Direct Epoxidation Of Propylene Research Articles

Direct propylene epoxidation with molecular oxygen over titanosilicate zeolites.

The direct epoxidation of propylene with molecular oxygen represents a desired route for propylene oxide (PO) production with 100% theoretical atomic economy. However, this aerobic epoxidation reaction suffers from the apparent trade-off between propylene conversion and PO selectivity, and remains a key challenge in catalysis. We report that Ti-Beta zeolites containing isolated framework Ti species can efficiently catalyze the aerobic epoxidation of propylene. Stable propylene conversion of 25% and PO selectivity of up to 90% are achieved at the same time, matching the levels of industrial ethylene aerobic epoxidation processes. H-terminated pentacoordinated Ti species in Beta zeolite frameworks are identified as the preferred active sites for propylene aerobic epoxidation and the reaction is initiated by the participation of lattice oxygen in Ti-OH. These results are expected to spark new technology for the industrial production of PO toward more sustainable chemistry and chemical engineering.

Enhanced catalytic performance for gas phase epoxidation of propylene with H2 and O2 by multiple pretreated Au/TS-1 catalysts

Direct epoxidation of propylene (C3H6) with hydrogen (H2) and oxygen (O2) over Au/TS-1 catalyst was promising way for the synthesis of propylene oxide (PO). However, high selectivity of PO (>95 %) and conversion of C3H6 (>5 %) cannot be achieved at the same time. The industrialization process was severely restricted. In this paper, Au/TS-1 catalysts were synthesized by deposition precipitation method with alkali metal promoter. And alkaline gases and silane coupling post-treatment were performed to promote the catalytic activity. The prepared samples exhibited a much better C3H6 conversion (6.5 %) and PO selectivity (95.1 %) compared with general catalyst using normal synthetic technology. Cs2CO3 was beneficial to increasing Au nanoparticles uptake efficiency. Both NH3 and triethoxyvinylsilane post-treatment were good for smaller and more uniform loading of Au nanoparticles. The decreasing in surface hydroxyl density effectively alleviated the occurrence of PO ring opening isomerization. The increased hydrophobicity of TS-1 surface was conducive to the rapid desorption of generated PO and significantly increased PO selectivity. Multiple pretreatment was also favorable for extending the lifetime of Au/TS-1 catalyst.

Intracrystalline diffusion regulation of Au/hierarchical TS-1 for simultaneously enhanced stability and selectivity in propene epoxidation with H2 and O2

Regulation of intracrystalline diffusion plays a pivotal role in ensuring the catalytic stability of metal-supported zeolite catalysts. Herein, we successfully achieved precise modulation of intracrystalline diffusion in Au-loaded hierarchical TS-1 catalysts by two-step recrystallization approach. This modulation was achieved by manipulating the length of mesoporogens, characterized by the hierarchical factor (HF). The HF could be used as the descriptor to reveal the reaction stability and deactivation process including blocking degree (VAu: gold volume / Vna: volume not accessible for N2) by the Au nanoparticles and coke formation in fresh and used catalysts, respectively. In addition, the enhanced selectivity was attributed to the improved hydrophobicity of Au/hierarchical TS-1 with gradually decreased terminal silanols by two-step recrystallization approach. This work not only provides the new strategy for the regulation of zeolite intracrystalline diffusion, but also could shed new lights on the industrial process of direct propene epoxidation with H2 and O2.

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.

Efficient formation of propylene oxide under low hydrogen concentration in propylene epoxidation over Au nanoparticles supported on V-doped TS-1

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 % H2 concentration (C3H6/H2 = 1/1) in the reaction atmosphere, but the PO selectivity was maintained. Interestingly, when the H2 concentration in the reaction atmosphere was lowered to 2 % (C3H6/H2 = 3/1), the propylene conversion in Au/TS-1 was significantly decreased compared to 10 % H2 concentration condition, while in Au/V-TS-1, the propylene conversion was maintained in 2 % H2 concentration condition compared to 10 % H2. Au/V-TS-1 also maintained its selectivity for PO even when the H2 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.

Effect on the introduction of hollow structure on Au/TS-1 to improve the stability for propylene epoxidation

In the direct gas-phase epoxidation of propylene, the channels of TS-1 are easily blocked by the carbon deposition which leads to poor stability and even deactivation of the catalyst. In this paper, H-TS-1 molecular sieve with hollow structure was successfully synthesized by the post-treatment of TS-1 molecular sieve with tetrapropylammonium hydroxide (TPAOH) solution. The experimental results show that the Au/H-TS-1 catalyst exhibits good stability compared with Au/TS-1. TG results show that the weight of carbon deposition is only ∼3.56 wt% in Au/H-TS-1 at the reaction time of 50 h compared with Au/TS-1 catalyst (∼8.55 wt%). The introduction of hollow structure improves the mass transfer ability of the catalyst which could reduce the formation of carbon deposition and improve the stability of Au/H-TS-1 in propylene epoxidation. The PO formation rate of Au/H-TS-1 only decreased by ∼10.3 % after 100 h, while that of Au/TS-1 decreased by ∼28.3 %.

Direct propylene epoxidation via water activation over Pd-Pt electrocatalysts.

Direct electrochemical propylene epoxidation by means of water-oxidation intermediates presents a sustainable alternative to existing routes that involve hazardous chlorine or peroxide reagents. We report an oxidized palladium-platinum alloy catalyst (PdPtOx/C), which reaches a Faradaic efficiency of 66 ± 5% toward propylene epoxidation at 50 milliamperes per square centimeter at ambient temperature and pressure. Embedding platinum into the palladium oxide crystal structure stabilized oxidized platinum species, resulting in improved catalyst performance. The reaction kinetics suggest that epoxidation on PdPtOx/C proceeds through electrophilic attack by metal-bound peroxo intermediates. This work demonstrates an effective strategy for selective electrochemical oxygen-atom transfer from water, without mediators, for diverse oxygenation reactions.

Uncalcined TS-1 supported Au catalyst via NaBH4 reduction method for propylene epoxidation: Insights into the H2 pretreatment effect on catalytic performance

Stable and efficient Au-supported titanium silicalite-1 (TS-1) catalysts are essential for the direct epoxidation of propylene (DEP) using O2 and H2. But the deposition-precipitation method for Au/TS-1 contains low Au capture efficiency and rapid deactivation rate. Herein, Au/TS-1-B (uncalcined TS-1) catalysts were synthesized through a NaBH4 reduction method. The impact of H2-thermal-pretreatment on DEP activities was studied. H2-pretreatment can enhance propylene oxide selectivity, shorten activation time and reduce deactivation rate of catalyst. Organic-templates on the surface of TS-1-B and stabilizers coated on Au-nanoparticles can be partially removed under H2-treatment, thus exposing more active sites for Au and Ti and facilitating their interactions. Furthermore, more porous structures with reduced acid sites and enhanced surface hydrophobicity of pretreated catalyst can promote DEP reaction. Solid stability in TS-1-B, high Au utilization resulting from the reduction method, and favorable modifications through H2-thermal treatment contribute to the exploration of more robust DEP catalyst performance.

Copper-doped LaCoO3 for direct propylene epoxidation: a DFT study

In the presence of Cl, doping Cu into LaCoO3 can improve the selectivity of propylene epoxidation to propylene oxide, and LaCuO–Cl surface shows the best activity and selectivity.

Coating hollow silicalite-1 with TS-1 shell as support of Au: preparation and enhanced catalytic stability for direct propylene epoxidation

Coating hollow silicalite-1 with TS-1 shell as support of Au: preparation and enhanced catalytic stability for direct propylene epoxidation

Epoxidation of Light Olefin Mixtures with Hydrogen Peroxide on TS-1 Catalyst

The direct epoxidation of mixed ethene and propene feedstocks using hydrogen peroxide over a titanium silicalite (TS-1) catalyst was investigated within a continuous trickle bed reactor operating in laboratory scale. Methanol was employed as the reaction solvent. This study aimed to streamline the epoxidation process by obviating the need for prior separation of alkenes, thereby enhancing process efficiency. An extensive array of operational parameters was explored in a trickle bed reactor, encompassing experimental parameters such as temperature, total pressure, hydrogen peroxide concentration, liquid flow rate, and gas composition. In contrast to prior investigations involving separate ethene and propene epoxidation, this study revealed a reduction in epoxide selectivity. The principal by-products observed were methoxy species, formed through the interaction between the epoxide and methanol, resulting in a ring-opening reaction. The influence of water on this ring-opening process was negligible. Notably, the tunability of the system was demonstrated, highlighting low temperature and elevated partial ethene pressure as pivotal factors for augmentingthe epoxide selectivity. The findings suggest that binary olefin mixtures exhibit diminished selectivity but improved stability. This behavior is potentially linked to the olefin solubility in methanol, or alterations in the surface species concentrations, typically associated with catalyst activity variations. These insights offer a valuable foundation for understanding and optimizing the direct epoxidation of mixed ethene and propene feedstock.Graphical

