysts are demonstrated to react cooperatively in the same system. Combination of a metathesis catalyst and a P450 enzyme lead to a dynamic equilibration of alkenes and a selective epoxidation of the cross-metathesis products. These results show the potential of combining the two classes of catalysts for synthetic transformations.

A new synthetic approach to oxidation organocatalysts supported on Merrifield resin using plasma-enhanced chemical vapor deposition

Described here is the first application of plasma-enhanced chemical vapor deposition (PE-CVD) to the synthesis of catalytically active materials, prepared by covering Merrifield resin beads with an oxygen-containing fluorocarbon thin film deposited in a hexafluoropropene-O2 plasma. Such modified resins catalyze both the selective epoxidation of trans-β-methylstyrene and its double oxidative cleavage to benzaldeyde in organic–water biphasic media at room temperature, using potassium monoperoxysulfate (KHSO5) as the terminal oxidant. Interestingly, the epoxide/benzaldehyde product ratio strictly depends on the conditions adopted for catalyst generation. This, coupled with evidence for the presence of carbonyl groups on the surface of treated resins, point to dioxirane and singlet oxygen (1O2)-mediated oxidations; 1O2 being produced in the decomposition of KHSO5. Compared to traditional synthetic procedures to obtain similar catalytic active materials, the application of PE-CVD is eco-sustainable and less expensive. Also, the activity of catalysts can be fully restored upon iterative re-treatment of the exhausted resin.

Calcium oxide supported gold nanoparticles as catalysts for the selective epoxidation of styrene by t-butyl hydroperoxide

Gold nanoparticles are deposited on basic CaO supports as catalysts for the selective conversion of styrene into styrene oxide. Synthetic methods, gold loading and calcination temperatures are varied to permit an understanding of their influence on gold nanoparticle size, the presence of cationic gold species and the nature of interaction between the gold nanoparticles and the CaO support. Based on these studies, optimal conditions are designed to make the Au/CaO catalyst efficient for the selective epoxidation of styrene.

Filled cobalt nanoparticles into carbon nanotubes as a rapid and high-efficiency catalyst for selective epoxidation of styrene with molecular oxygen

A new nanocatalyst has been synthesized by a simple and effective wet chemical method confined magnetic cobalt nanoparticles within carbon nanotubes (CNTs). We characterized the catalysts by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The TEM results demonstrated that the cobalt nanoparticles were filled into the CNTs successfully. In the meantime, the CNTs mesostructure were not destroyed in multistep synthetic procedure. In the catalytic activity test, liquid phase epoxidation of styrene by molecular oxygen was carried out at atmospheric pressure employed Co/CNTs (in) catalysts. The results showed that styrene conversion over 99% and epoxide selectivity of 93% for 1h reaction. The catalysts can be easily separated from the reaction mixtures by external magnet, and reused three times without significant loss in catalytic activity.

Synthesis, characterization and catalytic property of CuO and Ag/CuO nanoparticles for the epoxidation of styrene

CuO nanorodes, CuO nanoplates and Ag/CuO nanoparticles were synthesized in the presence of polyethylene glycol by depositional in alkaline environment. Oxide nanoparticles were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared absorption spectra (FT-IR). CuO and Ag/CuO nanoparticles show high catalytic activity for the selective epoxidation of styrene to styrene oxide by TBHP. Under the optimized reaction condition, the oxidation of styrene catalyzed by CuO nanorods gave 100% conversion with 60 and 35% styrene oxide and benzaldehyde, respectively. Ag/CuO gave 99% conversion and styrene oxide (71%) and benzaldehyde (12%) being the major product.

Electronic Tuning of Iron–Oxo‐Mediated CH Activation: Effect of Electron‐Donating Ligand on Selectivity

We have reported previously that an iron(III) complex supported by an anionic pentadentate monoamido ligand, dpaq(H) (dpaq(H) =2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido), promotes selective CH hydroxylation with H2 O2 with high regioselectivity. Herein, we report on the preparation of Fe(III) -dpaq derivatives that have a series of substituent groups at the 5-position of a quinoline moiety in the parent ligand dpaq(H) (dpaq(R) , R: OMe, H, Cl, and NO2 ), and examine them with respect to their catalytic activity in CH hydroxylation with H2 O2 . As the substituent group becomes more electron-withdrawing, both the selectivity and the turnover number increase, but the selectivity of epoxidation shows the opposite trend.

