Selective Oxidation Of Propane Research Articles

Microwave-assisted hydrothermal synthesis, characterization and catalytic performance of Fe2(MoO4)3 in the selective oxidation of propene

A method for the simple and efficient microwave radiation-assisted hydrothermal synthesis of Fe2(MoO4)3 has been developed. Several factors such as pH, addition rate, molybdenum precursor, type of solvent and various other microwave synthesis parameters are studied. The catalysts were characterized using X-ray diffraction, scanning electron microscopy, Mössbauer spectroscopy and specific surface area measurements. The results show that different morphologies can be obtained, depending on the conditions under which the iron molybdate is prepared. Nevertheless, in all cases the solid particles appear to be covered by an amorphous oxide layer, which is less rich in iron than in the case of a crystallized oxide layer. The presence of this amorphous layer was revealed on all facets of the molybdate, with approximately the same composition and thickness. In an effort to evaluate the relationship between the morphology of iron molybdate particles and their catalytic properties, several samples exhibiting different morphology were tested for the oxidation of propene to acrolein. These samples were tested for the oxidation of propene to acrolein. The catalysts with platelets morphology and exhibiting larger surface of (100) planes appeared more active. This was attributed to a faster re-oxidation due to the preferential diffusion of oxygen anions in the bulk structure channels perpendicular to these planes.

Gold-copper catalysts supported on SBA-15 with long and short channels – Characterization and the use in propene oxidation

Gold and copper were supported on short-channel SBA-15 and conventional long-channel SBA-15 for selective oxidation of propene with molecular oxygen. The obtained materials were characterized by several techniques (N2 adsorption, XRD, UV–vis, XPS, XAS) and tested as catalysts in propene oxidation with molecular oxygen. The influence of the following parameters on the catalytic activity and the product selectivity to acrolein was considered: i) the length of channels in SBA-15 which determined the particle size of active species and diffusion of the reaction substrates / products, ii) the conditions in which copper-gold alloy-like structure was formed, iii) the effect of gold and copper on the reaction pathway, iv) the changes on the catalyst surface caused by the oxidation of propene and v) the role of acidity on the reaction selectivity. One of the most important findings from this work was the evaluation of the conditions in which copper and gold species were introduced to be in vicinity which led to the effective oxidation of propene to acrolein. It was found that such a vicinity can be achieved by gold-copper alloy formation during hydrogen reduction and next separation of the two components upon exposure to the reaction conditions. The reaction pathway was also considered and discussed.

Cooperation between redox couples at the surface of molybdates based catalysts used for the selective oxidation of propene

FeMoTeO mixed oxides have been synthesized and tested as catalysts for the oxidation of propene into acrolein. In order to determine the role of their constituent metallic elements, these catalysts were characterized using bulk techniques such as TEM, XANES and Mössbauer spectroscopy, as well as surface techniques including XPS and LEIS. Our results confirmed that the active phase of these catalysts corresponds to an amorphous layer of molybdenum oxide containing Te and Fe, supported on the Fe2(MoO4)3 phase. They also show that the addition of tellurium significantly changes the redox dynamics of the catalytic surface, since it provides a fast re-oxidation route for molybdenum species. The study of MoTeO catalysts has shown that the presence of Te centers alone is not sufficient to fully re-oxidize the molybdenum. Although tellurium plays a role in the redox reaction mechanism, XPS on fresh and used FeMoTeO catalysts did not reveal any change in its oxidation state. However, the study of MoTeO samples in the early stage of their reduction by propene revealed the simultaneous formation of Te2+ and Te(0) species, indicating that the Te4+/Te2+ redox couple is involved in the redox cycle. Since Te(II) is unstable, it undergoes disproportionation, if not quickly re-oxidized, leading to metallic tellurium that is easily lost from the surface. The presence of iron provides an accessible route for the re-oxidation of tellurium, thereby restoring the Te(IV) oxidation. Since iron is unable to re-oxidize Mo(V) in the absence of tellurium, and although the usual role of tellurium is that of an α-hydrogen abstracting element, it acts as a redox bridge between iron and molybdenum. This behavior would account for the high efficiency of the FeTeMoO catalysts and shows that the three redox couples Fe3+/Fe2+, Te4+/Te2+ and Mo6+/Mo5+ cooperate in the re-oxidation step of the Mars and van Krevelen mechanism.

