Selective Oxidation Of Propane Research Articles

Microwave-Assisted Hydrothermal Synthesis, Characterization and Catalytic Performance of α-Bi2Mo3O12 in the Selective Oxidation of Propene

Microwave-Assisted Hydrothermal Synthesis, Characterization and Catalytic Performance of α-Bi2Mo3O12 in the Selective Oxidation of Propene

Insight into the role of metal support interface through the synergistic effect between Ag and α-Bi2Mo3O12 support for the selective oxidation of propylene to acrolein

Pre-synthesized morphologically tailored size and shape selective silver nanoparticles (AgNPs) decorated on bismuth molybdate support (AgNPs/α–Bi2Mo3O12) using simple wet impregnation method was employed. The prepared AgNPs/α-Bi2Mo3O12 nanocatalysts were tested in the selective partial oxidation of propylene to acrolein reaction. The introduction of well-defined size and shape of AgNPs on α–Bi2Mo3O12 is greatly promoted the strong metal support interactions (SMSI), creation of oxygen vacancies (Ov) and high propylene adsorption binding energy (calculated by DFT). The strong metal support interactions between Ag and α-Bi2Mo3O12 is clearly elucidated by the high quality HRTEM and STEM–HAADF microscopic images. In addition, surface atomic molar ratio measured by XPS analysis determined the key redox properties of AgNPs/α–Bi2Mo3O12 system and their influence on the overall catalytic efficiency in the oxidation of propylene via reduction of Mo6+ to Mo5+ initiated at low temperatures in Ag/α–Bi2Mo3O12 system. The Mo reduction is further confirmed by the activated oxygen removal from the MoO42– moieties after Ag incorporation on α–Bi2Mo3O12. Thus, confirmed the oxidation reaction pathway follows a Mars–van–Krevelen process. Further, DFT calculations supported the propylene adsorption is more favorable over Ag/α–Bi2Mo3O12 than bare α–Bi2Mo3O12 support. The propylene oxidation performance over Ag/α–Bi2Mo3O12 system was 5.5 times higher than bare α–Bi2Mo3O12 support, probably due to an enabled strong metal support interaction and increased oxygen vacancies.

Enhancing Efficiency and High-Value Chemicals Generation through Coupling Photocatalytic CO2 Reduction with Propane Oxidation.

Conversion of CO2 into high-value chemicals using solar energy is one of promising approaches to achieve carbon neutrality. However, the oxidation of water in the photocatalytic CO2 reduction is kinetically unfavorable due to multi-electron and proton transfer processes, along with the difficulty in generating O-O bonds. To tackle these challenges, this study investigated the coupling reaction of photocatalytic CO2 reduction and selective propane oxidation using the Pd/P25 (1 wt%) catalyst. Our findings reveal a significant improvement in CO2 reduction, nearly fivefold higher, achieved by substituting water oxidation with selective propane oxidation. This substitution not only accelerates the process of CO2 reduction but also yields valuable propylene. The relative ease of propane oxidation, compared to water, appears to increase the density of photogenerated electrons, ultimately enhancing the efficiency of CO2 reduction. We further found that hydroxyl radicals and reduced intermediate (carboxylate species) played important roles in the photocatalytic reaction. These findings not only propose a potential approach for the efficient utilization of CO2 through the coupling of selective propane oxidation into propylene, but also provide insights into the mechanistic understanding of the coupling reaction.

Comparison of Structure and Reactivity of Hydrothermally Prepared Bi−Mo−Co−Fe−O Catalysts in Selective Propylene and Isobutene Oxidation

