Propylene Oxide Formation Research Articles

Chemical and radiative transfer modeling of propylene oxide

Context.The recent identification of the first complex chiral molecule, propylene oxide (PrO), in space opens up a new window to further study the origin of homochirality on the Earth. There are some recent studies to explain the formation of PrO however additional studies on the formation of this species are needed for better understanding.Aims.We seek to prepare a complete reaction network to study the formation of propylene oxide in the astrophysically relevant conditions. Based on our results, a detailed radiative transfer modeling has been carried out to propose some more transitions that would potentially be targeted in the millimeter wave domain.Methods.A gas-grain chemical network was used to explain the observed abundance of PrO in a cold shell surrounding the high-mass star-forming region of Sgr B2. Quantum chemical calculations were employed to study various reaction parameters and to compute multiple vibrational frequencies of PrO.Results.To model the formation of PrO in the observed region, we considered a dark cloud model. Additionally, we used a model to check the feasibility of forming PrO in the hot core region. Some potential transitions in the millimeter wave domain are predicted that could be useful for the future astronomical detection. We used radiative transfer modeling to extract the physical condition that might be useful to know the properties of the source in detail. Moreover, we provided vibrational transitions of PrO, which could be very useful for the future detection of PrO by the upcomingJames WebbSpace Telescope.

Gas-Phase Epoxidation of Propylene to Propylene Oxide on a Supported Catalyst Modified with Various Dopants

In the present study, the production of propylene oxide (PO) from propylene via gas-phase epoxidation was investigated using various catalysts. Although Ag is known to be a highly active catalyst for the epoxidation of ethylene, it was not active in the present reaction. Both Al and Ti showed high levels of activity, however, which resulted in confusion. The present study was conducted to solve such confusion. Although the employment of MCM-41 modified with Ti and/or Al was reported as an active catalyst for epoxidation, the combination resulted in the formation of PO at a less than 0.1% yield. Since this research revealed that the acidic catalyst seemed favorable for the formation of PO, versions of ZSM-5 that were both undoped and doped with Na, Ti, and Ag were used as catalysts. In these cases, small improvements of 0.67% and 0.57% were achieved in the PO yield on H-ZSM-5 and Ti-ZSM-5, respectively. Based on the results of the Ti-dopant and acidic catalysts, Ag metal doped on carbonate species with a smaller surface area was used as a catalyst. As reported, Ag‒Na/CaCO3 showed a greater yield of PO at 1.29%. Furthermore, the use of SrCO3 for CaCO3 resulted in a further improvement in the PO yield to 2.17%. An experiment using CO2 and NH3 pulse together with SEM and TEM examinations for Ag‒Na/CaCO3 revealed that the greatest activity was the result of the greater particle size of metallic Ag rather than the acid‒base properties of the catalysts.

Propylene Oxide Formation on a Silica Surface with Peroxo Defects: Implications in Astrochemistry.

The formation of the chiral molecule propylene oxide (CH3CHCH2O) recently detected in the interstellar medium (ISM) is proposed to take place on an amorphous silicate grain surface where peroxo defects are present. A computational analysis conducted at the DFT and MP2-F12 levels of theory on a neat amorphous silica model supports such a hypothesis resulting in (a) strong thermodynamic driving forces and low activation energies allowing the synthesis of CH3CHCH2O at low temperatures, (b) chemical defects on silica surfaces promoting heterogeneous catalysis of the increasing molecular complexity found in interstellar and circumstellar medium, and (c) chemical defects that have implications on understanding how processing phases modify the nature of the reactive groups on a silica surface affecting the surface catalytic activity.