Correlation between Copper Oxide Particle Size and Selectivity towards Propylene Oxide in Selective Oxidation of Propene

AbstractCopper oxide catalysts supported on nanostructured silica SBA‐15 were synthesized with varying copper loadings in a range from 1.1 to 19.4 wt.%. Catalysts were structurally characterized by N2 physisorption, X‐ray diffraction and ex situ as well as in situ diffuse reflectance UV‐Vis spectroscopy. Copper oxide species contained Cu2+ in an octahedral configuration with a tetragonal distortion. Increasing the copper loading resulted in a decreasing optical band gap, which is associated with a growing particle size. Additionally, CuO/SBA‐15 samples were examined by temperature‐programmed reduction confirming a narrow particle size distribution. Structural evolution and catalytic activity were investigated during selective oxidation of propene focusing on product distribution of nucleophilic and electrophilic oxidation. Catalytic testing was carried out using an on‐line GC and MS coupled to a laboratory fixed‐bed reactor. All catalysts selectively oxidized propene to acrolein and propylene oxide (PO). With time on stream activity decreased while selectivity towards acrolein and PO increased. While selectivity towards PO increased with copper loading and, hence, copper oxide particle size, selectivity towards acrolein was largely independent of copper loading.

Decomposition of Large Cu Crystals into Ultrasmall Particles Using Chemical Vapor Deposition and Their Application in Selective Propylene Oxidation.

In this work, we report a novel application of chemical vapor deposition (CVD) in which the calcination and reduction of Cu(thd)2 deposited onto 4.9 wt % Cu/SiO2 induces significant decomposition of 28 nm crystalline Cu into ultrasmall ∼2 nm particles (5.1 wt % Cu/SiO2). The Cu loading slightly increased, but the particle size dramatically decreased. The deposition of Cu(thd)2 onto the Cu surface can initially affect the size reduction of the metallic Cu particles due to charge transfer between Cu(thd)2 and the Cu surface. Thermal treatments, including calcination in air and reduction in H2, can further influence the Cu particle decomposition. The mechanism of change in the Cu particle decomposition was investigated by a variety of experiments, such as X-ray diffraction and in situ X-ray absorption spectroscopy. CVD treatment of Cu/SiO2 can create Cu-rich sites, which effectively enhance the conversion and acrolein yield in selective propylene oxidation. The intermediate associated with propylene oxidation on the Cu catalysts was also examined by IR spectroscopy.

Structural Evolution of Highly Active Multicomponent Catalysts for Selective Propylene Oxidation

Multicomponent Bi-Mo-Fe-Co oxide catalysts prepared via flame spray pyrolysis were tested for selective propylene oxidation, showing high conversion (>70%) and selectivity (>85%) for acrolein and acrylic acid at temperatures of 330 °C. During extended time-on-stream tests (5–7 days), the catalysts retained high activity while undergoing diverse structural changes. This was evident on: (a) the atomic scale, using powder X-ray diffraction, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy; and (b) the microscopic scale, using synchrotron X-ray nanotomography, including full-field holotomography, scanning X-ray fluorescence, and absorption contrast imaging. On the atomic scale, sintering, coke formation, growth, and transformation of active and spectator components were observed. On the microscopic scale, the catalyst life cycle was studied at various stages through noninvasive imaging of a ~50-µm grain with 100-nm resolution. Variation of catalyst synthesis parameters led to the formation of notably different structural compositions after reaction. Mobile bismuth species formed agglomerates of several hundred nanometres and segregated within the catalyst interior. This appeared to facilitate the formation of different active phases and induce selectivity for acrolein and acrylic acid. The combined multiscale approach here is generally applicable for deconvolution of complex catalyst systems. This is an important step to bridge model two-component catalysts with more relevant but complex multicomponent catalysts.

Highly Efficient Removal of CO in Effluent Streams from Real‐Life Propane Oxidation Process over CuO−CeO2−Based Catalysts

AbstractSelective removal of CO from the effluent streams in real‐life chemical processes is of great importance in many different fields of industry. Until now it remains challenging to use non‐precious metal oxides to selectively remove CO at low temperatures from real‐life chemical effluent with complex compositions. Herein, a series of binary and ternary transition metal oxide catalysts were prepared by co‐precipitation method and employed for the preferential removal of CO in the effluent stream (off‐gas) of propane selective oxidation. It was found that the CuO−CeO2−based mixed oxide catalyst exhibited the best CO removal performance, wider operating temperature window and higher resistance to the inhibition of hydrocarbons, H2O and CO2. The presented catalyst showed high potential for selective removal of CO in the off‐gas stream containing CO2, H2O and various hydrocarbons.

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.

