Propylene Oxide Formation Research Articles

Cluster reactivity experiments: employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions.

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole-octopole-quadrupole (Q-O-Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO(3)(+), W(2)O(6)(+), and W(3)O(9)(+), exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C(3)H(6)), suggesting the formation of propylene oxide (C(3)H(6)O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO(2), a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W-O) with elongated metal-oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.

Copper-based efficient catalysts for propylene epoxidation by molecular oxygen

Among a series of SBA-15-supported transition metal oxides with and without modification, the CuO x /SBA-15 after K + modification exhibited the best catalytic performance for the epoxidation of propylene by molecular oxygen. Potassium was the best modifier among various alkali and alkaline earth metal ions examined, and potassium acetate was a superior precursor of K + for propylene oxide formation. The highest propylene oxide selectivity was obtained over a catalyst with copper content of 1 wt.% and K/Cu molar ratio of 0.7. Kinetic studies reveal that the allylic oxidation mainly proceeds over the CuO x /SBA-15 providing acrolein as the main partial oxidation product, and the K + modification switches the main reaction route from allylic oxidation to epoxidation. The characterizations suggest that copper species with content of ≤5 wt.% are located in the mesoporous channels of SBA-15 existing mainly as CuO x clusters and Cu 2+ ions, and there exists an interaction between K + and the copper species. This interaction is proposed to play pivotal roles in epoxidation of propylene. As compared with other reported Cu-based catalysts for propylene epoxidation, the present catalyst possesses several distinct features.

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.

Iron-containing heterogeneous catalysts for partial oxidation of methane and epoxidation of propylene

Mesoporous molecular sieves containing iron sites with different dispersions and environments were studied for the partial oxidation of methane and the epoxidation of propylene. The tetrahedral Fe(III) sites incorporated inside the framework of SBA-15 showed higher HCHO selectivity under a similar CH 4 conversion than the oligomeric FeO x clusters located in the mesopores in the partial oxidation of CH 4 by O 2. HCHO selectivity of the FeO x clusters in SBA-15 could be improved by the modification with either an acidic group (e.g., phosphate) or an alkali metal ion, which increased the dispersion or changed the local coordination environment of iron species. The catalyst containing FePO 4 clusters formed by the modification with phosphate exhibited the best catalytic performances for HCHO formation. This catalyst also showed uniquely high selectivities to HCHO and CH 3OH as N 2O was used as the oxidant for the partial oxidation of CH 4. For the epoxidation of propylene to propylene oxide (PO), the modification of the FeO x /SBA-15 with an alkali metal salt typified by KCl was effective. Among many SBA-15-supported transition metal oxides modified by KCl, the KCl-FeO x /SBA-15 showed the best catalytic performances for the epoxidation of propylene by N 2O. The comparative studies using N 2O and O 2 as the oxidant revealed that N 2O was necessary for PO formation over the KCl-FeO x /SBA-15 catalyst.

Copper-catalyzed propylene epoxidation by molecular oxygen: Superior catalytic performances of halogen-free K +-modified CuO x/SBA-15

A halogen-free K +-modified CuO x /SBA-15 exhibits significantly better catalytic performance than other Cu-based catalysts reported so far for the epoxidation of propylene by molecular oxygen. Moderate selectivity (ca. 15–50%) to propylene oxide (PO) can be sustained as propylene conversion increases from ca. 1 to 12%, and a PO formation rate of 2.1 mmol g −1 h −1 has been obtained over this catalyst. The current catalyst does not require prereduction, and the PO selectivity can be kept at a high O 2 partial pressure or a highly O 2-rich atmosphere.

Propylene Oxide Synthesis and Selective Oxidation over Supported Metal Oxide Photocatalysts with Molecular Oxygen

Abstract Recent progress of photocatalytic epoxidation of propylene with molecular oxygen is reviewed. Highly dispersed metal oxide photocatalysts, such as silica-supported V2O5, Nb2O5, and TiO2 with low loading, promote steady-state propylene oxide formation with high selectivity (>40%). The isolated tetrahedrally-coordinated species in the ligand-to-metal charge-transferred state is the photocatalytic active site. The photo-induced hole center localized on the lattice oxygen plays a significant role for the photocatalytic epoxidation. In the case of isolated TiO4 tetrahedra on silica, an electrophilic oxygen species is generated by the reaction between the hole center and molecular oxygen. The addition of alkali ions to surface VO4 tetrahedra improves the photocatalytic epoxidation ability of V2O5/SiO2 with low loading. The key step in the selective photocatalytic oxidation over isolated VO4 tetrahedra is the electron transfer from a hydrocarbon to the photo-induced hole center on the lattice oxygen, which is inserted in the photogenerated products.

