Heterogeneous Catalyst For Epoxidation Research Articles

Chromic Behaviors and Chemical Fixation of CO2 without Co-Catalysts of a Cationic Metal-Organic Framework

A cationic metal-organic framework (MOF) was constructed utilizing tetranuclear [Tb4(µ3-OH)4(COO)6]2− second building unit (SBU) and a tri(pyridinium)pyridine derivative linker, resulting in a three-dimensional (3D) framework. The MOF exhibits reversible ionochromic behavior through ion exchange and hydrochromic property via electron transfer (ET). Importantly, the MOF incorporates nucleophilic anion in the channel, thereby facilitating the cyclization reaction between carbon dioxide (CO2) and epoxides to yield carbonates without necessitating additional co-catalysts. Furthermore, the MOF exhibits Lewis acidic sites, contributing to its enhanced catalytic activity under mild conditions. The MOF functions as a heterogeneous catalyst for CO2 epoxidation, operating without needing co-catalysts or solvents, and exhibits chromic properties. The work presents a strategy for designing MOFs that can directly participate in the chemical fixation of CO2 without adding additional nucleophilic anions, and can also serve as chromic materials.

Facile encapsulation of a cobalt complex inside the micropores of ZIF-8 through the bottle around the ship method: A novel heterogeneous catalyst for epoxidation of alkenes

A novel and stable heterogeneous catalyst for alkene epoxidation reaction, named as CoDMG@ZIF-8, has been synthesized by facile encapsulation of Co(III) complex i.e. [Co(DMGH)(DMGH2)Cl2] (abbreviated as CoDMG, DMGH2 denotes for dimethylglyoxime and DMGH denotes for dimethylglyoxime mono anion) inside the ZIF-8 micropores via a solvothermal process. Based on the results of fourier transform-infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM), it was observed that the catalyst has maintained the structure, crystallinity and morphology of the parent ZIF-8 as well. Furthermore, CHN, DRS (diffuse reflectance spectra), energy dispersive X-ray (EDX), induced coupled plasma (ICP-OES), thermal gravimetric (TGA) and nitrogen adsorption-desorption (BET method) analyses have confirmed the encapsulation of the CoDMG complex. The epoxidation reaction was performed using tert‑butyl hydroperoxide (TBHP) in CH3CN solvent in the presence of catalyst. By optimizing the reaction parameters, a total conversion of 96 % with 82 % selectivity towards the epoxide product, was achieved. It was found that both CoDMG complex and ZIF-8 exhibited lower efficiency individually, while the encapsulation of this complex inside the ZIF-8 micropores has resulted to superior activity. Finally, the epoxidation of various alkenes such as cyclohexene, 2-norbornene, α-pinene, styrene and 1-hexene under the optimized reaction condition, have been studied and the catalytic mechanism was also proposed.

Tracking Coordination Environment and Reaction Intermediates in Homogeneous and Heterogeneous Epoxidation Catalysts via Ti L2,3-Edge Near-Edge X-ray Absorption Fine Structures.

Ti-based molecules and materials are ubiquitous and play a major role in both homogeneous and heterogeneous catalytic processes. Understanding the electronic structures of their active sites (oxidation state, local symmetry, and ligand environment) is key to developing molecular-level structure-property relationships. In that context, X-ray absorption spectroscopy (XAS) offers a unique combination of elemental selectivity and sensitivity to local symmetry. Commonly, for early transition metals such as Ti, K-edge XAS is applied for in situ characterization and subsequent structural analysis with high sensitivity toward tetrahedral species. Ti L2,3-edge spectroscopy is in principle complementary and offers specific opportunities to interrogate the electronic structure of five-and six-coordinated species. It is, however, much more rarely implemented because the use of soft X-rays implies ultrahigh vacuum conditions. Furthermore, the interpretation of the data can be challenging. Here, we show how Ti L2,3-edge spectroscopy can help to obtain unique information about both homogeneous and heterogeneous epoxidation catalysts and develop a molecular-level relationship between spectroscopic signatures and electronic structures. Toward this goal, we first establish a spectral library of molecular Ti reference compounds, comprising various coordination environments with mono- and dimeric Ti species having O, N, and Cl ligands. We next implemented a computational methodology based on multiplet ligand field theory and maximally localized Wannier orbitals benchmarked on our library to understand Ti L2,3-edge spectroscopic signatures. We finally used this approach to track and predict the spectra of catalytically relevant intermediates, focusing on Ti-based olefin epoxidation catalysts.