Constructing A Rational Kinetic Model of the Selective Propane Oxidation Over A Mixed Metal Oxide Catalyst

This research presents a kinetic investigation of the selective oxidation of propane to acrylic acid over a MoVTeNb oxide (M1 phase) catalyst. The paper contains both an overview of the related literature, and original results with a focus on kinetic aspects. Two types of kinetic experiments were performed in a plug flow reactor, observing (i) steady-state conditions (partial pressure variations) and (ii) the catalyst evolution as a function of time-on-stream. For this, the catalyst was treated in reducing atmosphere, before re-oxidising it. These observations in long term behaviour were used to distinguish different catalytic routes, namely for the formation of propene, acetic acid, acrylic acid, carbon monoxide and carbon dioxide. A partial carbon balance was introduced, which is a ‘kinetic fingerprint’, that distinguishes one type of active site from another. Furthermore, an ‘active site’ was found to consist of one or more ‘active centres’. A rational mechanism was developed based on the theory of graphs and includes two time scales belonging to (i) the catalytic cycle and (ii) the catalyst evolution. Several different types of active sites exist, at least as many, as kinetically independent product molecules are formed over a catalyst surface.

Efficient selective oxidation of propylene by dioxygen on mesoporous-silica-nanoparticle-supported nanosized copper

A direct synthetic route was developed to prepare mesoporous-silica-nanoparticle-supported copper catalysts featuring ordered mesostructures with extensive intraparticle voids, a high degree of silica condensation, and a high dispersion of copper species. After hydrogen reduction, the catalysts containing nanosized metallic copper showed superior and stable catalytic activity for the selective oxidation of propylene by dioxygen, with high conversion of both reactants and high yield and formation rate of acrolein. The best catalyst outperformed the state-of-the-art silica-supported copper catalysts, representing one of the most active acrolein-production catalysts. Results of in situ X-ray absorption spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, and other characterizations suggested that the metallic copper transformed and mainly stayed in the 1+ oxidation state under reaction conditions and that factors including the small size of copper and the presence of silanol groups on the silica support were crucial for the catalytic performance of the catalysts.

Controlled Generation of TiOx–Au Interface Using Titanium Molecular Complex Bearing Pyridyl Anchors: Synthesis, Characterization and Catalysis

Interfacial perimeter sites between metal and support are often important for catalysis. A mononuclear titanium siloxy complex III, Ti(acac)2[OSiC6H5N(OCH3)2]2 was synthesized to generate TiOx units of different degrees of clustering to decorate Au nanoparticles. Two different methods of preparation were examined; one was to deposit III onto Au/SiO2 and the other was to form III-covered Au nanoparticle first before deposition onto SiO2. The former method generated more highly dispersed TiOx units, while larger domains of TiO2 were formed with the latter method, as deduced by UV–vis and XAS characterization. A model was proposed to explain how TiOx dispersity could be related to the preparative procedures. These samples were further tested as catalysts in selective oxidation of propane in a stream of O2 and H2. They exhibited different product selectivities. The sample with more dispersed TiOx units were more selective for acetone formation versus propene formation. The results confirmed the important role of both the Au–TiOx interface at the perimeter and the extent of Ti isolation in the TiOx phase in the reaction.

Steam Influence and Effect of Oxidant Amount on Propane Oxidation over Multi Metal Oxide Catalyst Using High-Throughput Experiment

The high-throughput experimentation technique was used to verify the testing conditions due to the effects of catalyst structure modifications and/or due to reaction parameter variation in parallel. In this paper, the design of experiment and catalytic results are discussed in the development of selective oxidation catalyst, to demonstrate the importance and versatility of such technology. It is used for the automated parallel testing of selective oxidation of propane to acrylic acid over some types of multi metal oxide catalysts. The catalysts used for performance test were Mo (molybdenum), cat-1 (unsupported Mo1V0.3Te0.23Nb0.125O), and cat-2 (supported Mo1V0.3Te0.23Nb0.125O). All catalysts were dried using spray drier. The effect of some reaction parameters, such as the amount of oxidant, presence of steam and reaction temperature was also investigated during the test. The configuration of the ‘nanoflow’ is shown to be suitable to screen catalytic performance. The results obtained gave very good reproducibility.

Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene Probed by Correlative Operando Spectroscopies

α-Bi2Mo3O12, β-Bi2Mo2O9 , and γ-Bi2MoO6 as target bismuth molybdate phases were prepared by hydrothermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO42– entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius...

Carbon Dioxide as Feedstock in Selective Oxidation of Propane

Carbon dioxide is a promising nonconventional oxidant for catalytic dehydrogenation of lower alkanes to olefins. The lower catalytic activity of CO2 compared to oxygen, which is typically used in oxidative dehydrogenation (ODH), also means that more reactive olefins are less likely to undergo undesirable combustion in the presence of CO2. Supported vanadia catalysts were synthesized by incipient wetness impregnation of silica supports, and by varying the surface coverage of VOx species by means of sodium on the silica surface, the nature of the catalyst was investigated. Promotion with Na cations significantly improved the dispersion of surface VOx species but resulted in the formation of surface Na metavanadate or another reduced V3+/V4+ phase, which lacked catalytic activity in propane ODH in the presence of CO2.