The construction of Mo6−δO3−x-supported catalyst for low-temperature propylene gas-phase epoxidation by Cu modification

Selective epoxidation of propylene by molecular oxygen under mild conditions remains a great challenge to date. Here, we successfully designed and constructed molybdenum oxide in an electron-rich state by introducing the transition metal copper into MoO3–Bi2SiO5/SiO2 through a simple co-impregnation method. The new Mo species has the structure Mo6−δO3−x, and greatly improved propylene epoxidation performance at low temperatures. Propylene oxide formation rate increased from 106 to 336 g PO kgcat−1 h−1. X-ray photoelectron spectrometry (XPS) and UV–visible diffuse reflection (UV–vis DRS) characterizations suggest that the bridging oxygen bond between molybdenum and copper produces an oxygen vacancy, which facilitates the formation of the electron-rich state of molybdenum with lower 3d electronic binding energy. In addition, the incorporation of Cu effectively inhibits the formation of Bi2Mo3O12 and improves the dispersion of Mo species. Furthermore, kinetic studies show that Cu-modified catalysts maintain lower apparent activation energy for propylene epoxidation, which obviously reduced the selectivity of acrolein and then conspicuously improved the selectivity of propylene oxide. Our results not only offer a new solution for low temperature propylene epoxidation, but also provide a new strategy for the design of other types of low-temperature epoxidation catalysts.

Theoretical Study of Propylene Epoxidation over Cu2O(111) Surface: Activity of O2–, O–, and O2– Species

Propylene epoxidation catalyzed by cuprous oxide in the presence of oxygen species is important in both technology and scientific fields, and the active species—lattice oxygen (O2–), adsorbed atomic oxygen (O–), or adsorbed molecular oxygen (O2–)—plays a significant role in the catalytic reaction. In the present work, the mechanism of propylene epoxidation and dehydrogenation on a Cu2O(111) facet with different oxygen species has been studied through density functional theory (DFT) calculations with a Hubbard U correction in detail. The whole reaction processes of propylene oxidation includes two different routes: allylic hydrogen stripping (AHS) reactions and propylene epoxidation reactions. Acrolein can be generated by two H-stripping reactions in the AHS path, and propylene oxide (PO) is formed through the oxametallacycle propylene (OMP) intermediate. The calculated results show that the adsorbed atomic oxygen (O–) is the most active oxygen species for the selective oxidation of propylene because it has the strongest basicity among these three oxygen species, whereas the adsorbed molecular oxygen (O2–) has the highest selectivity for the PO formation among these three different oxygen species because of its relatively low basic properties and moderate oxidation compared to those of atomic oxygen (too active oxidation) or lattice oxygen (less oxidation due to the closed-shell nature of the oxide system). Moreover, a microkinetic simulation was used to confirm the above DFT calculation results. The aim of the present study is to give a guide in choosing the most efficient active oxygen species for PO formation, which should be the one with the moderate oxidant.

Mechanistic insights into the direct propylene epoxidation using Au nanoparticles dispersed on TiO2/SiO2

Direct propylene epoxidation is an important gas-phase reaction for the future industrial production of propylene oxide (PO). However, the mechanistic understanding of this reaction system is still elusive, including the role of side reactions. Herein, kinetic Monte Carlo (KMC) simulations are applied to explore the underlying mechanistic aspects of propylene epoxidation over Au/TiO2/SiO2, and consistent results are obtained with respect to experimental benchmarks. The present study systematically probes the reaction mechanism, which involves acrolein formation on Au nanoparticles as well as the formation of PO and several byproducts at the Au/Ti dual interface sites, and it highlights the synergistic effect of interface sites in heterogeneous catalysis. The origin of the negative effect of rising reaction temperature on PO selectivity has been clarified, as it is correlated with decreasing atomic oxygen coverage on the Au surface. The varying feed concentration of H2/O2/C3H6 leads to different coverages on the catalyst surface, emphasizing the importance of an optimum H2/C3H6 inlet concentration ratio for enhancing PO selectivity and hydrogen utilization efficiency. Based on our model results, we propose that supports with acidic groups may promote hydroperoxy species transferring to the adjacent Ti sites, thereby decreasing its decomposition to atomic oxygen on the Au sites, which leads to byproduct formation.