Supported AgCuCl Nanoclusters as Bimetal Catalysts for Propylene Epoxidation with Molecular Oxygen

Direct epoxidation of propylene (DEP) into propylene oxide using molecular oxygen is a promising approach due to high atom utilization and environmental sustainability. However, the absence of highly selective catalysts has been a challenge in this field. Herein, the ligand protected AgCuCl nanocluster was synthesized via a one-pot method and loaded onto BaCO3 to prepare a small size bimetal catalyst (AgCuCl/BaCO3-N2) via calcination under N2 atmosphere. A PO selectivity of 85.29% at a relatively mild temperature of 275 °C can be obtained. Characterizations revealed that AgCuCl/BaCO3-N2 with low-valence Cu+ species facilitated the formation of electrophilic oxygen species that preferentially attacked the C═C bond of propylene, resulting in the production of PO instead of attacking the C–H bond for the formation of byproduct. This study provides a clear example of using supported bimetal catalysts to achieve propylene epoxidation and offers guidance for selecting supports for supported nanoclusters as an efficient catalyst.

Investigation of the oxygen coverage effect on the direct epoxidation of propylene over copper through DFT calculations.

The direct epoxidation of propylene is one of the most important selective oxidation reactions in industry. The development of high-performance copper-based catalysts is the key to the selective oxidation technology and scientific research of propylene. The mechanism of propylene's partial oxidation catalyzed by Cu(111) under different oxygen coverage conditions was studied using density functional theory calculations and microkinetic modeling. We report here in detail two parallel reaction pathways: dehydrogenation and epoxidation. The transition states and energy distributions of the intermediates and products were calculated. The present results showed that propylene oxide (PO) selectivity was high under low oxygen coverage, and increasing the oxygen coverage would decrease the PO selectivity but increase the PO activity, and there was an inverse relationship between PO selectivity and activity. Increasing oxygen coverage would reduce the energy barrier for the C-O bond formation of C3H5O due to the weaker adsorption strength of C3H5, thus decreasing the PO formation selectivity. On the other hand, increasing oxygen coverage would reduce the energy barrier for the possible reaction steps of propylene epoxidation in general, and thus increasing the catalytic activity. It might be proposed that the active site for propylene epoxidation is the metallic copper or partially oxidized copper in terms of the change of PO formation selectivity with oxygen coverage.

Engineering sodium‐decorated bifunctional Au‐Ti sites to boost molecular transfer for propene epoxidation with H2 and O2

AbstractRegulating the microenvironment of active sites is crucial to boost the performance of direct propene epoxidation with H2 and O2. Herein, the enhanced surface transfer of H2O2 intermediates was first achieved by sodium‐decorated Au‐Ti bifunctional active sites. Combined with multi‐techniques (e.g., operando UV–vis–NIR system, DFT studies and quantitative model calculations), it is found that the sodium‐decorated silanols (Si‐ONa species) on the TS‐1 support enhance the formation of Ti‐OOH intermediates by restraining H2O2 decomposition. In addition, sodium‐decorated silanols improves the desorption of propylene oxide, suppressing its ring‐opening and formation for the carbonaceous deposits. Moreover, the sodium‐decorated Au nanoparticles with smaller diameter and higher electron density further strengthen O2 adsorption and boost the H2O2 formation. This work not only provides fundamental understandings on the microenvironment of Au‐Ti bifunctional catalysts but also sheds new light on enhancing the performance by boosting the surface molecular transfer.