Iron-catalyzed propylene epoxidation by nitrous oxide: Toward understanding the nature of active iron sites with modified Fe-MFI and Fe-MCM-41 catalysts

Alkali metal salt (KCl)-modified Fe-MFI and Fe-MCM-41 containing iron species in different locations have been studied for the epoxidation of propylene with nitrous oxide. In the allylic oxidation of propylene without modification, the samples with iron species in the framework positions showed slightly higher activity for the formation of acrolein and allyl alcohol. After modification with KCl, propylene oxide (PO) became the main product. Significant differences in PO formation activity were observed between the samples containing iron in different locations. For the Fe-MFI and the Fe-MCM-41 prepared by the direct hydrothermal (DHT) method, in which iron was located mainly in the framework position, much lower activity was obtained after KCl modification. In contrast, the Fe-MFI after steam treatment and the Fe-MCM-41 prepared by template ion-exchange (TIE) and conventional impregnation methods, which contained extra-framework iron species (e.g., FeO x clusters), exhibited higher PO formation activity after modification. A PO selectivity of 80% could be achieved at a propylene conversion of 3.3% over the KCl-modified Fe-MCM-41 prepared by the TIE or the impregnation method at 598 K (time on stream, 70 min). The modification with KCl increased the dispersion of the extra-framework iron species. Consequently, highly dispersed extra-framework iron species, probably in tetrahedral coordination, are proposed for the epoxidation of propylene with nitrous oxide.

Density functional study of propylene oxidation on Ag and Au surfaces. Comparison to ethylene oxidation

Density functional theory (DFT) calculations were carried out to investigate the reaction mechanisms for propylene oxide formation on Ag and Au catalysts. The energy profile and structural change along reactions were mutually compared between ethylene and propylene, and between Ag and Au catalysts using 5 and 7 atom cluster models. Ag is known as unique catalyst for ethylene epoxidation, but Au is not a good catalyst. For propylene oxidation, the influences of doping co-catalysts and supporting metal-oxides were investigated by varying the charge of clusters. The reaction profiles were calculated repeatedly using neutral, cationic and anionic clusters. In almost all cases considered on the Ag and Au clusters, propylene oxide formation is less favorable than hydrogen elimination which leads to π-allyl intermediates. The reaction with O atom as oxidizing agent results in both lower reaction barrier and lower selectivity in the products. For the interaction with O 2 molecule, cationic Ag cluster favors propylene oxide formation and disadvantages π-allyl formation. This unique result is also obtained for cationic Au cluster. As for propylene oxidation, the difference between Ag and Au metals is not so distinct compared to that for ethylene oxidation.

Photocatalytic Oxidation of Propylene with Molecular Oxygen over Highly Dispersed Titanium, Vanadium, and Chromium Oxides on Silica

Photocatalytic oxidation of propylene with molecular oxygen at room temperature was investigated over various silica-supported metal oxides with low loading. The photocatalytic active site is assumed to be the isolated tetrahedrally coordinated metal oxides in the ligand-to-metal charge-transferred state, such as (Mdelta- -OLdelta+). Photocatalytic epoxidation of propylene into propylene oxide was promoted over silica-supported V and Ti oxides at steady state. Over silica-supported Cr oxide, the propylene oxide formation rate was remarkably decreased with the time course in the reaction. The oxidation state and the coordination environment of the supported Ti, V, and Cr oxide species were determined by diffuse reflectance UV-vis spectroscopy (DRS) and electron spin resonance (ESR). During the photocatalytic oxidation, the oxidation state of the Ti4+ species was not varied. On the other hand, the V5+ species was partially reduced to V4+ and the Cr6+ species was successively reduced to Cr5+ and Cr3+. An isotopic tracer study of the C3H6-18O2 reaction suggests the difference of the active oxygen species between TiO2/SiO2 and V2O5/SiO2. The active oxygen species on TiO2/SiO2 is derived from molecular oxygen. On the other hand, the photogenerated products on V2O5/SiO2 incorporate the lattice oxygen of the surface metal oxide species. It is suggested that the kinds of terminal ligand (hydroxyl or oxo) of the tetrahedrally coordinated metal oxides on silica decide the active oxygen species in the photocatalytic oxidation. A photoinduced hole center on the monohydroxyl (SiO)3Ti-OH species activates molecular oxygen that reacts with propylene. In the case of the monooxo (SiO)3V=O and dioxo (SiO)2Cr=O2 species, the photoactivated lattice oxygen (OL-) directly reacts with propylene.