AbstractThe elucidation of structure‐activity correlations in selective oxidation of propylene and isobutene over mixed metal oxides (MMO) is attractive for knowledge‐based catalyst design and process optimisation. Particularly, 4‐component Bi−Mo−Co−Fe−O catalysts need to be studied since their complex metal oxide phase mixture leads to higher activity and selectivity than 2‐component Bi−Mo−O. Hence, three Bi−Mo−Co−Fe‐oxides with different metal ratios were prepared by hydrothermal synthesis and compared during selective oxidation tests with propylene and isobutene. The active phases after several days on stream were investigated by synchrotron X‐ray diffraction (XRD) and Raman spectroscopy, while the structural evolution under reaction conditions was followed by operando Raman spectroscopy, synchrotron XRD, and multi‐edge X‐ray absorption spectroscopy. Similar structural transformations were observed during selective oxidation of propylene and isobutene, with similar influence on catalytic performance. A phase mixture of β‐CoMoO4/β‐Co0.7Fe0.3MoO4, γ‐Bi2MoO6, Fe3O4 and Bi3FeMo2O12 was observed, whereby high amounts of β‐CoMoO4/β‐Co0.7Fe0.3MoO4 increased selectivity to acrolein/methacrolein. In contrast, high amounts of γ‐Bi2MoO6 and Fe3O4 favoured total oxidation to CO and CO2. The simultaneous presence of β‐CoMoO4/β‐Co0.7Fe0.3MoO4, Bi3FeMo2O12 and Fe2O3 increased selectivity to methacrolein in isobutene oxidation, whereas no comparative increase in acrolein selectivity was observed in propylene oxidation. This suggests different key active phases in both reactions.

Single-step propane transformation on vanadium-supported catalyst revealed by operando DRS UV–vis study

The present study investigated the influence of silica support texture on the catalytic performance of VOx/silica catalysts in the selective oxidation of propane. Two- and three-dimensional mesoporous materials, SBA-15 and MCF, were synthesized using hydrothermal methods to create different pore structures. Vanadium (approximately 5 wt%) was incorporated into the silica matrices (MCF, SBA-15) and SiO2 (used as a reference system) through impregnation. The surface composition, valence state, and reducibility of the VOx species accommodated in VOx/silica catalysts were characterized using X-ray diffraction (XRD), diffuse reflectance ultraviolet–visible spectroscopy (UV–vis DRS), temperature-programmed reduction with hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) techniques. The catalytic properties of the catalysts were evaluated in the propane selective oxidation conducted at 470 °C, utilizing a mixture of oxidants (N2O and O2). Propane oxidation resulted in propane conversion of 34 %, propene selectivity of 49 %, and propene oxide selectivity of 20 %, corresponding to space–time yield of propene reaching 37 gC3H6 kgcat−1·h−1and propene oxide equal to 20 gPO kgcat−1h−1. The textural properties of the support materials noticeably influenced the stability of the catalysts. The vanadium catalysts supported on three-dimensional mesocellular silica foams (MCF) demonstrated superior activity, achieving turnover frequency (TOF) values of 9.0 × 10−4 s−1 for propene and 6.67 × 10−4 s−1 for propene oxide (PO). The unique three-dimensional ultra-large pores of the MCF material facilitated the generation of significant amounts of propene oxide, along with propene, while minimizing the formation of CO, CO2, and other oxygen-bearing by-products. Operando UV–vis studies supported by two-dimensional correlation analysis (2D COS) were employed to explain the diversity in the catalytic performance of VOx/silica catalysts. It has been found that the oligomeric vanadium species are the active sites in propane oxidative dehydrogenation, whereas the monomeric forms participate in propene oxide formation. Moreover, the reduction of V5+ to V4+ and even V3+ was identified as a crucial step in the reaction.A reaction scheme has been proposed based on catalytic experiments and spectroscopic insights.

Cr-doped Mesoporous M1 Phase MoVTeNbOx Catalyze Selective Oxidation of Propane to Acrylic Acid

Cr-doped Mesoporous M1 Phase MoVTeNbOx Catalyze Selective Oxidation of Propane to Acrylic Acid

One‐pot synthesis of V doped mesoporous SiO2 catalysts for selective oxidation of ethane and propane

AbstractA series of vanadium‐doped mesoporous SiO2 catalysts were prepared using a one‐pot emulsion method. The effects of VOx species with different structures on C−H bond activation and product selectivity in ethane and propane selective oxidation were investigated using X‐ray diffraction (XRD), N2 adsorption‐desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman, ultraviolet‐visible diffuse reflectance spectroscopy (UV‐Vis DRS), hydrogen temperature programmed reduction (H2‐TPR), and ammonia temperature programmed desorption (NH3‐TPD) characterizations. The results revealed that the highly dispersed isolated tetrahedral VOx species on the catalyst gradually polymerized into highly dispersed oligomeric VOx species with the increased V loading. The former is beneficial for the formation of olefins and total aldehydes during the selective oxidation of propane. Oligomeric VOx species are conducive to improving the selectivity of ethylene and acetaldehyde in the selective oxidation of ethane. The highest catalytic performances for selective oxidation of ethane and propane were obtained at a temperature of 650 °C. The conversion of ethane was 41.6 %, with a corresponding yield of ethylene and acetaldehyde of 23.6 % over a 0.5 V−SiO2 catalyst. In contrast, the propane conversion was 61.5 %, with a corresponding total yield of olefins and aldehydes of 41.3 % over a 1.0 V−SiO2 catalyst.