New perspectives and insights into direct epoxidation of propylene using O2 and silver-based catalysts

A series of Ag-based catalysts were studied for direct epoxidation of propylene using molecular O2 as the oxidant. Ag supported on CaCO3 and α-Al2O3 were prepared as base materials to study. Different amounts of alkali earth metal promoter (K) and feed additives, ethyl chloride (EtCl) and nitric oxide (NO), were added to the catalyst and feed stream, respectively, to evaluate their effects on catalyst performance in terms of PO selectivity and propylene conversion. A conventional catalyst supported on α-Al2O3 performed similarly to the ARCO Ag (50 wt%) catalyst, supported on CaCO3, at a much lower Ag loading (12 wt%). EtCl inhibited consecutive oxidation presumably by Cl deposition on Ag sites and NO increased activity possibly by Ag-catalyzed oxidation of NO to NOx which was the selective oxidant. K+ promoter loadings as high as 1.9 wt% were essential to stabilize both Cl and NOx on the catalyst surface. PO Selectivities as high as 50 % were obtained.

Mechanistic insights into propylene oxidation to acrolein over gold catalysts

Direct epoxidation of propylene with H2/O2, being the dream reaction for propylene oxide (PO) production, has raised wide scientific and industrial interests. Fundamentally understanding the formation mechanism of acrolein, as the main by-product of this epoxidation process, is very important to achieve the high yield of PO. In this study, we perform the spin-polarized density functional theory (DFT) calculations to investigate the reaction pathway from propylene to acrolein over two representative Au surfaces, that is, Au(1 1 1) and Au(1 0 0), which incorporates propylene adsorption, methyl hydrogen activation and acrolein formation. The results show that the oxygenated species (mainly O*, OH* and OOH*) are able to stabilize the adsorption of propylene to decrease the energy barrier for its activation. It is demonstrated that the OOH* on Au(1 1 1) surface emerges as the most easily formed oxygenated species via the H-assisted O2 dissociation, which is also the most active for the cleavage of methyl CH bond in propylene. Furthermore, three pathways of acrolein formation activated by O*/OH*/OOH* are analyzed, in which O* is found as the key species to form acrolein. Finally, Bader charge analysis was conducted to explore the reasons behind the promotion effect of the oxygenated species. The insights reported here could be valuable in the design and optimization of gold catalysts for the direct epoxidation of propylene.

First-principles kinetic Monte Carlo modeling of propylene epoxidation on the Ti2CuO6/Cu(111) and Cu2O(111) surfaces

Direct propylene epoxidation by molecular oxygen is one of the most important commercial selective oxidation reactions. Although the mixed metal oxide (Ti2CuO6/Cu(111)) with highly dispersed and stabilized Cu+ has high selectivity toward propylene oxide (PO), the reaction mechanism and the physical nature of stable Cu+ involved in Ti2CuO6/Cu(111) remain unclear. To better understand the key role of stabilized Cu+ active sites, the systematic kinetic Monte Carlo (kMC) studies of the propylene epoxidation reaction on Ti2CuO6/Cu(111) and Cu2O(111) surfaces are carried out. It was found that O2 is weakly bound to Ti2CuO6/Cu(111) due to the less active of Cu+ ions compared to that of Cu+ ions in Cu2O(111), which can exist stably. The current simulation demonstrates that the relatively stable Ti2CuO6/Cu(111) mixed oxide exhibits better PO selectivity and activity than Cu2O(111). The reasons for the different catalytic performance are revealed: For the primary chemistry of propylene epoxidation mechanism, Cu2O(111) favors the first allylic hydrogen stripping, while Ti2CuO6/Cu(111) has an excellent open-shell electronic structure character, which makes it more reactive toward the propylene double bond to form the oxametallacycle (OOMMP or OMMP). For the secondary chemistry of propylene epoxidation mechanism, the energy barrier of PO formation is smaller than that of propanal(PA)/acetone(AC) due to the larger difference in bonding affinity toward the O and C atoms. In contrast, the opposite trend holds for Cu2O(111). In addition, the surface species can be balanced quickly on Ti2CuO6/Cu(111). The catalyst surface is not poisoned because the surface species coverage is moderate, which means that Ti2CuO6/Cu(111) remains highly stable during the reaction processes. The Cu2O(111) has fewer active sites due to higher surface species coverage (i.e., catalyst poisoning). One may rationally design an efficient propylene epoxidation catalyst based on the present work, i.e., doping Cu into the stable spinels or perovskites such as Cu-modified LaCoO3 perovskites (LaCoxCu1−xO3−δ) may show the similar catalytic properties like that of Ti2CuO6/Cu(111).

