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[Display omitted] • Synthesis and comprehensive characterization of two novel iron(II) NHC complexes. • Study of three iron NHC epoxidation catalysts with different electronic properties. • Olefin epoxidation with exceptionally high turnover frequencies (>700). • Remarkable high selectivity with high substrate adaptability. • Tolerance to functional groups (OH, Cl, C=O). A comparative study of three iron(II) NHC epoxidation catalysts with different electronic properties is performed to gain more profound insight into the influence of electronic variations on catalytic performance. One iron complex contains a pyridyl-NHC ligand, the other two are prepared with a modified ligand counterpart. The complexes are comprehensively characterized by various methods including Mössbauer, SQUID and DFT. While a lower electron density at the iron atom can be associated with a decline in epoxidation activity, a more electron rich iron center does not necessarily correspond with higher activity, due to reduced catalyst stability. Addition of Lewis acids increases both activity and stability significantly and is more effective than temperature variations. All three epoxidation catalysts achieve high selectivity, with a maximum TOF of 24 500 h−1 and TON of >700 for the unmodified complex. More nucleophilic alkenes promote higher activity and conversion. [ABSTRACT FROM AUTHOR]

[Display omitted] • The reaction rate or product selectivity for C 6 H 10 transformation varies with solvent applied. • C 6 H 10 epoxidation follows an Eley-Rideal mechanism, while C 6 H 10 O hydration follows a Langmuir-Hinshelwood mechanism. • The modulation of the C 6 H 10 concentration around Ti- by solvent affects epoxidation rate. • The difference in epoxidation rates is more kinetic control than being thermodynamically influenced. Alkene epoxidation is a very important reaction process for the selective generation of chiral products, usually performed in some solvent(s) with different polarity, solubility and proton properties. These physical properties are entangled and do not explain very well whether they are mainly against population or stability of the reaction intermediates during catalysis. Here, we investigate how different solvents with known polarity properties (CH 3 CN, (CH 3) 2 CO, CH 3 OH, etc.) affect the C 6 H 10 transformation over Ti-Beta catalysts. Combining kinetic measurements, thermodynamic investigations and isotope experiments, the solvent was found not to change the reaction path or mechanism, with C 6 H 10 epoxidation most likely following the Eley-Rideal mechanism and C 6 H 10 O hydration following a Langmuir-Hinshelwood mechanism, and both nucleophile and carbonium were found to be involved in the transition state (c.a. SN 2 process) of hydration. However, in the epoxidation reaction of C 6 H 10 with CH 3 CN and (CH 3) 2 CO as solvents, the reaction order of [C 6 H 10 ] is subtly different and the epoxidation rate increases with increasing volumetric fraction of CH 3 CN in the binary mixture, which indicates that the solvent affects the concentration of dissolved C 6 H 10 surrounding the active sites and leads to the corresponding differences in activity and product selectivity. Nevertheless, the measured apparent reaction barrier (energy difference between transition state and MASIs) spans the elementary step of solvent modification, thus allowing for the observation of negligible solvent thermodynamic modification. Collectively, these findings provide clear evidence of solvent effects on alkene epoxidation and can be easily extended to some other reactions in solvents. [ABSTRACT FROM AUTHOR]

[Display omitted] • Well-defined cyclic carbamoyl-Mn(I) complexes are prepared with improved stability. • Carbamoyl-Mn(I) accelerates alkenes epoxidation via Mn(III)=O and Mn(II)-O species. • CO 2 cycloaddition of epoxides was promoted by acidic N–H proton in carbamoyl unit. There is no precedent for the usage of transition metal carbamoyl complexes (typical hydrogenase models) as pre-catalysts for oxidation or CO 2 utilization purposes. In this work, we describe that carbamoyl-Mn(I) complexes are capable of selectively catalyzing both epoxidation of olefins and epoxides with carbon dioxide to form cyclic carbonates. The dual-role catalytic system was well adapted to a wide range of aromatic olefins, and the desired cyclic carbonates were obtained in moderate to excellent yields. Mechanistic investigations indicated that active Mn(II)-oxo and Mn(III)-oxo species were involved in the oxidation process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag particle size distribution over Ag/α-Al 2 O 3 follows a lognormal distribution. • A Gaussian rate function explains particle size effects in ethylene epoxidation. • Ag particle size distributions centered ∼ 140 nm facilitate high epoxidation rates. The post-reaction Ag surface area distribution of 16 Ag/α-Al 2 O 3 ethylene epoxidation catalysts with varying Ag weight loadings (6.9 – 35 wt%) containing different distributions of Ag particle sizes was correlated with measured ethylene oxide (EO) rates in presence of 3.5 ppm ethyl chloride via a particle size dependent Gaussian rate function. The Gaussian rate function model differs from a description based on average particle size as it accounts for the effects of the breadth of particle size distribution as well as the population of different particle sizes that constitute the Ag particle distribution on measured EO rates. This description of particle size effects in ethylene epoxidation catalysis facilitates accurate prediction of EO rates over a wide range of particle sizes (5–400 nm) and suggests that very small (5–50 nm) and very large Ag particles (>250 nm) have low EO rates (<3 μmol g Ag -1 s−1). A narrow Ag particle size distribution centered around ∼130–150 nm would enable operation of ethylene epoxidation with high EO rates and EO selectivity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag particles (40–170 nm) supported on α-Al 2 O 3 are composed of multiple crystallites. • Increasing Ag particle size increases the apparent TOF. • EO selectivity nearly constant with Ag particle size (at zero residence time). • Raman bands at 815 and 880 (Ag > 100 nm) cm−1 identified as active oxygen species. • 880 cm−1 band most likely responsible for higher apparent TOF in large Ag particles. In situ Raman spectroscopy and parallel fixed bed reactor studies were conducted under ethylene epoxidation conditions with O 2 at 1 atm and 200 ℃ on unpromoted Ag/α-Al 2 O 3 catalysts with different Ag particle sizes. It was found that for Ag particles of 20–50 nm, the weight normalized conversion rate decreased rapidly with increasing Ag particle size but remained almost constant above 50 nm. On the other hand, the apparent TOF increased with increasing Ag particle sizes in the 20–170 nm studied range, while ethylene oxide selectivity at zero residence time was nearly constant (55 ± 4%). Raman bands at 815 (all Ag sizes) and 880 (Ag sizes > 100 nm) cm−1 were identified and assigned to active molecular oxygen species. The 880 cm−1 species was assigned to a molecular oxygen complex structure stabilized by subsurface oxygen. The presence of the 880 cm−1 oxygen species likely explain the higher apparent TOF in larger Ag particles (>100 nm). [ABSTRACT FROM AUTHOR]