The positive role of cadmium in TS-1 catalyst for butadiene epoxidation

A series of Cd modified titanium silicalite 1 catalysts with different Cd content (xCd-TS-1, x=1–15) were successfully prepared by ultrasound impregnation. Epoxidation of butadiene over these catalysts were investigated using hydrogen peroxide as oxidant, which indicated that Cd greatly improve the catalytic performance of TS-1 and the selectivity of epoxide. Various characterization methods including quantum chemical calculation were employed to explore the specific roles of Cd in promoting TS-1 catalytic activity. Theoretical calculation consistently suggested TiO bond were weakened owing to the introduction of Cd, which resulted in the structure of Cd-TS-1 becoming more relaxant. As a consequence, it is favorable to methanol solvent and H2O2 interacting with the Ti active site to form five-member transition state during reaction. It was observed that catalysts modified with 1–5wt% Cd presented both high catalytic activity and good reusability. The highest yield of 0.63mol/L of vinyloxirane (VO) was obtained, while turnover number (TON, determined as the molar VO obtained per molar Ti atom) could reach to 1466.

Tuning the catalytic activity of the heteronuclear coordination polymers [CoxZn1 − x(tdc)(bipy)] and [CoxZn1 − x(Me2trz–pba)2] in the epoxidation of cyclooctene via isomorphous substitution

For two series of isostructural, heteronuclear 3D coordination polymers containing Co and Zn, i.e., [CoxZn1−x(tdc)(bipy)] and [CoxZn1−x(Me2trz–pba)2], with nCo/nZn-ratios from 0.00 to 1.00, the epoxidation of cyclooctene with tert-butyl hydroperoxide in liquid toluene at 348K was studied. In dependence of the Co-content in the material, the catalytic activity steadily increases with the Co-free materials being inactive and the Zn-free materials showing the highest catalytic activity (cyclooctene conversion of 32% and epoxide selectivity of 50%). Although the catalytic conversion occurs at the outer crystallite surface, the results prove that a sensitive tuning of the catalytic properties of coordination polymers and metal-organic frameworks can be achieved by isomorphous substitution with redox active elements.

Surface Functionalization of Supported Mn Clusters to Produce Robust Mn Catalysts for Selective Epoxidation

A robust heterogeneous Mn catalyst for selective epoxidation was prepared by the attachment of a Mn4 oxonuclear complex [Mn4O2(CH3COO)7(bipy)2](ClO4)·3H2O (1) on SiO2 and the successive stacking of SiO2-matrix overlayers around a supported Mn cluster. The structures of supported Mn catalysts were characterized by means of FT-IR spectroscopy, diffuse-reflectance UV/vis spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and Mn K-edge X-ray absorption fine structure. A ligand exchange reaction between the CH3COO ligand of 1 and surface silanol group produced a SiO2-supported Mn cluster (2), whose coordination structure was similar to 1. Subsequent heating of 2 under vacuum yielded supported Mn clusters (3, 4) through the partial elimination of CH3COO ligands. The surface-attached Mn clusters of 2, 3, and 4 were easily released to a reaction solution under epoxidation conditions (Mn leaching: approximately 50%), although they were active for epoxidation of trans-stilbene (the conversion of trans-stilbene, 99%, and the selectivity of trans-stilbene epoxid, 96%, for 6 h on 3). We found that the functionalization of the supported Mn cluster on 2 with surface SiO2-matrix overlayers altered the reactivity of the supported Mn cluster. Dimeric Mn species (5c) with reduced Mn oxidation state and coordination numbers was formed together with a reaction nanospace surrounded by the SiO2-matrix overlayers. By optimizing the stacking manner of the SiO2-matrix overlayers, the durability of the Mn catalyst was remarkably improved from leaching (the Mn leaching reached the minimum value of 0.01%), and active and stable epoxidation performances were successfully achieved in the heterogeneous phase (the conversion of trans-stilbene, 97%, and the selectivity of trans-stilbene epoxide, 91%, for 31 h on 5c).

