Selective Partial Oxidation Research Articles

Metal Incorporated Zeolites as Heterogeneous Catalysts in the Selective Partial Oxidation of Methane to Methanol: A Review

Metal Incorporated Zeolites as Heterogeneous Catalysts in the Selective Partial Oxidation of Methane to Methanol: A Review

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.

Design principles of operando ultraviolet-visible and electron paramagnetic resonance spectroscopy setups for active site characterization in ion-exchanged zeolites

Cu- and Fe-exchanged zeolites have been widely investigated for their applicability in selective partial oxidation of CH4 and abatement of environmentally harmful nitrogen oxides. However, the differentiation between spectator and active sites is cumbersome due to their dynamic co-existence, which aggravates the elucidation of the redox dynamics of the latter. Therefore, correlated multi-spectroscopic approaches carried out under operando conditions are needed in order to disentangle the inherent reactivity of specific species. Here we describe the conceptualization of complementary operando spectroscopic methods for the investigation of ion-exchanged zeolites employed in redox reactions. The potential and versatility of this approach are demonstrated by means of two exemplary case studies. First, we present the insight generated by two custom operando ultraviolet-visible and electron paramagnetic resonance spectroscopy setups operating in batch mode into CH4-to-CH3OH conversion over Cu-exchanged mordenite. In addition to recording the method-dependent spectroscopic fingerprints of distinct Cu centers, the overall CH4 consumption is simultaneously monitored in both setups. The extracted apparent activation energies of the reactive centers obtained from the two systems are in good agreement with each other allowing to link the bulk reactivity to the intrinsic site-specific kinetics of all Cu centers. Second, the EPR setup is equipped with a flow reactor designed to carry out modulated excitation experiments with phase-sensitive detection in order to improve the signal-to-noise ratio and time resolution. The cell is employed to study the conversion of NO via selective catalytic reduction over Fe-exchanged ferrierite. In combination with operando X-ray absorption spectroscopy, a small portion of active Fe species consisting of monomers in gamma-positions and oligomeric structures located in the main channel is identified, which are kept in a redox active state under relevant reaction conditions and thus sustain the chemical transformation of NO.

The Mechanism and Pathway of Selective Partial Oxidation of n-Butane to Maleic Anhydride Studied on Titanium Phosphate Catalysts

The Mechanism and Pathway of Selective Partial Oxidation of <i>n</i>-Butane to Maleic Anhydride Studied on Titanium Phosphate Catalysts

Electron-Delocalization-Stabilized Photoelectrocatalytic Coupling of Methane by NiO-Polyoxometalate Sub-1 nm Heterostructures.

The oxidative coupling of methane to C2 oxygenates merits great scientific and technological potential yet remains a challenge due to its inferior selectivity. Subnanomaterials (SNMs) with "p-n-p-n"-type heteroconstructions feature enhanced external field coupling properties and tunable electronic structures, serving as promising catalysts for the selective partial oxidation of methane. Here we develop NiO-polyoxometalate (POM) subnanocoils with a thickness of 1.8 nm, showing excellent catalytic activity toward photoelectrochemical coupling of methane into a C2 product under mild conditions (1 bar, 25 °C) with a notable productivity (up to 4.48 mmol gcat-1 h-1) and a high selectivity (>99%). Under photoelectrochemical coupling, C-H bonds can be activated by NiO, and the resulted *COOH intermediates are stabilized by the delocalized electrons in POM clusters. The contiguous active sites of NiO and POM at the molecular level allow the in situ coupling of *COOH into oxalate. This work points out an economic way for the oxidation of methane under mild conditions and may enlighten the design of functional SNMs from fundamental standpoints.

Methane against Methanol: The Tortoise and the Hare of the Oxidation Race.

The selective partial oxidation of methane to methanol has been a major chemistry challenge over the past several decades. The reason for this is that the weaker C-H bond of the desired product (methanol) is readily activated by the same catalyst used to activate the stronger C-H bond of methane. Quantum chemical calculations reveal how hydrogen-bonding interactions with the catalyst as well as other electronic and geometric effects slow the unwanted methanol oxidation reaction. Thus, the oxidation of methane (the tortoise in Aesop's fable) becomes faster than methanol (Aesop's hare), increasing the selectivity toward the desired product. Activation barriers are calculated for two different mechanisms (2+2 and radical), and reaction rates for the oxidation of the two molecules are obtained using semiclassical instanton theory to include tunneling effects for the proton transfers. The tunneling effects are shown to accelerate all reactions substantially but do not dramatically affect the selectivity.