Titanium dioxide functionalized silicon carbide phases as heterogeneous epoxidation catalysts.

We report silicon carbide (SiC) based epoxidation catalysts constituted of a silicon carbide core and a silica/titania (SiO2/TiO2) shell. The catalysts were obtained via surface modification of SiC microparticles and were used as heterogeneous catalysts for the epoxidation of cyclohexene using tert-butyl hydroperoxide or cumyl hydroperoxide as the oxidant. Conversions up to 83% and selectivities of more than 90% were obtained.

Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation

Developing efficient heterogeneous catalysts for cyclic olefins epoxidation is highly attractive for meeting the growing need for various cyclic epoxides. Herein, hierarchical TS-1 zeolite with relatively abundant mesopores and less amount of surface hydroxyl groups was obtained by hydrothermal modification of an as-synthesized TS-1 zeolite with a mixed solution of ammonia, tetrapropylammonium bromide (TPABr) and KCl. The post-modified TS-1 zeolite exhibited much higher catalytic activity (52% conversion) and epoxide selectivity (98%) for the epoxidation of cyclopentene than the conventional TS-1 zeolites. The excellent catalytic activity of the hierarchical TS-1 could be mainly assigned to the enhancement of the mass transport ability and the accessibility of the active Ti species, while the improvement of epoxidation selectivity may be mainly related to the introduction of a certain amount of K+ that can effectively modulate the coordination environment of Ti species as well as the polarity of the zeolite. This work demonstrated that a highly active and selective catalyst for the H2O2-mediated cyclopentene epoxidation could be obtained by concurrently generating mesopore and extinguishing the unfavorable defective hydroxyl groups through the simple hydrothermal treatment of the conventional TS-1 zeolite with a mixed base/salt solution.

Opportunities and Emerging Challenges of the Heterogeneous Metal-Based Catalysts for Vegetable Oil Epoxidation

Opportunities and Emerging Challenges of the Heterogeneous Metal-Based Catalysts for Vegetable Oil Epoxidation

Copper-complexed dipyridyl-pyridazine functionalized periodic mesoporous organosilica as a heterogeneous catalyst for styrene epoxidation.

A new heterogeneous catalyst has been synthesized by immobilization of a copper complex on dipyridyl-pyridazine functionalized periodic mesoporous organosilica (dppz-vPMO). This ordered support was first prepared by a co-condensation reaction between vinyltriethoxysilane and 1,2-bis(trimethoxysilyl)ethane and further post-functionalized through a hetero Diels-Alder reaction with 3,6-di-2-pyridyl-1,2,4,5-tetrazine. Techniques such as XRD, N2 isotherms, TEM, 13C NMR, XPS and DRIFT, among others, were employed to characterize the surface functionalized materials. These results have proven the ordered mesostructure of the materials as well as the presence of novel nitrogen-chelating heterocyclic compounds on the pore surface after the post-modification process. Additionally, the successful anchoring of a copper complex on the dipyridyl-pyridazine (dppz) ligands has been confirmed. The resulting material was evaluated as a heterogeneous catalyst in the epoxidation of styrene using tert-butylhydroperoxide (TBHP) as an oxidant. Under the optimized reaction conditions, Cu@dppz-vPMO showed a high styrene conversion (86.0%) and a remarkable selectivity to styrene oxide (41.9%). Indeed, this catalyst provided excellent catalytic results in terms of stability, reaction rate, conversion and selectivity compared to other bipyridine-like copper catalysts.