Practical and Highly Selective Sulfur Ylide-Mediated Asymmetric Epoxidations and Aziridinations Using a Cheap and Readily Available Chiral Sulfide: Extensive Studies To Map Out Scope, Limitations, and Rationalization of Diastereo- and Enantioselectivities

The chiral sulfide, isothiocineole, has been synthesized in one step from elemental sulfur, γ-terpinene, and limonene in 61% yield. A mechanism involving radical intermediates for this reaction is proposed based on experimental evidence. The application of isothiocineole to the asymmetric epoxidation of aldehydes and the aziridination of imines is described. Excellent enantioselectivities and diastereoselectivities have been obtained over a wide range of aromatic, aliphatic, and α,β-unsaturated aldehydes using simple protocols. In aziridinations, excellent enantioselectivities and good diastereoselectivities were obtained for a wide range of imines. Mechanistic models have been put forward to rationalize the high selectivities observed, which should enable the sulfide to be used with confidence in synthesis. In epoxidations, the degree of reversibility in betaine formation dominates both the diastereoselectivity and the enantioselectivity. Appropriate tuning of reaction conditions based on understanding the reaction mechanism enables high selectivities to be obtained in most cases. In aziridinations, betaine formation is nonreversible with semistabilized ylides and diastereoselectivities are determined in the betaine forming step and are more variable as a result.

Efficient and ecofriendly options for the chemoselective oxidation of alkenes using manganese porphyrin and dioxygen

Selective oxidation of styrene to benzaldehyde was carried out for the first time in homogeneous system by using a green and easily prepared manganese porphyrin as an efficient catalyst under elevated oxygen pressure in the absence of additive and co-reductant. Effect of temperature (80–130°C), pressure (0.6–1.0MPa), the amount of catalyst (2–15mg) and reaction media was studied to obtain the optimum conditions for the reaction. The best results were obtained at 110°C and 0.8MPa pressure using manganese deuteroporphyrin dimethyl ester (Mn-DPDME) with 98% styrene conversion and 82% selectivity of benzaldehyde in benzonitrile solvent. In order to elaborate the effect of meso substituents on porphyrin catalytic activity and stability, four different Mn-porphyrins were selected and compared under the identical condition. It was found that the natural Mn-DPDME which has no substituents in the meso-sites but does have the methyl and propionate groups in the β-sites exhibited the higher activity than the synthetic manganese tetraphenylporphyrin, however, the later showed the superior stability and high selectivity of benzaldehyde. Furthermore, the catalytic method was widely used in the oxidation of various cycloalkenes, which exhibited excellent selectivity of epoxide. An alternative mechanism for the aerobic oxidation of styrene has been proposed and discussed.

Nickel(II) complex anchored on MCM-41 for the epoxidation of styrene by oxygen

A heterogeneous catalyst, MCM-41-Ni, was synthesized by anchoring a nickel(II) Schiff base complex onto amino-modified MCM-41. The catalyst was characterized by small angle X-ray diffraction, nitrogen adsorption, infrared spectroscopy, thermal gravimetric analysis, inductively coupled plasma and C, H, N elemental analysis, and transmission electron microscopy. MCM-41-Ni showed excellent catalytic activity for the epoxidation of styrene with O 2 as oxidant with high styrene conversion (95.2%) and epoxide selectivity (66.7%). The influence of reaction temperature, catalyst amount, solvent, and reaction time was studied. MCM-41-Ni showed good recyclability and can be reused four times. A nickel(II) Schiff base complex functionalized MCM-41 catalyst was used to catalyze epoxidation of styrene using green oxidant molecular oxygen as oxidant with styrene conversion of 95.2% and epoxide selectivity of 66.7%.

Synthesis, characterization and photo-epoxidation performance of Au-loaded photocatalysts

Titanium silicalite-1 (TS-1) was synthesized by hydrothermal crystallization. Au-loaded TS-1 (Au/TS-1) was prepared by the photo-deposition method. The prepared photocatalysts have been fully characterized by Raman, Brunauer–Emmett–Teller (BET), transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet visible light spectroscopy to reveal their structure, surface morphology and chemical composition. Photocatalytic activity of these photocatalysts was ascertained by gas phase photo-epoxidation of propylene-to-propylene oxide in the presence of molecular oxygen only under light irradiation. No sacrificial reductant such as hydrogen was used during the photoreaction. Au/TS-1 photocatalyst remained highly stable under reaction conditions. It is found that the increased Ti3+ sites due to the incorporation of Au will not only enhance the selectivity of the photocatalyst to form epoxide but also its stability during C3H6 conversion. The photo epoxidation of propylene to propylene oxide was carried out over Au-loaded photocatalysts. Au/TS-1 photocatalyst performed highly stable selective epoxide under reaction conditions. The increased Ti3+ sites due to the incorporation of Au not only enhanced selectivity of epoxidation, but also maintained stability during C3H6 conversion.