Propylene Epoxidation Research Articles

Investigation of the oxygen coverage of propylene epoxidation on Ag(1 1 1) surfaces from DFT

The direct epoxidation of propylene on Ag(111) under different oxygen coverage conditions was systemically studied on the basis of density functional theory calculation and the general trends of oxygen coverage on the two parallel competitive reaction paths in propylene oxide (PO) formation: α-H stripping versus epoxidation and the generation of PO versus acetone (AC) were explored. Then, the PO formation selectivity was investigated. Results showed that increase in oxygen coverage enhanced the adsorption strength of adsorbed species, except the O* species. In the primary reaction of propylene epoxidation, high oxygen coverage facilitates the first α-H stripping process, and opposite trend was observed in epoxidation process. As for the secondary chemistry in propylene epoxidation, increase in oxygen coverage benefits PO and AC formation. The coverage of O* has considerable influences on the primary reaction but small influence on the secondary reaction. Although a high oxygen coverage benefits the first α-H stripping, the second α-H stripping has higher barrier than that in secondary chemistry, and thus the PO formation would be better than that of acrolein formation at higher oxygen coverage.

One‐pot Synthesis of Hierarchical, Micro‐macroporous Zeolites with Encapsulated Metal Particles as Sinter‐resistant, Bifunctional Catalysts

AbstractWe report a new one‐pot synthesis procedure for hierarchical zeolites with intracrystalline macropores and metal particles encapsulated within the zeolitic walls. The synthesis allows to prepare macroporous zeolites of MFI topology with different heteroatoms (silicalite‐1, ZSM‐5 and TS‐1) and different encapsulated noble metal particles, such as gold, platinum and palladium. The hierarchically structured zeolites contain large macropores with diameters around 400 nm, which are well distributed and interconnected and should significantly enhance mass transport properties. The encapsulation of metal nanoparticles within the zeolitic walls leads to remarkable sinter resistance of the particles. Encapsulated gold nanoparticles (2.6 nm) do not significantly change in size during an 18‐hour treatment at 600 °C under air, while non‐encapsulated gold particles sinter heavily during the same treatment. Catalytic experiments for the direct epoxidation of propene with hydrogen and oxygen show that both catalytic functions of a macroporous TS‐1 sample that encapsulates gold particles are accessible and active. This catalyst displays high activity, although PO selectivity could still be improved. These materials show great potential for use in catalytic applications, due to their bifunctional nature, high sintering resistance, shape selective properties and hierarchical structure.

Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene

Gas-phase epoxidation of propylene in the copresence of H2 and O2 was performed over the catalyst of Au on as-synthesized TS-1 that contained a small amount of anatase TiO2. The catalytic performance was studied by washing or nonwashing the catalyst precursor to modulate the content of purity (K, Cl) and then calcining the samples in O2 or H2 prior to reaction. The results show that the catalytic performance of Au/TS-1 can be improved without washing (more K+ and Au maintained) and O2 pretreatment. It was found that the calcination in O2 was able to maintain more metallic Au and form more surface-active oxygen species and thus providing a better yield of propylene oxide with the assistance of potassium. Interestingly, more acrolein can be produced over the catalysts with respect to the in situ calcination in O2 than that in H2 when the feed only contained 10% O2 and 10% propylene in argon, while there was no formation of propylene oxide. On the other hand, the catalyst precursor calcined in H2 prefers the formation of successive oxygenates of PO.

Effect of P modification on the structure and catalytic performance of Ti-MWW zeolite

Ti-MWW zeolite has attracted much attention because of its excellent catalytic oxidation performance. As one of the essential components in zeolites, phosphorus (P) has been widely used for modification to enhance the hydrothermal stability and catalytic performance of aluminosilicate zeolite. Herein, a series of P-modified Ti-MWW zeolite samples were prepared with a simple way by directly adding various amounts of phosphoric acid into the acid-treated system. The effect of P modification on both structure and catalytic performance of Ti-MWW zeolite was systematically investigated. We demonstrated that some highly active framework TiO4* species with Ti–O–P structure are formed after modification with moderate P, leading to the remarkable improvement of catalytic activity in 1-hexene epoxidation. While introducing excess amounts of phosphoric acid would result in the formation of much inactive titanium phosphate species, which block the channels and finally decrease the catalytic performance significantly. Density functional theory calculations confirmed that the new formed Ti–H2PO4 site is more active than the Ti–OH site in Ti-MWW zeolite for model propene epoxidation, revealing the promotional effect of P modification. This work would deepen our understanding of P modification at the atomic level and benefit to improve the catalytic performance of titanosilicate zeolite.