[Display omitted] • LaSrCoNiO 6 nanoparticles exhibit an unusual and tunable LSPR effect in the visible region. • LSCNi-N catalyst exhibits selectivity of up to 72.3% in a 93.4% conversion of cinnamyl alcohol. • LSCNi-N catalyst shows 91.8% selectivity of styrene oxide at almost 100% conversion of styrene. • The high epoxidation selectivity derives from the in-situ photochemical formation of H 2 O 2 mediated by the LSPR effect of LSCNi-N catalyst. Selective aerobic epoxidation of allylic alcohols and olefins presents a promising solution to the modern chemical industry. However, the development of non-noble metal catalysts with superior catalytic performance for this reaction remains a significant challenge. This study introduces a plasmonic photothermal-catalytic system centered around nano LaSrCoNiO 6 (LSCNi-N) catalyst, enabling the epoxidation of cinnamyl alcohol and styrene mediated by LSPR effect under visible light illumination (>420 nm). This catalyst exhibits superior epoxidation catalytic performance, with selectivities of up to 72.3 % in a 93.4 % conversion of cinnamyl alcohol and 91.8 % selectivity of styrene oxide at almost 100 % conversion of styrene. Mechanistic studies reveal that the high selectivity derives from the in-situ photochemical formation of H 2 O 2 mediated by the localized surface plasmon resonance effect of LSCNi-N and hole scavenger effect of cinnamyl alcohol. These findings highlight the potential of designing plasmonic transition-metal oxidic catalysts to overcome challenges in selectively synthesizing fine chemicals through visible light catalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • A simple reflux hydrolysis method to prepare MoO 3 /bipyridine hybrid (pre)catalysts. • Structure-directing effect of bipyridine (L) substituents in [MoO 2 Cl 2 (L)] precursors. • The hybrids promoted olefin epoxidation with H 2 O 2 or t -BuOOH as oxidant. • Heterogeneous, reaction-induced self-separating, and homogeneous catalytic features. • Bio-olefins and lignin-based isoeugenol gave useful products, in heterogeneous phase. Molybdenum catalysts have been industrially recognized for decades for liquid phase epoxidation, which is an important chemical reaction process, since epoxides are used for many industrial applications. In this work, molybdenum oxide hybrid catalysts, prepared by a reflux hydrolysis methodology, performed effectively as heterogeneous catalysts or reaction-induced self-separating catalysts under mild reaction conditions; in the two cases, the catalyst separation and reuse are facilitated. Specifically, catalysts with the general formula [MoO 3 (L)], possessing polymeric chain-like (L = 4,4′-dimethyl-2,2′-bipyridine (1)) or oligomeric (L = 4,4′-dinonyl-2,2′-bipyridine (2)) structures comprising corner-sharing {MoO 4 N 2 } units, were synthesized and characterized by various complementary techniques (ATR FT-IR, Raman, 13C{1H} CP MAS NMR spectroscopy, PXRD, SEM, TGA, elemental analysis, ICP-OES and N 2 sorption isotherms). Small chemical differences in the organic synthesis precursor had important structure directing effects on the type of hybrid material formed. The hybrids promoted olefin epoxidation with H 2 O 2 or tert -butylhydroperoxide (TBHP) as oxidant. For example, 1 catalyzed the conversion of biobased olefins (70 °C) and lignin-based isoeugenol (50 °C) with TBHP to useful bioproducts, in heterogeneous phase, leading to an epoxide yield of 100 % for DL -limonene (3:1 M molar ratio of 1,2-epoxy- p -menth-8-ene to 1,2:8,9-diepoxy- p -menthane), 81 % epoxide yield for fatty acid methyl esters, and 80 % Licarin A selectivity at 40 % isoeugenol conversion. For dienes (DL -limonene, methyl linoleate), kinetic modelling studies suggested that the formation of the monoepoxides was faster than that of diepoxides, accounting for enhanced monoepoxide selectivity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ti-Beta zeotypes with open Ti(OSi) 3 OH sites (OH-Ti-Beta zeotypes) synthesized via the treatment of Ti-Beta with NH 4 F. • OH-Ti-Beta zeotypes possess a stronger Lewis acidity than the parent Ti-Beta. • The resultant stronger Lewis acidity promotes the catalytic cyclohexene epoxidation. Ti-Beta zeotypes, possessing three-dimensional 12-membered ring pore-opening channels, have shown some superior catalytic performance in the epoxidation of bulky cycloalkenes with hydrogen peroxide (H 2 O 2) in comparison with other titanosilicate zeotypes. However, some further improvement on the catalytic activity and selectivity of Ti-Beta zeotypes is still demanded for the industrialization of cycloalkenes epoxidation. In the present work, the catalytic performance of Ti-Beta in cyclohexene epoxidation was significantly improved via the transformation of their closed Ti(OSi) 4 sites into the open Ti(OSi) 3 OH ones by a simple NH 4 F treatment. Based on the extensive characterization results, it is found that the NH 4 F-treated Ti-Beta zeotypes with open Ti(OSi) 3 OH sites, named OH-Ti-Beta, possess a stronger Lewis acidity that is favorable to the production of cyclohexene oxide (CHO) from the epoxidation of cyclohexene with H 2 O 2 , compared to the parent Ti-Beta with closed Ti(OSi) 4 sites. Moreover, OH-Ti-Beta catalysts are reusable after the calcination of the deactivated zeotypes. Hence, it is expected that the current developed NH 4 F treatment can obtain efficient OH-Ti-Beta catalysts industrially applied in cyclohexene epoxidation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The modified deposition–precipitation method can immobilize tiny Au particles (ca. 1.6 nm) on uncalcined TS-1. • The ammonia treatment facilitates the hydrolysis of partial ≡Ti-O-Si≡ bonds to defective Ti(OSi) 3 OH species. • The increase in defective Ti(OSi) 3 OH species can not only provide more Ti4+ active sites but also enhance the dispersion of Au. • The as-synthesized catalyst exhibits promising catalytic performance for the propylene epoxidation with H 2 and O 2. Reducing the Au particle size and increasing the number of defective Ti4+ species have been considered effective strategies to boost the catalytic performance of Au–Ti bifunctional catalysts used for propylene epoxidation utilizing H 2 and O 2. Consequently, the development of an effective method that can simultaneously achieve these goals is of significant scientific and industrial importance. Herein, we demonstrate, for the first time, that the simultaneous immobilization of small Au particles (approximately 1.6 nm) onto uncalcined titanium silicate-1 (i.e., TS-1-B) and facilitation of the formation of defective Ti4+ species can be achieved using a modified deposition–precipitation (DP) method utilizing ammonia solution as the precipitant. The as-synthesized catalyst exhibits a significantly enhanced propylene oxide (PO) formation rate combined with fascinating PO selectivity and hydrogen efficiency, which were better than the reference catalyst prepared via the DP urea (DPU) method and other reported Au/uncalcined TS-1 catalysts. The catalyst structure–performance relationship was established using multiple characterization studies. the ammonia treatment facilitates the hydrolysis of some of the ≡Ti-O-Si≡ bonds into defective Ti4+ species (i.e., Ti(OSi) 3 OH), which not only provides more active Ti4+ sites but also favors the uniform deposition of Au. This methodology and the insights stated herein may also be applicable for enhancing the dispersion of Au in a variety of zeolite-supported catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Gold-supported catalysts were prepared using TS-1 with a small amount of vanadium. • The added vanadium was incorporated into the molecular sieve framework of TS-1. • Propylene conversion was maintained in Au/V-TS-1 even at low H 2 concentrations. • Au/V-TS-1 maintained selectivity for propylene oxide at lower H 2 concentrations. The direct epoxidation of propylene to propylene oxide (PO) represents a unique feature of gold nanoparticle catalysts. Different amounts of vanadium were introduced into TS-1 using a hydrothermal method to give V-TS-1, and the effect of various vanadium loadings on the catalytic performance of Au/V-TS-1 was investigated in the epoxidation of propylene under different reaction conditions. The obtained characterization results indicate that the molecular sieve pore structure and crystalline phase structure of TS-1 were maintained after vanadium incorporation, and vanadium was incorporated into the molecular sieve framework. The introduction of an appropriate amount of vanadium resulted in the homogeneous dispersion of gold particles with a small average particle size, which facilitated the epoxidation reaction. When V was introduced into TS-1, the propylene conversion decreased under the condition of 10 % H 2 concentration (C 3 H 6 /H 2 = 1/1) in the reaction atmosphere, but the PO selectivity was maintained. Interestingly, when the H 2 concentration in the reaction atmosphere was lowered to 2 % (C 3 H 6 /H 2 = 3/1), the propylene conversion in Au/TS-1 was significantly decreased compared to 10 % H 2 concentration condition, while in Au/V-TS-1, the propylene conversion was maintained in 2 % H 2 concentration condition compared to 10 % H 2. Au/V-TS-1 also maintained its selectivity for PO even when the H 2 concentration was lowered. Therefore, under low hydrogen concentration conditions, the PO formation rate of Au/V-TS-1 was much higher than that of Au/TS-1. More, the same trend was observed when Mo was added in place of V. Our strategy will guide the design of future catalysts as a way to utilize hydrogen more effectively while maintaining PO productivity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Chlorine coverage varies as a function of Ag particle size. • Small Ag particles (<30 nm) are fully chlorided resulting in low epoxidation rates. • Ag particle size distribution determines Cl coverage and ethylene oxide selectivity. Measured chlorine coverages over Ag/α-Al 2 O 3 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 C 2 H 5 Cl. These assessments of Cl coverages were made possible by correlating the cumulative Cl coverage to the lognormal surface area distribution of 22 Ag/α-Al 2 O 3 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-Al 2 O 3 ethylene epoxidation catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Turnover rates for epoxidation on dispersed Ti atoms vary by 1000-fold with identity of oxide support. • Apparent activation enthalpies (ΔH‡ epox) span 85 kJ mol−1 across Ti-based catalysts. • In situ spectroscopy does not detect differences among electronic structures of reactive intermediates (Ti-OOH/O 2). • Isoelectric points and water uptakes on oxides correlate with ΔH‡ epox. • Acidic supports concentrate water near the solid-liquid interfaces and lower ΔH‡ epox values. Atomically disperse Ti sites on metal oxides (MO x , including SiO 2 , γ-Al 2 O 3 , ZnO, GeO 2) activate H 2 O 2 to create intermediates active for alkene epoxidations. Turnover rates for 1-hexene epoxidation in acetonitrile vary 1000-fold at identical conditions due to differences in apparent activation enthalpies (ΔH‡ epox) and entropies (ΔS‡ epox). Ligand-to-metal charge transfer energies and vibrational frequencies of reactive species assessed by in situ UV–Vis and Raman spectroscopy, respectively, indicate supports do not detectably change electronic properties of H 2 O 2 -derived intermediates. However, isoelectric points and solution-phase water uptakes for these metal oxides correlate with ΔH‡ epox and suggest that non-covalent interactions at the solid-liquid interface influence the stability of epoxidation transition states. Supports with lower pK a values concentrate water near the solid-liquid interface and enthalpically stabilize the transition state. These findings illustrate that outer sphere interactions impact epoxidation reactions upon metal oxide catalysts including titanium silicates. [ABSTRACT FROM AUTHOR]

