Selective Partial Oxidation Research Articles

Additive-free photo-assisted selective partial oxidation at ambient conditions of 5-hydroxymethylfurfural by manganese (IV) oxide nanorods

The valorization of natural and renewable resources, like lignocellulosic biomass, towards value-added chemicals by low energy and economically viable processes still remains a global research and technological challenge. 5-hydroxymethylfurfural (HMF) is an important platform chemical that can be easily derived from biomass, and it can be further used as a feedstock for the production of building blocks for polymers or fuels. In this context, the partial oxidation of the hydroxyl group on the HMF molecule leads to the formation of the corresponding aldehyde, 2,5-diformylfuran (DFF), which may find multiple applications in bio-chemical industries. Herein, we present the synthesis and characterization of novel manganese (IV) oxide nanorods as catalyst for the HMF to DFF partial oxidation at ambient conditions. This 1D nanocatalyst operates at low energy light irradiation and without the addition of chemicals (bases or oxidants) as a highly selective photo-assisted catalyst. Under optimized experimental conditions, the HMF conversion was found to be above 99%, while the DFF selectivity was almost 100%. The presence of molecular O2 played a key role in triggering the selective oxidation, while the use of an aprotic and less polar organic solvent, such as acetonitrile, compared to water, further enhanced the reactivity of the catalyst.

Misconceptions and challenges in methane-to-methanol over transition-metal-exchanged zeolites

Direct methane functionalization and, in particular, the selective partial oxidation to methanol, remains an eminent challenge and a field of competitive research. The conversion of methane to methanol over transition-metal-containing zeolites using molecular oxygen is a promising and extensively studied process. Herein, we scrutinize some oft-cited assumptions in this topic—which include the labelling of the process as biomimetic, the debate regarding the industrial viability of direct methane-oxidation systems and the claim that methane is difficult to activate—and delineate the extent to which these are scientifically robust. We highlight both the merits and pitfalls of such statements and point out the hazards associated with their improper use. By examining these misconceptions, we build an outlook for future research, highlighting the need to optimize materials and process conditions for the stepwise approach and to further explore catalytic processes that explicitly employ strategies for the preservation of methanol. While converting methane to methanol is an attractive process, making a catalytic—and commercially viable—route has presented severe difficulties. Here van Bokhoven and co-workers discuss the successes, problems and misconceptions in the field, focusing on the reaction with molecular oxygen over zeolites.

Bromine and iodine for selective partial oxidation of propane and methane

The oxidative dehydrogenation of propane to propylene can be enhanced through the addition of a halogen to a feed of oxygen and propane. In this paper, we examine halogen-assisted oxidative dehydrogenation by a mixture of I2 and Br2. We find that adding a small amount of Br2 to a feed of I2, C3H8, and O2 produces propylene with 71% propane conversion and 83% propylene selectivity. Good performance is also obtained if a small amount of I2 is added to a feed of Br2, C3H8, and O2. In both cases, the propylene yield is greater than the sum of the yields obtained with iodine and bromine separately. A variety of studies were performed to help understand the reaction mechanism. Methane halogenation was also investigated by using combinations of iodine and bromine to produce methyl iodide, and a microkinetic model was also constructed for methane halogenation in the gas-phase to help understand the mechanism.

La1-xSrxFeO3 perovskite-type oxides for chemical-looping steam methane reforming: Identification of the surface elements and redox cyclic performance