Enhancing catalytic performance and hot electron generation through engineering metal-oxide and oxide-oxide interfaces

Interfaces are of utmost importance in catalytic reactions, influencing reaction kinetics and electron transfer processes. However, investigations in combined interfaces of metal-oxide and oxide-oxide at heterogeneous catalysts still have challenges due to their complex structure. Herein, we synthesized well-defined Co3O4 and CeO2 cubes with distinct facets and investigated their catalytic performance when deposited on a Pt-thin film, focusing on the influence of metal-oxide and oxide-oxide interfaces. Catalytic measurements demonstrated that the CeO2/Pt interface significantly enhanced turnover frequency (TOF) and selectivity for partial methanol oxidation compared to Co3O4/Pt and bare Pt. Notably, the CeO2/Co3O4/Pt nanodevice exhibited improved partial oxidation selectivity, highlighting the role of the CeO2/Co3O4 interface in methyl formate production. Chemicurrent measurements demonstrate enhanced hot electron generation due to increased overall TOF and partial oxidation production. We also conducted near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) analysis, revealing a higher concentration of Ce3+ ions and increased oxygen vacancies in the CeO2/Co3O4/Pt catalyst, suggesting oxygen migration from CeO2 to Co3O4, leading to methoxy species stabilization and promoting methyl formate formation.

Selective Partial Oxidation of Methane with CO2 Using Mobile Lattice Oxygens of LSF

Selective Partial Oxidation of Methane with CO₂ Using Mobile Lattice Oxygens of LSF

A Selective Oxidation Strategy towards the Yolk-Shell Structured ZnS@C Material for Ultra-Stable Li-Ion Storage.

Metal chalcogenides are attractive anode materials for lithium-ion batteries (LIBs) due to their high theoretical capacities. With the advantages of low cost and abundance reserves, ZnS is regarded as the prime candidate anode material for future generations, but its practical application is hindered by the large volume expansion during repeated cycling processes and inherent poor conductivity. Rational design of the microstructure with large pore volume and high specific surface area is of great significance to solve these problems. Here, a carbon-coated ZnS yolk-shell structure (YS-ZnS@C) has been prepared by selective partial oxidation of a core-shell structured ZnS@C precursor in air and subsequent acid etching. Studies show that the carbon wrapping and proper etching to bring cavities can not only improve the material's electrical conductivity, but can also effectively alleviate the volume expansion problem of ZnS during its cycles. As a LIB anode material, the YS-ZnS@C exhibits an obvious superiority in capacity and cycle life compared to ZnS@C. The YS-ZnS@C composite shows a discharge capacity of 910 mA h g-1 at the current density of 100 mA g-1 after 65 cycles, compared to only 604 mA h g-1 for ZnS@C after 65 cycles. Notably, at a large current density of 3000 mA g-1, a capacity of 206 mA h g-1 can still be maintained after 1000 cycles (over three times of the capacity for ZnS@C). It is expected that the synthetic strategy developed here is applicable to designing various high-performance metal chalcogenide-based anode materials for LIBs.

Nanosponge-C3N4 composites as photocatalysts for selective partial alcohol oxidation in aqueous suspension.

A set of four composite materials was prepared, consisting of a nanosponge matrix based on β-cyclodextrin in which carbon nitride was dispersed. The materials were characterized by the presence of diverse cross-linker units joining the cyclodextrin moieties, in order to vary the absorption/release abilities of the matrix. The composites were characterized and used as photocatalysts in aqueous medium under UV, visible and natural solar irradiation for the photodegradation of 4-nitrophenol, and for the selective partial oxidation of 5-hydroxymethylfurfural and veratryl alcohol to the corresponding aldehydes. The nanosponge-C3N4 composites showed higher activity than the pristine semiconductor, which can probably be attributed to the synergic effect of the nanosponge, capable of increasing the substrate concentration near the surface of the photocatalyst.

Tuning the physicochemical features of titanium oxide nanomaterials by ultrasound: Elevating photocatalytic selective partial oxidation of lignin-inspired aromatic alcohols

The research for “green” and economically feasible approaches such as (photo)catalysis especially for biomass valorization such as selective oxidation of biomass derived compounds like aromatic alcohols to corresponding aldehyde by avoiding the harsh reaction conditions and the addition of reagents concentrate the focus of attention the last years. Hence, design and development of novel photocatalyst for the partial selective oxidation is highly desirable. In this research work, ultrasonication of different frequencies (22, 40, 80 kHz) and different amplitudes was utilized as synthesis tool in order to obtain novel materials by precipitation method. The synthesized samples were characterized by using different techniques such as N2 sorption, TEM, XPS, XRD, thermal analysis, and diffuse reflectance spectroscopy. The synthesized sample by using low ultrasound frequency (22 kHz) and amplitude showed a mixed morphological and structural nature consisting of asymmetric 1-dimensional (nanorods-like), layered nano-structures and not well-defined areas, leading to elevate for metal oxide specific surface areas up to 155 m2/g. The observed 1-D nanostructures have diamentions in the range of 20–60 nm. This sample revealed the highest photo-oxidation efficiency for the selective conversion of two biomass-derived, and more specifically lignin-inspired model compounds, benzyl alcohol and cinnamyl alcohol to benzaldehyde and cinnamyl aldehyde, respectively, and hence the highest yield towards the desired aldehydes. The selective photo-oxidation activity was retained even after 5 photocatalytic cycles, while no leaching of Ti was recorded.