TS-1 supported highly dispersed sub-5 nm gold nanoparticles toward direct propylene epoxidation using H2 and O2

We report a simple and efficient method for the preparation of highly dispersed Au nanoparticles (< 5 nm) on TS-1 substrate. The synthesis relies on the use of NaBH4 as a reductant for rapid Au atom generation, as well as PVA as a capping agent confining the particle size and dispersion. The samples were characterized by N2 physisorption, inductively coupled plasma mass spectrometry, power X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, CO pulse chemisorption and thermogravimetric analysis. The size of Au particles can be controlled in the range of 3–5 nm. The supported catalyst shows both good activity and selectivity for propylene oxide (PO) generation from direct propylene epoxidation. An optimal performance with PO formation rate of 102.94 gPO h−1 kg−1cat and selectivity of 84.83% is achieved over 2.0 wt% Au/TS-1 catalyst, which is prepared by controlling PVA/Au3+ mass ratio of 1.5/1 and NaBH4/Au3+ mole ratio of 5/1. After 50 h test at 200 °C, no significant decrement of both catalytic activity and PO selectivity can be observed, indicating the excellent thermally stability of the catalyst. Furthermore, a possible reaction mechanism is described on basis of the previous researches and our experimental results.

A computational study of the catalytic aerobic epoxidation of propylene over the coordinatively unsaturated metal-organic framework Fe3(btc)2: formation of propylene oxide and competing reactions.

The aerobic epoxidation of propylene over the metal-organic framework Fe3(btc)2 (btc = 1,3,5-benzentricarboxylate) as catalyst has been investigated by means of density functional calculations. The mechanisms of the reaction towards propylene oxide, carbonylic products (acetone and propanal) and a pi-allyl radical were investigated to assess the efficiency of Fe3(btc)2 for the selective formation of propylene oxide. Propylene oxide and carbonylic products are formed on Fe3(btc)2 by proceeding via propyleneoxy intermediates in the first step. Subsequently, the intermediates can then either be transformed to propylene oxide by way of ring closure of the intermediate or to the carbonylic compounds of propanal and acetone via 1,2-hydride shift. The results show that the formation of propylene oxide is favored over the formation of carbonylic products mainly due to the activation barriers being 2-3 times smaller. The activation barriers for the formation of the propyleneoxy intermediates on the Fe3(btc)2 catalyst for the first and second reaction cycle are also lower than the barriers obtained for the formation of the pi-allyl radical that acts as the precursor to combustion products. On the basis of these computational results, we therefore expect a high catalytic selectivity of the Fe3(btc)2 catalyst with respect to the formation of propylene oxide. We also compared the catalytic activities of Fe3(btc)2 and Cu3(btc)2. The activation energy of the rate-determining step is almost 2 times lower for Fe3(btc)2 than that for Cu3(btc)2, due to a larger charge transfer from the catalytic site to the O2 molecule in the case of Fe3(btc)2.

A density functional theory study of propylene epoxidation mechanism on Ag2O(001) surface.

Propylene oxide (PO) is one of the 50 most produced chemicals according to the production volume. Environmental and economic drawbacks of conventional PO production processes necessitate new production methods. Among the new production alternatives, direct epoxidation of propylene to propylene oxide by molecular oxygen is a highly desired method and seen as the holy grail of propylene epoxidation studies. In this study, the propylene epoxidation mechanism on an Ag2O(001) surface is investigated computationally by means of density functional theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP). A perfect Ag2O(001) surface and a surface with one O vacancy are utilized for this purpose. It is found that propylene oxide can be directly formed on an Ag2O(001) surface whether there is an oxygen vacancy or not. The rate controlling step is PO desorption from both surfaces. PO isomers, i.e. acetone and propanal, can also be formed on these surfaces. However, activation barriers do exist for these molecules. Direct allyl formation on the Ag2O(001) surface is found to be unfavorable unlike what is proposed in the literature. On the other hand, it is observed that an allyl radical can be formed either via an oxametallocycle path or after the formation of propylene oxide. In fact, the discovered allyl radical formation pathway from propylene oxide is found as the most probable successive reaction pathway because of the high desorption barrier of PO.