We report a family of perovskite-type oxides La1-xSrxFeO3 (x = 0.1, 0.3, 0.5, 0.7, 1.0) prepared by combustion method as effective redox catalysts for methane partial oxidation and thermochemical water splitting in a cyclic redox scheme. The effect of Sr-doping on the characterizations and properties of these perovskite-type oxides were studied by means of X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM). All the as-prepared and regenerated samples with various Sr substitutions exhibited pure crystalline perovskite structure. The oxygen carrying capacity of the La1-xSrxFeO3 perovskites was improved by doping Sr into the La-site. Besides, Sr-substitution has obvious effects on the valences of the Fe cations in the B-site and the oxygen species distribution of the La1-xSrxFeO3 perovskites. We recommend La0.7Sr0.3FeO3 as the optimal oxygen carrier in the series because it gives the maximum Ola/Oad (Ola and Oad stand for lattice oxygen and adsorbed oxygen species, respectively.) ratio of 3.64:1, which can be regarded as a criterion for the reactivity and selectivity of partial oxidation of methane into syngas of the oxygen carriers. Up to 80% CH4 conversion in the methane partial oxidation step and 96% of H2 concentration in the water splitting step were achieved in ten successive redox tests conducted in a fixed bed reactor at 850 °C with La0.7Sr0.3FeO3 as a redox catalyst. The electronic properties of the original LaFeO3 cell and its lattice substituted by Sr were calculated based on the density functional theory method. Electronic structure analysis demonstrates that doping of Sr makes LaFeO3 more electric conductive and its electron is prone to be excited. This is in agreement with the test results that La0.7Sr0.3FeO3 exhibited better performance in chemical looping reactions.

Selective Partial Oxidation of Hydrogen Sulfide by the BrOx Cycle

AbstractNeutralization of the toxic gas hydrogen sulfide is an important step in almost every natural gas processing plant. A novel method for the conversion of hydrogen sulfide is a combination of bromination and oxidation reactions (BrOx). The first of those, the bromination of hydrogen sulfide, is investigated experimentally. An analysis of the potential by‐products reveals that their formation during the reaction can be excluded. Furthermore, a parameter study showed that reaction yields of up to 94 % are achievable by suitable reaction conditions.

Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins.

Oxygenase reactivity toward selective partial oxidation of CH4 to CH3OH requires an atomic oxygen-radical bound to metal (M-O•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH4. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O• intermediate. However, no experimental data of M-O• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O•, its reactivity for CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temperature. The resultant CH3• species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol-1. On the basis of the findings in gas-phase experiments regarding the CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-O• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O• bond and thereby enables to create effectively the highly reactive M-O• bond required for low-temperature activation of CH4. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

Mars van Krevelen Mechanism for the Selective Partial Oxidation of Ethane

Abstract Ethylene is the most important olefin in the petrochemical context, since it is the main raw material for the production of many polymers. Traditional production of ethylene via thermal cracking and catalytic dehydrogenation consumes large amounts of energy; hence selective partial oxidation of ethane has been considered as an attractive alternative production path. Recently, development of a promising catalyst for selective partial oxidation of ethane, which consists of multi-metallic mixed oxides of Mo, Te, V, and Nb, has been published. It is also noteworthy that this catalytic system starts to be active at temperatures below 400 °C, substantially lower than the one required by commercial thermal processes, >800 °C. In this work, a kinetic mechanism based on Mars van Krevelen formalism is proposed for the selective partial oxidation of ethane, considering the surface itself as active protagonist of the reaction. RedOx steps on active sites are considered as the controlling ones, and the rest of transformations are considered as pseudo-steady steps. It is noticed that there are side reactions, which produces CO and CO2 as combustion by-products. Additionally, there is competition for reduced sites on the catalytic surface, mainly between oxygen and water molecules, which adsorb strongly on these sites. Adjusted results by the mechanism proposed are in agreement with experimental observations of reaction rates diminishing proportionally to partial pressure of water, caused by competition of reduced sites on catalyst surface.

Heterogeneous Catalytic Oxidation of Amides to Imides by Manganese Oxides

Herein, we report a one-step peroxide mediated heterogeneous catalytic oxidation of amides to imides utilizing a series of manganese oxides. Among them, Cs/Mn2O3 was found to be the most active catalyst for the selective partial oxidation of N-benzylbenzamide to diphenyl imide. We have been able to apply an optimized oxidation method to other aromatic substrates. The feasibility of using air as an oxidant, the heterogeneous nature, inexpensive catalytic materials, respectable turnover numbers, and chemoselectivity to imides make this methodology an attractive choice for functional group transformations of amides to imides.

Photo‐Electrochemical C−H Bond Activation of Cyclohexane Using a WO3 Photoanode and Visible Light

AbstractThe photo‐electrochemical C−H bond activation of cyclohexane to produce cyclohexanol and cyclohexanone (KA oil) with high partial oxidation selectivity (99 %) and high current utilization ratio (76 %) was achieved in air at room temperature at atmospheric pressure. The production rate of KA oil was accelerated by applying a bias. The incident photon to current efficiencies at 365 and 420 nm were 57 % and 24 %, respectively.

Photo-Electrochemical C-H Bond Activation of Cyclohexane Using a WO3 Photoanode and Visible Light.

The photo-electrochemical C-H bond activation of cyclohexane to produce cyclohexanol and cyclohexanone (KA oil) with high partial oxidation selectivity (99 %) and high current utilization ratio (76 %) was achieved in air at room temperature at atmospheric pressure. The production rate of KA oil was accelerated by applying a bias. The incident photon to current efficiencies at 365 and 420 nm were 57 % and 24 %, respectively.

Spinel‐Type Mixed Oxides for Stable and Selective Partial Oxidation of Benzyl Alcohol

AbstractBenzaldehyde is an important chemical intermediate, which need to be prepared through an eco‐friendly process. Manganese oxides are active as catalysts for its preparation through partial oxidation of benzyl alcohol. In this study, Cu, Co and Mn containing spinel based mixed oxides (CuxMn3−xO4) were prepared and tested for partial oxidation of benzyl alcohol to get benzaldehyde with high selectivity. The catalytic activity strongly depended on chemical composition, acid‐base properties and oxygen uptake. The influence of chemical composition, reaction temperature, oxygen partial pressure and the weight hourly space velocity (WHSV) were investigated to optimize benzaldehyde yield and to stabilize the catalyst activity. A high benzaldehyde selectivity of 98%, accompanied with benzyl alcohol conversion of 84.5% was obtained over Cu0.25Mn2.75O4 catalyst at 300 °C. It was evident from temperature programmed desorption (TPD) of CO2 that this catalyst was found to have optimum basicity and high oxygen uptake, thus imparting high activity and stability. Activity and catalyst life could be attributed to appropriate acid‐base properties.

Control of selectivity in methane conversion reactions in RF plasma: the influence of reaction conditions

RF plasma excitation of methane has been studied in an effort to optimize the reaction conditions for a selective partial oxidation of methane. The reaction products of RF-excited methane are C2 hydrocarbons such as ethane and acetylene when O2 is not used. The introduction of a few percent of O2, however, is found to switch the selectivity in favor of CO while CO2 formation is suppressed down to a level below a few percent. Interestingly, in the low O2 ratio regime (0–0.6), the selectivity between CO and C2 hydrocarbons is observed to vary systematically in response to the detailed reaction conditions, including flow rate, pressure and applied RF power, which are explained by the competition between coupling and partial oxidation reactions. Variation in the density and the residence time of the active species in the plasma is suggested to determine the overall reaction pathways. The present results suggest a possibility of a selective production of the partial oxidation products of methane such as CO with a high selectivity and a high conversion efficiency using controlled RF plasma from methane and O2.

Selective photocatalytic oxidation of 5-hydroxymethyl-2-furfural in aqueous suspension of polymeric carbon nitride and its adduct with H2O2 in a solar pilot plant

This work reports a study on the reactivity of polymeric carbon nitride (PCN) and PCN-H2O2 adduct samples for sunlight photocatalytic selective oxidation reactions. The characterization of these materials was reported in a previous paper where the reactivity towards the partial oxidation of 5-hydroxymethylfurfural (HMF) was studied by using two different laboratory scale photoreactors; one irradiated by UV lamps and the other one by natural sunlight. In the present study it has been confirmed the effectiveness of these C3N4 based materials for the selective partial oxidation of HMF to FDC (2,5-furandicarboxaldehyde) in aqueous medium in a pilot plant photoreactor irradiated by natural sunlight. The reactivity results and, in particular, the selectivity to FDC formation have been very encouraging, mostly by considering that the reaction was carried out in water. Moreover, they are comparable with those obtained in the laboratory scale photoreactors irradiated by both UV lamps and natural sunlight. Interestingly, the pristine PCN sample has shown a higher HMF conversion with respect to that of the PCN-H2O2 adduct, but the last one is more selective to the FDC formation. A kinetic study indicates that the pseudo-first-order rate constant of HMF oxidation is higher in the case of pristine photocatalyst and that the equilibrium adsorption constants of HMF is higher in the case of PCN-H2O2 adduct catalyst. Finally, the partial oxidation of two aromatic alcohols, i.e. benzyl alcohol (BA) and 4-methoxy benzyl alcohol (4-MBA) to benzaldehyde (BAL) and 4-methoxy benzaldehyde (4-MBAL), respectively, has also been studied.

Effect of synthesis solution pH of Co/γ-Al2O3 catalyst on its catalytic properties for methane conversion to syngas

The cobalt nanoparticles over γ-Al2O3 support were prepared via chemical reduction of CoCl2·6H2O using NaBH4 with various values of pH in the range of 11.92–13.80. Synthesized catalysts were studied through X-ray diffraction (XRD), N2 adsorption/desorption (BET), H2-temperature programmed reduction (H2-TPR), H2-chemisorption, O2 pulse titration and temperature programmed oxidation (TPO) methods. Obtained results exhibited the synthesis solution pH showed a significant influence on the activity and selectivity in partial oxidation of methane reaction. The methane conversion, CO selectivity and H2 yield were enhanced by increasing of the synthesis solution pH. Compared to other catalysts, the catalyst that synthesized at pH of 13.80, showed a superior ability in syngas production with a H2/CO ratio of near 2 and also a proper stability against deactivation during the partial oxidation of methane.

Recent development of heterogeneous catalysis in ring-opening, biocatalysis, and selective partial oxidation reactions on metal oxides

Abstract In this review article, we analyze the state of the art and future developments in three important domains of heterogeneous catalysis, namely ring opening, biocatalysis, and partial oxidation on metal oxides. After recollecting the scientific bases of each domain, we consider several examples, some recent improvements/developments, and some prospective views.

Phase Coexistence of Multiple Copper Oxides on AgCu Catalysts during Ethylene Epoxidation

Alloy catalysts under reaction conditions are complex entities. In oxidizing atmospheres, multiple phases can coexist on a catalyst’s surface as a result of phase segregation and preferential oxidation. Such a scenario can result in unusual substoichiometric and metastable phases that could play important roles in catalytic processes. For instance, AgCu alloys—known to exhibit enhanced epoxide selectivity in partial oxidation of ethylene—form an oxide-like surface structure under reaction conditions. Under these conditions, copper oxides are stable, while silver oxides are not. Consequently, copper segregates to the alloy’s surface and forms an oxide overlayer. Little is known about the structure or function of such overlayers, and it is unknown whether they play an active role in the catalyst’s enhanced selectivity. In order to develop a clearer picture of such catalysts, the current work utilizes several in situ spectroscopic and microscopic techniques to examine the copper oxide phases that form when AgCu is exposed to epoxidation conditions. It is found that several forms of oxidic Cu coexist simultaneously on the active catalyst’s surface, namely, CuO, Cu2O, and some previously unreported form of oxidized Cu, referred to here as CuxOy. Online product analysis, performed during the in situ spectroscopic measurements, shows that increased epoxide selectivity is correlated with the presence of mixed copper oxidation states and the presence of the CuxOy species. These results support previous theoretical predictions that oxidic copper overlayers on silver play an active role in epoxidation. These results furthermore emphasize the need for in situ spectromicroscopic methods to understand the complexity of alloy catalysts.

Effect of Solvent on Conversion and Selectivity during the Selective Oxidation of Cyclohexane by Nano-V2O5/Ti Membrane Electrode

An electrocatalytic membrane reactor (ECMR) with nano-V2O5 loading porous Ti electrode (V2O5/Ti) as an anode was conducted to the selective partial oxidation of cyclohexane (CHA). The ternary phase diagram of CHA, H2O and solvents was determined. The effect of the solvent system on the conversion rate of CHA and selectivity toward cyclohexanone (K) and cyclohexanol (A) during the selective oxidation of CHA was explored. Cyclic voltammetry (CV) and electron spin resonance (ESR) were employed to monitor the electrochemical behavior of CHA-H2O-solvent (acetone and acetic acid) system. Response surface methodology (RSM) was used to optimize the operating parameters of ECMR. Results showed that the conversion of CHA improved with an increasing solvent polarity. The selectivity to K was related to solvent solubility parameter. 16.8% conversion of CHA, 90.2% selectivity to K and 99.9% selectivity to KA oil were obtained from the reaction system with acetone as solvent and acetic acid as co-solvent (CHA concentration 20 mmol L−1). In particular, acetic acid promoted the production of hydroxyl radicals which played a critical role in the selective oxidation of CHA by ECMR.

Partial oxidation of alkanes by dioxiranes formed in situ at low temperature.

Partial oxidation catalysts capable of efficiently operating at low temperatures may limit the over-oxidation of alkane substrates and thereby improve selectivity. This work focuses on examining alkane oxidation using completely metal-free organocatalysts, dioxiranes. The dioxiranes employed here are synthesized by oxidation of a ketone using a terminal oxidant, such as hydrogen peroxide. Our work generates the dioxirane in situ, so that the process can be catalytic with respect to the ketone. To date, we have demonstrated selective partial oxidation of adamantane using ketone catalysts resulting in yields upwards of 60% towards 1-adamantanol with greater than 99% selectivity. Furthermore, we have demonstrated that changing the electrophilic character of the ketone R groups to contain more electron-donating ligands facilitates the dioxirane ring formation and improves overall oxidation yields. Isotopic labelling studies using H218O2 show the preferential incorporation of an 18O label into the parent ketone, providing evidence for a dioxirane intermediate formed in situ The isotopic labelling studies, along with solvent effect studies, suggest the formation of peracetic acid as a reactive intermediate.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

The Direct Catalytic Oxidation of Methane to Methanol-A Critical Assessment.

Despite the large number of disparate approaches for the direct selective partial oxidation of methane, none of them has translated into an industrial process. The oxidation of methane to methanol is a difficult, but intriguing and rewarding, task as it has the potential to eliminate the prevalent natural gas flaring by providing novel routes to its valorization. This Review considers the synthesis of methanol and methanol derivatives from methane by homogeneous and heterogeneous pathways. By establishing the severe limitations related to the direct catalytic synthesis of methanol from methane, we highlight the vastly superior performance of systems which produce methanol derivatives or incorporate specific measures, such as the use of multicomponent catalysts to stabilize methanol. We thereby identify methanol protection as being indispensable for future research on homogeneous and heterogeneous catalysis.

Heterogeneous Catalysis on Metal Oxides

This review article contains a reminder of the fundamentals of heterogeneous catalysis and a description of the main domains of heterogeneous catalysis and main families of metal oxide catalysts, which cover acid-base reactions, selective partial oxidation reactions, total oxidation reactions, depollution, biomass conversion, green chemistry and photocatalysis. Metal oxide catalysts are essential components in most refining and petrochemical processes. These catalysts are also critical to improving environmental quality. This paper attempts to review the major current industrial applications of supported and unsupported metal oxide catalysts. Viewpoints for understanding the catalysts’ action are given, while applications and several case studies from academia and industry are given. Emphases are on catalyst description from synthesis to reaction conditions, on main industrial applications in the different domains and on views for the future, mainly regulated by environmental issues. Following a review of the major types of metal oxide catalysts and the processes that use these catalysts, this paper considers current and prospective major applications, where recent advances in the science of metal oxide catalysts have major economic and environmental impacts.