Synthesis, characterization, and function of Au nanoparticles within TS-1 zeotypes as catalysts for alkene epoxidation using O2/H2O reactants

A synthetic procedure was developed for the encapsulation of Au nanoparticles within titanium silicalite-1 (TS-1). Encapsulation was achieved via crystallization of TS-1 synthesis gels containing ligated Au cations, resulting in TS-1 framework assembly around Au coordination complexes. X-Ray diffraction and micropore volume measurements indicated that TS-1 could be crystallized at much milder conditions than previously reported, allowing the assembly of TS-1 without concomitant decomposition of added Au complexes. Infrared (IR) and UV–vis spectroscopy showed that Ti was incorporated into the frameworks as tetrahedrally coordinated atoms. Changes to established crystallization procedures for TS-1 included lower temperature (393 K vs. 448 K) compensated by longer time (120 vs 48 h) and the use of seed crystals. Post-synthetic thermal treatments led to the formation of small Au nanoparticles (2.8–3.8 nm) visible in electron micrographs and free of organic debris, as indicated by IR spectroscopy. Such nanoparticles are protected from bulky titrants, and thus reside within TS-1 crystallites (>96% encapsulation). The Au particles were active for propene epoxidation with H2O/O2 even without promotion by alkali cations (e.g., Cs+), though alkali doping led to enhanced rates. These Au/TS-1 systems were less active for epoxidation than Au/TS-1 prepared by deposition–precipitation, likely because the latter contains a disproportionately active minority population of exceedingly small (<1 nm) Au clusters.

Influence of the Support on Propene Oxidation over Gold Catalysts

The epoxidation of propene without forming a substantial amount of byproducts is one of the holy grails of catalysis. Supported Cu, Ag and Au catalysts are studied for this reaction and the activity of the supported metals is generally well understood. On the contrary, limited information is available on the influence of the support on the epoxide selectivity. The reaction of propene with equal amounts of hydrogen and oxygen was tested over gold nanoparticles deposited onto CeO2, TiO2, WO3, γ-Al2O3, SiO2, TiO2-SiO2 and titanosilicate-1. Several metal oxide supports caused further conversion of the synthesized propene oxide. Strongly acidic supports, such as WO3 and titanosilicate-1, catalyzed the isomerization of propene oxide towards propanal and acetone. Key factors for achieving high PO selectivity are having inert or neutralized surface sites, a low specific surface and/or a low density of surface -OH groups. This work provides insights and practical guidelines to which metal oxide support properties lead to which products in the reaction of propene in the presence of oxygen and hydrogen over supported gold catalysts.

Micropore blocking strategy for mitigating adsorption and diffusion limitations in the direct epoxidation of propylene

The direct epoxidation of propylene has been a hot topic in the academic field for years. Researchers have spent extensive effort on the development of catalyst, however, only limited attention on the effect of reaction conditions, especially pressure. In this paper, we found for the first time that elevated pressure should favor the forward epoxidation reaction, it actually led to a drop of reaction rate in this system. We proposed explanation by considering the difficulty of gas molecules diffusing into and out of the micropores on the TS-1 zeolite surface. Higher pressure would inhibit the escaping of the gas molecules from the micropores and lead to carbon deposition inside the channel. To validate this hypothesis, we designed and compared catalysts with micropores blocked (BTS-1) and unblocked (TS-1) and found that the BTS-1 showed higher resistance to pressure change and significantly longer life time than the TS-1.

Opportunities and Challenges in Electrolytic Propylene Epoxidation.

Propylene oxide (PO) is an important chemical. So far, its synthesis protocol relies on expensive oxidants. In contrast, direct epoxidation of propylene (DEP) using molecular oxygen is considered ideal for PO synthesis. Unfortunately, DEP has not met industrial demands due to the low propylene conversion and high side-product selectivity for known catalysts. Instead of a thermal process using molecular oxygen, electrolytic propylene oxidation can synthesize PO at room temperature, using the atomic oxygen generated from water-splitting. Herein, using density functional theory, surface Pourbaix analysis, scaling relation analysis, and microkinetic modeling, we show that (i) propylene epoxidation is facile on weak-binding catalysts if reactive atomic oxygen preexists; (ii) electrolytic epoxidation is facile to provide atomic oxygen for epoxidation, while hydroperoxyl formation does not overwhelm the epoxidation process at the potential of interest; (iii) propylene dehydrogenation is a competing step that forms side products. Finally, we discuss the opportunities and challenges of this green PO synthesis method.

Theoretical comparative study on the mechanism of gas phase epoxidation of propylene about complete and defective dimer MoOx

Density Functional Theory is used to calculate the epoxidation of propylene on the dimer MoOx by Gaussian 09 software. We calculate the dehydrogenation process of propylene and the formation process of propylene oxide. It is found that the activity of O in Mo-O-Si, which is beneficial to the dehydrogenation process of propylene, is high. Furthermore, we establish a dimer MoOx model with defect sites, and find that the defect sites in the dimer MoOx can activate O2 (O22-) effectively. Consequently, the catalyst containing defect sites is conducive to the selectivity of propylene oxide.

Direct Propylene Epoxidation with Molecular Oxygen over Cobalt-Containing Zeolites.

Direct propylene epoxidation with molecular oxygen is a dream reaction with 100% atom economy, but aerobic epoxidation is challenging because of the undesired over-oxidation and isomerization of epoxide products. Herein, we report the construction of uniform cobalt ions confined in faujasite zeolite, namely, Co@Y, which exhibits unprecedented catalytic performance in the aerobic epoxidation of propylene. Propylene conversion of 24.6% is achieved at propylene oxide selectivity of 57% at 773 K, giving a state-of-the-art propylene oxide production rate of 4.7 mmol/gcat/h. The catalytic performance of Co@Y is very stable, and no activity loss can be observed for over 200 h. Spectroscopic analyses reveal the details of molecular oxygen activation on isolated cobalt ions, followed by interaction with propylene to produce epoxide, in which the Co2+-Coδ+-Co2+ (2 < δ < 3) redox cycle is involved. The reaction pathway of propylene oxide and byproduct acrolein formation from propylene epoxidation is investigated by density functional theory calculations, and the unique catalytic performance of Co@Y is interpreted. This work presents an explicit example of constructing specific transition-metal ions within the zeolite matrix toward selective catalytic oxidations.

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

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

Preparation and Catalytic Performance in Propylene Epoxidation of Hydrophobic Hierarchical Porous TS-1 Zeolite

Preparation and Catalytic Performance in Propylene Epoxidation of Hydrophobic Hierarchical Porous TS-1 Zeolite

Effects of potassium on propylene epoxidation by molecular oxygen on Cu2O (111): a DFT study

Propylene epoxidation catalyzed by cuprous oxide (Cu2O) with molecular oxygen is significant in the industrial field, and strategies to improve the selectivity of the target product propylene oxide (PO) are highly desired.

Au supported defect free TS-1 for enhanced performance of gas phase propylene epoxidation with H2 and O2

A facile strategy for the synthesis of a high quality TS-1 zeolite without anatase Ti species via a hydrothermal method using urea as a mineralizer was reported, which exhibits excellent performance in the direct gas phase epoxidation of propylene.

Activation of molecular oxygen over Mn-doped La2CuO4perovskite for direct epoxidation of propylene

The synergetic interaction between manganese and copper in LaMn0.5Cu0.5O3significantly promoted the epoxidation of propylene at lower temperature by converting the active sites from oxygen vacancies to Cu active sites of Cu–O–Mn.

New Penta- and Hexa-Coordinated Titanium Sites in Titanium Silicalite-1 Catalyst for Propylene Epoxidation

New Penta- and Hexa-Coordinated Titanium Sites in Titanium Silicalite-1 Catalyst for Propylene Epoxidation

Direct propylene epoxidation with oxygen using a photo-electro-heterogeneous catalytic system

Propylene oxide is a crucial feedstock in the plastic industry. The direct epoxidation of propylene using O2 is considered among the most promising means of producing propylene oxide. Here we report an integrated photo-electro-heterogeneous catalytic system for propylene epoxidation with O2. Bismuth vanadate (or TiO2) photocatalyst and a Co-based electrocatalyst produces H2O2. A titanium silicalite-1 heterogeneous catalyst subsequently epoxidizes propylene to propylene oxide with the in situ-generated H2O2. The proposed system enables propylene oxide production with O2 as the sole oxidizing agent under light irradiation without using H2, a sacrificial agent, or external bias. It stably produces propylene oxide for 24 h with high selectivity (≥98%) under ambient conditions. These results demonstrate the potential of this catalytic system to produce chemical compounds in an environmentally benign manner.

Enhanced oxygen transfer over bifunctional Mo-based oxametallacycle catalyst for epoxidation of propylene

Activation of inert propylene to produce propylene oxide (PO) is critical, but still faces some challenges in realizing higher PO selectivity and productivity. Herein, a temperature-controlled phase transfer catalyst (MoOO·DMF) is prepared for the liquid-phase epoxidation of propylene with tert-butyl hydroperoxide (TBHP) as oxidant, which exhibit the selectivity of 90.6% and the productivity of 1286.42·h−1 for PO (catalyst/propylene = 0.77 mol‰). Some experimental factors (solvent types, reaction temperature, contact time, the dosage of catalyst, TBHP and substrate) were investigated, and the reaction kinetics and thermodynamics are discussed. MoOO·DMF has the characteristic of both homogeneous and heterogeneous catalysts, which can be dissolved in the solvent at higher temperatures and separated from the solvent after reaction by lowering the temperature. Importantly, MoOO·DMF has a wonderful epoxidation performance for many olefins (e.g., light olefins, linear α-olefins, cyclic olefins and others). The mechanisms are proved by in-situ FT-IR, ESR and HRMS spectrum to be the selective oxygen transfer from tert-butyl peroxide radical and the MoOO bridge in MoOO·DMF to propylene. Density functional theory (DFT) calculations show that the MoOO bridge in catalyst is the key role for the activation of both the OH bond in TBHP and the CC bond in propylene, thus enhanced the epoxidation of propylene.

Enhanced catalytic performance of trimethylsilylated Ti-MWW zeolites for the liquid-phase epoxidation of propylene with H2O2

The titanosilicate zeolite catalyzed epoxidation of propylene with hydrogen peroxide (H2O2, HP) to propylene oxide (PO), the so-called HPPO, is more eco-efficient, compared to the chlorohydrin process and the epoxidation using organic hydroperoxides. However, the zeolite catalysts used in the HPPO process are easily deactivated, because the main byproduct propylene glycol (PG) and other heavy byproducts with high boiling points, derived from the solvolysis of PO, are deposited inside the zeolite micropores. In the present work, trimethylsilylated Ti-MWW zeolites, named Si–Ti-MWW, were developed for the HPPO process. In combination with the results from extensive characterizations for the prepared zeolites, it is found that the trimethylsilylation merely occurs on the external surface of Ti-MWW crystals and can significantly enhance the hydrophobicity, but not alter the coordination states of the Ti species and textural properties of the zeolites. The catalytic properties of Ti-MWW and Si–Ti-MWW were compared in propylene epoxidation. The results show that the solvolysis of PO to the byproducts can be effectively inhibited over the hydrophobic zeolite surface, making Si–Ti-MWW exhibit a much higher selectivity for PO and a much better stability and reusability than the parent Ti-MWW in the HPPO process. The current study, therefore, provides a new approach to develop efficient catalysts for the HPPO process.

Fine cubic Cu2O nanocrystals as highly selective catalyst for propylene epoxidation with molecular oxygen

Propylene epoxidation with O2 to propylene oxide is a very valuable reaction but remains as a long-standing challenge due to unavailable efficient catalysts with high selectivity. Herein, we successfully explore 27 nm-sized cubic Cu2O nanocrystals enclosed with {100} faces and {110} edges as a highly selective catalyst for propylene epoxidation with O2, which acquires propylene oxide selectivity of more than 80% at 90–110 °C. Propylene epoxidation with weakly-adsorbed O2 species at the {110} edge sites exhibits a low barrier and is the dominant reaction occurring at low reaction temperatures, leading to the high propylene oxide selectivity. Such a weakly-adsorbed O2 species is not stable at high reaction temperatures, and the surface lattice oxygen species becomes the active oxygen species to participate in propylene epoxidation to propylene oxide and propylene partial oxidation to acrolein at the {110} edge sites and propylene combustion to CO2 at the {100} face sites, which all exhibit high barriers and result in decreased propylene oxide selectivity.