[Display omitted] • Method to prepare Au nanoparticles encapsulated within titanium silicalite-1. • Synthesis circumvents harsh conditions that otherwise make Au confinement difficult. • Au particles are uniform in size and selectively reside within crystallites. • Au/TS-1 system is active for propylene epoxidation with H 2 O/O 2. • Alkali promoters (Cs, K) are not required for epoxidation activity, but boost rates. 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 H 2 O/O 2 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electroless deposition increases Ag particle size w/o high temp sintering. • H 2 titration combined with SEM to characterize Ag size and site conc following eval. • TOF has minimal sensitivity to Ag size where EO selectivity greatly increases for larger particles. • Small particles sinter and favor foulant formation. Catalysts have been prepared on a low surface area α-Al 2 O 3 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 AgNO 3 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-Al 2 O 3 using Ag 2 C 2 O₄ 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 H 2 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 CO 2 /H 2 O formation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrophilic Ti-zeolite gives higher epoxidation rates than their hydrophobic counterparts. • Rate expressions and identity of reactive intermediates depend weakly on solvent identity. • CH 3 CN gives greater rates in Ti-BEA, while CH 3 OH provides greater rates in Ti-MFI. • Stabilization of epoxidation surface intermediates depends on solvent identity and pore architecture. Ti-silicates activate H 2 O 2 to form Ti-hydroperoxo and Ti-peroxo intermediates that can react with alkenes to form epoxide products. Comparisons of kinetics for 1-octene epoxidation with H 2 O 2 on Ti-BEA and Ti-MFI catalysts with different hydrophilicities in methanol (CH 3 OH) or acetonitrile (CH 3 CN) solvents show the significance of the solvent for stabilizing catalytically-relevant species and the complex interdependencies between solvent, catalyst topology, and hydrophilicity. Epoxidation turnover rates are higher in CH 3 CN than CH 3 OH for Ti-BEA, but the opposite trend is observed for Ti-MFI. Ti-silicates with greater silanol densities, however, give greater epoxidation turnover rates than their hydrophobic counterparts in both solvents. Kinetic, spectroscopic, and thermodynamic analyses show that differences in turnover rates mainly arise from changes in the stabilization of reactive surface species by solvent mediated interactions, because the mechanism of the reaction and stability of the fluid-phase reactants remain similar in CH 3 CN and CH 3 OH. Specifically, apparent activation free energy values (Δ G App ‡) indicate that surface intermediates responsible for alkene epoxidation are stabilized to a greater extent in CH 3 CN on Ti-BEA and in CH 3 OH on Ti-MFI. Hydrophilic Ti-silicates present lower Δ G App ‡ values regardless of solvent identity, which suggests that these differences correspond to the number of hydrogen-bonding solvent molecules found near reactive species bound to Ti active sites. Taken together, these findings demonstrate the role of solvent molecules in allowing reactive intermediates to recognize the properties of active sites beyond the length-scale of covalent bonds, which carry implications for epoxidation but also other reactions within solvent-filled pores of microporous materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ethylene epoxidation occurs over reconstructed surface oxides that encase metallic Ag particles. • Oxide-like surfaces contain O–Ag–O motifs represented by a Raman scattering feature at 600 cm−1. • Raman bands near 810 – 840 cm−1 correspond to dioxygen complexes bound at oxygen vacancies. The partial oxidation of ethylene to ethylene oxide (EO) proceeds over supported Ag particles. Adsorption of molecular oxygen upon Ag forms the reactive oxygen species responsible for both epoxidation and combustion reactions, however, the catalyst surface structure and the molecular structure and identity of the reactive oxygen species present remain debated despite past use of appropriate forms of in situ spectroscopy. Specifically, there is no consensus for the molecular origin of widely reported Raman features that appear on contact with only O 2 or in reactant mixtures of C 2 H 4 and O 2 at pressures and temperatures relevant for the industrial process, in part, due to the lack of concerted ab initio studies that compute vibrational frequencies for oxygen-containing Ag surfaces. Here, we elucidate the molecular structure of the catalyst surface and reactive oxygen species by coordinating spectral deconvolution of transient and steady-state surface-enhanced Raman spectroscopy of Ag catalysts exposed to oxygen (2 – 101 kPa O 2 , 523 – 673 K) and mixtures of oxygen and ethylene (2 – 101 kPa O 2 , 0.5 – 9.8 kPa C 2 H 4 , 523 K) with ab initio thermodynamic modeling and vibrational frequency calculations. These comparisons suggest that during EO catalysis reconstructed surface oxides form and partially or completely encase metallic Ag particles. Raman features near 600 cm−1 that persist in O 2 or cofed C 2 H 4 represent O–Ag–O structural motifs that form only on surface oxides and high oxygen-coverage reconstructed Ag x O y surfaces. We assign features centered near 810 – 840 cm−1 and at higher frequencies, which are ubiquitous throughout the literature, to dioxygen complexes partially embedded within an oxide-like overlayer that forms during exposure either to O 2 or during steady-state epoxidations. Taken together our results implicate the presence of a combination of monatomic and diatomic surface oxygen species, which emerge with appreciable quantities of subsurface oxygen at temperatures and O 2 pressures representative of EO catalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ostwald ripening dominates particle growth of supported silver particles. • Particle growth on α-alumina depends on atmosphere and temperature. • Restricting silver particle movement does not limit particle growth. • Intraparticle distance is a crucial parameter for silver particle stability. • Higher surface area α-alumina provides better silver catalyst stability. The stability of supported metal particles is an important parameter in heterogenous catalysis. For silver catalysts supported on α-alumina, industrially used in ethylene epoxidation, the loss of silver surface area as result of particle growth is one of the most important deactivation mechanisms. In this work, the growth of silver particles was investigated by exposing catalysts to thermal treatments. The presence of oxygen during heating strongly enhanced particle growth, and the interparticle distance was a crucial parameter. However, restricting movement of complete silver particles using cage-like α-alumina did not limit particle growth. These findings indicate that Ostwald ripening was the dominant mechanism behind particle growth, with the diffusion of oxidized silver species being a rate limiting factor. Finally, higher surface area α-alumina provided better silver stability during ethylene epoxidation, with only limited decrease in selectivity. This makes silver supported on high surface area α-alumina promising candidates for ethylene epoxidation catalysis. [ABSTRACT FROM AUTHOR]

[Display omitted] • Direct propane epoxidation was performed over V-containing mesoporous silica. • N 2 O or O 2 separately and their mixture were used as oxidants. • Mixed oxidants provide stable conditions of propane epoxidation over 5V-SBA-3. • Reaction pathways for propane epoxidation with N 2 O/O 2 have been proposed. Propane to propene oxide (PO) oxidation over V-containing mesoporous silica of SBA-3 structure has been studied using different oxidants (nitrous oxide, oxygen, and their mixture) in the temperature range 673–773 K. Electron spin resonance spectroscopy, ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy (XPS), as well as X-ray diffraction, temperature-programmed reduction with hydrogen (H 2 TPR), and low-temperature N 2 adsorption/desorption, were applied for characterization of fresh and spent catalysts. XPS spectra and H 2 TPR profiles revealed a significant reduction of V-species as a result of propane oxidation with N 2 O alone, which leads to a decrease in both propane conversion and the space–time yield (STY) of PO. The use of an N 2 O–oxygen mixture as an oxidant of propane allows the vanadium valence to be stabilized at a level similar to the initial sample, which results in stable activity with time on stream. Propane conversion of 40%, propylene selectivity of 45%, and propylene oxide selectivity of 11%, corresponding to a STY of propylene oxide of about 15 g kg cat -1h−1, have been obtained, which makes these results very promising compared with the data reported in the literature. Vanadium catalyst used with only oxygen results in stable propane conversion with high total oxidation and stable propene selectivity, although the STY of PO is 10 times lower. N 2 O applied as the only oxidant results in rapid catalyst deactivation, and after 2 h on stream, STY of PO is only 2.5 g kg cat -1h−1. [ABSTRACT FROM AUTHOR]