Kinetics of Propylene Epoxidation over Extracrystalline Gold Active Sites on AU/TS-1 Catalysts

To determine whether the catalytic roles of extracrystalline and intracrystalline gold nanoparticles supported on titanosilicate-1 (TS-1) for direct propylene epoxidation are intrinsically different, the kinetics of direct propylene epoxidation were measured in a gas-phase continuous stirred tank reactor (CSTR) over polyvinylpyrrolidone-coated (PVP) gold nanoparticles (Au-PVP/TS-1) deposited on TS-1. The as-made PVP-coated gold nanoparticles were too large to fit into the micropores of TS-1, even after ligands were removed in situ by a series of pretreatments, as confirmed by both TEM and TGA-DSC. The activation energy (51 kJ mol–1) and reaction orders for H2 (1.3), O2 (0.4), propylene (0.4), propylene oxide (−0.6), carbon dioxide (0), and water (0) for propylene epoxidation measured on Au-PVP/TS-1 were consistent with those reported for Au/TS-1 prepared via deposition-precipitation (Au-DP/TS-1) (52 kJ mol–1, H2, 1; O2, 0.4; C3H6, 0.4; C3H6O, −0.6; CO2, 0; H2O, 0). However, while the reaction orders for hydrogen oxidation on Au-PVP/TS-1 (H2, 0.8; O2, 0; C3H6, −0.3; C3H6O, −0.1; CO2, 0; H2O, −0.2) were similar to those measured on Au-DP/TS-1 (H2, 0.9; O2, 0.3; C3H6, −0.3; C3H6O, 0; CO2, 0; H2O, −0.1), a decrease in activation energy from approximately 30 kJ mol–1 for Au-DP/TS-1 to 4–5 kJ mol–1 for Au-PVP/TS-1 suggests there is a change in rate-limiting step or active site for hydrogen oxidation. Additionally, an active site model was developed which determines the number of Ti within an interaction range of the perimeter of extracrystalline Au nanoparticles (i.e., the number of Au–Ti active site pairs). This model was used to estimate catalytic turnover frequencies over solely proximal Au–Ti pairs, assuming that hydrogen peroxide does not desorb and migrate from Au sites to Ti sites and instead propylene oxide forms in a concerted mechanism previously termed the “simultaneous” mechanism. Turnover frequencies estimated for this active site model for a data set containing both Au-DP/TS-1 and Au-PVP/TS-1 were ∼20× higher than the highest previous reported estimates (∼80 s–1 vs 1–5 s–1 at 473 K) for catalytic oxidation on noble metals, suggesting that the simultaneous mechanism occurring over proximal Au–Ti sites alone is incapable of explaining the observed rate of propylene epoxidation and that short-range migration of hydrogen peroxide is kinetically relevant. The agreement of reaction orders, activation energy, and active site model for propylene epoxidation on both Au-DP/TS-1 and Au-PVP/TS-1 suggests a common mechanism for propylene epoxidation on both catalysts containing small intraporous gold clusters and catalysts with exclusively larger extracrystalline gold nanoparticles. Rates of hydrogen oxidation were found to vary proportionally to the amount of surface gold atoms. This is also consistent with the hypothesis that the observed decrease in hydrogen efficiency and PO site-time-yield per gold mass with increasing gold loading are driven primarily by the gold dispersion in Au/TS-1 catalysts.

Liquid-phase epoxidation of propylene with molecular oxygen by chloride manganese meso-tetraphenylporphyrins

Propylene molecule owns two active sites, the direct epoxidation of propylene by dioxygen is still a challenge due to the limitation of selectivity. In this work, the direct liquid-phase propylene aerobic epoxidation protocol by chloride manganese meso -tetraphenylporphyrin (MnTPPCl) was developed. The conversion of propylene was 12.7%, and the selectivity towards PO (propylene oxide) reached up to 80.5%. The formation of PO was attributed to the mechanism via high-valent Mn species, which was confirmed by means of in situ UV–vis spectrum.

Effect of urea on the morphology of TS-1 and catalytic performance of Au/TS-1 for direct gas phase epoxidation of propylene

Effect of urea on the morphology of TS-1 and catalytic performance of Au/TS-1 for direct gas phase epoxidation of propylene