NiO/ZnO heterojunction nanorod catalyst for high-efficiency electrochemical conversion of methane

Electrochemical conversion is promising for selective partial oxidation of methane. We present a NiO/ZnO shell/core nanorod catalyst used to enhance electrochemical conversion. The built-in potential at the NiO/ZnO interface and the 1D morphology of the nanorods allow for fast charge transfer, thereby enhancing electrochemical methane conversion. We achieved a high ethanol production rate of 1084.2 μmol/gNiO/hr with a selectivity of 81 % for a 600-nm long NiO/ZnO nanorod catalyst. Mechanistic analyses with isotopic labeling reactions suggested ethanol production pathways involving generation of active oxygen by dissociative adsorption of the anionic CO32- oxidant, formation of methanol by methane activation, and coupling of deprotonated methanol and methane. Furthermore, we improved the ethanol yield by improving methane solubility in sulfolane cosolvent mixtures. We also demonstrated conversion with in situ generation of CO32- from a CH4/CO2 mixture.

The role of promoters on the catalytic performance of MxV2O5 bronzes for the selective partial oxidation of H2S

Metal-containing vanadium oxide bronzes (MxV2O5; M= Cu, Ag and Ca) have been prepared, characterized (before and after reaction) by various physicochemical techniques, and tested in the partial oxidation of H2S. The catalysts were prepared hydrothermally at 175 °C (from gels containing M/V molar ratios of 0.17 or 0.33) and heat-treated at 500 °C/2 h in N2. The most effective catalysts, showing a sulfur selectivity greater than 95 % for a H2S conversion beyond 90 %, were those presenting vanadium oxide bronze (β-Cu0.261V2O5 or Ag0.333V2O5) as the main crystalline phase. Cu- and Ag-containing vanadium oxide bronzes were stable under reaction conditions. For calcium containing materials (mainly presenting the Ca0.17V2O5 bronze phase), the formation of CaSO4 has been observed during the reaction, which resulted in a negative effect on both activity and selectivity. The nature of active and selective sites in this type of catalysts, as well as the role of promoters, are also discussed.

Photocatalytic conversion of methane selectively into oxygenated products in the presence of chloride ions

Selective partial oxidation of methane to methanol under mild conditions remains a great challenge in activating the first CH bond of CH4 and inhibiting overoxidation. Chlorine-mediated pathway arises as a promising strategy to obtain high CH4 conversion under mild conditions. Herein, a considerable CH3OH production rate of 180.75 μmol·gcat−1·h−1 with 80.07 % selectivity is achieved when the BiOCl with oxygen vacancies is irradiated by visible light in NaCl solution with H2O2 addition. Experimental studies reveal that chlorine radicals are in situ produced and constrained on the surface of the catalyst, which inhibits the over halogenation. Reactive oxygen species produced by H2O2 dissociation were utilized to be the receptors of CH3. The high productivity of methanol was achieved by the synergy effect of chlorine radicals and H2O2. This work provides a new feasible method to convert inert methane into high-value-added products via photocatalysis.

Mechanistic and kinetic studies of benzyl alcohol photocatalytic oxidation by nanostructured titanium (hydro)oxides: Do we know the entire story?

Selective upgrade of lignocellulosic biomass-derived aromatic compounds by photocatalytic oxidation is assumed as a prosperous, environmentally friendly, and cost-effective process. We present for the first time the application of various advantageous titanium oxide nano-photocatalysts for the additives-free selective partial oxidation of benzyl alcohol to benzaldehyde at ambient conditions. The two best-performing materials were found titanate nanotubes and nanocores of anatase surrounded by amorphous titanium hydroxide phase. The results obtained by scavenger tests on top of the DFT calculations led us to conclude that various reactions and active species are responsible by a complex way for materials’ photoreactivity. Depending on nanocatalysts’ physicochemical features as well as the light irradiation (ultraviolet vs. royal-blue), different mechanisms/oxidation-pathways are photo-catalyzed. Finally, we show that utilizing organic compounds like benzoquinone as scavenger hides risks due to the interactions with the targeted to be converted organic that results in elevated photolytic decomposition even under royal-blue light.

