Propylene Epoxidation Research Articles

Selective Oxidation by TS-1 coupled with in-situ Synthesised H2O2

Feedstock valorisation with preformed H2O2 in conjunction with titanosilicate zeotypes is particularly attractive due to the increased selectivity, lower operating temperatures and improved process efficiency (particularly lower energy usage and enhanced atom economy) compared to aerobic pathways or alternative stoichiometric oxidants. However, numerous environmental and financial concerns associated with the application of commercial H2O2 remain. These concerns have led to a growing interest in the application of the in-situ synthesized oxidant for a range of chemical transformations. Herein, we provide an overview of the key contributions to this emerging field of oxidative catalysis, with a particular focus on propene epoxidation and ketone ammoximation, which are currently two crucial industrial-scale processes that utilise preformed H2O2 with the titanosilicate catalyst TS-1 for chemical synthesis. This contribution will highlight the potential for in-situ H2O2-mediated feedstock upgrading to replace existing technologies and contribute to the declared sustainability goals of the chemical synthesis sector.

Kinetic insights into reaction pathways of acrolein formation in propylene epoxidation by H2 and O2 over Au/TS-1 catalyst

Propylene epoxidation by H2 and O2 to propylene oxide (HOPO), as a novel method for PO production, has attracted widespread attention due to its economic and environmental advantages. Acrolein is a notable byproduct arising in HOPO reaction over Au/TS-1 catalyst, and is traditionally thought to be exclusively formed on Au site through propylene oxidation. However, on the intraporous and external surface Au sites of Au-Ti catalysts, the HOPO reaction have been shown to proceed by different mechanisms, yet the effect of active sites distribution on acrolein formation is far from being understood. Herein, we found that the reaction orders in acrolein formation on Au/TS-1 catalyst (H2, 0.19; O2, 0.44; C3H6, 0.44) relative to Au/TS-1 with blocked pores (Au/TS-1-B) catalyst (H2, −0.36; O2, 0.61; C3H6, 0.52) were quite different, suggesting a variation in the reaction pathway on the catalysts with different Au locations. Moreover, it is found that PO can be converted to acrolein by O2 on TS-1 zeolite, which is absent on TS-1-B zeolite without micropores. Consequently, acrolein formation on Au/TS-1 catalyst can proceed through two parallel pathways: propylene hydro-oxidation on Au site, and PO conversion by O2 inside micropores. Furthermore, by designing and preparing the Au/TS-1 catalysts with different structures, the Au locations are closely related to acrolein formation mechanism and kinetic behaviors. This work provides kinetic insights into the acrolein formation pathways in the HOPO reaction on Au/TS-1 catalyst, and is helpful to the rational design and preparation of efficient catalysts with high selectivity.

A new stratagem to efficiently synthesize monolithic TS-1 via dry gel conversion in the atmosphere of tripropylamine-water and revealing insight into the structure–activity relationship for propylene epoxidation

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.

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 %.

Photocatalytic propylene epoxidation with O2 over 4,4′-dibromobenzophenone modified carbon nitride coupled with TS-1

The utilization of O2 for propylene epoxidation is a highly desirable reaction due to its 100 % atom economy. However, achieving this reaction under mild conditions presents a significant challenge. In this paper, we present a tandem reaction system that combines 4,4′-dibromobenzophenone (BP) modified carbon nitride (g-C3N4) with titanium silicalite zeolite (TS-1) to enable the selective photooxidation of propylene into propylene oxide at room temperature, using O2 as the oxygen source. The achieved production rate of propylene oxide was an impressive 1195 μmol/gcat./h, with a selectivity exceeding 99 %. Furthermore, the catalyst exhibited no significant deactivation after 5 cycles. Spectra and in-situ capture experiments revealed that the reaction involved a two-step system, with photocatalytic production of H2O2 and propylene epoxidation facilitated by H2O2·H2O2 was generated through O2− and 1O2 pathways, with an in-situ utilization efficiency of H2O2 reaching 82 %. This work presents a novel approach for the direct epoxidation of propylene using O2.

Direct electrochemical propylene epoxidation driven by water-activation intermediates

Direct electrochemical propylene epoxidation driven by water-activation intermediates

Mechanistic control of surface chlorine within propylene epoxidation

Propylene epoxidation over K-promoted Ag/CaCO3 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/CaCO3 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 C3H5Cl) and correspond with substantial improvement in steady state propylene oxide selectivity (S ∼ 5% to 55%) and rates (2 to 33 nmol gcat-1 s−1). Varying oxygen (∼2–8 kPa O2) and propylene (∼5–20 kPa C3H6) 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 O2 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.