Chiral porous poly(ionic liquid)s: Facile one-pot, one-step synthesis and efficient heterogeneous catalysts for asymmetric epoxidation of olefins

Ionic liquids are potential media/solvents for asymmetric synthesis when combined with chiral catalysts, while most reported catalysts are homogenous, making them difficult to separate from the reaction systems. Herein, chiral porous poly(ionic liquid)s (CPPILs) containing chiral salen Mn(III) complex have been successfully synthesized through a simple and one-pot route involving simultaneously occurred Friedel-Crafts alkylation and quaternization. These CPPILs possess large specific surface area, plentiful micro/meso-porosity, and abundant ionic active sites. Consequently, CPPILs can serve as highly efficient heterogeneous catalysts for asymmetric epoxidation of styrene. Excellent catalytic performance with yield of epoxides and enantiomeric excess value comparable with or even higher than that of the homogeneous system was achieved. The high catalytic activity can be further expanded to various substrates of unfunctionalized olefins. These catalysts are stable and could be recycled at least five times without significant loss of reactivity and enantioselectivity.

A 5-(2-Pyridyl)tetrazolate Complex of Molybdenum(VI), Its Structure, and Transformation to a Molybdenum Oxide-Based Hybrid Heterogeneous Catalyst for the Epoxidation of Olefins

There is a considerable practical interest in discovering new ways to obtain organomolybdenum heterogeneous catalysts for olefin epoxidation that are easier to recover and reuse and display enhanced productivity. In this study, the complex salt (H2pytz)[MoO2Cl2(pytz)] (1) (Hpytz = 5-(2-pyridyl)tetrazole) has been prepared, structurally characterized, and employed as a precursor for the hydrolysis-based synthesis of a microcrystalline molybdenum oxide/organic hybrid material formulated as [MoO3(Hpytz)] (2). In addition to single-crystal X-ray diffraction (for 1), compounds 1 and 2 were characterized by FT-IR and Raman spectroscopies, solid-state 13C{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR, and scanning electron microscopy (SEM). Compounds 1 and 2 were evaluated as olefin epoxidation catalysts using the model reaction of cis-cyclooctene (Cy8) with tert-butyl hydroperoxide (TBHP), at 70 °C, which gave 100% epoxide selectivity up to 100% conversion. While 1 behaved as a homogeneous catalyst, hybrid 2 behaved as a heterogeneous catalyst and could be recovered for recycling without showing structural degradation or loss of catalytic performance over consecutive reaction cycles. The substrate scope was broadened to monoterpene DL-limonene (Lim) and biobased unsaturated fatty acid methyl esters, methyl oleate (MeOle), and methyl linoleate (MeLin), which gave predominantly epoxide products.

Co(II) Schiff base Complexes Encapsulated in the Nanopores of Zeolite Y as Heterogeneous Catalysts for Selective Epoxidation of Alkenes with Molecular Oxygen

Co(II) Schiff base Complexes Encapsulated in the Nanopores of Zeolite Y as Heterogeneous Catalysts for Selective Epoxidation of Alkenes with Molecular Oxygen

Assembly of SBA‐15 Derived Hybrid Heterogeneous Catalysts for Liquid Phase Cyclopentene Epoxidation

AbstractThree two‐dimensional SBA‐15‐based hybrid materials, SBA‐15‐ImCl‐PF6, SBA‐15‐NH(PO3H2), and SBA‐15‐N+, were synthesized employing a post‐synthetic functionalization strategy, and peroxo‐W and PW species were successfully immobilized on the surface of the corresponding organic moieties‐modified SBA‐15 via electrostatic interaction or covalent linking. The TEM and XRD results presented that the structure and morphology of SBA‐15 were maintained after multiple transformations. The final catalysts SBA‐15‐ImCl‐PF6‐W, SBA‐15‐NH(PO3H2)‐W, and SBA‐15‐N+(PW4O24)3− all showed considerable activity for the epoxidation of cyclopentene to cyclopentene oxide using H2O2 as the oxygen source without any co‐solvent. It was noteworthy that SBA‐15‐ImCl‐PF6‐W catalyst with increased hydrophobicity via altering counteranions of imisazolium ions exhibited excellent cyclopentene oxide selectivity (99 %), but a good cycle performance (10 times) was achieved via SBA‐15‐NH(PO3H2)‐W owing robust covalent bonds. Besides, the use of a low dose of heterogeneous catalyst (W=20 μmol) and a green oxidant of H2O2, along with the absence of co‐solvent or other additives make cyclopentene epoxidation reaction an environmenttally benign chemical process.

Iron (II) complex supported on graphene nanosheet: An efficient and heterogeneous catalyst for epoxidation of alkenes

A novel heterogeneous catalyst was prepared by chemical attachment of iron Schiff-base complex on functionalized graphene oxide as stable support (GO–EBD/Fe). The GO–EBD/Fe catalyst was characterized by different techniques such as Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray, thermal gravimetric analysis, X-ray diffraction, N2 adsorption–desorption isotherms, and inductively coupled plasma optical emission spectroscopy. These analyses confirmed the high chemical and thermal stability of the designed catalyst. The GO–EBD/Fe was used for epoxidation of linear and cyclic alkenes giving corresponding epoxides in high yields. It could be readily reused consecutive five times without selectivity deterioration and discernible activity, displaying potential for practical applications.

Immobilized some of vanadium compounds on modified graphene oxide as nanofiber network for epoxidation of allyl alcohols

VO(acac)2 and the in situ generated VO(Salen) complex were immobilized on the covalently modified graphene oxide (GO) with ethylenediamine (en) and designated as VO(acac)2@en‐GO and VO@Salen@en‐GO, respectively. Characterization of the prepared compounds was carried out with FT‐IR, XRD, ICP, CHN, Raman, SEM, TEM, and BET techniques. The SEM results confirmed the formation a nanofiber network and nanointerconnected needles. The prepared immobilized complexes were then used as heterogeneous catalysts for epoxidation of some allyl alcohols such as geraniol, trans‐2‐hexene‐1‐ol, and 1‐octen‐3‐ol. Based on the results obtained by GC and GC–mass analyses, epoxidation of geraniol with 97% conversion and 100% selectivity seemed promising. Readily recycling of the catalyst and reusing for four successive catalytic runs without significant loss of activity revealed that this catalysis system deserves nomination for practical application.

Heterogeneous catalysis with an organic–inorganic hybrid based on MoO3 chains decorated with 2,2′-biimidazole ligands

Polymeric [MoO3(2,2′-biimidazole)]·H2O outperforms other one-dimensional MoO3-ligand hybrid materials as a heterogeneous and recyclable catalyst for (bio)olefin epoxidation and sulfoxidation.

Preparation of zeolitic bismuth vanadomolybdate using a ball-shaped giant polyoxometalate for olefin epoxidation

Bismuth vanadomolybdate-based zeolitic octahedral metal oxide was synthesized from {Mo72V30} and used as a heterogeneous catalyst for olefin epoxidation.

WO3–SiO2 nanomaterials synthesized using a novel template-free method in supercritical CO2 as heterogeneous catalysts for epoxidation with H2O2