Phosphorus modified MoO3–Bi2SiO5/SiO2 catalyst for gas-phase epoxidation of propylene by molecular oxygen

The phosphorus (P) modified MoO3–Bi2SiO5/SiO2 catalyst was prepared by a simple co-impregnation method and investigated in the epoxidation of propylene by molecular oxygen. The catalyst was characterized by X-ray diffraction (XRD), N2 adsorption–desorption analysis, NH3-temperature-programmed desorption (NH3-TPD), transmission electron microscopy, and Raman spectroscopy. It was found that the P-modified MoO3–Bi2SiO5/SiO2 catalyst with a P/Mo molar ratio of 0.5 exhibits the best catalytic performance for epoxidation of propylene by O2, the TOFs for propylene oxide (PO) formation was four times higher than that of the unmodified one at 633 K. The modification by P could promote the dispersion of MoO3 nanoparticles and increase the number of weak and moderate acid sites with respect to the phosphorus-free MoO3–Bi2SiO5/SiO2 catalyst, which were beneficial to the formation of PO. Moreover, the introduction of P also could protect the mesoporous structure by inhibiting the formation of Bi2Mo3O12, which was beneficial to the dispersion of active species. We suppose that the phosphorus, bismuth and molybdenum species of P-modified MoO3–Bi2SiO5/SiO2 catalyst play important roles for propylene epoxidation by molecular oxygen.

Direct Propylene Epoxidation over RuO2–CuO–NaCl–TeO2–MnOx/SiO2 Catalysts: Optimized Operating Conditions and Catalyst Characterization

Multimetallic catalysts of RuO2–CuO–NaCl–TeO2–MnOx supported on SiO2 were investigated for the direct-gas-phase epoxidation of propylene to propylene oxide (PO) using molecular oxygen under atmospheric pressure. Four operating variables (reactor temperature, O2 to propylene feed gas volume ratio, reactants to carrier gas volume ratio, and total feed gas flow rate) were studied using the Box–Behnken design, which enabled the achievement of a relatively high PO formation rate at 1507 gPO h–1 kgcat–1 with PO selectivity of 25.3% and propylene conversion of 5.9% at 1 h of time on stream. The optimized conditions were a reactor temperature of 297 °C, a O2/C3H6 ratio of 11.95, a (O2 + C3H6)/He ratio of 0.24, and a total feed gas flow rate of 36 cm3 min–1. The physical and chemical properties were characterized using various techniques that revealed that the crystallinity of RuO2 and CuO, the close proximity of RuO2 and CuO, and the surface’s acidity are crucial for PO production. NaCl plays a key role in reducing CO2 combustion. TeO2 and MnOx enhanced the active sites that facilitate PO production.

Direct synthesis of propylene oxide using hydrogen peroxide in a membrane reactor

The epoxidation of propylene to propylene oxide (PO) with hydrogen peroxide was carried out in a titanium silicalite-1 (TS-1) catalytic membrane reactor. The TS-1 catalytic membrane with a thickness of 4–5 μm was fabricated on a tubular porous α-Al2O3 substrate. The structure and morphology of the membranes were characterized by scanning electron microscope, X-ray diffractometer, FT-IR, and a UV–vis spectrometer. Reaction conditions including temperature, pressure and H2O2 concentration were investigated for their effects on H2O2 conversion, PO selectivity and PO formation rate. The PO formation rate on the TS-1 catalytic membrane is more than twice as high as that on the TS-1 powder catalyst. Moreover, the regeneration tests of the TS-1 membrane reactor showed that the catalytic membrane kept relatively stable H2O2 conversion of and PO selectivity for eight reactions.