The competition between propylene epoxidation and hydrogen oxidation and the effect of water on Au/TS-1/MOx catalysts

Water is the only by-product of propylene epoxidation theoretically and there is not much research on the effect of water at present. In the paper, through encapsulating metal oxides with the difference on the ability to adsorb O2 into titanium silicalite-1 (TS-1) shell, the TS-1/MOx carriers (MOx = CeO2, MnO2, Fe2O3 and Co2O3) with core-shell structure were prepared. After loading Au, they were used for propylene epoxidation. With the reactant mixture of C3H6: H2: O2: N2 = 1:1:1:7 Au/TS-1/Fe2O3 shows better performance than Au/TS-1 and Au/TS-1/CeO2 performs badly because of the strong ability of CeO2 to adsorb and activate O2. A large amount of O2 is adsorbed on Au/TS-1/CeO2 but only a part can be used for propylene epoxidation, and the excess part of O2 will react with H2 to form water. Thus the hydrogen oxidation is dominant. Interestingly, reducing the oxygen concentration in the reactant mixture can weaken the hydrogen oxidation, and the amount of water generated can be reduced. Besides, the PO formation rate and H2 efficiency of Au/TS-1/CeO2 can be greatly increased. XPS and in-situ DRIFTS are employed to study the mechanism of the formation of water and its effect on the reaction. The results show that the generation of excessive water results in the competitive adsorption with propylene, affecting the catalytic performance. Additionally, some of the adsorbed water increases the amount of hydroxyl structure in the catalyst, leading to the ring-opening of the PO and impacting the PO selectivity of the catalyst.

Fabrication of gold nanoparticles supported on hollow microsphere of nanosized TS-1 for the epoxidation of propylene with H2 and O2

The TS-1 microsphere was facilely prepared via a direct spray drying of the mixture containing the nanocrystal TS-1 and mother liquor. The calcined TS-1 zeolites possess a hollow structure and numerous intercrystalline mesopores, which may facilitate the diffusion properties. Au nanoparticles were loaded on TS-1 support by utilizing a deposition–precipitation process to prepare the Au/TS-1-M catalysts for propylene epoxidation in the presence of H2 and O2 conditions. The effects of the Ti and Au loadings on the structure, porosity, morphology, and catalytic performances of the Au/TS-1 catalysts have been investigated by various techniques. Under the optimized synthesis conditions (Si/Ti molar ratio = 57, 0.49 wt% Au), a propylene conversion of 6.1 % and a PO selectivity of 91.7 % were obtained on the Au/TS-1 catalyst. A PO formation rate of 186.8 gPO·kgcat.−1·h−1 was achieved at the 20th hour.

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.

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.

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.

Investigating the mechanism of propylene epoxidation over halogen (X = F, Cl, Br, I) modified Cu2O(110) surfaces: a theoretical study

A comprehensive atomic-scale understanding of how F, Cl, Br,and I ions influence direct propylene epoxidation over Cu2O(110) surfaces.

Selective Oxidation Using In Situ-Generated Hydrogen Peroxide.

ConspectusHydrogen peroxide (H2O2) for industrial applications is manufactured through an indirect process that relies on the sequential reduction and reoxidation of quinone carriers. While highly effective, production is typically centralized and entails numerous energy-intensive concentration steps. Furthermore, the overhydrogenation of the quinone necessitates periodic replacement, leading to incomplete atom efficiency. These factors, in addition to the presence of propriety stabilizing agents and concerns associated with their separation from product streams, have driven interest in alternative technologies for chemical upgrading. The decoupling of oxidative transformations from commercially synthesized H2O2 may offer significant economic savings and a reduction in greenhouse gas emissions for several industrially relevant processes. Indeed, the production and utilization of the oxidant in situ, from the elements, would represent a positive step toward a more sustainable chemical synthesis sector, offering the potential for total atom efficiency, while avoiding the drawbacks associated with current industrial routes, which are inherently linked to commercial H2O2 production. Such interest is perhaps now more pertinent than ever given the rapidly improving viability of green hydrogen production.The application of in situ-generated H2O2 has been a long-standing goal in feedstock valorization, with perhaps the most significant interest placed on propylene epoxidation. Until very recently a viable in situ alternative to current industrial oxidative processes has been lacking, with prior approaches typically hindered by low rates of conversion or poor selectivity toward desired products, often resulting from competitive hydrogenation reactions. Based on over 20 years of research, which has led to the development of catalysts for the direct synthesis of H2O2 that offer high synthesis rates and >99% H2 utilization, we have recently turned our attention to a range of oxidative transformations where H2O2 is generated and utilized in situ. Indeed, we have recently demonstrated that it is possible to rival state-of-the-art industrial processes through in situ H2O2 synthesis, establishing the potential for significant process intensification and considerable decarbonization of the chemical synthesis sector.We have further established the potential of an in situ route to both bulk and fine chemical synthesis through a chemo-catalytic/enzymatic one-pot approach, where H2O2 is synthesized over heterogeneous surfaces and subsequently utilized by a class of unspecific peroxygenase enzymes for C-H bond functionalization. Strikingly, through careful control of the chemo-catalyst, it is possible to ensure that competitive, nonenzymatic pathways are inhibited while also avoiding the regiospecific and selectivity concerns associated with current energy-intensive industrial processes, with further cost savings associated with the operation of the chemo-enzymatic approach at near-ambient temperatures and pressures. Beyond traditional applications of chemo-catalysis, the efficacy of in situ-generated H2O2 (and associated oxygen-based radical species) for the remediation of environmental pollutants has also been a major interest of our laboratory, with such technology offering considerable improvements over conventional disinfection processes.We hope that this Account, which highlights the key contributions of our laboratory to the field over recent years, demonstrates the chemistries that may be unlocked and improved upon via in situ H2O2 synthesis and it inspires broader interest from the scientific community.