A series of tungsten oxide-silica (WO3–SiO2) composite nanomaterials were synthesized through a novel, template-free sol-gel method, in which supercritical-CO2 (scCO2) was utilized as synthesis medium. The efficacy of the synthesis method stems from a tailored reactor design that allows the contact of the reactants only in the presence of scCO2. Selected synthetic parameters were screened with the purpose of enhancing the performance of the resulting materials as heterogeneous catalysts in epoxidation reactions with H2O2 as environmentally friendly oxidant. A cyclooctene conversion of 73% with epoxide selectivity of > 99% was achieved over the best WO3–SiO2 catalyst under mild reaction conditions (80 °C), equimolar H2O2 amount (1:1) and low WO3 loading (~2.5 wt%). The turnover number achieved with this catalyst (TON = 328), is significantly higher than that of a WO3–SiO2 prepared via a similar sol-gel route but without supercritical CO2, and that of commercial WO3. A thorough characterization with a combination of techniques (ICP-OES, N2-physisorption, XRD, TEM, STEM-EDX, SEM-EDX, FT-IR and Raman spectroscopy, XPS, TGA and FT-IR analysis of adsorbed pyridine) allowed correlating the physicochemical properties of the WO3–SiO2 nanomaterials with their catalytic performance. The high catalytic activity was attributed to: (i) the very high surface area (892 m2/g) and (ii) good dispersion of the W species acting as Lewis acid sites, which were both brought about by the synthesis in supercritical CO2, and (iii) the relatively low hydrophilicity, which was tuned by optimizing the tetramethyl orthosilicate concentration and the amount of basic solution used in the synthesis of the materials. Our optimum catalyst was also tested in the reaction of cyclohexene with H2O2, resulting in cyclohexane diol as main product due to the presence of strong Brønsted acid sites in the catalyst, whereas the reaction with limonene yielded the internal epoxide as the major product and the corresponding diol as side product. Importantly, the catalyst did not show leaching and could be reused in five consecutive runs without any decrease in activity.

Co supported on interparticle porosity dominated hierarchical porous TS-1 as highly efficient heterogeneous catalyst for epoxidation of styrene

A convenient and easily large-scale production strategy for the synthesis of interparticle porosity dominated hierarchical porous TS-1 (IPD-mesoTS-1) based on sufficiently small TS-1 nanosized crystals was designed and Co nanoparticles were in situ dispersed and deposited on the as-prepared IPD-mesoTS-1 (Co/IPD-mesoTS-1). The catalytic performances for epoxidation of styrene were systematically evaluated. Outstanding catalytic activity has been obtained owing to the highly dispersed Co nanoparticles and the hierarchical porous structure of IPD-mesoTS-1. In addition, the Co concentration, temperature, solvents, oxidants and reaction time also play the important roles in the epoxidation.

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.

The direct assembly of metalloporphyrin and Mg–Al layered double hydroxides nanosheets: a highly efficient catalyst for the green epoxidation of olefins

Co 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (CoTPPS) anions were successfully immobilized into the lamellar space of Mg–Al LDH through the spontaneous flocculation method by using exfoliated LDH nanosheets and guest molecules as building blocks. A variety of techniques including XRD, IR, UV–Vis, SEM, TEM and TG–DSC have been applied to characterize the morphology and microstructure of the obtained hybrids. It was found that the ratio between the metalloporphyrin molecules and LDH nanosheets has an important effect on the structure of restacking products. The interlayer spacing determined from XRD result suggests that a perpendicular orientation of the CoTPPS anions in the lamellar space. The surface charge changes associated with the flocculation process were traced through zeta potential measurements. The immobilized CoTPPS could act as an efficient and reusable heterogeneous catalyst for selective epoxidation of various alkenes including different cycloalkenes, styrene derivatives and long-chain alkenes. Cyclohexene undergoes up to 99% conversion within 2 h and 90% selectivity with molecular oxygen as environmentally friendly oxidant and isobutyraldehyde as co-reductant at room temperature. The CoTPPS/LDH2.0 can be easily recovered and reused for at least 5 times without obvious decrease of activity. This work provides a facile route to synthesize a highly efficient catalyst for the green epoxidation of olefins.

Heterogeneous epoxidation of menadione with hydrogen peroxide over the zeolite imidazolate framework ZIF-8.

The zeolite imidazolate framework ZIF-8 exhibits superior catalytic performance in the epoxidation of the electron-deficient C[double bond, length as m-dash]C bond in menadione using aqueous hydrogen peroxide as the oxidant. The catalysis has a truly heterogeneous nature and the framework structure remains intact. This is the first example of oxidation catalysis with ZIF-8.