This work proposes an efficient strategy to promote the enantioselectivity of manganese-catalyzed epoxidation, which simply utilizes the nanosheets of layered double hydroxides (LDHs) to modify amino acid derivatives as chiral ligands. [Display omitted] • Promoting the enantioselectivity of heterogeneous manganese-catalyzed epoxidation. • Layered double hydroxides (LDHs) nanosheets to modify amino acid derivatives as chiral ligands. • Modulating the orientation and the arrangement order of the nanosheet modified amino acid Schiff base anions to intensify the enantioselectivity enhancement. • Achieving an enantioselectivity of more than 91.0% ee value. • Achieving an enantioselectivity enhancement of 75% (from 20% to 95%). Catalytic enantioselective epoxidation of olefins plays an important role in the production of optically-active epoxy. Transition-metal complexes prove efficient for the catalytic epoxidation of un-functionalized olefins by employing privileged chiral ligands with rotational symmetry and bulky substituents, which usually require complicated multistep synthesis and are often very expensive. Here, this work proposes an efficient strategy to promote the enantioselectivity of Mn(III)-catalyzed indene epoxidation, which simply utilizes the nanosheets of layered double hydroxides (LDHs) to modify α-amino acid derivatives as chiral ligands, achieving an enantioselectivity of more than 91.0%, much higher than that (<35%) in the previous reports using chiral ligands without any rotational-symmetry. By virtue of "flexible" interlayer spaces of LDHs, intercalated L- tert -leucine, L-valine, L-leucine, and L-phenylalanine anions have further been derived in situ to L- tert -leucine, L-valine, L-leucine, and L-phenylalanine Schiff base anions. Over the Mn(III) coordinated with L-phenylalanine Schiff base anion intercalated LDHs, an indene conversion of 81.0 ± 1.0%, a chem-selectivity of 89.0 ± 1.0%, and an enantiomeric excesses (ee) value of 94.5 ± 0.5% for (1S,2R)-1,2-epoxyindane have been afforded. The spatial effects of LDH nanosheets on the enantioselectivity have been investigated, focusing on the interlayer orientation and arrangement order of intercalated amino acid derivative anions. The strategy has been proved valid for the epoxidation of functionalized olefins, which is more challenging, including allylic alcohols, chalcone, and chromene derivatives. [ABSTRACT FROM AUTHOR]

[Display omitted] • This study aimed at the complete elucidation of the mechanism of the Mukaiyama-type alkene epoxidation reaction using the [(salophen)CrIIICl] precatalyst, by combining spectroscopic (UV–visible and EPR) studies, catalytic reactivity and quantum chemical modeling. These studies performed in parallel allowed us to propose a detailed reaction scheme, in which two consecutive active species incorporating both {salophen(CrV = O)} moieties were formed in situ , resulting in two distinct catalytic regimes. This study aimed at the complete elucidation of the mechanism of the Mukaiyama-type alkene epoxidation reaction using the [(salophen)CrIIICl] precatalyst, by combining spectroscopic (UV–visible and EPR) studies, catalytic reactivity and quantum chemical modeling. These studies performed in parallel allowed us to propose a detailed reaction scheme, in which two consecutive active species incorporating both {salophen(CrV = O)} moieties were formed in situ , resulting in two distinct catalytic regimes. [ABSTRACT FROM AUTHOR]

[Display omitted] • The crystallization time of monolithic TS-1 by dry gel conversion in tripropylamine-water atmosphere was 12 h, which was greatly shorter than that of powder TS-1 synthesized by classical hydrothermal crystallization (72 h). • The effect mechanism of tripropylamine and water on the structure of monolithic TS-1 was investigated, and the Lewis acid sites and Bronsted acid sites in monolithic TS-1 were effectively adjusted to make it possess excellent catalytic performance in propylene epoxidation. • How the structure parameters of TS-1 reciprocally affected the catalyst properties were proposed for the first time. The epoxidation of propylene catalyzed by TS-1 possesses the advantages of high atomic utilization and mild reaction conditions. To explore the structure–activity relationship of the catalyst, carbon nanotubes grown on the surface of nickel foam were used as the carrier to synthesize monolithic TS-1 by dry gel conversion in the tripropylamine-water gas environment. Compared with the powder TS-1 by hydrothermal crystallization, the crystallization time of monolithic TS-1 was shortened from 72 h to 12 h. Tripropylamine promoted the dissociation of dry gel by increasing the basicity of liquid film on the surface of dry gel. Water provided the necessary pressure for crystallization and was also the medium of mass transfer in the process of crystallization. Stepwise regression analysis showed the increase of Lewis acid content and pore volume improved the conversion and utilization of hydrogen peroxide and the selectivity of propylene to propylene oxide. [ABSTRACT FROM AUTHOR]

• Synthesis and characterization of novel macrocyclic tetradentate iron-NHC complexes. • In-depth catalytic studies in the epoxidation of olefins. • Tolerance of elevated temperatures in catalytic reactions. • DFT calculation investigation of the influence of different ligand substituents on catalytic properties. Two sets of bio-inspired non-heme iron complexes, each comprising of an FeII and FeIII species, bearing 16-membered macrocyclic tetradentate N -heterocyclic carbene ligands are reported. The complexes exhibit trans labile coordination sites, are characterized by means of NMR, ESI-MS, elemental analysis, SC-XRD and cyclic voltammetry and assessed as olefin epoxidation (pre-)catalysts applying H 2 O 2 as oxidant. Sc(OTf) 3 and AcOH are evaluated as Lewis and Brønsted acidic additives, respectively, resulting in partially noticeable improvement in catalytic performance. Hereby, complex 2b shows high stability (TON = 1000 at 20 °C), high temperature tolerance and advances in the more challenging epoxidation of terminal and functionalized olefins. Furthermore, in-depth DFT calculations are conducted to put the catalysts' structural and electronic features into relation with the catalytic results. [ABSTRACT FROM AUTHOR]

• Mesoporous silica nanoparticles as a proper support for limonene epoxidation. • Catalytic epoxidation of limonene carried out on Mn centres dispersed onto silica. • Photocatalytic epoxidation performed onto binary TiO 2 -silica photocatalysts. • Green and sustainable (photo)catalytic epoxidation of limonene. Mesoporous silica nanoparticles (MSN) were used as a platform to design novel active materials for the catalytic and photocatalytic epoxidation of limonene. Binary systems comprised of TiO 2 and MSN were used for the catalytic reaction when doped with manganese, and for the photocatalytic reaction when functionalised with hexadecyl chains or imidazolinyl groups. All of the MSN based systems were synthesized by condensation in emulsion. A thorough characterization of the powders has been performed by means of Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES), X-ray diffraction (XRD), FT-IR, Raman and EPR Spectroscopy, Fluorescence and diffuse reflectance UV–vis (DRS) Spectroscopy, gas Porosimetry, and Transmission Electron Microscopy (TEM). The obtained morphological and structural information have been related to the catalytic and photocatalytic performances for the epoxidation of limonene. MSN support finely distributes the manganese active centres in the case of the catalytic epoxidation, while acts as functional co-catalyst in the case of the photocatalytic reaction. Upon careful optimization of the experimental conditions, the catalytic process afforded conversion and selectivity values up to 90 and 84%, respectively, while the photocatalytic process provided in the best case conversions up to 80% and selectivity values of ca. 50%. Both the catalytic and photocatalytic approaches were performed under relatively mild experimental conditions and use molecular oxygen as the oxidant. Therefore, both of them represent promising green alternatives to traditional methods for the industrially relevant production of limonene epoxide. This compound, in fact, is the starting material for the production of poly(limonene carbonates), biopolymers with outstanding properties which are promising substitutes of oil derived polycarbonates. [ABSTRACT FROM AUTHOR]

• EO forms ethylene glycol, acetaldehyde, 1,4-dioxane, p-dioxanone, or combusts over α-Al 2 O 3. • Ethylene oxide (EO) forms acetaldehyde or combusts over Ag. • Primary EO degradation products react much (≥30x) faster than EO over Ag. • Areal EO degradation rates over Ag are ~3x larger than rates over α-Al 2 O 3. Ethylene oxide (EO) degradation over Ag/α-Al 2 O 3 catalysts involves bifunctional pathways, whereby α-Al 2 O 3 surfaces catalyze hydration, isomerization, dimerization, and oxidation while Ag surfaces facilitate combustion and isomerization of ethylene oxide. EO degradation reactions run over Ag/α-Al 2 O 3 , α-Al 2 O 3 , and intrapellet mixtures thereof with co-feeds of O 2 and oxygenate products show that Ag surfaces are more reactive than α-Al 2 O 3 and that oxygenate products generated over α-Al 2 O 3 combust over Ag much more (>30x) readily than EO. These observations suggest that additives which mitigate surface hydration of α-Al 2 O 3 and combustion activity of Ag will most effectively mitigate EO degradation. [ABSTRACT FROM AUTHOR]