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

Decomposition of hydrogen peroxide during catalytic epoxidation of propylene

Decomposition of hydrogen peroxide during catalytic epoxidation of propylene

Theoretical insights into the thermal behaviors and coalescence of truncated octahedral Au nanoparticles

Truncated octahedral Au nanoparticles (NPs) dispersed on Ti-containing supports has been demonstrated as a promising catalyst for the propylene epoxidation with H2 and O2, while the catalytic stability for the industrial application is still challenging. Herein, thermodynamics analysis and molecular dynamics calculations are employed to address the thermal behaviors and coalescence of truncated octahedral Au NPs. The melting temperature of Au NPs is found to decrease with the decreasing particle size, and the melting initiates from the outermost edge and (100) atoms with lower coordination numbers followed by the (111) atoms and proceeds inwards. The coalescence of two Au NPs deforms smaller sized Au NP while remains the larger sized one at low temperatures, but the two nanoparticles melt into a fully mixed spherical one at high temperatures. Moreover, the (111) facet is more facile for the coalescence than other exposed facets on Au NPs.

Tandem photocatalytic production of H2O2 and propylene oxide on 5-Bromoisatin modified carbon nitride

Propylene oxide (PO) is an important raw material and synthetic intermediate for organic chemical synthesis. The direct in-situ synthesis of hydrogen peroxide (H2O2) using molecular oxygen to produce PO is considered as one of the most promising methods. Herein, we propose a photocatalytic multiphase system for the production of PO with oxygen. The ligand 5-bromoindoline-2,3-dione doped graphitic carbon nitride (CN/Bd) was employed as a photocatalyst for the in-situ production of H2O2, in tandem with epoxidation catalyst titanium silicalite-1 (TS-1) to achieve the epoxidation of propylene. This system achieved H2-free and efficient production of PO at room temperature with a high yield of 1.33 mmol·g−1·h−1, the selectivity of PO over 99% and H2O2 utilization efficiency over 95%. The combination of in-situ H2O2 generation and selective oxidation reaction realizes the efficient utilization of H2O2 and provides guidance for safer industrial production of PO.

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

Synthesis strategy of atomically dispersed Au clusters induced by NH3 on TS-1: Significantly improve the epoxidation activity of propylene

The epoxidation of propylene (C3H6) over TS-1 molecular sieve supported with nano-Au clusters by deposition–precipitation (DP) method has always been the focus of the catalytic field. A quantity of atomically dispersed Au clusters can be synthesized induced by NH3 as an assistant in the DP system. These smaller Au clusters show 14.3% C3H6 conversion, 91.7% propylene oxide (PO) selectivity, 10.6% H2 efficiency and 477.4 gPO·kgcat-1·h-1 PO productivity in the epoxidation reaction of C3H6. The interaction between NH3 and [AuCl4]- in aqueous solution is analyzed by means of experiments and Density Functional Theory (DFT), and it is concluded that at least 2 times of NH3 is needed to make Au precipitate completely. Similarly, the mechanism of C3H6 epoxidation catalyzed by Aun (n = 2, 3, 4) clusters in coordination with Ti sites is calculated with the help of DFT. It is believed that the in-situ formation of Ti-OOH structure at Ti sites through H2 and O2 is the key to initiate C3H6 epoxidation. The general rules of catalytic mechanism under different size of Au clusters are also analyzed.