Silylation enhances the performance of Au/Ti–SiO2 catalysts in direct epoxidation of propene using H2 and O2

The effect of silylation on a series of Au/Ti-SiO2 catalysts for the direct epoxidation of propylene in the presence of H2 and O2 was studied. It was found that silylation significantly improved catalyst performance: propylene conversion, propylene oxide (PO) selectivity, and H2 efficiency increased. The extent of improvement depended on the Au and Ti content of the catalysts. The catalyst showing the best activity (Au(0.1)/Ti(1)–SiO2) exhibited an average PO formation rate of 121gPOkgcat−1h−1 and a PO selectivity of 92% at 473K, while the catalyst having the maximum Au and Ti loading (Au(1)/Ti(5)–SiO2) showed the most significant improvement in performance with a 78% increase in the rate of PO formation upon silylation. The catalysts were characterized by contact angle measurements, FTIR, TGA, TEM, ICP-OES and these observations were used to elucidate the key factors governing the enhanced catalytic performance upon silylation. It was found that the silylated catalyst exhibited superior performance due to increased hydrophobicity, which aids product desorption, a decrease in acidic sites that are responsible for side-product formation, and a possible redistribution of the Au particles.

Direct epoxidation of propylene to propylene oxide over RuO2-CuO-NaCl-TeO2-MnOx/SiO2 catalysts

Multi-metallic RuO2-CuO-NaCl-TeO2-MnOx/SiO2 catalysts were investigated for the epoxidation of propylene to propylene oxide (PO) using O2 under atmospheric pressure. The metallic weight ratio and the total metal loading on SiO2 support were optimized at Ru/Cu/Na/Te/Mn=7.14/3.57/1.79/0.175/1.00 and 25.0wt%, respectively. RuO2 and CuO were determined to exhibit a critical bifunctional role for PO synthesis. These active sites were assisted by MnOx and TeO2 to increase PO formation rate and by NaCl to reduce CO2 formation. With using the Box-Behnken design, a maximized PO formation rate of 1258gPOh−1kgcat−1 was obtained by varying O2/propylene feed ratio and reaction temperature.

Direct synthesis of propylene oxide in the liquid phase under mild conditions

Here, we study the direct synthesis of propylene oxide (PO) on Pd-based catalysts operating under mild conditions (40°C, 5.5bar) and in a continuous flow microreactor. We show that the PO yield can be improved by a factor of two, with respect to the values present in literature, by using a Pd–Pt/TS-1 catalyst in excess of oxygen. Moreover, we compare the PO reaction with the hydrogen peroxide (H2O2) synthesis, being H2O2 (or OOH species) the intermediate and rate limiting step of the direct PO formation. We found that the optimal conditions for the PO synthesis are not advantageous for the H2O2 productivity. In this respect, the presence of Pt, which improves the PO selectivity by lowering the hydrogenation of propylene, negatively affects the H2O2 productivity due to an acceleration of its side reactions. This effect is more pronounced for the Pd–Au/TS-1 catalyst, which shows a high performance for H2O2 production. However, the PO formation remains relatively poor due to a very fast hydrogenation of propylene to propane. We conclude that the optimization of the H2O2 synthesis is not sufficient to improve the direct PO formation. Indeed, the hydrogenation of propylene needs also to be considered.

Theoretical mechanistic insights into propylene epoxidation on Au-based catalysts: Surface O versus OOH as oxidizing agents

Propylene oxide (PO) is an important bulk chemical used for synthesis of many value-added products. Much effort has been devoted to finding a simple and “green” process for PO production. Promising results in terms of activity and selectivity have been achieved for propylene oxidation on supported gold catalysts with a mixture of O2 and H2 or with O2 and H2O. In this work a detailed transformation network of competitive reaction pathways following the initial steps of oxidation has been studied theoretically using density functional theory (DFT). The results of calculations question some of the earlier assumptions regarding the mechanism of PO formation. Surface hydroperoxo species (OOH) formed in situ are shown to be responsible for the high selectivity of propylene epoxidation with O2 on gold-based catalysts using hydrogen or water as co-reactants.