• Ni-based catalysts have been prepared on titanosilicates by a simple route. • The prepared catalysts show outstanding propylene epoxidation performance. • DFT calculations and HRTEM have helped understand the catalysts behavior. • Formation of Ni-O-Ti interfaces is crucial to obtain efficient catalysts. • The catalysts are extremely cost-effective compared to Au-based systems. An efficient gas-phase production of propylene oxide (PO) with noble metal-free catalysts could modify the industrial output of this valuable compound. We present a novel catalyst based on well-dispersed Ni nanoparticles loaded on a Ti-SiO 2 support for the propylene epoxidation reaction using H 2 /O 2 mixtures. XPS, High Resolution Transmission Electron Microscopy (HRTEM), and UV–Vis corroborate both the small size of Ni particles and the excellent dispersion and incorporation of Ti as tetrahedral single site species into the silica framework. The catalytic results under steady-state conditions at low temperature (200 °C) show high PO selectivity (around 85%) with a substantial propylene conversion (over 6%) and excellent H 2 efficiency (~37%) using only 0.5 wt% of Ni on Ti-SiO 2 (Ti/Si = 0.01 M ratio). In this work, we have also carried out a preliminary DFT study to gain understanding of the characteristics that make this nickel catalyst active and selective for the propene epoxidation reaction. [ABSTRACT FROM AUTHOR]

• Synthesis of novel bio-inspired iron(II) NHC/1,2,3-triazole complexes. • Olefin epoxidation with exceptionally high turnover frequencies (up to 76,000 h−1). • Tolerance with regard to functional groups (OH, Cl, C O). Two bio-inspired non-heme iron complexes bearing mixed tetradentate N -heterocyclic carbene/1,2,3-triazole ligands with cis labile coordination sites are reported. The compounds are studied in olefin epoxidation catalysis using H 2 O 2 as oxidant. Sc(OTf) 3 , HClO 4 and HOAc are applied as additives resulting in significant improvement of catalytic performance. Under optimized conditions the most active catalyst exhibits activities of 76,000 turnovers per hour, which is the highest reported value for an iron(II) catalyst. The complexes reveal comparably high stability and enable the challenging epoxidation of functionalized olefins. These results prove 1,2,3-triazoles to be promising and tunable ligands for iron catalyzed oxidation reactions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Heterogenous electrocatalytic epoxidation of cyclooctene directly using water as oxygen source was achieved. • Co 4 P 2 W 18 shows higher Faradic efficiency (∼36 %) for epoxide than CoPW 11 and other typical electrocatalysts. • The O atom directly transfer from water to adsorbed cyclooctene on the surface of Co 4 P 2 W 18. • The adjacent terminal O to Co site plays key role in adsorption and activation of water and cyclooctene. The electrocatalytic epoxidation directly using water as oxygen source is considered as a sustainable strategy to achieve epoxide production under ambient conditions. However, it is challenging to develop non-noble metal based heterogeneous electrocatalysts for efficient epoxidation with a deep understanding of the structure–activity relationships. In this work, we report a Weakley-polyoxometalate Na 10 [Co 4 (H 2 O) 2 (PW 9 O 34) 2 ] (Co 4 P 2 W 18) with high efficiency toward electrocatalytic epoxidation of cyclooctene via directly transfer oxygen atom from water. A significantly higher faradaic efficiency of cyclooctene oxide was achieved over Co 4 P 2 W 18 than a Keggin type Co substituted polyoxometalate Na 5 [Co(H 2 O)PW 11 O 39 ] (CoPW 11) and some other typical electrocatalysts including RuO 2 , IrO 2 , and Co 3 O 4 in our system. Experimental results and theoretical calculation indicate that the oxygen atom directly transfers from water for the epoxidation of cyclooctene via the cleavage of O−H of pre-adsorbed water as the rate-determined step. Further calculation demonstrates that the water and cyclooctene were adsorbed by Co site and their adjacent terminal oxygen atom during the epoxidation reaction. With the assistance of this terminal oxygen atom for adsorption and as an electron capture center, lower energy barriers for activation of water and cyclooctene are observed on Co 4 P 2 W 18 than on CoPW 11. Our study provides atomic insight into how structure affects catalytic activity, which benefits designing efficient electrocatalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • Allyl chloride deposits Cl over silver surface(s) during propylene epoxidation. • Oxidation-relevant Cl coverages scale linearly with allyl chloride pressure. • Cl coverages are not significantly impacted by pressures of O 2 or C 3 H 6. • Oxidation rates are linear with O 2 pressure at all Cl coverages investigated. Propylene epoxidation over K-promoted Ag/CaCO 3 yields maximally reported selectivity to propylene oxide (S ∼ 55%) only in the presence of trace allyl chloride. Steady state plug flow propylene epoxidation over K-Ag/CaCO 3 reveals co-fed allyl chloride decomposes readily over Ag, rendering significant bed-scale gradients which are eliminated herein via the use of a gradientless flow reactor with continuous stirred tank reactor hydrodynamics. Conversion of allyl chloride is measured on-stream to enumerate coverages of surface-deposited Cl* in-situ during propylene epoxidation, revealing oxidation-relevant Cl coverages that range linearly (∼0–0.15 mol Cl per mole surface Ag) with allyl chloride partial pressure (∼0.2–2 Pa C 3 H 5 Cl) and correspond with substantial improvement in steady state propylene oxide selectivity (S ∼ 5% to 55%) and rates (2 to 33 nmol g cat -1 s−1). Varying oxygen (∼2–8 kPa O 2) and propylene (∼5–20 kPa C 3 H 6) partial pressure yields no significant change to oxidation-relevant Cl coverages, revealing allyl chloride controls such Cl* coverages without significant bias from reactant partial pressures. Oxidation rates at controlled chlorinating conditions feature near-linear dependence on O 2 partial pressure (∼0.7–0.9 order) and are independent of propylene partial pressure while epoxidation selectivity under these conditions is independent of both oxygen and propylene partial pressure, providing mechanistic insight into oxidation and chlorination pathways within promoted Ag-catalyzed propylene epoxidation. [ABSTRACT FROM AUTHOR]

• Selectivies are quantitatively correlated to the coordination number of Ti O. • The single-site, 4-fold coordinated TiO 2 favors high PO selectivity and H 2 efficiency. • Atomic layer deposition was used to precisely deposit a uniform layer of TiO 2. • Uniform single-site Ti facilitates alternation of Ti O coordination using calcination. Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO 2 atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO 2 framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and Ti O coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the Ti O coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single Ti SiO 2 sites scale linearly as a function of the coordination number. Also, the predicted Ti O bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships. [ABSTRACT FROM AUTHOR]

• Heterolytic C Cl cleavage enthalpy governs Cl deposition by alkyl chlorides. • Homolytic C H cleavage enthalpy governs efficacy in Cl moderation by alkanes. • Ethylene oxide formation rate and selectivity are not a function of Cl source or Cl moderator identity. • Oxygen species participate in heterolytic C Cl cleavage, homolytic C H cleavage, and proton abstraction from gas-phase acids. The selectivity of Ag ethylene epoxidation catalysts correlates with surface coverage of the promoter chlorine. Cl coverage, in turn, is a function of co-fed organic chloride and alkane moderator pressures. Organic molecules can be classified based on their deprotonation enthalpy, as those that are inert, those capable of selective oxidation, or those that act as gas-phase acids. The influence of alkane moderator and alkyl chloride promoter identities on Cl coverage was assessed by titration of Cl by ethane in a recirculating gas-phase batch reactor immediately following steady-state reactions with various alkane and alkyl chlorides included in ethylene oxidation feedstreams. The faculty of organic molecules for Cl removal correlates with the weakest C H bond dissociation enthalpy, such that molecules with C H bonds susceptible to homolysis remove Cl more effectively than those with stronger C H bonds. Cl deposition likely occurs via heterolytic cleavage of C Cl bonds in alkyl chlorides, such that molecules with weak C Cl bonds deposit multiple monolayers of Cl. Cl coverage affects ethylene epoxidation rates identically regardless of the organic chloride or the aliphatic hydrocarbon employed, demonstrating that these molecules only impact catalysis by moderating Cl coverage. These findings implicate the concurrent prevalence of homolytic and heterolytic pathways during ethylene epoxidation on promoted Ag/α-Al 2 O 3 catalysts and provide context to examine additional roles of surface oxygen beyond involvement in oxidation pathways. [ABSTRACT FROM AUTHOR]

