Catalyst For Epoxidation Of Olefins Research Articles

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.

Synthesis, Structural and Redox Properties of Vanadyl β‐Cyanoporphyrin and its Utilization as Efficient Catalyst for Epoxidation of Olefins and Oxidative Bromination of Phenols

Abstract2‐Cyano‐5,10,15,20‐tetraphenylporphyrin [H2TPPCN] (1) having cyano group at one of β‐pyrrolic positions using nucleophilic substitution reaction of tetrabutylammonium cyanide (TBACN) on free‐base 2‐nitro‐5,10,15,20‐tetraphenylporphyrin, and its oxidovanadium(IV) complex [VIVOTPPCN] (2) were synthesized in good yields. Both the porphyrins 1 and 2 have been characterized by UV‐Vis spectroscopy, mass spectrometry, and cyclic voltammetric techniques. Single crystal X‐Ray crystallography revealed quasi‐planar geometry for 1. Compounds 1 and 2 exhibited a red shift (λmax=7–9 nm) in the Soret band relative to [MTPP], where M=2H, VIVO owing to the electron‐withdrawing effect of the cyano group at the β‐position. The first oxidation (ΔE1/2=110–140 mV) and reduction potentials (ΔE1/2=220–260 mV) of 1 and 2 are anodically shifted relative to [MTPP] where M=2H, VIVO. Porphyrin 2 has been utilized as a catalyst for two reactions: epoxidation of olefins and bromination of phenols. The bromination of phenol using KBr/H2O2/HClO4 in water resulted in 100 % conversion with a TOF value as high as 19.6 s−1 in 0.5 h. Using H2O2/NaHCO3 in a CH3CN/H2O solvent mixture at 60 °C, epoxidation was carried out, and the highest conversion rate with a turnover frequency of 1.9 s−1 was achieved in the case of cyclohexene. Catalyst 2 was recovered successfully at the end of the reaction up to 5 cycles and had good thermal stability, indicating its industrial viability and applicability. Moreover, these cyano‐functionalized porphyrins could be further utilized for the generation of molecular self‐assemblies, post‐functionalization of porphyrin core and various applications. The current findings in this work present insights for a facile approach for the synthesis of β‐cyano functionalized porphyrin 1 and further shed light on the utility of its vanadyl complex 2 as an efficient catalyst for olefin epoxidation and phenol bromination reactions.

Epoxidation of Olefins Catalyzed by Sulfate‐based Supramolecular Ion Pairs

AbstractThe development of inexpensive and effective catalysts for the epoxidation of olefins to epoxides, which are key commodities for the chemical industry, is a continuing challenge. In this context, we present a supramolecular solution with the development of new host‐guest assemblies of sulfate ions and amidoammonium receptor cations that, for the first time, are shown to act as catalysts for olefin epoxidation by hydrogen peroxide under biphasic conditions. Analysis of the reaction mechanism shows that the reactive and oxidizing peroxymonosulfate is formed in the organic phase. Furthermore, a variety of readily available precursors may be used to form the supramolecular ion pairs (SIPs), which is enabling a large‐scale synthesis of the catalysts while maintaining catalytic control and effectiveness.

Construction of a highly stable natural silicate-supported molybdenum catalyst for efficient epoxidation of olefins

Epoxides are important bulk chemicals, playing irreplaceable role in the chemical industry, but facing serious pollution and low productivity in the production process. Therefore, the development of green and efficient epoxidation of olefins by stable catalysts with low prices is of great significance. In this study, a Mo-MATP catalyst was prepared by modifying Mo(CO)₆ on attapulgite through Si-O bonding. Mo-MATP exhibits excellent performance (99% yield of cyclooctane oxide, CYCO) and stability (80% selectivity of CYCO after 17 cycles), highly tert-butyl hydroperoxide (TBHP) utilization, and extensive substrate scalability. Furthermore, the in-situ Fourier Transform Infrared Spectroscopy (FT-IR), Electron Spin-resonance Spectroscopy (ESR) and High Resolution Mass Spectrometry (HRMS) spectra suggest that TBHP would be activated by Mo-MATP to generate peroxyl radicals, which then oxidize alkenes to their corresponding epoxides. In this study, the stable loading of Mo would largely solve the problem of Mo loss during the catalytic process, thus providing a stable and dispersed Mo active center, enabling the catalyst to possess high catalytic performance and recycling stability.

Characterization of Ag/α-Al2O3 olefin epoxidation catalysts containing promoters and co-promoters using pulse hydrogen titration methods

A series of Cs and Cs + Re promoted 12 wt% Ag/α-Al2O3 catalysts were prepared and characterized by H2 pulse titration of O-precovered Ag to measure active Ag site concentrations. Temperature programmed reduction (TPR) at temperatures up to 500 °C was performed on α-Al2O3 supported 3–4 wt% Cs, Re, or Mo only catalysts to confirm the metal–oxygen bonds of the promoters were not reactive during titration of Ag-O surface at 170 °C; absence of H2 uptake during H2 titration for these samples confirmed the TPR results. H2 pulse titration results of 0 to 7.5 μmol Cs promoted, 12 wt% Ag/α-Al2O3 catalysts showed non-preferential deposition of Cs on Ag and α-Al2O3 surfaces. However, when Cs (2.6 μmol)–12 wt% Ag/α-Al2O3 catalyst was promoted with ≥1.3 μmol of Re, titration results showed preferential deposition of Re on Ag sites, presumably to lower overall surface free energy of the Re-Ag-Al2O3 system.

A novel POM/LDH/GO nanocomposite as highly efficient heterogeneous catalyst in green epoxidation of alkenes with hydrogen peroxide

In the present work, various transition metal substituted Keggin-type polyoxometalates/Ni-Al layered double hydroxide/graphene oxide (PWM/Ni-Al LDH/GO; M = Fe, Ni, Cu, Mn, Co, Cr, Zn) nanocomposites have been synthesized and utilized as catalysts in the epoxidation of alkenes. Results confirmed that in the epoxidation of alkenes, PWFe/Ni-Al LDH/GO nanocomposite could perform its role as the best catalyst. The selected nanocomposite evaluated as a heterogeneous catalyst in the green oxidation of various alkenes into the corresponding epoxides while aqueous hydrogen peroxide was used as an environmentally benign oxidant. The results of recovery and reusability experiments on PWFe/Ni-Al LDH/GO nanocomposite revealed that even after five times it could be easily reused without any considerable decrease in catalytic efficiency.

Ultrasound-promoted metal-free homogenous olefins epoxidation and direct conversion of alkenes to cyclic carbonates using catalytic TAIm[X] (X = WO42-, HSO5-) recyclable ionic liquid under mild conditions

TAIm[WO4] and TAIm[HSO5] ionic liquids were prepared and used as efficient solvent and recyclable catalysts for the mild epoxidation of olefins and the domino preparation of cyclic carbonate from alkenes under ultrasound irradiation. H2O2 was used as an oxidant. The physical properties and structure of the ionic liquids were studied by various analytical methods including FTIR, EDX, viscosity, and density measurements. High to excellent yields were obtained for a wide variety of olefins whether aliphatic or aromatic types for the epoxides as well as the conversion of cyclic and terminal olefins to cyclic carbonates at low CO2(g) pressure (∼1 atm.). Moderate CO2(g) solubility in the ionic liquids was found. Also, excellent epoxide-selectivity was obtained for all olefins. Scalability of the method demonstrated a direct scalability for the promising gram scale epoxidation of olefins with at least efficiency loss. The TAIm[WO4] and TAIm[HSO5] ionic liquids could be recycled from the homogenous mixture by a simple solvent-solvent extraction, and reused for at least several times with insignificant activity loss.

Systematic Development of Vanadium Catalysts for Sustainable Epoxidation of Small Alkenes and Allylic Alcohols.

The catalytic epoxidation of small alkenes and allylic alcohols includes a wide range of valuable chemical applications, with many works describing vanadium complexes as suitable catalysts towards sustainable process chemistry. But, given the complexity of these mechanisms, it is not always easy to sort out efficient examples for streamlining sustainable processes and tuning product optimization. In this review, we provide an update on major works of tunable vanadium-catalyzed epoxidations, with a focus on sustainable optimization routes. After presenting the current mechanistic view on vanadium catalysts for small alkenes and allylic alcohols' epoxidation, we argue the key challenges in green process development by highlighting the value of updated kinetic and mechanistic studies, along with essential computational studies.

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

AbstractThe titanium complex of the cis‐1,2‐diaminocyclohexane (cis‐DACH) derived Berkessel‐salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide (“Berkessel‐Katsuki catalyst”). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C−H bonds with hydrogen peroxide. Mechanism‐based ligand optimization identified a novel nitro‐salalen Ti‐catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro‐salalen Ti‐catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1‐decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol‐% only.

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation.

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide ("Berkessel-Katsuki catalyst"). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C-H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.

Fe-substituted Polyoxometalate-based Spherical Assemblies as Catalysts for Olefin Epoxidation

Fe-substituted Polyoxometalate-based Spherical Assemblies as Catalysts for Olefin Epoxidation

Molybdenum(VI) complexes with a chiral l-alanine bisphenol [O,N,O,O’] ligand. Synthesis, structure, spectroscopic properties and catalytic activity

Dioxidomolybdenum(VI) compound [MoO2Cl2(dmso)2] reacts with a chiral tetradentate O3N-type l-alanine bisphenol ligand precursor (Et3NH)H2Lala to form an oxidochloridomolybdenum(VI) complex [MoOCl(Lala)] (1) as two separable geometric isomers with phenolate groups in cis or trans positions. The single crystal X-ray and NMR analyses of cis- and trans-1 reveal that the complexes are formed of monomeric molecules, in which the ligand has a tetradentate coordination through three oxygen donors and one nitrogen donor. The reaction of Na2MoO4·2H2O with the same ligand precursor in an acidic methanol solution leads to the formation of an anionic dioxido complex (Et3NH)[MoO2(Lala)] (2) with a trans coordination of the tetradentate ligand. Trans-1 and 2 were studied as active catalysts for olefin epoxidation: i.e. styrene, cyclohexene, S(–)-limonene and (–)-α-pinene using H2O2 and tBuOOH as oxidants.

A Molybdenum(VI) Complex of 5-(2-pyridyl-1-oxide)tetrazole: Synthesis, Structure, and Transformation into a MoO3-Based Hybrid Catalyst for the Epoxidation of Bio-Olefins

The discovery of heterogeneous catalysts synthesized in easy, sustainable ways for the valorization of olefins derived from renewable biomass is attractive from environmental, sustainability, and economic viewpoints. Here, an organic–inorganic hybrid catalyst formulated as [MoO3(Hpto)]·H2O (2), where Hpto = 5-(2-pyridyl-1-oxide)tetrazole, was prepared by a hydrolysis–condensation reaction of the complex [MoO2Cl2(Hpto)]∙THF (1). The characterization of 1 and 2 by FT-IR and Raman spectroscopies, as well as 13C solid-state NMR, suggests that the bidentate N,O-coordination of Hpto in 1 (forming a six-membered chelate ring, confirmed by X-ray crystallography) is maintained in 2, with the ligand coordinated to a molybdenum oxide substructure. Catalytic studies suggested that 2 is a rare case of a molybdenum oxide/organic hybrid that acts as a stable solid catalyst for olefin epoxidation with tert-butyl hydroperoxide. The catalyst was effective for converting biobased olefins, namely fatty acid methyl esters (methyl oleate, methyl linoleate, methyl linolenate, and methyl ricinoleate) and the terpene limonene, leading predominantly to the corresponding epoxide products with yields in the range of 85–100% after 24 h at 70 °C. The versatility of catalyst 2 was shown by its effectiveness for the oxidation of sulfides into sulfoxides and sulfones, at 35 °C (quantitative yield of sulfoxide plus sulfone, at 24 h; sulfone yields in the range of 77–86%). To the best of our knowledge, 2 is the first molybdenum catalyst reported for methyl linolenate epoxidation, and the first of the family [MoO3(L)x] studied for methyl ricinoleate epoxidation.

Well-dispersed Mn/g-C3N4 as bifunctional catalysts for selective epoxidation of olefins and carbon dioxide cycloaddition

Epoxidation is an important chemical process for the production of epoxides, key building blocks in chemical industry. Despite great efforts being made to facilitate this process, it remains a significant challenge to develop cost-effective, environmental-friendly, and selective catalysts. Herein, we reported a highly dispersed Mn supported by g-C3N4 (Mn/g-C3N4) with Mn loading up to 2.56 wt%. The Mn/g-C3N4 exhibited satisfied catalytic performance for olefin epoxidation with excellent conversion (91%), high selectivity (93%) as well as outstanding recycling stability. Further analysis revealed the importance of Mn-N structure for the generation of active oxo-containing species and subsequent oxygen atom transfer. Besides, an efficient synthesis of cyclic carbonates from styrene epoxide and CO2 has been achieved (88% conversion, 89% selectivity) based on the polar Mn-N coordinated characteristics of Mn/g-C3N4 catalyst.

A novel MOFs-induced strategy for preparing anatase-free hierarchical TS-1 zeolite:synthesis routes, growth mechanisms and enhanced catalytic performance

Titanosilicate-1 zeolites (TS-1) as one of the most commonly used catalysts for alkene epoxidation, construction of hierarchical pores as well as elimination of anatase to promote mass transportation and avoid invalid decomposition of hydrogen peroxide are always desirable yet challenging goals. Here, a novel and unique Ti-based metal organic frameworks (MOFs)-induced synthetic strategy for fabricating anatase-free hierarchical TS-1 was first proposed. All the components of MOFs perform different functions: the uniformly distributed Ti nodes replace conventional tetrabutyl titanate (TBOT) to serve as sole Ti source for constructing zeolite crystal; the separated ligands can be embedded in the zeolite framework and act as template to in situ build hierarchical pore structure; the coordination interaction between Ti nodes and ligands can efficiently avoid the anatase generation by balancing the forming rates of Ti-OH and Si-OH. This synthetic strategy is of general applicability, and two different synthetic routes including traditional hydrothermal process and steam assisted crystallization (SAC) procedure are successfully adopted. The obtained hydrothermal TS-1 and SAC anatase-free samples all possess abundant intercrystalline mesopores of 20–50 nm and even macropores between 50 and 150 nm, improving the conversion over 25 % for 1‑hexene epoxidation than TS-1 sample prepared by conventional route. The influences of the amount of Ti MOFs precursor and the crystallization process are studied in detail, and possible synthesis mechanisms are proposed. This MOFs-induced strategy might open up an avenue for the rational design of ideal and hierarchical zeolite to boost the catalytic efficiency.

Effect of hydrolysis of tetraethyl orthosilicate (TEOS) on titanium distribution of TS-1 zeolite

The hydrolysis process of tetraethyl orthosilicate (TEOS) was tracked by FT-IR analysis, and the effect of hydrolysis time on the distribution of titanium species in TS-1 zeolites was discussed through XRF and UV-vis. The catalytic oxidation performance of TS-1 zeolite was tested by 1-hexene epoxidation as a model reaction. The results show that moderate hydrolysis of tetraethyl orthosilicate can effectively inhibit the formation of extra-framework Ti and increase the content of framework Ti. However, shorter or longer hydrolysis time is not conducive to the entry of titanium into the skeleton and reduce the catalytic performance of TS-1 zeolites. TS-1 prepared with 2 h TEOS hydrolysis time has the best catalytic performance for epoxidation of 1-hexene. Under the reaction conditions of atmospheric pressure and 60 °C, the conversion of 1-hexene can reach 37.5% and the selectivity of 1,2-epoxy hexane can maintain at about 85%, and the result has a guiding role for the development of olefin epoxidation catalyst.

Dehydrogenation Based Asymmetric Epoxidation of Arylalkanes to Chiral Epoxides

Comprehensive SummaryWe report herein a formal asymmetric epoxidation of arylalkanes enabled by a one‐pot dual‐catalysts system comprising a pincer Ir catalyst for alkane dehydrogenation‐alkene isomerization (AD‐ISO) and a chiral ketone catalyst for asymmetric alkene epoxidation. This protocol provides a catalytic method for the synthesis of aryl epoxides in useful yields with high regio‐ and stereoselectivity directly from readily available alkyl arenes. Stereospecific derivatizations of the epoxidation product allow access to various enantioenriched compounds.

Synthesis, characterization, and function of Au nanoparticles within TS-1 zeotypes as catalysts for alkene epoxidation using O2/H2O reactants

A synthetic procedure was developed for the encapsulation of Au nanoparticles within titanium silicalite-1 (TS-1). Encapsulation was achieved via crystallization of TS-1 synthesis gels containing ligated Au cations, resulting in TS-1 framework assembly around Au coordination complexes. X-Ray diffraction and micropore volume measurements indicated that TS-1 could be crystallized at much milder conditions than previously reported, allowing the assembly of TS-1 without concomitant decomposition of added Au complexes. Infrared (IR) and UV–vis spectroscopy showed that Ti was incorporated into the frameworks as tetrahedrally coordinated atoms. Changes to established crystallization procedures for TS-1 included lower temperature (393 K vs. 448 K) compensated by longer time (120 vs 48 h) and the use of seed crystals. Post-synthetic thermal treatments led to the formation of small Au nanoparticles (2.8–3.8 nm) visible in electron micrographs and free of organic debris, as indicated by IR spectroscopy. Such nanoparticles are protected from bulky titrants, and thus reside within TS-1 crystallites (>96% encapsulation). The Au particles were active for propene epoxidation with H2O/O2 even without promotion by alkali cations (e.g., Cs+), though alkali doping led to enhanced rates. These Au/TS-1 systems were less active for epoxidation than Au/TS-1 prepared by deposition–precipitation, likely because the latter contains a disproportionately active minority population of exceedingly small (<1 nm) Au clusters.

Effects of the method of active site characterization for determining structure-sensitivity in Ag-catalyzed ethylene epoxidation

Catalysts have been prepared on a low surface area α-Al2O3 support used commercially for previous generation olefin epoxidation catalysts. Prescreening of the low surface area alumina (0.73 m2/g) indicated the absence of acid catalyzed isomerization of EO at an evaluation temperature of 210 °C. A 0.1 wt% Ag base material synthesized by incipient wetness impregnation of AgNO3 was used as a base material for electroless deposition (ED) of additional Ag to increase particle sizes by controlled reduction of Ag+ directly onto the preexisting Ag surface to form weight loadings between 0.3 and 5.0 wt% metal. A 12 wt% Ag/α-Al2O3 using Ag2C2O₄ as the Ag precursor was also prepared to compare performance of the ED samples with a catalyst more typical of industrial formulations. Characterization by SEM, STEM, and hydrogen titration of oxygen precovered Ag characterized before and after catalytic evaluation indicated that microscopy is required to accurately represent distributions of Ag particle sizes, but H2 titration of O-precovered Ag gives the best representation of active sites since it directly counts the number of Ag surface sites. Larger particles >100 nm are resistant to both Ag sintering and carbon foulant; TOF values were relatively insensitive to particle size with only a 2.2 × difference between the best and worst performing samples. Selectivity, which is not a function of TOF, shows the most significant structure sensitivity effect where particle sizes follow the trend 67 nm ≈ 92 nm (58% EO) < 157 nm (67% EO) < 211–542 nm (73% EO). The lower EO selectivities were also correlated with increased fouling for the smaller Ag sizes, suggesting that more strongly bound EO precursor(s) leads to combustion and CO2/H2O formation.