Effect of preparation condition on performance of Ag–MoO 3/ZrO 2 catalyst for direct epoxidation of propylene by molecular oxygen

The Ag–MoO 3/ZrO 2 catalyst for an epoxidation of propylene by molecular oxygen was prepared by the precipitation/impregnation method, and characterized by N 2 adsorption at low temperature, XRD, NH 3-TPD and CO 2-TPD. The effects of preparation condition of ZrO 2 support on its physicochemical properties and catalytic performance were studied. The results show that suitably low surface area, big pore size, and modest and weak surface acidity–basicity for the Ag–MoO 3/ZrO 2 catalyst are in favor of the selective formation of propylene oxide. High surface area, abundant micro-pore structure and strong acidity–basicity or acidity-free are liable to the deep oxidation of propylene to CO 2 and H 2O. As the support of the Ag–MoO 3 catalyst for an epoxidation of propylene, the monoclinic phase of ZrO 2 is preferable to tetragonal one. Suitably larger particles of Ag and highly dispersed MoO 3 on the Ag–MoO 3/ZrO 2 catalyst are beneficial to improve the selectivity to propylene oxide.

Gas-Phase Epoxidation of Propylene and Ethylene

New results from the catalyst-free epoxidation route for small alkenes in the homogeneous gas phase using O 3 as the oxidizing agent (Berndt, T.; Boge, O.; Heintzenberg, J.; Claus, P. Ind. Eng. Chem. Res. 2003, 42, 2870.) are presented. The formation of propylene oxide was studied in a wide range of experimental conditions such as total pressure, temperature, feed composition, residence time, and reactor dimension. In the absence of dilution gas, for a propylene conversion of 91.8%, a selectivity to propylene oxide of 93.3% was found. An optimal O 3 utilization of ∼95% (reacted propylene/initial O 3 = 0.95) was achieved by limiting the propylene conversion to 20-30%. The epoxidation of ethylene yielded a selectivity to ethylene oxide of 90% and better in a wide range of conditions. The maximum formation rates of propylene oxide and ethylene oxide measured were 3640 and 2870 g per hour and per liter of reaction volume, respectively.

Direct gas-phase epoxidation of propene with nitrous oxide over modified silica supported [formula omitted] catalysts

Vapour phase epoxidation of propene with nitrous oxide over alkaline containing silica supported iron oxide catalysts was experimentally investigated. A reaction network is deduced from the trends of product selectivities as function of propene conversion using different catalysts. Important side reactions are parallel propanal formation and consecutive isomerisations of propylene oxide (PO) to propanal, acetone and allyl alcohol. Rates of these reactions were affected by catalyst composition, which influenced acid-base properties, number of silanol groups and surface area. A PO selectivity of ∼ 75 % and PO space–time yields of 0.02– 0.03 g PO g cat - 1 h - 1 were obtained with a catalyst containing ironoxo-species and caesium ions/-oxides. These results can only be obtained at a high surplus of N 2 O versus propene concentration in the reactor feed. Likely reasons for that is an inhibition of propylene oxide's formation rate by propene and propylene oxide itself.

Effect of surface chemical properties and texture of mesoporous titanosilicates on direct vapor-phase epoxidation of propylene over Au catalysts at high reaction temperature

Two kinds of organically modified mesoporous titanosilicate supports, Ti-MCM-48- and Ti-MCM-41-type, with different surface chemical properties and texture were prepared by two-step and one-step synthesis, respectively. The direct vapor-phase epoxidation of propylene with O 2 and H 2 was performed over Au nanoparticles deposited on the above supports. Both the supports and the Au catalysts were characterized by XRD, UV-Vis, N 2 adsorption–desorption isotherms, FT-IR, ICP, TEM, and ADF-EELS. Over Au-deposited on trimethylsilylated Ti-MCM-48 supports, an optimum reaction temperature for the highest yield of propylene oxide (PO) while maintaining selectivity above 80% was 523 K. This temperature was 100 K higher than over the non-silylated Au catalyst. Catalyst deactivation with time-on-stream was appreciably depressed for the silylated ones, which could be explained by the change of surface property from hydrophilic to hydrophobic. Over Au-deposited on Ti-MCM-41-type amorphous supports, but not on pure Ti-MCM-41, excellent catalyst stability with time-on-stream was observed from the very early stage of reaction at 473 K. In terms of PO formation rate per unit weight of Au, Au/Ti-MCM-41-type amorphous support with a very low Au loading of 0.015 wt.% and Ti/Si molar ratio of 1/100 was found to be more active by one order of magnitude than Au/Ti-MCM-48 with a high Au loading of 0.57 wt.% and Ti/Si molar ratio of 2/100; the former also presented higher H 2 efficiency. Organic modification did not change this feature. FT-IR spectra showed strong adsorption of water even on the Au catalyst composed of trimethyl silylated Ti-MCM-48 and the partial regeneration of hydroxylated surfaces during reaction at 523 K. This may partly explain why the silylated catalysts still deactivate in the reaction mixture.

