Catalysts For Epoxidation Research Articles

Modulating Ti coordination environment in Ti-containing materials by sulfation for synergizing with Au sites to facilitate propylene epoxidation

Modulating the coordination environment of Ti sites in Ti-containing materials to synergize with Au sites is a crucial issue to boost the process of propylene epoxidation with H2 and O2 to produce propylene oxide (i.e., HOPO), but still remains a great challenge. Herein, we report a facile strategy to tailor the coordination states and electronic properties of Ti sites. The sulfur-decorated Ti-SiO2 (i.e., Ti sites anchored onto silica) with moderate sulfur loading (Ti-SiO2-S-M) can synergize well with Au sites for the HOPO process with high activity, propylene oxide (PO) selectivity, and H2 efficiency, which are superior to those achieved on the referenced Ti-SiO2 synergizing with Au sites. Multiple characterizations, in-situ diffuse reflectance infrared Fourier transform spectroscopy of propylene and PO, PO conversion experiment, and kinetics behaviors demonstrate that the superior advantages of Ti-SiO2-S-M against Ti-SiO2 result from the increased Ti binding energy, improved hydrophobicity, and weakened acidity, which simultaneously facilitate the adsorption of propylene and promote the desorption of PO. This insight reported here could guide the rational design and optimization of Ti-containing materials to synergize with Au catalysts for propylene epoxidation.

Catalytic performances of Ag/SiC catalysts for styrene epoxidation in O2 atmosphere

SiC supported Ag catalysts were prepared by an impregnation-reduction method for styrene epoxidation in O2 atmosphere. Under the condition of 100 °C, 0.5 MPa O2 and 4 h, 5 wt% Ag/SiC catalyst achieved styrene conversion of 98.5 % and styrene oxide selectivity of 69.7 %. Further studies show that O2 was activated on the Ag surface and then formed reactive oxygen species, 1O2 and •O2-. Based on the O2 adsorption results, the atomic ratio of O/Ag larger than 0.5 indicated that more oxygen species had spilled over to the SiC surface. In-situ diffuse reflectance Fourier transform infrared spectroscopy and Density Functional Theory calculation results showed that styrene was adsorbed on the SiC surface. The active oxygen species generated on the Ag surface could spill over to the SiC surface, reacting with the adsorbed styrene to produce styrene oxide or benzaldehyde. In addition, the catalyst exhibits good stability.

Characterization of Fresh, Deactivated, and Regenerated TS‐1 for Propylene Epoxidation: Investigating Surface Deactivation Species

AbstractTitanium silicalite‐1 (TS‐1) serves as an effective catalyst for propylene epoxidation in the hydrogen peroxide propylene oxide (HPPO) process, attracting wide attention from researchers and industry. Nonetheless, the TS‐1 catalyst experienced inevitable deactivation during industrial applications, which adversely impacts its lifespan. To enhance reaction stability of catalyst, the present work conducted a systematic investigation into the deactivation reason of the TS‐1 catalyst. The fresh, deactivated, and regenerated catalysts were characterized by using various analytical techniques to compare their physicochemical properties. The surface coke species of the deactivated catalysts were extracted by the Soxhlet extraction method and subsequently analyzed using GC–MS. Additionally, the pathways for by‐product formation were simulated through Gauss software. The results indicated that the so‐called coking deposits formed during the reaction predominantly consist of propylene glycol, ethers, and oligomers. These coke species covered the partial active sites on the catalyst surface, blocked the pores of the catalyst, and accordingly led to the deactivation of the catalyst, although the crystalline structure of the catalyst hardly changed after deactivation. These results provide a theoretical basis and novel insights for the rational design and development of efficient catalysts for the HPPO process.

Surface-decorated magnetite nanoparticles with dioxidomolybdenum(VI) complexes as a catalyst for alkene epoxidation

Two dioxidomolybdenum(VI) complexes, denoted as [(MoO2)2(L1)] 1 and [(MoO2)2(L2)] 2, were successfully synthesized and characterized. Subsequently, these complexes were immobilized onto the surface of Fe3O4@SiO2-NH2, yielding supported compounds [(MoO2)2(L1)]@Fe3O4@SiO2-NH23 and [(MoO2)2(L2)]@Fe3O4@SiO2-NH24. A range of analytical techniques was employed to thoroughly characterize all synthesized compounds. The effectiveness of the developed supported catalysts in catalyzing the epoxidation of alkenes with hydrogen peroxide (H2O2) as the oxidant was assessed. Moreover, the magnetic properties of the supported compounds were assessed before and after catalytic reactions. It was observed that the magnetic properties exhibited slight alteration following the catalytic cycles. Remarkably, the supported catalysts demonstrated remarkable performance, affording excellent conversion rates along with favorable selectivity towards epoxide products. In addition, the catalysts were found to be reusable for up to seven successive cycles in the transformation of olefins and could be effectively recovered from the reaction mixture using an external magnet.