Dendritic Mesoporous Silica Nanoparticle-Supported Molybdena Catalysts for Propane Selective Oxidation: Catalytic Performance versus Molybdena Molecular Structure

Molybdena catalysts supported on dendritic mesoporous silica nanoparticles (Mo/DMSNs) were prepared by the impregnation method and evaluated for the selective oxidation of propane. The dendritic channels and high specific surface area of the DMSN support isolated the active components, which was beneficial for improving the dispersion of the Mo species. The identities of the active Mo species on the Mo/DMSN catalysts were determined using a combination of characterization techniques, and the relationship between the Mo loading and the structures of these Mo species was examined. Upon increasing the Mo loading, the mono-oxo O═Mo(O–Si)4 and di-oxo (O═)2Mo(O–Si)2 species underwent gradual polymerization to afford crystalline MoOx, where the mono-oxo and di-oxo species led to higher target product selectivities compared with the crystalline phase. In situ Raman spectroscopy results indicated that the highly dispersed monomolybdenum species were highly stable and resistant to carbon deposition during the reaction. Consequently, the 0.1Mo/DMSN, 0.5Mo/DMSN, and 1.0Mo/DMSN catalysts with monomolybdenum species exhibited higher propane conversions and target product selectivities than the catalysts bearing crystalline MoOx species. The highest yield (37.6%) of the target products (olefins and total aldehydes) was obtained over the 0.5Mo/DMSN catalyst at a temperature of 625 °C.

Liquid Phase Epoxidation of Propylene to Propylene Oxide with Hydrogen Peroxide on Titanium Silicalite-1: Spatially Resolved Measurements and Numerical Simulations

The spatial profile measurement technique was applied for the first time to a liquid phase reaction to study the selective oxidation of propylene to propylene oxide with hydrogen peroxide on titanium silicalite-1 catalyst, also known as the HPPO process. A spatial profiling reactor based on minimal invasive capillary sampling measurements was used to obtain the experimental data. The reactor is able to measure spatially resolved concentration and temperature profiles at industrially relevant conditions, that is, 40 °C and 20–35 bar pressure, with propylene in the liquid phase. The reaction was carried out at different contact times of LHSV 3.0, 6.0, and 12.0 h–1, respectively, achieving high conversion and selectivity. The experimental data show that the concentration profiles of the main epoxidation reaction follows a pseudo-zeroth-order behavior. In contrast, the side product concentration profiles exhibit a nonlinear trend. The primary side products of the reaction 1-methoxy-2-propanol, 2-methoxy-1-propanol, and propylene glycol were quantified. Furthermore, hydroxacetone and methoxyacetone from the consecutive reaction of 1-methoxy-2-propanol, 2-methoxy-1-propanol, and propylene glycol with hydrogen peroxide were detected as secondary side products. Formic acid, methylformate, dipropylene glycol methyl ether isomers, and dipropylene glycol isomers were detected in trace amounts. Quantitative data for the formation of hydroxyacetone is provided for the first time. The experimental concentration profiles were simulated using an axial 1D-pseudo-homogeneous dispersion model with two kinetic models reported in the literature. Additionally, a kinetic model based on the Eley–Rideal mechanism is proposed. Upon linearization, the derived model shows a correlation of R2 = 0.99 and R2 = 0.98 with the linearized form of the experimental differential rate. Statistical analysis of the models in this work shows that the derived Eley–Rideal mechanism has the highest correlation with the concentration gradients measured within the catalyst bed.

An Advanced Characterization Toolbox for Selective Olefin Oxidation Catalysts

AbstractThe selective oxidation of propylene and isobutene form a central cornerstone in chemical industry, typically catalysed by bismuth molybdate based multicomponent systems. Over the past decades, a detailed knowledge has been gained on the mechanism by theoretical and experimental studies including in situ and operando characterization techniques, using simplified model catalysts. These have provided excellent insight, but in industrially applied selective oxidation typically multi‐component catalysts are used, leading to higher activity and selectivity. New concepts and characterization methods allow now studies of the more complex multicomponent catalysts covering the various length and time scales and potentially providing new insights into the dynamics and cooperation during catalysis. Starting with an overview of past concepts and milestones, we report on latest developments and future trends in the field of selective oxidation of lower olefins. Thereby we focus both on conventional spectroscopic and advanced characterisation methods and discuss the importance of integral and spatially‐resolved approaches. We conclude that fundamental understanding of such complex oxidation reactions requires combined and interdisciplinary research, now possible with the new characterization tools, data analysis schemes and reactor modelling approaches. This allows bridging the various complexity scales.

Kinetics and mechanistic insights into the active sites of Au catalysts for selective propylene oxidation

Kinetics and mechanistic insights into the active sites of Au catalysts for selective propylene oxidation

Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects.

Keggin-type phosphomolybdic acid (PMo12O40), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo12O40 and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

High-pressure hydrothermal synthesis of MoVTeNbOx with high surface V5+ abundance for oxidative conversion of propane to acrylic acid

A series of MoVTeNbOx-mixed metal oxides were synthesized by high-pressure hydrothermal method. The effects of temperature and time during the preparation process on the structure and catalytic performance of MoVTeNbOx were investigated. It was demonstrated that high content of M1 phase (94.5%) MoVTeNbOx with typical needle-shape morphology and high surface abundance of V5+ (68.9%) could be fabricated by high-pressure hydrothermal method in a short time (30–60 min) without hydrogen peroxide post-treatment. The catalytic results of propane selective oxidation showed that the MoVTeNbOx sample synthesized at 200 °C for 30 min possessed the highest surface V5+ abundance and largest proportion of terminating (001) plane, displaying the highest selectivity (74.1%) for acrylic acid. Moreover, the prepared MoVTeNbOx showed excellent stability in high temperature continuous reaction.

Complementary operando insights into the activation of multicomponent selective propylene oxidation catalysts

Two Bi–Mo–Co–Fe–O catalysts were synthesized by flame spray pyrolysis and tested for their catalytic performance in selective oxidation of propylene to acrolein. Pronounced structural changes during temperature–programmed oxidation and reaction were observed by operando X–ray absorption spectroscopy, X–ray diffraction, Raman spectroscopy, and thermogravimetric analysis. During oxidative treatment, mainly binary oxide phases (α–Bi2Mo3O12, β–CoMoO4, Fe2(MoO4)3) were observed, but single (MoO3) or ternary (Bi3(FeO4)(MoO4)2) oxides also formed depending on the relative elemental catalyst composition. During propylene oxidation, the reduction of Fe3+ to Fe2+ led to a strong rise in activity and induced further phase transformations. MoO3 was found to be unselective towards acrolein but was essential in binding other single oxides. The formation of β–Co0.7Fe0.3MoO4 and Bi3(FeO4)(MoO4)2 as well as their synergistic interplay with α–Bi2Mo3O12 are key factors for high performance. The combination of complementary operando methods was crucial to reveal new structure–activity/selectivity correlations, therefore bridging the knowledge gap between simplified model systems and complex applied catalysts.

Selective oxidation of propylene to acrolein over silver-promoted hexagonal molybdates and derivative Ag/Ag2Mo4O13/α-MoO3 composites

Hexagonal molybdates (h-MoO3) include abundant Mo vacancies that are charge-compensated by cations residing within one-dimensional channels. Hence, they present an intriguing structural motif for Mo catalysis which has been hardly explored for selective oxidation of olefins. We discovered that NH4+ and Ag+-exchanged h-MoO3 display surprisingly varied behavior during propylene oxidation. Acrolein and acrylonitrile are the main partial oxidation products with the latter forming due to NH4+ consumption. The catalytic activity more than doubles upon introduction of Ag+ which also enhances selectivity for the main products up to ∼60 %. Unexpectedly, the hexagonal frameworks transform into orthorhombic α-MoO3 40−60 °C below their reported stability limits already. In addition, Ag2Mo4O13 and Ag nanoparticles segregate in case of Ag+-containing h-MoO3 frameworks. The resulting Ag/Ag2Mo4O13/α-MoO3 composites catalyze propylene conversion into acrolein with even higher selectivity (∼80 %). Detailed analyses indicate that Ag2Mo4O13 drives formation of this oxygenate over these composites.

The active role of the support in propane ODH over VOx/CeO2 catalysts studied using multiple operando spectroscopies

Because of the relevance of supported vanadia catalysts for the oxidative dehydrogenation (ODH) of alcohols and short alkanes, a detailed mechanistic understanding, including the role of the support, is of great importance. In this work, we address the ODH of propane over ceria-supported vanadia (VOx/CeO2) catalysts, especially regarding the active participation of ceria and the synergy effect between ceria and vanadia. A combination of operando multi-wavelength Raman-, operando UV-Vis- and operando IR spectroscopy is applied, including the targeted use of different Raman excitation wavelengths to selectively enhance vibrational features through resonance from vanadia (514 nm) and the ceria support (385 nm). Our operando results show that surface lattice oxygen from ceria is crucial for the selective oxidation of propane while the structure of vanadia seems to stay unchanged during the reaction. Rather vanadia structures play an indirect role in increasing the overall selectivity of the reaction. The surface oxygen defects of ceria are affected by the interaction with vanadia monomeric species in direct proximity, owing to an irreversible occupancy of the vacancies by vanadia monomers, which slows down the overall oxygen dynamics and thereby decreases the amount of CO2 produced. Dimeric and oligomeric vanadia species weaken the adsorption of propane onto the ceria surface, further increasing the selectivity. Our mechanistic insights demonstrate the synergy effect between vanadia and ceria in oxidation reactions and provide an experimental basis for a detailed understanding of the role of the support in VOx/CeO2 and other supported vanadia catalysts.

Chemical Imaging of Mixed Metal Oxide Catalysts for Propylene Oxidation: From Model Binary Systems to Complex Multicomponent Systems

AbstractIndustrially‐applied mixed metal oxide catalysts often possess an ensemble of structural components with complementary functions. Characterisation of these hierarchical systems is challenging, particularly moving from binary to quaternary systems. Here a quaternary Bi−Mo−Co−Fe oxide catalyst showing significantly greater activity than binary Bi−Mo oxides for selective propylene oxidation to acrolein was studied with chemical imaging techniques from the microscale to nanoscale. Conventional techniques like XRD and Raman spectroscopy could only distinguish a small number of components. Spatially‐resolved characterisation provided a clearer picture of metal oxide phase composition, starting from elemental distribution by SEM‐EDX and spatially‐resolved mapping of metal oxide components by 2D Raman spectroscopy. This was extended to 3D using multiscale hard X‐ray tomography with fluorescence, phase, and diffraction contrast. The identification and co‐localisation of phases in 2D and 3D can assist in rationalising catalytic performance during propylene oxidation, based on studies of model, binary, or ternary catalyst systems in literature. This approach is generally applicable and attractive for characterisation of complex mixed metal oxide systems.

Cover Feature: Roles of Enhancement of C−H Activation and Diminution of C−O Formation Within M1‐Phase Pores in Propane Selective Oxidation (ChemCatChem 3/2021)

Cover Feature: Roles of Enhancement of C−H Activation and Diminution of C−O Formation Within M1‐Phase Pores in Propane Selective Oxidation (ChemCatChem 3/2021)

Enhanced catalytic performance for selective oxidation of propene with O2 over bimetallic Au–Cu/SiO2 catalysts

Enhanced catalytic performance for selective oxidation of propene with O2 over bimetallic Au–Cu/SiO2 catalysts

DFT studies of selective oxidation of propene on the MoO3(010) surface.

Selective oxidation of propene to acrolein and acrylic acid has been applied in industry for many years. In this work, the density functional theory plus U (DFT+U) method was performed to study the hydrogen abstraction of propene on the MoO3(010) surface. From the most stable chemisorbed propene (di-σ propene), the allyl intermediate is difficult to produce on a perfect MoO3(010) surface because of the high barrier. In general, the barriers of the second hydrogen abstraction are much lower than those of the first one. The conclusion from our slab model calculations is consistent with the experimental results. It is found that the (3 + 2) mechanism exhibits lower barriers than the (5 + 2) mechanism. Oxygen defects facilitate the first dehydrogenation significantly, and π-allyl converts to σ-allyl favorably on defects, in agreement with a previous experimental study. The present study indicates that increasing the surface oxygen defects might be an effective way to promote the activity of MoO3 to propene oxidation.