From Atmospheric Research to an Industrial Process: The Formation of Propylene Oxide

Initiated from experimental results in the field of alkene oxidation in atmospheric chemistry, a novel production route for propylene oxide on an industrial scale is described. The proposed route represents a simple one-step process in the homogeneous gas phase under mild reaction conditions in the absence of a catalyst. The oxidizing agent is O3. A selectivity to propylene oxide of up to 98% was achieved. This process is not afflicted with the production of coproducts, and only a negligible environmental burden is expected. Technical parameters, such as propylene conversion, selectivity, and formation rate of propylene oxide, are comparable to the corresponding data of the established, Ag-catalyzed ethylene oxide route. The homogeneous gas-phase route given is also applicable for the formation of ethylene oxide and butene oxides.

Selective oxidation of propylene to propylene oxide by molecular oxygen over Ti-Al-HMS catalysts

Ti- and Al-containing hexagonal mesoporous silicas (HMS) were synthesized at ambient temperature using dodecylamine as surfactant, and the resultant compounds were used as catalysts for the oxidation of propylene by molecular oxygen. Ti-Al-HMS, which was prepared by reaction of Ti(iso-OC3H7)4, Al(iso-OC3H7)3 and SiOC2H5)4 in an alcohol-aqueous solution in the presence of dodecylamine, showed 47.8 of conversion and 30.6 of selectivity for propylene oxide (PO) at 523K. The yield of PO over Ti-Al-HMS (14.6) was much higher than those over Ti-HMS (1.3) and Al-HMS (0.9) at the same reaction conditions. Ti-Al-HMS was also found to be more effective for PO formation than the supported catalysts (Ti/Al-HMS and Al/Ti-HMS) prepared by impregnation method. These results indicated that the simultaneous existence of Ti and Al in HMS was very important for improving the yield of PO from the oxidation of propylene, and the synthesis method influenced the catalytic activity of the Ti- and Al-containing HMS catalysts.

Synthesis of titanium silicalite (TS-1) from the TPABr system and its catalytic properties for epoxidation of propylene

Titanium silicalite (TS-1) was successfully synthesized by using tetra-propylammonium bromide (TPABr) and n-butylamine as the template and the base, respectively. XRD, IR, SEM, 13 C CP/MAS NMR, UV–VIS, and UV-Raman techniques were used to characterize the synthesized products. The results show that TS-1 has MFI structure with high crystallinity and large crystal size, and with two kinds of titanium species. Using epoxidation of propylene as the probe, the catalytic properties of TS-1 in the present work were investigated. The crystal size of TS-1 affects much the initial reaction rate of propylene oxide (PO) formation. The effects of reaction conditions on PO reveal that the PO formation is greatly influenced by the solvent and the solution pH value. Methanol is the most preferable solvent, and a portion of water (<30%) in solution does not have much impact on the reaction. Moreover, the addition of base used to neutralize the residual acidity of the catalyst favor the reaction, the catalyst shows high activity and selectivity upon the adequate amount of base and the catalyst can be used repeatedly without regeneration. In the fixed-bed reactor, using ammonium water as the neutralizer, after 200 h of reaction, both the conversion and the utilization of H 2O 2 is about 95%, the selectivity of PO is about 90%.

