Catalysts For Epoxidation Research Articles

In situ discovery on the formation of supported silver catalysts for ethylene epoxidation

Silver particles supported on alumina are the main industrial catalysts for ethylene epoxidation. Silver catalysts are usually produced by an impregnation method followed by thermal decomposition. Although the decomposition process determines the structures and properties of silver catalysts, we are short of information on this high temperature process. Herein we employed a confocal laser scanning microscope (CLSM) equipped with a heating stage to disclose the thermal decomposition process of silver precursor towards the preparation of high selective silver catalysts. It is found that the formation of silver catalysts experiences two steps: crystallization and decomposition. The crystallization occurs at the temperature below 80℃, resulting in the formation of silver complex. Further heating the sample to 140℃, leads to the formation of silver particles at alumina wafers. The atmosphere has a significant effect on the formation and performance of catalysts. When the samples are heated in the nitrogen, small silver particles with similar size are formed, which leads to a high selectivity of ethylene epoxidation. The role of thermal atmospheres on the size and property of silver catalysts is discussed in this paper.

Design of CuCs-doped Ag-based catalyst for ethylene epoxidation

Our recent theoretical studies have screened out CuCs-doped Ag-based promising catalysts for ethylene epoxidation [ACS Catal. 11, 3371 (2021)]. The theoretical results were based on surface modeling, while in the actual reaction process Ag catalysts are particle shaped. In this work, we combine density functional theory (DFT), Wulff construction theory, and micro kinetic analysis to study the catalytic performance of Ag catalysts at the particle model. It demonstrates that the CuCs-doped Ag catalysts are superior to pure Ag catalysts in terms of selectivity and activity, which is further proved by experimental validation. The characterization analysis finds that both Cu and Cs dopant promote particle growth as well as particle dispersion, resulting in a grain boundary-rich Ag particle. Besides, CuCs also facilitate electrophilic atomic oxygen formation on catalyst surface, which is benefitial for ethylene oxide formation and desorption. Our work provides a case study for catalyst design by combining theory and experiment.

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.

Effective Regulation of the Au Spatial Position in a Hierarchically Structured Au/HTS-1 Catalyst: To Boost the Catalytic Performance of Propene Epoxidation with H2 and O2

The traditional deposition–precipitation (DP) method is common but not universal in manipulating Au nanoparticles’ spatial locations in propene epoxidation catalysts, especially for hierarchical hydrophilic TS-1 supports. Directional loading of Au nanoparticles to hierarchical hydrophilic TS-1 pores is challenging because the strong hydrophilicity of the hierarchical TS-1 zeolite would make Au nanoparticles loaded by the DP method tend to be deposited on the hydrophilic support surface, thus blocking the pore mouths and leading to deactivation. Therefore, manipulating Au nanoparticles’ spatial locations on hierarchical hydrophilic TS-1 supports is of great significance to enhance the catalytic performance but is a bottleneck problem. In this work, taking hierarchical hydrophilic HTS-1 supports as an example, we propose a new modified isometric impregnation (NIMG) method to manipulate the spatial location of Au nanoparticles. Moreover, the Au spatial location inside HTS-1 pores is quantitatively reflected by introducing the Vna parameter. Combined with the N2 physisorption, high-angle annular dark-field scanning transmission electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy, it is found that the Au nanoparticles loaded by the NIMG method tend to be uniformly distributed inside the pores of HTS-1 due to the capillary effect. As expected, the Au/HTS-1(NIMG) catalyst (with the Au nanoparticles inside the pores) exhibits a much higher propylene oxide (PO) formation rate, PO selectivity, and H2 efficiency than the Au/HTS-1(DP) catalyst (with the Au nanoparticles on the outer surface). This is possibly because loading Au nanoparticles inside the pores is conducive to the transfer of H2O2 from Au to the nearby abundant Ti active sites, thus weakening the decomposition of H2O2 to H2O, avoiding the ring-opening side reactions, and promoting propene epoxidation into PO. This work sheds new light for controlling the spatial location of Au in hierarchically structured hydrophilic catalysts for direct propene epoxidation.

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

Styrene epoxidation in aqueous over triazine

Epoxies are a popular kind of industrial chemical molecule used as an intermediate in agricultural and medicinal research.Metal-free catalyst for styrene epoxidation was found to be trazine-based microporous polymeric (TMP) network. H2O2 was used as a green oxidant in water as an ecologically friendly medium. The reactions were carried out at room temperature. In 6 hours, the process produced a greater yield of styrene oxide with a 90% conversion and 98% selectivity. Up to four times, the catalytic activity and selectivity of triadzine-based micro porous polymeric networks may be restored and reused.

Highly Hydrophilic Ti−Beta Zeolite with Ti−Rich Exterior as Efficient Catalyst for Cyclohexene Epoxidation

Nanocrystalline Ti−Beta zeolite with high hydrophilicity and a Ti−rich exterior was successfully prepared via a dissolution–recrystallization method. With the post−treatment of tetraethylammonium hydroxide (TEAOH) solution at elevated temperature, the Si and Ti species within the Ti−Beta matrix were partially dissolved and recrystallized on the outer surface of crystals, resulting in the Ti−rich exterior and higher hydrophilicity, which improved the accessibility of the active Ti sites and the enrichment of H2O2. Simultaneously, some of the closed Ti(OSi)4 species were transformed to more active open Ti(OSi)3OH or Ti(OSi)2(H2O)2(OH)2 species. The modified Ti−Beta zeolite exhibited greatly enhanced catalytic performance in the epoxidation of cyclohexene in comparison to the parent Ti−Beta.

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.

Aquivion perfluorosulfonic superacid as an effective catalyst for selective epoxidation of vegetable oils.

The acid-promoted epoxidation of vegetable oils was studied using a variety of acidic ion exchange resins as heterogeneous acid catalysts. Quantitative and selective epoxidation of a series of vegetable oils with different composition of saturated, mono-, di- and tri-unsaturated fatty acids was obtained upon identification of the more efficient catalyst and experimental conditions. Furthermore, optimized reaction conditions were successfully applied to the epoxidation of a waste cooking oil, thus extending our procedure to the valorization of a biowaste, an area of increasing importance within a more sustainable society. The use of quantitative 1HNMR besides making accurate evaluation of the amounts of reagents to be employed and of the selectivity, allowed facile and rapid quantification of mono-, di- and tri-epoxides, thus providing an indirect indication on the fatty acid composition of the vegetable oils, even in the presence of very low quantities of linolenic acid.

Porous core–shell Fe3O4@CuSiO3 microsphere as effective catalyst for styrene epoxidation

The porous core–shell Fe3O4@CuSiO3 catalyst were synthesized via a multi-step preparation method including StÖber method and hydrothermal process. The morphological, magnetic and physicochemical studies demonstrated a successful fabrication of the targeted material. This catalyst possessed a spherical morphology with a magnetic core and protective porous shell. The Fe3O4 core not only offered a convenient way for magnetic recycling, but also worked as an electron donor to increase the electron density on the CuSiO3 shell. The catalytic performance of the synthesized material was investigated in the styrene epoxidation with tert-butyl hydroperoxide as oxidant. On account of the synergy effect between Fe3O4 and CuSiO3, the porous core–shell Fe3O4@CuSiO3 catalyst showed styrene conversion significantly higher compared to those of pure Fe3O4, pure CuSiO3 and physically mixed catalyst (Fe3O4 + CuSiO3). In particularly, the catalytic activity of the Fe3O4@CuSiO3 was retained after the recyclability test and re-calcination, which confirmed that the Fe3O4@CuSiO3 catalyst was easily separated, highly stable and could be used six times without significantly affecting its structural order. Moreover, the kinetic study revealed the activation energy of Fe3O4@CuSiO3 in styrene epoxidation was 45.4 kJ/mol. The possible free radical mechanism for the styrene epoxidation based on the Fe3O4@CuSiO3 catalyst was proposed.

Oxidation of Cyclopentene with Manganese Oxides Octahedral Molecular Sieves with Layer and Tunnel Structures

AbstractManganese oxides type octahedral molecular sieves OL‐1, OMS‐1, and OMS‐2 were synthesized, impregnated with sodium tungstate, and evaluated as catalysts for the selective epoxidation of cyclopentene (CPE) to cyclopentene oxide (CPO) in the sodium bicarbonate/hydrogen peroxide (BAP) system. OL‐1 was the best catalyst with a conversion of 78.6 % and a selectivity to CPO of 31.4 %. Impregnation with sodium tungstate affected the structure and activity of OL‐1 and OMS‐1. Mn2+ is required for the epoxidation of CPE. FTIR spectra provides evidence that tungstate ions react, during impregnation, with Mg2+ and/or Mn2+ present in OMS‐1 and OL‐1 producing an amorphous compound that is not active in the epoxidation reaction. The crystalline and chemical structure of OMS‐2 was not affected by tungstate impregnation, since most of the manganese occurs as Mn3+ and Mn4+, which in presence of hydrogen peroxide, tend to form radicals, active in the total oxidation of organic compounds.

Lipase catalyzed green epoxidation of oleic acid using ultrasound as a process intensification method

Epoxidized fatty acids are important green intermediates for chemical industry. This work was focused on the direct epoxidation of oleic acid (OA) using immobilized lipase Novozym® 435 as the catalyst and hydrogen peroxide (HP) as the epoxidation agent. The impacts of reactant molar ratio (HP:OA = 1…2), reaction temperature (30–60 °C), catalyst loading (7 wt-%) and stirring rate (500–1000 rpm) on the fatty acid conversion and epoxide selectivity were evaluated Acoustic irradiation and an advanced stirring system were implemented for reaction intensification and different ultrasound amplitudes (0–90%) were screened. Novozym® 435 revealed to be an efficient and durable catalyst for fatty acid epoxidation. The reaction was promoted by an excess of HP. The highest epoxide selectivity was about 80% at full conversion of oleic acid reached within 200 min. Ultrasound irradiation (90% amplitude) enhanced the initial reaction rate by a factor two compared to silent conditions and higher oleic acid conversions were reached compared to silent experiments. The catalyst preserved its activity and selectivity rather well, which was confirmed by catalyst recovery experiments conducted in the absence and presence acoustic irradiation. The main ring-opening agent is the fatty acid itself, which resulted in the formation of esters.

Synthesis, characterization, DFT calculations, and catalytic epoxidation of two oxovanadium(IV) Schiff base complexes

The present paper reports the synthesis, characterization, and DFT calculations of two oxovanadium (IV) Schiff base complexes symbolized as VOL1 and VOL2, which prepared by the reaction of bivalent tridentate Schiff base ligands (E)-2-((5-chloro-2-hydroxybenzylidene)amino)acetic acid and (E)-2-((2-hydoxy-5-nitrobenzylidene)amino)acetic acid and VO(acac)2 as vanadium source. The Schiff base ligands and its oxovanadium (IV) complexes were characterized by the elemental analysis (C, H, N), FTIR, 1H NMR and 13C NMR, DFT calculations were performed to derive some of their molecular properties. Schiff base ligands coordinated to vanadium center via nitrogen from the azomethine group and one oxygen from the hydroxyl attached to the benzene ring and one oxygen from hydroxyl of carboxyl group. The catalytic activity of the two complexes were tested against cyclooctene and found that both complexes were highly effective and selective in optimized conditions when used as cyclooctene epoxidation catalysts with the conversion percentage of 91.85% (with VOL1) and 87.40% (with VOL2) at 78 °C within a period of ten hours. To understand the structural properties of the two complexes, the two complexes were well optimized at B3LYP/6-31G(d,p) level of theory, structural parameters such as electron affinity, global electrophilicity, global hardness, electronegativity, ionization potential, and electron chemical potential based on HOMO and LUMO energy values were calculated.

