Surface-Initiated Gas-Phase Epoxidation of Propylene with Molecular Oxygen by Silica-Supported Molybdenum Oxide: Effects of Addition of C3H8 or NO and Reactor Design

The gas-phase epoxidation of propylene was studied over MoO _x /SiO₂ catalysts in a reaction system with a post-catalytic bed volume. In the reaction of a mixture of propylene and propane with oxygen below 578 K, propylene oxide (PO) was mainly formed from the oxidation of propylene. It was found that the oxidation reaction was very sensitive to the temperature of the post-catalytic space more than the temperature of the catalyst bed, strongly indicating that radical reactions occurring in the post-catalytic bed free space were responsible for the PO formation. The addition of NO increased propylene conversions and PO selectivity at low conversions, confirming that radical reactions were involved in the propylene reactions.     

Direct epoxidation of propylene to propylene oxide on various catalytic systems: A combinatorial micro-reactor study

A combinatorial approach is used to investigate several bimetallic catalytic systems and the promoter effect on these catalysts to develop highly active and selective catalysts for direct epoxidation of propylene to propylene oxide (PO) using molecular oxygen. 2%Cu/5%Ru/c-SiO₂ catalyst yielded the highest performance with high propylene conversion and PO selectivity among the bimetallic catalytic systems including silver, ruthenium, manganese and copper metals. On the other hand, the most effective catalyst and promoter in the epoxidation reaction was determined to be sodium chloride promoted Cu–Ru catalyst supported over SiO₂ with 36% selectivity & 9.6% conversion (3.46% yield) at 300 °C and 0.5 feed gas ratio (propylene/oxygen).     

Direct gas-phase epoxidation of propylene over nanostructured molybdenum oxide film catalysts

Mixed molybdenum oxides are widely used catalysts for propylene oxidation to acrolein or acrylic acid or for methanol oxidation in the gas phase. In the present study, however molybdenum oxides exhibited an epoxidation activity in the gas phase. To achieve this activity, the oxidation state, particle sizes and accessibility of molybdenum oxides are needed to be controlled. Such active molybdenum species can be prepared by PVD deposition of molybdenum nanorods and their subsequent oxidation in a controlled atmosphere. Accordingly, monoclinic MoO₂ particles with a diameter of < 10 nm can be prepared, which exhibit 42% selectivity to propylene oxide (the byproducts of the oxidation being mainly acrolein and acetone). In comparison, molybdenum-containing catalysts prepared by traditional impregnation methods exhibited practically zero epoxidation activity in gas-phase epoxidation of propylene.     

钛硅分子筛TS-1催化环氧丙烷异构反应的机理探究

丙烯氢氧环氧化一步法制备环氧丙烷（PO）相比于传统的PO工业生产方法在经济和环保方面具有不可比拟的优势。Au/TS-1双功能催化剂在该反应中展现出较优的PO性能,针对其中TS-1催化PO开环异构生成副产物进行了研究,结合PO在堵孔TS-1分子筛（TS-1-B）和Au/TS-1-B催化剂上的反应性能和红外表征结果,采用理论计算探究了Ti-Defect位点上丙醛和丙酮的生成路径以及涉及的能量变化。结果显示PO在TS-1上的异构化主要经历碳氧键断裂和氢原子转移重排两个过渡态,以及具有五元环结构的双配位丙氧基物种中间体。相比于丙醛,丙酮由于生成过程中氢原子重排的过渡态能垒较高而具有更低的异构化选择性。所揭示的TS-1上PO吸附及异构化反应机制将为钛基丙烯环氧化催化剂的结构改性以增强PO脱附从而提高PO选择性提供理论依据。     

Mono-atomically dispersed Pd on TiO2 as a catalyst for epoxidation of light olefins at low temperatures in the presence of H2 and O2

Palladium supported at extremely low loading on TiO₂ catalyzed epoxidation of light olefins (except ethylene) in the presence of O₂ and H₂ at ambient temperature. Propylene oxide (PO) was obtained from propylene (1–2% yield; 30–60% selectivity) together with propane. Simple oxidation of H₂ to water was slow, and the ratio of H₂ used for PO formation in the reacted H₂ was 10–25%. A Pd concentration of 0.005–0.01 atom-Pd/nm²–TiO₂ was optimum for epoxidation, and the major product at 0.02 atom-Pd/nm² was acetone. That the distributions for a Pd/TiO₂ catalysts prepared from Pd nitrate and from a Pd-tetraphenylporphyrin complex at the same Pd loading were similar suggests that Pd was supported mono-atomically on TiO₂. We propose that PO formed via reaction between propylene adsorbed on Pd and an active species derived from reaction of H₂ and O₂ on TiO₂ (perhaps ·OOH).     

Gas-phase epoxidation of propylene through radicals generated by silica-supported molybdenum oxide

It was found that silica-supported molybdenum oxide was high effective for the epoxidation of propylene among various silica-supported metal oxides. The post-catalytic bed volume played an important role in its formation. On a MoO_x/SiO₂ with 0.255 mmol/g-SiO₂, a propylene conversion of 17.6% and a PO selectivity of 43.6% were obtained at 5 atm, 573 K and flow rates of C₃H₆/O₂/He = 10/5/10 cm³ min⁻¹. The characterization studies indicated that crystalline MoO₃ nano-particle species was more effective for propylene epoxidation to PO than molecularly dispersed Mo oxide species. The reaction mechanism of propylene epoxidation on MoO_x/SiO₂ catalysts is hypothesized to involve gas-phase radicals generated at relatively low temperature by the dispersed molybdenum oxide species. These radicals participated in homogeneous reactions with molecular oxygen to produce propylene oxide.     

Direct Oxidation of Propylene to Propylene Oxide with Molecular Oxygen: A Review

The direct oxidation of propylene to propylene oxide (PO) using molecular oxygen has many advantages over existing chlorohydrin and hydroperoxide process, which produce side products and require complex purification schemes. Recent advances in liquid-phase and gas-phase catalytic oxidation of propylene in the presence of only molecular oxygen as oxidant and in absence of reducing agents are summarized. Liquid-phase PO processes involving soluble or insoluble Mo, W, or V catalysts have been reported which provide moderate conversions and selectivities, but these likely involve autoxidation by homogeneous chain reactions. Gas-phase PO catalysts have been mostly Ag-, Cu-, or TiO₂-based substances, although other compositions such as Au-, MoO₃-, Bi-based catalysts and photocatalysts have also been suggested as possibilities. The Ag catalysts differ from those used for ethylene oxide production in having high Ag contents and numerous additives. The additives are solid-phase alkali metals, alkaline earth metals, and halogens, with the most common substances being NaCl and CaCO₃. Nitrogen oxides in the form of gas-phase species or nitrates have also been found to be effective in enhancing PO production. Direct epoxidation by surface nitrates is a possibility. Titania catalysts supported on silicates have also been reported. These have higher PO selectivities at high conversion than silver catalysts.     

Gas-phase hydrogen-transfer reduction of acrolein using 2-propanol over MgO/B2O3 and SiO2/AlPO4 catalysts

Two MgO/B2O3 and SiO2/AlPO4 catalysts designated BM50 and PM2, respectively, were used in the gas-phase hydrogen transfer between acrolein and 2-propanol to obtain allyl alcohol and propanal. The acid-base properties and catalytic activity of the two systems were found to be rather different. Thus, the MgO/B2O3 catalyst is more selective towards allyl alcohol than is the SiO2/ AlPO4 catalyst (conversion to the alcohol was 28% with the former and 0% with the latter). This special selectivity is discussed in terms of the different ways in which acrolein can be adsorbed on the catalytic surface as revealed by temperature-programmed desorption profiles and DRIFT spectra for pre-adsorbed acrolein. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective conversion of allyl alcohol to oxygenates and hydrocarbons using ion exchanged zeolite Y

The reaction of allyl alcohol using zeolite Y as catalyst has been investigated and it is shown that it can be converted into a range of products, including hydrocarbons, acrolein and diallyl ether. Control of product selectivity can be achieved by careful selection and manipulation of the charge balancing cation, a series of catalysts can be prepared which, for the conversion of allyl alcohol, lead almost exclusively to the initial formation of either (a) C₂-C₆ hydrocarbons and coke (H-NaY), (b) acrolein (H-CsY), (c) propene (Li-NaY) or (d) diallyl ether (Cs-NaY). The effects of addition of H₂ and H₂O to the reactant are described and discussed with respect to the reaction mechanism and the reaction of potential intermediates (2-propanol and propene oxide) is also described. Mechanisms of formation of the major products are proposed that involve the concerted action of Brønsted acid and basic sites within the zeolite. In particular, since the addition of H₂O does not affect the product distribution, it is considered that the mechanism of hydrocarbon formation does not involve the allyl cation as an intermediate.     

Vanadium, niobium and tantalum modified mesoporous molecular sieves as catalysts for propene epoxidation

SBA-3 mesoporous molecular sieves doped with transition metal ions (Fe, V, Nb and Ta) have been applied for selective oxidation of propene towards propylene oxide in the presence of N₂O as an oxidant. The kind and amount of applied modifiers significantly affected the catalytic activity. V/SBA-3 was found to be the most active among the catalyst under study. In spite of relatively high selectivity towards propylene oxide (reaching up to 23%), the main oxidation product was still propionaldehyde. Surprisingly, CO_x was not formed over V, Nb and Ta modified SBA-3 catalysts. Additional modification of V containing samples (V/SBA-3) with iron complexes resulted in the further increase in the catalysts activity for epoxidation reaction. A PO selectivity of about 20% could be achieved at a propylene conversion of 17% over mixed Fe/V/SBA-3 catalytic system.     

Selective oxidation of propylene by O2 with visible light in a zeolite

Propylene and O₂ loaded into zeolite BaY were found to react upon irradiation with green or blue light in a single photon process. In situ monitoring by FT-infrared spectroscopy showed that allyl hydroperoxide is the predominant product at –100°C. At room temperature, acrolein, propylene oxide, and allyl hydroperoxide were observed in comparable amounts. The aldehyde and epoxide are shown to emerge from secondary thermal chemistry of the allyl hydroperoxide photoproduct. The selectivity in terms of the hydroperoxide photoproduct is very high (98% at room temperature) even at high propylene conversion. Diffuse reflectance spectra show that access to the mild oxidation path is made possible by a zeolite-induced shift of the propylene O₂ charge-transfer absorption from the UV into the visible region.     

Effect of support and preparation on silver-based direct propylene epoxidation catalyst

The effect of catalyst support and preparation method on silver-based catalysts for the direct vapor phase epoxidation of propylene to propylene oxide (PO) was studied. The shape of the CaCO₃ support is critical for optimum activity and selectivity of the catalyst. Among the CaCO₃ materials investigated as supports, the scalenohedral shape gives the best performance. A simplified method of catalyst preparation (referred to as “slurry method”) was developed. With this method, it is not necessary to dissolve Ag₂O into a solution as in the conventional incipient wetness method for preparing silver-based catalysts for propylene and ethylene epoxidation. The key to the slurry method is the addition of a small amount of an effective organic compound, such as ethanolamine, during the formation of the slurry.     

Epoxidation of Propylene on a [Ag<Subscript>14</Subscript>O<Subscript>9</Subscript>] Cluster Representing Ag<Subscript>2</Subscript>O (001) Surface: A Density Functional Theory Study

Abstract  

Density functional theory calculations were employed to study partial oxidation of propylene on a [Ag₁₄O₉] cluster representing Ag₂O (001) surface for which positive effect for ethylene oxide formation has been reported in our earlier work at the same level of theory (Fellah et al., Catal Lett 141:762, 2011). Propylene oxide (PO), propanal, acetone and П-allyl radical formation reaction mechanisms were investigated. Π-allyl formation path and two propylene adsorption paths resulting in PO formation are competing reactions on silver oxide (001) surface because of their comparable activation barriers (9, 8 and 9 kcal/mol, respectively) while Π-allyl formation path is generally a more favorable path on Ag (111) surface as reported in previous theoretical literature. SO₂ adsorption calculations indicate that silver oxide has lower Lewis basicity relative to oxygen atom adsorbed on silver. Calculations also showed that surface oxygen atom of Ag₂O (001) has a higher spin density compared to that of oxygen atom adsorbed on Ag (111), which indicates that oxygen atom on Ag₂O (001) cluster has a more radical character.     

Modification of Ag-MoO3/ZrO2Catalyst with Metallic Chloride for Propylene Epoxidation by Molecular Oxygen

The modification of the Ag-MoO₃/ZrO₂catalyst with metallic chloride was studied for a gas-phase epoxidation of propylene by molecular oxygen. As a modifier, the presence of NaCl, CsCl or CaCl₂can effectively regulate an electronic property of the Ag-MoO₃/ZrO₂catalyst and improve obviously its catalytic epoxidation performance. When NaCl (1000 ppm), CsCl (1500 ppm) or CaCl₂(1000 ppm) was used to modify the 20%Ag-4%MoO₃/ZrO₂catalyst, the selectivity to propylene oxide (PO) increased to 63. 5%, 66. 7% and 62. 9%, from 47. 9%, respectively, and the corresponding O₂conversion was 7. 3%, 7. 7% and 8. 1%, respectively. The studies of XPS indicates that the electron-interaction between the active component of catalyst and metallic chloride has occurred obviously, and the presence of chloride can enhance the electron-deficient property of Ag to improve the electrophilic property of adsorbed oxygen and inhibit an isomerization and deep oxidation of PO to improve the selectivity to PO.     

Gas-phase conversion of glycerol over mixed metal oxide catalysts

A series of aluminophosphates (APO) catalysts with Ce, Cu, Cr, Fe, Mn, Mo, V, and W oxide loading at a constant ratio M: Al = 1: 10 and PO₄: Al = 1: 12 were prepared and characterized by N₂ physisorption, XRD and NH₃-TPD. Gas-phase dehydration of glycerol to produce acrolein and acetol was investigated at 280°C in presence of water. Conversion and product distribution depended on the intrinsic acidity and the type of transition metal oxide. Best selectivity to acrolein (52–58%) was obtained for W- und Mo-APO catalysts. Cr-, Mn- and W- oxide containing catalysts enhanced the formation of phenol, acetaldehyde and CO _x . The catalysts containing V- and Fe-oxide promoted the formation of allyl alcohol. All catalysts showed long term stability, which can be attributed to the redox ability of the metal oxides that enhances the removal of coke deposits. The investigated catalyst a specially W-APO and Mo-APO can be recommended for further controlled trials on a pilot plant for selective conversion of water solution of glycerol to acrolein and/or acetol.     

Regeneration of Lamina TS-1 Catalyst in the Epoxidation of Propylene with Hydrogen Peroxide

The deactivation and regeneration of the lamina titanium silicalite (TS-1) catalyst for the epoxidation of propylene with dilute H₂O₂was investigated in a fixed-bed reactor. In the scale-up experiment, the dosage of the lamina TS-1 catalyst is 2. 5 kg, after 1000 h reaction the catalyst still exhibits good performance and further increases the reaction time, the conversion of H₂O₂begins to decrease. TG and BET analyses of the deactivated catalysts show that the main species occluded within the zeolite pore are propylene oxide oligomers, and these species occupying the active Ti site and blocking the pores of the lamina TS-1 are the main reason for the deactivation of catalyst. The deactivated catalyst can be regenerated by different regeneration methods. The activity of deactivated catalysts regenerated by dilute H₂O₂or heat treatment by using air or nitrogen as a calcination media can be fully recovered, but a decline in propylene oxide (PO) selectivity of the regenerated catalyst has been observed during the first hours of reaction. However, water vapor treatment of the deactivated catalyst can improve the PO selectivity with the same activity as that of the fresh lamina TS-1 catalyst.     

Epoxidation of propylene in the gas phase

The gas-phase epoxidation of propylene using N₂O, air and air-ammonia mixture as an oxidants was studied. Propylene can be epoxidized by nitrous oxide with a yield as high as 13.3% over silica supported iron oxide catalysts modified by amines. The iron oxide dispersion, the acidity of the support and the nitrogen-containing modifiers are the key factors determining the catalytic performance. We suggest a reaction pathway involving two concurrent mechanisms: the radical oxidation of propylene to acroleine, hexanediene, etc., and a non-radical oxidation leading to epoxide. Propylene is epoxidized with air over silica-supported iron oxide catalysts at a conversion of about 0.2%. Using air as an oxidizing agent, the presence of gaseous ammonia improves the propylene conversion by 10-fold preserving the considerable selectivity (up to 60%). This observation suggests a reaction mechanism involving the oxidation of ammonia to nitrous oxide in the first step, which subsequently produces active oxygen species, which selectively oxidize propylene to propylene oxide (PO).     

A study of the abatement of VOC over V2O5–WO3–TiO2 and alternative SCR catalysts

The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO₂ commercial SCR catalysts differing in the V content and over Mn–TiO₂ alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO₂. The sample higher in V is more active. Mn–TiO₂ is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO₂ as the only detectable product.

V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO₂ can be proposed for DeNO_x and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.     

The study of the uncalcined Au catalyst and inorganic salts on direct gas-phase epoxidation of propylene

This paper presents the results of the propylene epoxidation in co-presence of hydrogen and oxygen on both dried and calcined Au catalysts, which were prepared by liquid grafting method using Me₂Au(acac) as a precursor and trimethylsilylated Ti-MCM-48 as a support. The calcined catalyst shows PO activity at temperatures much higher and wider than the dried one. Addition of inorganic salts greatly modified the catalytic behaviors. CsNO₃ could be a promising promoter whereas KBr led to the production of propionaldehyde instead of propylene oxide.     

Struktur-Aktivitäts/Selektivitäts-Beziehungen von Oxidationskatalysatoren

Structure-Activity/Selectivity Correlation of Oxidation Catalysts Various reactive forms of oxygen are active in the selective oxygen functionalization of olefins and aromatic compounds. These forms are different on different crystal faces of transition metal oxides as shown by their different cation-oxygen bond lengths. Therefore, face specifity of transition metal oxides as catalysts can be expected for the selective oxidation of hydrocarbons. Furthermore, the reactivity of the framework oxygen in the transition metal oxides for selective oxygen functionalization of hydrocarbons is dependent on the nature of the catalytic cycle (one- or two-electron-processes). Therefore, structure sensitivity of oxidation catalysts on the selectivity of transition metal oxides as oxidation catalysts is possible. Starting from this concept the following phenomena of the activity/selectivity relations are discussed for the most important industrial oxidation catalysts:

• —ensemble effects on the selectivity of supported Ag catalysts for the oxidation of ethylene to ethylenoxide.
• —face specifity of multicomponent Bi/Mo-oxide catalysts in the selective oxidation of propylene to acrolein and in the ammoxidation of propylene.
• —structure sensitivity of V₂O₅-containing catalysts in the oxidation of aromatic compounds.