Effects of structures of molybdenum catalysts on selectivity in gas-phase propylene oxidation

Molybdenum-based catalysts for the gas-phase oxidation of propylene with air were investigated. Various types of silica-supported molybdenum oxide and molybdenum-bismuth mixed oxide catalysts were prepared from inorganic and organometallic molybdenum precursors using wet impregnation and physical vapor deposition methods. The epoxidation activities of the prepared catalysts showed direct correlations with their nanostructures, which were identified using transmission electron microscopy. The appearance of a partly or fully crystalline molybdenum oxide phase, which interacted poorly with the silica support, decreased the selectivity for propylene oxide formation to below 10%; non-crystalline octahedrally coordinated molybdenum species anchored on the support gave propylene oxide formations greater than 55%, with 11% propylene conversion. Electrochemical characterization of molybdenum oxides with various morphologies showed the importance of structural defects. Direct promotion by bismuth of the epoxidation reactivities over molybdenum oxides is disputed.

Synergetic photo-epoxidation of propylene over V[sbnd]Ti/MCM-41 mesoporous photocatalysts

The photo-epoxidation of propylene has gained preference among many alternative processes because the current processes for propylene oxide (PO) production are neither economical nor environmentally sustainable. We synthesized various single and binary metal oxides (Ti and V) constructed within the MCM-41 framework and demonstrated their favorable efficiency for the photo-epoxidation of propylene. The results indicated that highly dispersed tetrahedral coordination states played a role in photo-epoxidation. The maximum PO formation rate was 634±17 (μmolg−1h−1) on V0.2Ti0.3/MCM-41 under UV-light irradiation of 0.3mWcm−2 at 323K. The Delplot method revealed two pathways to produce PO. One is indirectly from propionaldehyde via either V or Ti oxides, and the other is directly from a C3H6 reactant via Ti oxides. This paper clearly illustrates how V and Ti oxides affect the photo-epoxidation pathways. Furthermore, a possible photo-epoxidation mechanism via the metal oxide active site is proposed to elucidate the synergetic photoactivity.

Epoxidation of propylene to propylene oxide with molecular oxygen over Sb2O3–CuO–NaCl/SiO2 catalysts

Co-impregnated Sb2O3–CuO–NaCl/SiO2 catalysts were investigated for the direct gas-phase synthesis of propylene oxide (PO) from propylene and molecular oxygen at atmospheric pressure. The trimetallic system was optimized for PO formation by varying the Sb:Cu:Na ratios, total metal loadings and calcination temperatures. At a reaction temperature of 250°C and propylene/oxygen molar ratio=1:4, the highest performing catalyst achieved 40–45% PO selectivity with 0.4–0.7% propylene conversion without a co-feeding gas. The coexistence of Sb2O3 and CuO crystals was determined to play a vital role in propylene epoxidation, while NaCl helped to suppress combustion.

Enhanced catalytic performance in the gas-phase epoxidation of propylene over Ti-modified MoO3–Bi2SiO5/SiO2 catalysts

It was found that Ti-modified MoO3–Bi2SiO5/SiO2 exhibited excellent catalytic performance for the epoxidation of propylene by molecular oxygen. The effect of Ti on the performance of the catalysts was investigated, and the results showed that the high surface area, greater pore size, and moderate acidity of the Ti-modified catalyst were favorable to the selective formation of propylene oxide (PO). Interactions between MoO3, bismuth oxide clusters, and Ti species were proposed to favor PO formation. The presence of Ti not only enhanced the dispersion of MoO3 nanoparticles but also inhibited the reactivity of the lattice oxygen and reduced the strong acidity of the catalyst, contributing to the increase in C3H6 conversion and PO selectivity, respectively. Kinetic studies indicated that a high space velocity or low reaction temperature was beneficial for improving the selectivity to PO by decreasing the deep oxidation of PO.