Shape-controlled nanostructured MoO3/CeO2 catalysts for selective cyclohexene epoxidation

This study reported a vital role of CeO2 shape on the structure-activity properties of MoOx/CeO2 catalysts for cyclohexene epoxidation. Uniform dispersion of MoOx on the surface of shape-controlled CeO2 nanocubes (average particle size: 23.5 nm) and nanorods (average width: 6.3 nm and length: 15–95 nm) was found in MoOx/CeO2 nanomaterials. The MoOx/CeO2 nanocubes showed higher epoxide selectivity (97.3%) with 98.9% cyclohexene conversion, although it has lower BET surface area (30.2 m2/g) compared with MoOx/CeO2 nanorods (72.8 m2/g). The presence of more electropositive Mo6+ species and the strong Mo-Ce interaction led to improved catalytic efficacy of MoOx/CeO2 nanocubes in cyclohexene epoxidation.

High-Performance Styrene Epoxidation with Vacancy-Defect Cobalt Single-Atom Catalysts.

Exploring highly active and cost-effective catalysts for styrene epoxidation is of great significance, but it remains challenging to simultaneously achieve excellent conversion and selectivity toward styrene oxide. In this work, the structures and performance of Co, Fe, and Cu single-atom catalysts (SACs) in styrene epoxidation with tert-butyl hydroperoxide (TBHP) are predicted using density functional theory (DFT) calculations. The results reveal that the Co-N structure prefers that of styrene oxide over Fe-N and Cu-N structures. This predicted result is verified via catalytic evaluations, where the Co SACs displayed significantly higher styrene oxide selectivity than Fe and Cu SACs. Moreover, the activity of Co SAC can be further improved by the construction of unsaturated vacancy-defect cobalt single sites. As a result, excellent performance with styrene conversion of 99.9% and styrene oxide selectivity of 71% is achieved after a reaction time of 8 h on the optimal Co SAC.

New terpyridine W(VI) complex on magnetite as a recoverable catalyst in epoxidation of olefins

New terpyridine W(VI) complex on magnetite as a recoverable catalyst in epoxidation of olefins

Insight into the effect of mass transfer channels and intrinsic reactivity in titanium silicalite catalyst for one-step epoxidation of propylene

One-step epoxidation of propylene to propylene oxide (PO) in TS-1/H2O2 system is a promising green and sustainable formation route. However, the TS-1 catalyst is limited by the small pore channels and inadequate intrinsic reactivity. Herein, tetrapropylammonium hydroxide (TPAOH) is used to modify the TS-1 skeleton structures to improve the mass transfer and intrinsic reactivity. Comparative catalytic performance tests indicate that the optimized temperature for TPAOH treatment to modify TS-1 was 90 °C, and the achieved H2O2 utilization efficiency and the PO selectivity can be over 80%. Different TPAOH treatment temperatures significantly influenced the dissolution and recrystallization behavior of TS-1, which altered the composition and microstructure in terms of the pore volume and size, surface properties, and active sites. Our findings indicated that the enlarged pore channels for favorable mass transfer, improved hydrophobic surfaces, enriched framework titanium, and reduced acid strength of titanium silicalite zeolite should be the primary reasons for the enhanced conversion and yields.

Correction to: Solid sulfonated silica acid catalyst for epoxidation of podocarpus falcatus seed oil

Correction to: Solid sulfonated silica acid catalyst for epoxidation of podocarpus falcatus seed oil

Synthesis of new chiral Mn(iii)–salen complexes as recoverable and reusable homogeneous catalysts for the asymmetric epoxidation of styrenes and chromenes

New chiral Mn(iii)–salen complexes 1a–e and 2a–e were synthesized from the reaction of C2-symmetric chiral salen ligands and Mn(CH3COO)2·4H2O under an inert atmosphere followed by aerobic oxidation.