Environmental benefits of valorising food waste into bio-based polyols for the production of polyurethane rigid foams

Under the global pursuit of sustainable development, waste streams are being recognised as renewable feedstocks to produce value-added products. Given this, food waste (FW) was explored to synthesise bio-based polyols to further develop polyurethane rigid foams (PURF). However, relevant environmental aspects are yet to be examined to support this biorefinery scheme as a green and sustainable solution. In this work, we examined the environmental performance associated with the production of PURF using polyols derived from a FW biorefinery scheme by life cycle assessment (LCA). Analysis was first conducted at the polyol level. Different allocation and offset options were examined to configure the LCA model. Based on mass allocation, compared with fossil-based production, the production of FW-derived polyols achieved reductions of 24.30 % and 34.19 % in global warming potential (GWP) and cumulative energy demand (CED), respectively. At the midpoint level, FW-derived polyols had reduced impacts on human carcinogenic toxicity, freshwater eutrophication, and fossil resource scarcity but caused additional burdens on freshwater and marine ecotoxicity. Key environmental hotspots at this level included diethylene glycol, ion exchange resin (epoxidation catalyst), and hydrogen peroxide. The lipid content in FW also played a significant role. It was demonstrated that reducing the use of enzymes for FW hydrolysis to a cost-effective level remarkably mitigated the overall impacts of FW-derived polyol production. At the next level, we examined the production of FW-derived PURF using the obtained polyols. When 70 % of the polyols were replaced with bio-based products, the resultant PURF production achieved a GWP and CED of 5.67 kg CO2eq and 110.66 MJ/kg, respectively. In general, FW-derived PURF leads to environmental benefits compared to fossil-based ones. However, isocyanate used for foam formulation was the dominant contributor, causing almost two-thirds of the total impacts. The flame retardant also caused considerable impacts. Through the systematic examination of FW-derived polyols and PURF, this study demonstrated that FW-derived PURF could benefit the sustainable development of FW biorefineries and bio-based plastic industries, while the identified environmental hotspots need to be further studied and replaced with greener substitutes. AcronymsUnlabelled TableCEDCumulative energy demandDEGDiethylene glycolDALYDisability-adjusted life yearsFWFood wasteGWPGlobal warming potentialLCALife cycle assessmentLCILife cycle inventoryMDIMethylene diphenyl diisocyanateNIPUNon-isocyanate polyurethanespMDIPolymeric 4,4′-methylene diphenyl isocyanatePURFPolyurethane rigid foamsPUsPolyurethanesTPhPTriphenyl phosphateTCPPTris(1-chloro-2-propyl) phosphate

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.

Impact of Ligand Design on an Iron NHC Epoxidation Catalyst.

An open-chain iron pyridine-NHC framework is expanded utilizing a benzimidazole moiety to deepen the understanding of the impact of electronic variations on iron NHC epoxidation catalysts, especially regarding the stability. The thereby newly obtained iron(II) NHC complex is characterized and employed in olefin epoxidation. It is remarkably temperature tolerant and achieves a TOF of ca. 10 000 h-1 and TON of ca. 700 at 60 °C in the presence of the Lewis acid Sc(OTf)3, displaying equal stability, but lower activity than the unmodified iron pyridine-NHC (pre-)catalyst. In addition, a synthetic approach towards another ligand containing 2-imidazoline units is described but formylation as well as hydrolysis hamper its successful synthesis.

Thiamine-modified octamolybdate catalyzes solvent-free stereoselective olefin epoxidation with TBHP

Simple stirring of an acidic aqueous solution (pH=3) containing sodium molybdate and thiamine hydrochloride (VB1) at room temperature resulted in an octamolybdate-containing VB1 (MoB1) material as an efficient and cost-effective epoxidation catalyst using tert‑butyl hydroperoxide (TBHP) under solvent-free condition. The stereospecific stilbene oxidation (>99 %) observed in the presence of MoB1 can be originated from the steric hindrance of some groups on the thiamine molecule and π–π stacking interactions of its pyrimidine and/or thiazole rings with phenyl rings of stilbenes. Further olefins such as cyclooctene, indene, 1-octene, and styrenes are efficiently oxidized using MoB1/TBHP catalytic system. The catalyst is composed of two thiaminium dications (C12H18N4OS)2+ and one Mo8O264– tetraanion based on different characterization techniques and chemical compositional analyses. Field Emission Scanning Electron Microscope (FE-SEM) images shows a sheet-like nanostructure for MoB1 comprising layers with thicknesses ranging from 50 to 500 nm. It exhibits a heterogeneous nature in catalysis evidenced by hot filtration test and ICP-OES analysis. X-ray photoelectron spectroscopy (XPS) indicates minor contributions of Mo in the oxidation states +V and +IV in addition to MoVI attributed to partial electron transfer from VB1 to Mo8O264–. The scavenging experiments support the presence of radical species during catalysis, while, they do not damage the structural integrity of the MoB1 catalyst based on the recycling experiments and FT-IR spectra.

Dynamic Phase Behavior of Surface-Active Fluorinated Ionic Liquid Epoxidation Catalysts.

We report on the synthesis of amphiphobic fluorinated surface-active ionic liquid (FSAIL) epoxidation catalysts, which show reversible temperature-controlled solubility in water. The solubility of FSAILs containing the catalytically active perrhenate- and tungstate anions was studied in both the aqueous and the substrate phase, showing a significant solubility decrease in both media compared to their non-fluorinated congeners. It was shown that both the epoxide product and the catalyst additive phenylphosphonic acid (PPA) are efficient in transferring the FSAIL catalyst into the organic phase, rendering the reaction homogeneous. The FSAILs were used as catalysts for the epoxidation of olefins using aqueous H2O2 as oxidant, showing an exceptionally high catalytic activity at mild conditions. Catalyst recycling was demonstrated over ten consecutive runs by phase separation and subsequent product distillation.

Models for Single‐Site Heterogeneous Catalysts on Carbon: MoO2 Epoxidation Catalyst Anchored to a Fullerene

Single‐site molybdenum dioxo catalysts, fullerenol/MoO2, are prepared via grafting precursor (DME)MoO2Cl2 onto a highly polyhydroxylated fullerene (ful) and a isomerically pure and well‐defined fullerene (ful*). These catalyst structures are characterized by ICP‐OES, XPS, XANES, EXAFS, DRIFT, Raman, and NMR spectroscopy, and DFT. Mo 3d5/2 XPS and Mo K‐edge XANES assign the oxidation state as Mo(VI). Mo EXAFS data fitting reveals two Mo=O double and two Mo‐O single bonds at distances of 1.7 and 1.9 Å, respectively, while an Mo=O stretchingl mode is observed at ~950 cm‐1 by DRIFT and Raman spectroscopy. These data align well with computational results, supporting the proposed catalyst structure as Fullerene(‐µ‐O‐)2M(=O)2. Additionally, DFT provides insight into the energetically favorable grafting sites for an isomerically pure fullerenol. The scope of fullerenol/MoO2 mediated alkene epoxidation includes abiotic alkenes, natural occurring terpenes, and conjugated olefins. For cyclooctene the rate law is first‐order in [Mo], near first order in [olefin] and zero‐order in [t‐butyl hydroperoxide]. A plausible reaction mechanism involves peroxide addition first and then cyclooctene addition directly across the peroxo bond forming the epoxide product, consistent with DFT computation. Overall, fullerenol/MoO2 shows promise as a sustainable and structurally well‐defined system with versatile catalytic activity and good epoxidation recyclability.

The fabrication of potassium ion modification on Cu2O(111) for enhanced propylene oxide selectivity: Insights from density functional theory

Utilizing the alkali metal effect to enhance catalyst performance is an effective approach in the development of highly efficient catalysts. Herein, we report a K+ ion-modification strategy on Cu-based catalysts for the direct epoxidation of propylene (DEP) using molecular oxygen, a reaction deemed as one of the “dream reactions” in heterogeneous catalysis. By elaborately engineering of K+-modified Cu2O(111) catalyst, we unveil the versatile roles played by K+ ions in promoting catalytic performance for DEP. Firstly, K+ ions stabilize the surface coordination structures of Cu2O(111). Secondly, K+ ions make the dissociation of adsorbed O2* less likely, thus facilitating the reaction of propylene with O2*. Furthermore, both K+ and two-coordinated Cu+ serve as active centers in this reaction. Lastly, K+ ions create unique surface coordination structures favorable for propylene oxide (PO) formation. The density functional theory (DFT) calculations demonstrate that the effective utilization of the alkali metal effect can engineer highly efficient Cu-based catalysts for DEP.

Low Concentration Template Preparation of Microsized Au/TS-1 as an Efficient Catalyst for Gas-Phase Propylene Epoxidation with H2 and O2

Low Concentration Template Preparation of Microsized Au/TS-1 as an Efficient Catalyst for Gas-Phase Propylene Epoxidation with H₂ and O₂

Impact of POM's coordination mode and Mo-hybrid constituents on the binding, stability, and catalytic properties of hybrid (pre)catalysts.

The assembly of MoVIO2 2+ and methoxy-substituted salicylaldehyde nicotinoyl hydrazone ligands afforded two classes of hybrid polyoxometalates (POMs). In the Class I architectures, [MoO2(HL1-3)(D)]2[Mo6O19]·xCH3COCH3 (D = CH3COCH3 or H2O, x = 0 or 2, and L1-3 = ligands bearing the OMe group at position 3, 4 and 5, respectively), the main driving force for self-assembly is the electrostatic interaction between the components. Class II architectures are composed of a POM anion covalently linked to two Mo-complex units through the terminal Ot or bridging μ2-OPOM oxygen atoms, as found in Lindqvist-based hybrids [{MoO2(HL1-3)}2Mo6O19]·xCH3CN (x = 0 or 2) and the asymmetrical β-octamolybdate-based hybrid [{Mo2O4(HL2)(H2L)}{MoO2(HL2)}2Mo8O26]·CH3CN·H2O. Quantum chemical calculations were applied to evaluate the impact of the POM hybrid constituents on the hybrid-type stability, showing that it strongly depends on the ligand substituent position and ancillary ligand nature. Hybrids were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP in water or decane) and with or without the addition of acetonitrile (CH3CN) as an organic solvent. The catalytic results provided by the use of TBHP in decane are the best ones and classify all the prepared catalysts as very active, with the conversion of cyclooctene >90%, and high selectivity towards epoxide, >80%. We also examined the influence of the ligand structure, POM's hybrid type, and coordination mode on the Mo-hybrid activity and selectivity.

Selective Oxidation of Cyclohexene over the Mesoporous H-Beta Zeolite on Copper/Nickel Bimetal Catalyst in Continuous Reactor.

The copper/nickel-metal on commercial H-Beta zeolite supports was synthesized with different wt % (Ni) of 5, 10, 15, and 20, and was used in the cyclohexene epoxidation process. The synthesized catalyst has been used in a continuous reactor for the cyclohexene epoxidation process, with mild conditions and H2O2 as an oxidant. The catalytic performance was ascertained by adjusting parameters such as the temperature, pressure, WHSV, reaction time, and solvents. The catalytic performance showed the resulting yield in both cyclohexene conversion and selectivity was more than 98.5%. The catalyst's textural attributes, morphology, chemical composition, and stability were determined using FT-IR, XRD, BET, HR-SEM, and TPD. The most active catalyst among those that were synthesized was evaluated, and the reaction parameters were selected to optimize yield and conversion. The H-Beta/Cu/Ni (15%) catalyst has the best conversion (98.5%) and selectivity (100%) for cyclohexene among the catalysts examined. Cu and Ni(15%) metals were successfully added to the H-Beta zeolite, causing little damage to the crystalline structure and resulting in good reusability over five cycles, as well as little loss of catalytic selectivity. Acetonitrile was the solvent that provided the highest conversion and selectivity among the others. These findings show that H-Beta/Cu/Ni bimetallic catalysts have the potential to be effective epoxidation catalysts. Because of their outstanding conversion and selectivity, the continuous reaction technique used in this work makes them appropriate for industrial production-level applications.

Synthesis of titanium silicalite-1 supported zinc catalysts for efficient and clean epoxidation of 1-hexene and methyl oleate

To improve the catalytic epoxidation activity of conventional TS-1 zeolite under acid-free conditions, a series of TS-1-supported zinc (xZn/TS-1) catalysts were prepared and evaluated in the epoxidation of 1-hexene and methyl oleate (MO) with H2O2 oxidant. The microstructure of xZn/TS-1 was investigated using various characterization techniques, and the results showed that the introduction of Zn species was not only well dispersed on the surface of TS-1 but also did not disrupt the crystal structure of TS-1 and tetrahedral framework titanium. Furthermore, the Ti-O-Zn structure can be formed between Zn and Ti species through O-atomic coordination, which improves the vulnerability of the Ti-O bond to attack the H2O2 molecule, and ultimately facilitates the epoxidation reaction between 1-hexene and MO. In addition, the 2% Zn/TS-1 catalysts showed optimal conversions of 45.7 % and 75.41 % for 1-hexene and MO, respectively, and exhibited good catalytic stability and regeneration during the epoxidation process.

The role of chlorine promoters in mediating particle size effects in silver-catalyzed ethylene epoxidation

Measured chlorine coverages over Ag/α-Al2O3 catalysts with varying Ag weight loadings (1.3–35 wt%) and different Ag particle size distributions when parsed in terms of Cl coverages over particles of varying size constituted in the distribution reveal that Ag particles of size below ∼30 nm are covered in more than 1 monolayer (ML) of chlorine in presence of 3.5 ppm C2H5Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al2O3 samples using a single exponential decay function. Fully chlorided small Ag particles exhibit low rates of epoxidation and only large Ag particles which exhibit sub-monolayer Cl coverages catalyze ethylene epoxidation with high ethylene oxide (EO) rates and selectivity. The Ag particle size dependence of Cl coverages plausibly explains why ≥ 100 nm Ag particles are typical in promoted Ag/α-Al2O3 ethylene epoxidation catalysts.

Oxidovanadium(V) complexes with tridentate hydrazone ligands as oxygen atom transfer catalysts

Four isostructural oxovanadium(V) complexes with hydrazone ligands have been synthesised, characterised, and evaluated as epoxidation and sulfoxidation catalysts. The reactions between [VO(acac)2] (acac– = acetylacetonate) and H2Ln (n = 1–4), precursors for monoanionic tridentate hydrazone ligands, afford complexes formulated as [VO(Ln)(bzh)·MeOH] (1–4) when bidentate benzohydroxamic acid (Hbzh) is included as a co-ligand. Single crystal X-ray structure analyses showed that complexes 1–3 have a distorted octahedral coordination geometry with an O5N coordination environment. Cyclic voltammetry showed that all complexes undergo two quasi-irreversible reduction peaks and a single irreversible oxidation peak. The bonding in 1 has been investigated by electronic structure calculations, and these data are discussed with respect to the electrochemical results. Complexes 1–4 were tested as catalysts for the epoxidation of cis-cyclooctene at 50 °C and sulfoxidation of methyl-p-tolylsulfide at room temperature using tert-butyl hydroperoxide (tBuOOH) and aqueous H2O2 as the terminal oxidants.

Cascade Sequence of Photooxygenation-Epoxidation for the Flow Synthesis of Epoxy Alcohols.

A photooxygenation-epoxidation cascade sequence converting alkenes to epoxy alcohols was developed and evaluated in batch and continuous-flow systems. In the batch system, the undesired interactions between the photooxygenation and epoxidation catalysts resulted in suboptimal yields, whereas the fine control of reaction parameters in the flow system allowed the allyl hydroperoxides produced through photooxygenation of alkenes to be rapidly converted to epoxy alcohols in yields of up to 93%. The developed procedure allows one to avoid an important synthetic bottleneck, works well where traditional batch synthesis fails, and can be scaled up to meet the needs of industrial production, thus presenting a valuable addition to the toolbox of practicing organic chemists.

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.

Nanoparticles as an antidote for poisoned gold single-atom catalysts in sustainable propylene epoxidation

The development of sustainable and anti-poisoning single-atom catalysts (SACs) is essential for advancing their research from laboratory to industry. Here, we present a proof-of-concept study on the poisoning of Au SACs, and the antidote of Au nanoparticles (NPs), with trace addition shown to reinforce and sustain propylene epoxidation. Multiple characterizations, kinetics investigations, and multiscale simulations reveal that Au SACs display remarkable epoxidation activity at a low propylene coverage, but become poisoned at higher coverages. Interestingly, Au NPs can synergistically cooperate with Au SACs by providing distinct active sites required for H2/O2 and C3H6 activations, as well as hydroperoxyl radical to restore poisoned SACs. The difference in reaction order between C3H6 and H2 (nC3H6-nH2) is identified as the descriptor for establishing the volcano curves, which can be fine-tuned by the intimacy and composition of SACs and NPs to achieve a rate-matching scenario for the formation, transfer, and consumption of hydroperoxyl. Consequently, only trace addition of Au NPs antidote (0.3% ratio of SACs) stimulates significant improvements in propylene oxide formation rate, selectivity, and H2 efficiency compared to SACs alone, offering a 56-fold, 3-fold, and 22-fold increase, respectively, whose performances can be maintained for 150 h.