A photocatalyst was synthesized by assembling the photosensitizer DPNDI, electron donor PYI and electron acceptor polyanion within a single framework. By the conPET process and the intact multiple charge transfer circle, it exhibited high activity in photocatalytic coupling of primary amines and selectivity in olefins epoxidation with air under visible-light. • A ternary supramolecular system, ZnW–DPNDI– PYI , was achieved for photocatalysis. • Photosensitizer, polyoxometalate and electron donor are introduced simultaneously. • ZnW–DPNDI– PYI possesses the conPET process and long-lived charge-separated states. • ZnW–DPNDI– PYI is well explored in the photocatalytic amines coupling with air. Developing new photocatalysts for settling the tightest bottlenecks that lower efficiency, inferior selectivity and harsh conditions in organic oxidation reactions with air is very significant. Herein, ZnW–DPNDI– PYI was achieved by assembling a functional photosensitizer, an oxidation catalyst [BW 12 O 40 ]5− , and an organocatalyst into one single framework. The anion⋯π and N H⋯O hydrogen bonds interactions facilitate the consecutive photo-induced electron transfer (conPET) process for the photocatalytic oxidation by stabilizing the radical-anion intermediate and catalyst-substrate interacted moiety. Besides, N,N′-bis(4-pyridylmethyl)naphthalene diimide (DPNDI) can be excited by visible light, the photogenerated electron and hole would further activate oxygen and substrates, respectively, to give the corresponding redox products. High conversion and selectivity of ZnW–DPNDI– PYI in photocatalytic coupling of primary amines and olefins epoxidation with air under visible-light have been demonstrated. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Iron containing carbon catalyst (Fe-N/C) is active for olefin epoxidation using O 2 and 2 eq aldehyde. • Mild conditions required for excellent conversion – room temperature and 1 atm O 2. • Activity shows same ordering as electrochemical oxygen reduction: Fe-N/C > Co-N/C > N/C. • For Fe-N/C catalyst TOF > 2700 h−1, TON > 16,000, catalyst can be recycled >5 times. Abstract Pyrolized transition metal and nitrogen containing carbon materials (M-N/C) have shown promising activities as electrocatalysts for oxygen reduction reactions (ORR) in fuel cell cathodes. Similar materials have recently gained interest as heterogeneous catalysts. We report that ORR-active heterogeneous M-N/C materials can catalyze the chemical epoxidation of olefins with molecular oxygen and two equivalents of aldehyde at room temperature and ambient pressure. The observed yield and selectivity is higher than that for homogeneous analogues and the catalysts achieve TOF > 2700 h−1 and TON > 16,000. The ability to recycle the catalyst several times is also demonstrated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Kinetics for alkene epoxidation depend upon silanol density and solvent composition. • Spectroscopy and calorimetry reveal intrapore solvent reorganization in MFI catalysts. • Non-monotonic changes in barriers reflect short and long range solvation effects. • Entropic transition state stabilization leads to greater rates in hydrophilic Ti-MFI. Kinetics of alkene epoxidation in Ti incorporated MFI (Ti-MFI) zeolites that contact liquid methanol (CH 3 OH) depend strongly on the bulk concentration of water ([H 2 O]) and the density of silanol groups ((SiOH) x) within the framework, because these species influence excess free energy contributions (Gε) that stabilize transition states for epoxidation and hydrogen peroxide derived reactive intermediates. These effects rely on hydrogen bonds among intrapore molecules that couple catalytic events to the structure of the surrounding solvent molecules. Turnover rates for 1-hexene (C 6 H 12) epoxidation with hydrogen peroxide over Ti-MFI zeolites increase by ten-fold both when either [H 2 O] or (SiOH) x densities increase. Activation enthalpies and entropies of hydrophilic and hydrophobic Ti-MFI materials are similar at the lowest [H 2 O] (0.005 M H 2 O), implying solvation environments near active sites do not depend on defect density in the near-absence of H 2 O. However, kinetic barriers for these same Ti-MFI materials increase and diverge from one another as non-monotonic functions of [H 2 O] (0.005–5 M H 2 O), and these differences reflect stabilizing contributions from discrete solvation shells both adjacent (near) and further (or far) from active sites. The structure of these solvation shells must reorganize to accommodate H 2 O 2 activation and formation of epoxidation transition states. Adsorption enthalpies for the epoxide product become more exothermic with larger values of [H 2 O] over each Ti-MFI catalyst, and the apparent activation enthalpies generally increase as epoxide adsorption enthalpies become more exothermic. Comparisons to trends previously obtained for Ti-BEA catalysts suggest the negative correlation indicates that excess contributions (Hε, Gε) impact reactive species (Ti-OOH) within the 10-membered ring pores of MFI to a greater extent than 12-membered ring pores due to distinctions among hydrogen bonding interactions among bound reactive species, spectating solvent molecules, and extended surfaces of MFI pores. [ABSTRACT FROM AUTHOR]

[Display omitted] • Acid-modulated isomorphous substitution synthesis of Ti-MWW. • Nanosheet thicknesses of the developed Ti-MWW are extremely thin. • Absence of extra-framework Ti species in the synthesized Ti-MWW. • Catalyst development for the efficient epoxidation of cycloalkenes. The Ti-MWW zeolites with a layered morphology, synthesized by a conventional method, usually show low catalytic activities in the epoxidation of bulky alkenes due to the serious steric constraint imposed by the zeolite micropores and the adverse effects from some extra-framework octahedral Ti species. In the present work, the Ti-MWW zeolite was prepared by the reaction of an ultra-thin layered borosilicate MWW-type zeolite (D-ERB-1) with tetrabutyl orthotitanate (TBOT) in a HNO 3 aqueous solution, wherein D-ERB-1 was hydrothermally synthesized by introducing an organosilane into the synthetic gel. In combination with the results from extensive characterizations for the synthesized zeolites, it is found that the currently synthesized Ti-MWW zeolite without extra-framework octahedral Ti species has thinner nanosheets that are very helpful to the epoxidation of cycloalkenes with tert-butyl hydroperoxide (TBHP), and is more active than the conventional Ti-MWW and other titanosilicate zeolites. Moreover, the developed catalyst is recyclable after the regeneration of the deactivated zeolite by washing with acetonitrile (MeCN). Therefore, the present work provides some guidance in the development of an efficient titanosilicate catalyst for the epoxidation of bulky alkenes. [ABSTRACT FROM AUTHOR]