Sol–Gel Synthesis of Bismuth Molybdate Catalysts for the Selective Oxidation of Propylene to Acrolein: Influence of pH Value and Theoretical Molar Atomic Ratio

Different bismuth molybdate catalysts for the selective oxidation of propylene to acrolein were prepared by the sol–gel method, starting from bismuth nitrate, ammonium molybdate, and citric acid. The influence of pH value and theoretical molar Bi/Mo atomic ratio on the complexation and gelation is surveyed using IR spectroscopy, X‐ray diffraction, and BET. Their catalytic activities for the conversion propylene to acrolein are examined.

Controlled silylation of MoVTeNb mixed oxide catalyst for the selective oxidation of propane to acrylic acid

Acrylic acid is an important industrial chemical, and efficient catalysts for its direct preparation by propane oxidation are highly desirable. For this purpose, neutral silica networks were introduced on the surface of MoVTeNb mixed oxide catalysts by controlled silylation using a methyl silicate oligomer (MS-51). The modified catalysts gave ∼56.5% yield of acrylic acid with a selectivity of 77.1% in the oxidation of propane at 380°C. The catalysts were characterized by X-ray fluorescence, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller specific surface area, X-ray diffraction (Rietveld analysis), pyridine desorption, and scanning electron microscopy. MoVTeNb mixed oxide was found to be composed of 90.9% M1 phase and 2.3% M2 phase, and upon silylation, the surface was uniformly covered by a thin SiO2 layer with 0.14 molar ratio with respect to Mo and an estimated thickness of 2.4nm. The amount of acid sites decreased after the first three silylation cycles, but was not affected by repeated cycles. The results of the kinetic study based on the comparison of the simulated contribution of each side reaction were consistent with those of the model reactions using acrylic acid and other reactants: the controlled silylation effectively suppressed acrylic acid oxidation, especially after repeated silylation cycles, which is responsible for the superior performance of the silylated catalysts. Considering the relatively large size of acrylic acid compared to propane and the efficient propane activation by silica-covered catalysts, the controlled silylation was proposed to have two roles, by which further consecutive oxidation is prevented effectively to exhibit excellent performance in oxidation of propane: i) to block the unfavorable acidic sides, ii) to generate a silica layer with pore mouth openings on the surface of MoVTeNb mixed oxide, which allow the entrance of propane but inhibit re-entrance of the produced acrylic acid.

Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches

The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity ...

Selective oxidation of propene to acrolein on FeMoTeO catalysts: determination of active phase and enhancement of catalytic activity and stability

Multicomponent FeMoTeO catalysts have been synthesized and studied for mild propene oxidation to acrolein.

Molybdenum/Bismuth Based Mixed Metal Oxide Catalysts for Selective Propylene Oxidation and Zeolite Membrane Protected Palladium/Alumina Catalysts for Selective Carbon Monoxide Oxidation and Application in a Process Loop Using a Propane Feed

As part of a process loop to make acrylic acid from propane, catalysts for two reactions have been investigated. Mo/Bi based mixed metal oxides for selective oxidation of propylene in a propane feed have been prepared by a sol-gel method which increased activity over the standard catalyst. Modifications of the molecular formula led to an increase in selectivity towards acrolein whilst maintaining the improved conversion. X-ray diffraction (XRD) analysis showed the sol-gel method helped incorporate the various mixed metal oxides and prevented segregation during reaction conditions. A Pd-loaded 4A zeolite membrane catalyst was developed on the surface of 1 mm beads γ-Al2O3 support. The membrane was employed in the selective oxidation of CO in a propane-rich mixture. The zeolite coating was developed by using a new modification of the hydrothermal synthesis (dilution method). Catalytic test results confirmed that the selective oxidation of CO in the presence of propane is possible by using a 4A zeolite membrane coated on a Pd based catalyst. Temperature and time during zeolite preparation are key parameters in the stability and reproducibility of the zeolite membrane. The seeding method improves the growth of the zeolite on the catalyst surface and does not affect the stability and reproducibility of catalyst. By using this catalyst it is possible to get a temperature window for the selective oxidation of CO (65°C).

Identifying the Unique Properties of α-Bi2Mo3O12 for the Activation of Propene

In order to understand the remarkable activity of α-Bi2Mo3O12 for selective oxidation and ammoxidation of propene, the propene activation ability of four molybdenum-based mixed metal oxides—Bi2Mo3O12, PbMoO4, Bi2Pb5Mo8O32, and MoO3—was investigated using density functional theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C–H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5- or 6-coordinate geometries at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that, in 5- and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ∼15 kcal/mol more favorable than that on 4-coordinate molybdate oxo atoms. Bi3+ cations in Bi2Mo3O12 thus promote catalytic activity by providing both strong adsorption sites for propene and forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE.