Propylene Epoxidation Research Articles

Size-Dependent Structures and Catalytic Performances of Au/TiO2-{001} Catalysts for Propene Epoxidation

Aiming at a detailed understanding of the size-dependent Au active structure and reaction mechanism for the low-temperature propene epoxidation with H2 and O2, we have prepared and studied the Au c...

Zeolite crystal size effects of Au/uncalcined TS-1 bifunctional catalysts on direct propylene epoxidation with H2 and O2

Understanding the nature of Au-Ti bifunctional catalysis is important yet challenging for improving direct propylene epoxidation with H2 and O2 to propylene oxide (PO). Herein, we report a facile strategy to fabricate four different crystal sized TS-1 with the presence of highly active isolated Ti4+ species by adjusting the amount of TPAOH template. The resultant uncalcined zeolite samples, i.e., TS-1-B, are employed to load Au via deposition–precipitation with urea for obtaining similar sized Au immobilized on TS-1-B catalysts and thus excluding the size effects of Au active sites. The zeolite crystal size effects are further probed, and then decreasing the crystal size gives rise to the simultaneous promotion in PO formation rate and selectivity, as well as H2 efficiency. These are attributed to the Au-Ti electronic synergy based on UV–vis and XPS measurements. The insights reported here could benefit the development of high-efficiency Au-Ti bifunctional catalysts for this reaction.

Effect of Na, Cs and Ca on propylene epoxidation selectivity over CuOx/SiO2 catalysts studied by catalytic tests, in-situ XAS and DFT

This research focuses on epoxidation of propylene over pristine, Na, Ca and Cs modified CuOx/SiO2 catalysts using O2. The selectivity of the reaction is analyzed using a combination of catalytic tests, in-situ XAS and DFT calculations. The initially present subnanometer CuO clusters are present in all catalysts which re-disperse/flatten during reaction. During catalytic reaction, the Cu1+ becomes the predominant oxidation state. There is no correlation between propylene oxide (PO) selectivity and copper oxidation state. DFT analysis of the propylene reaction pathway revealed that Na, Cs, and Ca addition decreases the bonding strength of propylene to CuO and decreases the O2 activation barrier, while simultaneously increase the exothermicity of O2 dissociation. The Na induced Cu–O bond modification decreases the activation barrier from 0.87 to 0.71 eV for the oxametallacycle (OMC) ring closure (first step in the reaction pathway favoring selectivity towards PO) compared to pristine 5Cu catalyst. At the same time, we observed an increase (from 0.45 to 0.72 eV) of the barrier for the abstraction of allylic hydrogen. The opposite effect is achieved by Ca addition: the activation barrier for OMC ring closure increases to 1.08 eV and that for allylic hydrogen stripping decreases to 0.16 eV.

Low-Temperature Propylene Epoxidation Activity of CuO–CeO2 Catalyst with CO + O2: Role of Metal–Support Interaction on the Reducibility and Catalytic Property of CuOx Species

Epoxidation of propylene into propylene oxide (PO) in the gas phase is a highly challenging reaction. Cu-based catalysts have been active for this reaction, but the state of Cu as an active species is still debatable. In this paper, we report the propylene epoxidation activity of solution combustion synthesized Cu/CeO2 catalysts with the CO + O2 mixture at low temperatures (50–100 °C) peaking at ∼80 °C. The highest PO yield was obtained with 20–25% Cu loading in CeO2. In contrast, the reaction over the catalyst containing nonreducible support such as Cu/SiO2 occurred above 170 °C. Detailed structural characterization indicated two types of Cu species such as Cu2+ partly (∼3%) dissolved in CeO2 forming a CuxCe1–xO2−δ phase and the remaining amount formed highly dispersed CuO as a separate phase. Thus, the highest activity relates to the optimum presence of CuO along with Ce1–xCuxO2−δ. The reducibility of the Cu species in two phases was significantly shifted toward lower temperatures, indicating strong electronic interaction between the two phases. The substituted Cu2+ was reduced first, and then, the bulk CuO reduction was initiated. In situ spectroscopic studies showed Cu+ formation from Cu2+ over Cu/CeO2 catalysts even at room temperature unlike in CeO2 or CuO + CeO2 physical mixtures, indicating strong electronic interaction between Ce1–xCuxO2−δ and CuO phases on CO adsorption in the Cu/CeO2 catalyst. It is proposed that substituted Cu2+ along with Ce4+ is reduced easily, and then, Ce3+ promotes the reduction of the interfacial CuO phase that might donate active oxygen species for epoxidation reaction.

Propylene Epoxidation to Propylene Oxide Over RuO₂, CuO, TeO₂, and TiO₂ Supported on Modified Mesoporous Silicas.

Direct gas phase epoxidation of propylene to propylene oxide (PO) using O₂ is a challenging problem in catalysis research. Silica-supported ruthenium-copper-based catalysts have been recently reported to be promising for propylene epoxidation. In this work, mesoporous silica supports modified with RuO₂, CuO, and TeO₂ with and without TiO₂ were investigated for propylene epoxidation to PO. The prepared catalysts were divided into two groups. The first group consisted of mesoporous silica supports modified with RuO₂, CuO, and TeO₂, and the second group consisted of the same components as the first group but adding TiO₂. The prepared supports and catalysts were characterized using BET surface area analysis and other advanced instrument techniques. It was found that the catalyst made with RuO₂ and TeO₂ impregnated onto porous silica modified with CuO and TiO₂ (denoted as RuTe/CuTiSi) exhibited an excellent PO formation of 344 gPO h-1 kg-1cat, which was superior to that of the other prepared catalysts. Moreover, the addition of TiO₂ into the catalyst greatly improved the PO formation rate and the arrangement of active components in the catalyst and strongly influenced catalytic performance.

Effect of TS-1 Crystal Planes on the Catalytic Activity of Au/TS-1 for Direct Propylene Epoxidation with H2 and O2

In this work, we demonstrated that titanium silicalite-1 (TS-1) crystal planes affected the activity of nanogold supported on TS-1 (denoted as Au/TS-1) for direct propylene epoxidation with H2 and ...

The Construction of Au–Fe–TS-1 Interface Coupling Structure for Improving Catalytic Performance of Propylene Epoxidation with H2 and O2

The gas-phase propylene epoxidation for noble metal system as a green process has confronted a great challenging task in metal utilization and stability. In this work, we realize the enhancement of Au capture efficiency and high-dispersed Au nanoparticles by supports modification and interfacial regulation to construct Au–Fe–TS-1 interface coupling structure. The Au capture efficiency of the optimum catalyst arrived at 69% and it displayed a 7.3% C3H6 conversion with a PO selectivity of 89.9%, giving a H2 efficiency of about 25.7%. Then some typical characterizations systematically elucidate that Fe is not only incorporated into TS-1 framework and captures Au species with high dispersion and stability, but also inhibits the losing of Ti species. Furthermore, Fe-modified catalysts change the property of Au active site and maintain lower apparent activation energy. This report may provide a new strategy to design advanced catalysts for enhancing metal utilization efficiency by the construction of interface coupling structure.

Enhancing Catalytic Activity and Selectivity by Plasmon-Induced Hot Carriers.

SummaryPlasmon-assisted chemical transformation holds great potential for solar energy conversion. However, simultaneous enhancement of reactivity and selectivity is still challenging and the mechanism remains mysterious. Herein, we elucidate the localized surface plasmon resonance (LSPR)-induced principles underlying the enhanced activity (∼70%) and selectivity of photoelectrocatalytic redox of nitrobenzene (NB) on Au nanoparticles. Hot carriers selectively accelerate the conversion rate from NB to phenylhydroxylamine (PHA) by ∼14% but suppress the transformation rate from PHA to nitrosobenzene (NSB) by ∼13%. By adding an electron accepter, the as-observed suppression ratio is substantially enlarged up to 43%. Our experiments, supported by in situ surface-enhanced Raman spectroscopy and density functional theory simulations, reveal such particular hot-carrier-induced selectivity is conjointly contributed by the accelerated hot electron transfer and the corresponding residual hot holes. This work will help expand the applications of renewable sunlight in the directional production of value-added chemicals under mild conditions.

Advances in Designing Au Nanoparticles for Catalytic Epoxidation of Propylene with H2 and O2

Au nanoparticles, which can be used in various industrial and environmental applications, have drawn substantial research interest. In this review, a comprehensive background and some insights are provided regarding recent studies concerning the use of Au nanoparticles for catalytic propylene epoxidation with H2 and O2. Over the last two decades, substantial progress has been made toward the efficient production of propylene oxide (PO); this includes the design of highly dispersed Au catalysts on Ti-modified mesoporous silica supports, the optimization of catalytic epoxidation, and the determination of the mechanisms and reaction pathways of epoxidation. Particularly, the critical roles of catalyst synthesis, the types of material support, Au nanoparticle sizes, and the dispersion amounts of Au nanoparticles are emphasized in this review. In future studies, novel, practical, robust, and highly PO-selective Au nanoparticle catalyst systems are expected to be continually designed for the enhanced catalytic epoxidation of propylene.

Kinetics of the Ag/KNO3/CaCO3 Catalyzed Aerobic Propylene Epoxidation and Effects of CO2

AbstractIn this contribution we investigate the aerobic propylene epoxidation over a Ag/KNO3/CaCO3 catalyst. The catalytic performance and surface speciation of the catalyst depends on the concentrations of feed modifiers such as ethyl chloride and nitric oxide, added to the feed to improve the selectivity by suppressing total combustion to CO2. After a kinetic characterization of the system, we investigate the kinetics in the presence of CO2. We demonstrate that CO2 alters the rate dependence in both reactants, affects the activation of O2, and results in the same maximum rate of propylene oxide formation.

Novelty without nobility: Outstanding Ni/Ti-SiO2 catalysts for propylene epoxidation

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-SiO2 support for the propylene epoxidation reaction using H2/O2 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 H2 efficiency (~37%) using only 0.5 wt% of Ni on Ti-SiO2 (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.

Dual-Component Sodium and Cesium Promoters for Au/TS-1: Enhancement of Propene Epoxidation with Hydrogen and Oxygen

Titanosilcalite-1 (TS-1) supported gold catalysts (Au/TS-1) are promising for propene oxide (PO) synthesis through gas-phase propene epoxidation with hydrogen and oxygen. Herein, we report an effic...

Recovery of Au Nanoparticles via High‐Solubility Carboxylic Starch and its Significantly Improved Catalysis of Propylene Epoxidation

AbstractRecovery of Au nanoparticles (Au NPs) catalysts via bioreduction method has attracted much attention due to its cost‐effective and eco‐friendly. However, it is a great challenge to recover Au NPs by starch due to its poor solubility. Herein, a novel biomolecule, carboxyl corn starch (CCS), exhibited high solubility and low viscosity is successfully fabricated and utilized to reduce HAuCl4 in the homogeneous phase environment. Effects of CCS concentration on the structure and performances of Au NPs and its loading in the TS‐1 support (Au/TS‐1) are investigated. The results reveal that both the particle size and Au NPs recovery rate increased with the increase of CCS concentration, while the BET data of Au/TS‐1 decreased. The Au/TS‐1 catalyst achieved 353.01 m2g−1 surface area and 0.293 cm3g−1 pore volume with 2.0 ± 0.3 nm average diameter of loaded Au NPs. When this catalyst is employed to catalyze the vapor phase propylene epoxidation with H2/O2 mixture, the propylene conversion rate (19.2%) achieved 67% and 5.8 fold increases when in data reported (11.5%) and soluble starch are used as the control, respectively. The study develops a green and facile method for Au recovery via carboxyl starch bioreduction and Au/TS‐1 exhibited significantly improves catalysis of propylene epoxidation.

Ab initio studies of propene oxide formation at gold nanocatalysts supported on anatase-TiO2

Small gold particles supported on TiO2 are promising catalysts for the direct epoxidation of propene. Some experimental studies suggest a critical role of the oxide/gold interfacial region, where a bidentate propoxy species is formed during the reaction. However, it is not yet clear that such complex is a true reaction intermediate, and not a mere spectator or even a deactivating species. In order to clarify this issue, we have performed extensive ab initio simulations of the propene oxide formation on a model catalyst formed by small gold clusters (Au4, Au5 and Au8) supported on the anatase-TiO2(101) surface. In a variety of situations (with or without the supported gold cluster, with a varying amount of coadsorbed water or hydroxyl groups, etc.) we have investigated the stability of the bidentate propoxy species adsorbed at TiO2, as well as the barrier for its desorption into propene oxide. In all cases, the bidentate propoxy bonded at two surface Ti sites is highly stable, with a binding energy of the order of 1.5 eV. Such stability leads to very high energy barriers for propene oxide formation, of almost 2 eV. This suggests that such species is likely to be an spectator. Alternative reaction pathways, with the propoxy species formed in direct contact with the gold catalyst, are a much more probable candidate for a true reaction intermediate towards propene oxide formation.

Изучение молибденсодержащих растворов методами ИК спектроскопии

Molybdenum compounds are widely used as catalysts for various chemical reactions, such as: oxidation, epoxidation, hydrogenation, reduction, etc. In particular, molybdenum compounds are used as a catalyst in the epoxidation of propylene with ethylbenzene hydroperoxide during the joint production of styrene and propylene oxide introduced into the industry as part of PJSC "Nizhnekamskneftekhim". We are working to verify the possibility of using glycol solutions, oxygen-containing inorganic compounds of molybdenum, such as molybdenum acid (MK), molybdenum anhydride (MA) and ammonium paramolybdate (PMA) as an epoxidation reaction catalyst. In the initial molybdenum compounds used to prepare the catalyst solutions, molybdenum is in the hexavalent state. The temperature treatment of these compounds in coordinating solvents, glycols, leads to their partial depolymerization and reduction to Mo(VI). Moreover, the more coordinating the solvent, the deeper these processes are flowed. There are reports in the literature on the study of various molybdenum compounds in aqueous media and crystalline state by IR spectroscopy and Raman scattering. It was found that the dissolution of MA, MK, and PMA in monoethylene glycol (MEG) is associated with the formation in the solution of compounds Mo(V), Mo(VI) of various molecular composition in the form of monomers, dimers, tetramers, and also in the form of the main components of hept- and octamers and higher molecular weight compounds. As the temperature rises, initially larger monomers of the octa- and hepta-form type transform into monomers of lower aggregation. All established patterns are analyzed by the example of PMA solutions in MEG.

Impact of Titanium in Controlling Silver Particle Size on Enhancement of Catalytic Performance of AgMoO3/Ti-HMS for Direct Epoxidation of Propylene.

In this work, direct gas-phase epoxidation of propylene (DPO) to propylene oxide by molecular oxygen has been studied by using Ag–MoO3 supported on titanium-containing hexagonal mesoporous silica (Ti-HMSn) of different Si/Ti molar ratios. The promotion effect of NaCl on the synthesized catalysts has also been investigated. Among the studied supports, the hexagonal mesoporous silica (HMS) with a Si/Ti ratio of 10 was the most suitable one for production of propylene oxide (PO). The optimal performance of the AgMo/Ti-HMS10 catalyst in DPO exhibited a selectivity to PO of 43.2% with a propylene conversion of 14.1%, at 400 °C, 0.1 MPa, and a space velocity of 12,000 h–1. The catalyst verified good stability over at least 20 h on stream. Only 2.7% PO selectivity with a propylene conversion of 10.1% was achieved over the AgMo/HMS sample. The incorporation of Ti into the HMS frame could optimize the particle size distribution of Ag, producing Ag nanoparticles with an average size of 6.8 nm compared with that of Ag/HMS (24.3 nm). The in situ Raman spectrum of AgMo/Ti-HMS10 heated in a stream of C3H6/He at 400 °C showed new bands at 616, 390, and 210 cm–1, characteristic of the AgxMoyOz intermediate phase. The obtained results suggested that this formed AgMo/Ti-HMS10 phase could most likely be relevant for selective epoxidation of propylene. However, during the reaction of C3H6 with AgMo/HMS, the formation of this intermediate was hardly detected. On the other hand, the hydrogen temperature-programmed reduction measurements indicated improved reducibility of MoO3 in the AgMo/Ti-HMS10 catalyst, which acknowledged the role of Mo6+ in gaining electrons from silver to form positively charged Ag. This could reduce the effective charge of the adsorbed oxygen on silver sites and in turn favor the epoxidation path of propylene rather than the combustion route. Also, during the reaction of C3H6 with NaAgMo/Ti-HMS10 at 400 °C, two new Raman bands were detected at 277 and 350 cm–1, characteristic of Ag2MoO4.

A DFT Study and Microkinetic Simulation in Propylene Oxidation on the “29” CuxO/Cu(111) Surface

The propylene epoxidation reaction in a rich variety of other industrially important catalytic processes was used for synthesis of many value-added products. As an attractive and environmentally friendly process for direct epoxidation of propylene by Cu-based catalysts, the active oxidation states of Cu (Cu0, Cu+, or Cu0/Cu+) in the epoxidation reaction were widely studied. Herein, “29” CuxO/Cu(111) was chosen to act as a precursor to bulk oxidation, as it can be observed in special oxide phases of Cu (Cux+). In our work, the crucial competitive reactions of dehydrogenation versus the epoxidation using the lattice oxygen (Olatt*) and adsorbed molecular oxygen as the oxidants were investigated on “29” CuxO/Cu(111) surface by virtue of the periodic density functional theory computational method. Because of the higher energy barriers of the key steps for the lattice oxygen (Olatt*) as the oxidant, there are two active species in the whole reaction processes of propylene oxidation with O2 as oxidant: the molecular O2* species and the atomic O* species. It was found that the molecular oxygen is difficult to dissociate on the surface directly, while the atomic O* species can be produced via the C3H6* + Olatt* + OO* = C3H5* + Olatt* + OOH* = C3H5Olatt* + O* + OH* mechanism or by the mechanism of C3H6* + OO* = dioxametallacycle = oxametallacycle + O*. It was found that the O* mechanism is preferred for the PO formation as compared to that of O2* on pure “29” CuxO/Cu(111) surface. Promising results in terms of selectivity were achieved for PO formation, which can reach 49% with the apparent activation energy of only 0.62 eV by microkinetic simulation. Therefore, “29” CuxO/Cu(111) surface involving the Cux+ active phase is expected to show an efficient PO selectivity, which is much higher that of Cu+ of Cu2O(111) and similar to that of metallic Cu0. It is hoped that the present study could provide a theoretical guide for the further development of catalysts for propylene epoxidation.

Grafting Ti Sites on Defective Silicalite-1 via TiCl4 Chemical Vapor Deposition for Gas-Phase Epoxidation of Propylene and H2O2 Vapor

The gas-phase epoxidation of propylene and hydrogen peroxide (H2O2) vapor to produce propylene oxide (G-HPPO) is a good way to bypass the methanol solvent problem faced by the liquid-phase HPPO pro...

Insight into Deactivation Reasons for Nanogold Catalysts Used in Gas-Phase Epoxidation of Propylene

Gas-phase epoxidation of propylene over nanogold catalysts has attracted extensive attention in recent years. Among the Au/Ti-based catalysts, Au/titanium silicalite-1 (denoted as Au/TS-1) shows excellent activity of PO generation. However, Au/TS-1 is reported to suffer from rapid deactivation. To prolong its lifetime, the main deactivation reason was investigated. In this work, Au/TS-1 and Au/TS-1 coated with silicalite-1 (denoted as Au/TS-1@S-1) were prepared. Compared to the traditional Au/TS-1, Au/TS-1@S-1 exhibits better catalytic stability during the same reaction time. This is most likely due to the confinement effect of TS-1@S-1 channels on Au clusters. Multiple types of characterization further confirm that the deactivation of Au/TS-1 is mainly caused by the agglomeration of Au clusters. Moreover, higher reaction temperature as well as longer reaction time results in greater Au particles and the gradual catalyst deactivation.

Recent Advances in Selective Photo-Epoxidation of Propylene: A Review

The epoxidation of propylene to produce propylene oxide (PO) has a vital role in the industrial production of several commercial compounds and the synthesis of numerous intermediates, fine chemicals, and pharmaceuticals. However, the current PO production processes pose significant problems regarding the environment and economy. The direct photo-epoxidation of propylene using molecular oxygen (an ideal oxidant with active oxygen of 100 wt %) under light irradiation is a promising technology to produce PO. This process offers numerous advantages, including the use of simple technologies, low-cost methods, and environmental friendliness. Many efforts have focused on the design of new photocatalyst systems, optimizing the conditions for a photocatalytic reaction, and elucidating the mechanisms of photo-epoxidation. This review is expected to serve as a comprehensive background, providing researchers with insight into the recent developments regarding the direct photo-epoxidation of propylene.