[Display omitted] • H 2 pulse titration over O-precovered surface is capable of measuring active surface Ag site concentrations for both unpromoted and ppmw level loading Cs, Re promoted Ag catalyst. • Temperature programmed reduction showed the metal–oxygen bonds of alkali metal (Cs) and high valent oxide promoters (Re, Mo) were not reactive during H 2 titration of the Ag-O surface at 170 °C. • Particle counts from scanning electron microscopy (SEM) imaging gave higher Ag site concentration and dispersion over H 2 pulse titration method. • H 2 pulse titration results showed Cs was randomly deposited on Ag and α-Al 2 O 3 surfaces. • At high loading (≥1.3 μmol), Re was preferentially deposited on Ag to lower overall surface energy of the system. A series of Cs and Cs + Re promoted 12 wt% Ag/α-Al 2 O 3 catalysts were prepared and characterized by H 2 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-Al 2 O 3 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 H 2 uptake during H 2 titration for these samples confirmed the TPR results. H 2 pulse titration results of 0 to 7.5 μmol Cs promoted, 12 wt% Ag/α-Al 2 O 3 catalysts showed non-preferential deposition of Cs on Ag and α-Al 2 O 3 surfaces. However, when Cs (2.6 μmol)–12 wt% Ag/α-Al 2 O 3 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-Al 2 O 3 system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pure silica MWW zeolite was synthesized by dry gel method. • Ti implantation was completed by TiCl 4 acid solution treatment. • Boron-free Ti-MWW has double 1-hexene conversion at lower Ti content. The pure silicon MWW were synthesized for the first time by means of a dry-gel conversion method utilizing N, N, N-trimethyl-1-adamantyl ammonium hydroxide and hexamethyleneimine as the structural directing agents. A vapor transport assisted crystallization method for obtaining complete crystallinity was developed to replace the traditional dry-gel conversion methods. TiCl 4 acid solution treatment was cleverly used for the implantation of titanium in MWW. Compared with Ti-ERB-1, the Ti content of boron- free Ti-MWW is only 20% of the former, the conversion rate of 1-hexene epoxidation is 2.3 times of the former and nearly 100% of H 2 O 2 efficiency. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Mo6−δO 3− x with lower Mo electron binding energy is successfully constructed by copper doping. • Molybdenum oxide with oxygen vacancies and electron-rich state is synthesized. • Propylene oxide formation rate in gas phase epoxidation is greatly improved at relatively low reaction temperatures. • Doping with Cu effectively suppresses the generation of bismuth molybdate, thus reducing the formation of acrolein. Abstract Selective epoxidation of propylene by molecular oxygen under mild conditions remains a great challenge to date. Here, we successfully designed and constructed molybdenum oxide in an electron-rich state by introducing the transition metal copper into MoO 3 –Bi 2 SiO 5 /SiO 2 through a simple co-impregnation method. The new Mo species has the structure Mo6−δO 3− x , and greatly improved propylene epoxidation performance at low temperatures. Propylene oxide formation rate increased from 106 to 336 g PO kg cat −1 h−1. X-ray photoelectron spectrometry (XPS) and UV–visible diffuse reflection (UV–vis DRS) characterizations suggest that the bridging oxygen bond between molybdenum and copper produces an oxygen vacancy, which facilitates the formation of the electron-rich state of molybdenum with lower 3 d electronic binding energy. In addition, the incorporation of Cu effectively inhibits the formation of Bi 2 Mo 3 O 12 and improves the dispersion of Mo species. Furthermore, kinetic studies show that Cu-modified catalysts maintain lower apparent activation energy for propylene epoxidation, which obviously reduced the selectivity of acrolein and then conspicuously improved the selectivity of propylene oxide. Our results not only offer a new solution for low temperature propylene epoxidation, but also provide a new strategy for the design of other types of low-temperature epoxidation catalysts. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Atomically dispersed gold species stabilized by the framework oxygens. • Alkali ions play a crucial role in nucleation and stabilization of Au precursor. • Structure of atomic Au species can be elucidated using advance X-ray techniques. • Anchored single Au upon controlled reduction in H 2 forms uniform gold clusters. Abstract Deposition of gold on supports can produce catalytically active forms of gold as well as spectators, but previous understanding of the nature of active immobilized precursors is poor. By using synchrotron X-ray powder diffraction (SXRD) and X-ray absorption spectroscopy (XAS) techniques, we report a novel synthesis and structural elucidation of atomically dispersed gold species anchored to the internal surface of TS-1 as K+Au(OH) 2 Na+(O f) 3 ("f" signifies the framework atoms of TS-1 in the formula.). It is found that the choice of alkali ions plays a crucial role in nucleation and stabilization of the atomic precursor. These anchored single Au upon controlled reduction in H 2 can form uniform gold clusters in direct contact with the TS-1 surface containing isolated Ti sites: their interface exhibits excellent specificity and stability towards epoxidation of propylene in H 2 /O 2 due to synergetic effect. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • O 2 dissociation is quasi-equilibrated during C 3 H 6 -O 2 reactions. • O∗ selectively inserts into the C C double bond of C 3 H 6 to generate C 3 H 6 O. • O 2 ∗ and H 2 O∗ reactions generate OOH∗ that oxidizes C 3 H 6 to CO x. • Weaker O 2 and H 2 O adsorption on larger Ag clusters leads to higher rates. Abstract Rate measurements in the kinetically controlled regime, kinetic fittings, and isothermal C 3 H 6 and O 2 uptake experiments lead to a proposed mechanism for C 3 H 6 epoxidation and combustion reactions on predominantly O∗ covered Ag cluster surfaces. Epoxidation occurs via kinetically relevant reactions between chemisorbed oxygen adatoms (O∗) and gas phase C 3 H 6. In contrast, combustion occurs via kinetically relevant hydroperoxyl (OOH∗) formation, formed from H transfer from H 2 O∗ to O 2 ∗. C 3 H 6 oxidation reactions with C 3 H 6 -O 2 -H 2 O-H 2 O 2 mixtures show that H 2 O 2 derived OOH∗ species are more effective oxidants than O∗ for C 3 H 6 combustion, as confirmed from H 2 O 2 /D 2 O 2 kinetic isotope effects. Both C 3 H 6 ∗ and O∗ coverages are near or at chemical equilibrium during steady-state reactions, as confirmed from in-situ chemical titrations. C 3 H 6 O and CO x site-time-yields and CO x selectivities increase with increasing Ag cluster diameters and Ag surface coordination, because O∗, O 2 ∗, H 2 O∗, and OOH∗ oxidants remain more weakly bound and therefore much more reactive. [ABSTRACT FROM AUTHOR]

Catalytic redox cycles involve dioxygen activation via peroxo (OO ∗ ) or H 2 O 2 species, denoted as inner-sphere and outer-sphere routes respectively, for metal-oxo catalysts solvated by liquids. On solid oxides, O 2 activation is typically more facile than the reduction part of redox cycles, making kinetic inquiries difficult at steady-state. These steps are examined here for oxidative alkanol dehydrogenation (ODH) by scavenging OO ∗ species with C 3 H 6 to form epoxides and by energies and barriers from density functional theory. Alkanols react with O-atoms (O ∗ ) in oxides to form vicinal OH pairs that eliminate H 2 O to form OO ∗ at O-vacancies formed or react with O 2 to give H 2 O 2 . OO ∗ reacts with alkanols to re-form O ∗ via steps favored over OO ∗ migrations, otherwise required to oxidize non-vicinal vacancies. C 3 H 6 epoxidizes by reaction with OO ∗ with rates that increase with C 3 H 6 pressure, but reach constant values as all OO ∗ species react with C 3 H 6 at high C 3 H 6 /alkanol ratios. Asymptotic epoxidation/ODH rate ratios are smaller than unity, because outer-sphere routes that shuttle O-atoms via H 2 O 2 (g) are favored over endoergic vacancy formation required for inner-sphere routes. The relative contributions of these two routes are influenced by H 2 O, because vacancies, required to form OO ∗ , react with H 2 O to form OH pairs and H 2 O 2 . OO ∗ -mediated routes and epoxidation become favored at low coverages of reduced centers, prevalent for less reactive alkanols and lower alkanol/O 2 ratios, because H 2 O 2 then reacts preferentially with O ∗ (forming OO ∗ ), instead of vacancies (forming O ∗ /H 2 O). Such kinetic shunts between two routes compensate for lower barriers required to form H 2 O 2 than OO ∗ . These re-oxidation routes prefer molecular donor (H 2 O 2 ) or acceptor (alkanol) to perform stepwise two-electron oxidations by dioxygen, instead of kinetically demanding O-atom migrations. The quantitative descriptions, derived from theory and experiment on Mo-based polyoxometalate clusters with known structures, bring together the dioxygen chemistry in liquid-phase oxidations, including electro-catalysis and monooxygenase enzymes, and oxide surfaces into a common framework, while suggesting a practical process for epoxidation by kinetically coupling with ODH reaction. [ABSTRACT FROM AUTHOR]

Ethylene epoxidation is one of the most important selective oxidation reactions in industry and is almost invariably carried out on silver catalysts. Empirically it is known that moderate selectivity of silver can be greatly enhanced with cesium and chlorine dopants, whereas a mechanistic understanding of the underlying reason remains elusive. In this work, we present a first-principles explanation of the dipole effect on reaction selectivity, which could, in principle, be effected by alkali metals. We show that, on a silver surface, alkali metals work in a similar fashion thanks to their low electronegative character. The selectivity is impacted by inducing electric dipoles on the catalytic surface, whereas the electronic properties of the catalyst remain largely unperturbed. Cesium, as the least electronegative metal, shows the most pronounced effect. This effect is shown to persist when alkali metals are treated as “naked” atoms or, more realistically, when oxidized. Additionally, alkali metals increase activity of the catalyst by favoring the rate-determining step of oxygen dissociation. Chlorine, which is strongly electronegative, acts differently. Subsurface chlorine would increase selectivity through the same mechanism, while surface chlorine stabilizes alkali metals and moderately increases selectivity. Interplay of both dopants is needed to precisely tailor the catalyst for the best performance. [ABSTRACT FROM AUTHOR]

Chemical looping epoxidation of ethylene was demonstrated, whereby the sole oxidant was a solid oxygen carrier, 15 wt% Ag supported on SrFeO 3 . Ethylene reacted with a bed of carrier particles, without any O 2(g) in the feed, to produce ethylene oxide (EO) and CO 2 . Following the reduction by the C 2 H 4 of the SrFeO 3 , it was regenerated by passing air through the bed. The rate of reoxidation was slow, with full regeneration being achieved only by prolonged oxidation at elevated temperatures. A striking synergy between Ag and SrFeO 3 was observed solely when they were in intimate contact, suggesting a basis for a proposed reaction mechanism. [ABSTRACT FROM AUTHOR]

Niobium-containing mesoporous silicates reveal superior activity and selectivity in epoxidation of alkenes using hydrogen peroxide as a green oxidant and, in contrast to mesoporous titanium silicates, catalyze epoxidation of both electron-rich and electron-deficient C C bonds. This report describes a kinetic and mechanistic investigation of epoxidation of two representative substrates, cyclooctene (CyO) and 2-methyl-1,4-naphthoquinone (MNQ), over two mesoporous niobium silicates with predominantly di(oligo)meric or isolated Nb(V) sites. The observed kinetic regularities did not depend on the state of Nb but were strictly determined by the nature of the organic substrate. The rate law established for CyO is consistent with a mechanism that involves interaction of H 2 O 2 with Nb(V) sites to give a hydroperoxo species NbOOH and water, followed by oxygen transfer to a nucleophilic C C bond, producing an epoxide and regenerating the initial state of the catalyst. This mechanism is strongly supported by stereospecificity in epoxidation of cis -alkenes and high heterolytic pathway selectivity in the oxidation of cyclohexene. The NbOOH species is manifested by an absorption feature at 307 nm in diffuse reflectance UV–vis spectra. The addition of a base (NaOAc) causes a shift of the absorption band to 293 nm and produces a rate-retarding effect on the epoxidation reaction. Several lines of evidence, including zero reaction order in MNQ, rate-accelerating effect of base, detection of acetamide in the reaction mixture, negligible reaction rate in ethyl acetate, and recognition of weak basic sites in the niobium silicates using infrared spectroscopy of adsorbed CDCl 3 , all indicate that MNQ epoxidation proceeds by another mechanism that involves rate-limiting oxidation of the solvent molecule (MeCN) to generate peroxycarboximidic acid, which reacts with electron-deficient C C bonds, producing an epoxy derivative and acetamide. [ABSTRACT FROM AUTHOR]