Epoxidation of propylene over gold catalysts supported on non-porous silica

Non-porous silica was used as a carrier of Au/Ti-SiO 2 catalysts for the direct vapor-phase epoxidation of propylene in the copresence of H 2 and O 2, because the smooth surfaces were expected to facilitate the desorption of propylene oxide (PO). Their catalytic performances were inferior to those of Au/TS-1 and Au/Ti-MCM catalysts, however, an interesting feature was that propylene epoxidation over Au/Ti-SiO 2 appreciably depended on calcination temperature of the Ti-SiO 2 support. Higher calcination temperature benefits PO production at reaction temperatures above 100°C. IR and HRTEM analyses showed that the interaction between titanium oxides and silica was intensified to form more isolated TiO 4 units with tetrahedral coordination on the surfaces of Ti-SiO 2 supports with an increase in heat treatment temperature. These isolated TiO 4 sites incorporated in silica surface layers are assumed to be an important structural factor for the selective deposition of Au particles and the selective PO formation.

Continuous epoxidation of propylene with oxygen and hydrogen on a Pd–Pt/TS-1 catalyst

Propylene epoxidation over a Pd–Pt/TS-1 catalyst with in situ formed hydrogen peroxide was carried out in a fixed bed reactor under high pressure conditions. The continuous operation allowed the study of catalyst deactivation and changes in product distribution with time-on-stream. The initial propylene oxide selectivity was very high, 99% at 3.5% conversion, but the catalyst deactivated rapidly with time-on-stream and successively the formation of methyl formate became the prevalent reaction. Using carbon dioxide, instead of nitrogen, had a beneficial effect on the formation of propylene oxide, and even higher yields were obtained when increasing the pressure from 50 to 120 bar (supercritical fluid phase). Thermal analysis (TA-MS and TA-FTIR) indicated that catalyst regeneration requires oxidation at elevated temperature; washing with an organic solvent is less efficient. The serious catalyst deactivation and the striking shift in the selectivity pattern of the catalyst is traced to competing alcohol oxidation on platinum metal.

Characterization of Gold–Titania Catalysts via Oxidation of Propylene to Propylene Oxide

Propylene oxidation to propylene oxide (PO) was performed with an H2/O2 mixture in the temperature range of 373–473 K using Au/TiO2 catalysts synthesized by a variety of techniques, including one which produced Au–Ti nanoclusters with controlled composition. The two most PO-active catalysts were one prepared by deposition–precipitation of gold onto titania-modified silica and one consisting of silica-supported Au–Ti nanoclusters with a 200:1 gold-to-titanium ratio. Most Au/TiO2 catalysts had a small spread in PO activity and selectivity at 373 K, but gave a much wider distribution in performance at temperatures above 413 K. With very few exceptions, these catalysts exhibit a maximum in PO turnover frequency over the experimental temperature range, resulting from sequential reaction of PO to other oxidation products. The largest observed losses in PO selectivity with increasing temperature and decreasing WHSV were to the oxidative cracking products ethanal and CO2, suggesting that maximizing the number of PO active sites is not the only solution to increase PO yields over these catalysts. The amount, phase, and method of contacting titania with gold controlled both PO oligomerization, as seen in the time on stream studies, and H2-assisted oxidative cracking rates. Catalysts that have isolated Ti atoms, such as the silica-supported Au–Ti nanoclusters, generally maintain a higher selectivity to PO with temperature. A D2 kinetic isotope effect was observed for PO formation which strongly suggests that a hydroperoxy intermediate is involved in the rate-limiting step for PO formation.

Selective Oxidation of Propene with Air to Propylene Oxide, a Case Study of Autoxidation Versus Catalytic Oxidation with AMM-Catalysts

The formation of propylene oxide from propene by direct oxidation with air has been studied with a series of new microporous amorphous catalysts containing isolated active sites. Although the materials showed good activities as heterogeneous oxidation catalysts, the formation of acroleine dominated in the gas phase. Oxidation selectivities of the various catalysts are reported. Significant PO-formation was only observed under reaction conditions involving pressures larger 5 bar. Promoting effects of solvents or additives like acetaldehyde could be excluded. Best results were observed in continuous gas phase flow experiments. All catalysts studied proved detrimental to the PO-yield and control experiments pointed to catalyst free autoxidation conditions. A factorial design followed by an optimization of the reaction conditions provided PO-selectivities of 60 % at propene conversion of 11.4 % (T = 290 °C; pressure = 25 bar; propene content = 75 %; total feed = 39.5 ml/min) at continuous reaction conditions. By passivating the inner reactor surfaces with gold, a PO-selectivity of 62.8 % at a propene conversion of 15.1 % could be obtained, indicating, that autoxidation of propene may become a serious contender in the search for improved production processes. This work is a case study of attempted catalyzed selective oxidation with air illustrating potential pitfalls and misleading interpretations.