A chiral Mn III -porphyrin complex covalently bonded with mesoporous SBA-15 (SBA15-[Mn(TCPP-R ∗ )Cl]) was synthesized and fully characterized. The heterogeneous SBA15-[Mn(TCPP-R ∗ )Cl] exhibited remarkable catalytic activity toward enantioselective olefin epoxidation using O 2 as terminal oxidant in the presence of isobutyraldehyde. The catalyst showed higher enantioselectivity than its homogeneous counterpart in the oxidation of α-methylstyrene. SBA15-[Mn(TCPP-R ∗ )Cl] catalyzed epoxidation of styrene was achieved within 8 h and optically active styrene oxide was obtained in 89% ee and 90% yield. Likewise, styrene derivatives ( trans -β-methylstyrene, indene), conjugated cis - and trans -disubstituted olefins (e.g. cis - and tans -stilbene) and terminal olefins were converted effectively to their corresponding epoxides in 63–99% ee under the Mn(III)-catalyzed conditions. The catalyst could be recycled five times without any significant loss in activity. [ABSTRACT FROM AUTHOR]

The selective epoxidation of olefins with hydrogen peroxide (H 2 O 2 ) over transition metal substituted zeolites is less environmentally impactful than epoxidation schemes that use chlorinated or organic oxidants. The structure and reactivity of reactive intermediates derived from H 2 O 2 and the mechanism for olefin epoxidation on such materials are debated. Here, cyclohexene oxide formation and H 2 O 2 decomposition rates (measured as functions of reactant and product concentrations) and in situ infrared (IR) and UV-vis spectroscopy are used to probe the intervening elementary steps for cyclohexene (C 6 H 10 ) epoxidation and the identity of the reactive intermediates on a Nb-β catalyst. IR and UV-vis spectra acquired in situ show that the reactive intermediates are predominantly superoxide species (Nb IV -(O 2 ) − , observed also by X-ray photoelectron spectroscopy), which form by the irreversible activation of H 2 O 2 over Nb centers. Similar M-(O 2 ) ∗ (M = Ti or Ta) intermediates were previously assumed to form via reversible processes; however, in situ IR and UV-vis measurements directly show that Nb IV -(O 2 ) − forms irreversibly in both H 2 O and acetonitrile. Activation enthalpies (Δ H ‡ ) for C 6 H 10 epoxidation are 27 kJ mol −1 higher than for H 2 O 2 decomposition, while activation entropies (Δ S ‡ ) for epoxidation are 56 J mol −1 K −1 lower than for H 2 O 2 . These comparisons show that the selectivities for epoxidation, via primary reaction pathways, increase with increasing reaction temperatures. Collectively, these results provide a self-consistent mechanism for C 6 H 10 epoxidation that is also in agreement with previously published data. These findings will aid the rational design and study of alternative metal oxide catalysts for olefin oxidation reactions. [ABSTRACT FROM AUTHOR]

The influence of potassium ions on the structure and catalytic properties of silica-supported vanadium oxide catalysts was studied in the epoxidation of propene using N 2 O as an oxidant. The combination of different techniques (XRD, DR UV–vis, Raman, FTIR, and H 2 TPR) for the characterization of potassium-doped V/SBA-15 catalysts reveals the formation of various types of vanadate species, including polyvanadates. Potassium changes the acid–base characteristics of the system and decreases the acidic character of surface vanadia. A decrease in the acidic character accounts for better selectivity in propene epoxidation; however, neutralization of active vanadium forms results in lower propene conversion. Textural, XRD and SEM/TEM, results indicate that the ordered hexagonal mesoporous structure with large pore diameters of the support is retained upon vanadium incorporation, although further modification with potassium ions results in a partial structure damage. [ABSTRACT FROM AUTHOR]

The effect of silylation on a series of Au/Ti-SiO 2 catalysts for the direct epoxidation of propylene in the presence of H 2 and O 2 was studied. It was found that silylation significantly improved catalyst performance: propylene conversion, propylene oxide (PO) selectivity, and H 2 efficiency increased. The extent of improvement depended on the Au and Ti content of the catalysts. The catalyst showing the best activity (Au(0.1)/Ti(1)–SiO 2 ) exhibited an average PO formation rate of 121 g PO kg cat −1 h −1 and a PO selectivity of 92% at 473 K, while the catalyst having the maximum Au and Ti loading (Au(1)/Ti(5)–SiO 2 ) showed the most significant improvement in performance with a 78% increase in the rate of PO formation upon silylation. The catalysts were characterized by contact angle measurements, FTIR, TGA, TEM, ICP-OES and these observations were used to elucidate the key factors governing the enhanced catalytic performance upon silylation. It was found that the silylated catalyst exhibited superior performance due to increased hydrophobicity, which aids product desorption, a decrease in acidic sites that are responsible for side-product formation, and a possible redistribution of the Au particles. [ABSTRACT FROM AUTHOR]

The liquid-phase epoxidation of propylene to propylene oxide (PO) over formed titanosilicate catalysts was investigated in a fixed-bed reactor. The effects of reaction temperature, n (C 3 = )/ n (H 2 O 2 ) molar ratio, and weight hourly space velocity (WHSV) of H 2 O 2 or solvent on the catalytic performance of the formed Ti-MWW catalyst have been extensively studied. Adding an appropriate amount of ammonia to the reaction mixture prolonged the catalyst lifetime effectively. The main byproduct of propylene glycol (PG) and other heavy byproducts with high boiling points were deposited inside zeolite micropores, which corresponded to the main reason for the catalyst deactivation. The high-temperature calcination in air recovered readily the reactivity of the deactivated catalyst. Fluorine implantation remarkably enhanced the reactivity and lifetime of the catalyst in the hydrogen peroxide propylene oxide (HPPO) process, exhibiting a PO selectivity of >99%. [ABSTRACT FROM AUTHOR]

Well dispersed iron catalysts were synthesized in silica (Fe 0.0 X SiO 2 ) by a one-step synthesis procedure. These materials were tested in the propylene epoxidation reaction with gaseous O 2 . The influence of the iron metal loading on the iron incorporation and distribution in the support (both influenced by the synthetic procedure) were thoroughly studied (conversion, generation and selectivity). Electron Microscopy and UltraViolet–Visible (UV–VIS), Raman and Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy techniques were used to analyze the iron distribution in the catalysts and to probe its incorporation into the silica framework. In situ FTIR was also used to analyze the interaction between propylene and iron-based catalysts. Computational calculations considering a single-site iron catalyst incorporated into the silica structure show a possible interaction between O 2 and the incorporated iron atom and the olefin bond and the acidic proton neighboring the iron species which favor the reaction between the two molecules near the iron atom. [ABSTRACT FROM AUTHOR]

Significant ethylene epoxidation activity was observed over niobium (Nb) incorporated mesoporous silicate materials Nb-KIT-5, Nb-MCM-48, and Nb-TUD-1, with hydrogen peroxide (H 2 O 2 ) as oxidant and methanol (MeOH) as solvent under mild operating conditions (35 °C and 50 bars). No CO 2 as by-product was detected at these conditions. The measured ethylene oxide (EO) productivity over Nb-TUD-1 materials (342–2539 g EO h −1 kg −1 Nb) spans a greater range than those observed with Nb-KIT-6 (234–794 g EO h −1 kg −1 Nb), Nb-KIT-5 (273–867 g EO h −1 kg −1 Nb) and Nb-MCM-48 (71–219 g EO h −1 kg −1 Nb) materials at similar operating conditions. However, significant H 2 O 2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Brønsted acidity may lead to these detrimental pathways. Indeed, lowering the metal loading to significantly reduce the Brønsted acidity results in a dramatic increase in H 2 O 2 utilization toward EO formation (4304 g EO h −1 kg −1 Nb). The increased EO productivity either matches or surpasses what was observed on the conventional Ag-based heterogeneous catalyst (with O 2 as oxidant) as well as a Re-based homogeneous catalyst (with H 2 O 2 as oxidant). These results are paving the way for further computational and experimental investigations aimed at the rational design of improved epoxidation catalysts that reduce H 2 O 2 decomposition and metal leaching to practically viable levels. [ABSTRACT FROM AUTHOR]