Rational Catalyst Design for Higher Propene Partial Electro-oxidation Activity by Alloying Pd with Au

Selective partial oxidation of hydrocarbons to oxygenates plays a large role in the chemical industry, while falling prices for electricity from renewable sources make electrification of such industrial chemical processes relevant. The oxidation of propene is an interesting model system as propene can be oxidized in two different positions, allowing for insights into the reaction mechanism. On Pd, a layer of adsorbates formed in situ governs the reaction by steering reactant adsorption to achieve high selectivity for allyl oxidation, albeit largely inhibiting the reaction rate. Through rational catalyst design, we demonstrate that alloying reactive Pd with inert Au influences the adsorbate layer formation, enhancing activity while maintaining high selectivity toward allyl oxidation. We obtain mechanistic insights with a combination of ab initio computational modeling and electrochemical measurements with ex situ product quantification and online mass spectrometry. Using a statistical approach, we explore the correlation of the Au:Pd ratio with Pd surface cluster size and density, which determine the properties of the adsorbate layer and thus the reaction outcome. We report an activity enhancement by a factor 2.4 with 10% Au in Pd and propose that (i) activity is maximized at potentials just before Pd cluster oxidation and (ii) the optimal catalyst surface contains approximately one Au every six Pd atoms, statistically most frequent at the nominal alloy composition Au14Pd86.

Collision dependent silver nucleation regulated by chemical diffusion and reaction

Nucleation is widely present in nature, from food production to haze generation. Owe to the spatiotemporal limitation in catching the dynamic characters of nucleation process, the theoretical development and the control technique of nucleation is far behind the demand. Here we report a strategy of controlling nucleation process via manipulating the collision of nucleation units, with the aim of disclosing the mechanism dominating the nucleation process. Silver nucleation is simulated at different collision conditions which is regulated by chemicals diffusion and reaction rate. It is found that the nucleation rate has a good linear relationship with the collision frequency, suggesting a collision dependent nucleation model. This model is evaluated by particles-based silver aggregation and validated by experiments conducted in an in-situ electron microscopy. Furthermore, the model is applied to the improved synthesis of silver catalysts for selective partial oxidation of ethylene.

Carbon-Based Nanocatalysts (CnCs) for Biomass Valorization and Hazardous Organics Remediation.

The continuous increase of the demand in merchandise and fuels augments the need of modern approaches for the mass-production of renewable chemicals derived from abundant feedstocks, like biomass, as well as for the water and soil remediation pollution resulting from the anthropogenic discharge of organic compounds. Towards these directions and within the concept of circular (bio)economy, the development of efficient and sustainable catalytic processes is of paramount importance. Within this context, the design of novel catalysts play a key role, with carbon-based nanocatalysts (CnCs) representing one of the most promising class of materials. In this review, a wide range of CnCs utilized for biomass valorization towards valuable chemicals production, and for environmental remediation applications are summarized and discussed. Emphasis is given in particular on the catalytic production of 5-hydroxymethylfurfural (5-HMF) from cellulose or starch-rich food waste, the hydrogenolysis of lignin towards high bio-oil yields enriched predominately in alkyl and oxygenated phenolic monomers, the photocatalytic, sonocatalytic or sonophotocatalytic selective partial oxidation of 5-HMF to 2,5-diformylfuran (DFF) and the decomposition of organic pollutants in aqueous matrixes. The carbonaceous materials were utilized as stand-alone catalysts or as supports of (nano)metals are various types of activated micro/mesoporous carbons, graphene/graphite and the chemically modified counterparts like graphite oxide and reduced graphite oxide, carbon nanotubes, carbon quantum dots, graphitic carbon nitride, and fullerenes.

Oxo dicopper anchored on carbon nitride for selective oxidation of methane

Selective conversion of methane (CH4) into value-added chemicals represents a grand challenge for the efficient utilization of rising hydrocarbon sources. We report here dimeric copper centers supported on graphitic carbon nitride (denoted as Cu2@C3N4) as advanced catalysts for CH4 partial oxidation. The copper-dimer catalysts demonstrate high selectivity for partial oxidation of methane under both thermo- and photocatalytic reaction conditions, with hydrogen peroxide (H2O2) and oxygen (O2) being used as the oxidizer, respectively. In particular, the photocatalytic oxidation of CH4 with O2 achieves >10% conversion, and >98% selectivity toward methyl oxygenates and a mass-specific activity of 1399.3 mmol g Cu−1h−1. Mechanistic studies reveal that the high reactivity of Cu2@C3N4 can be ascribed to symphonic mechanisms among the bridging oxygen, the two copper sites and the semiconducting C3N4 substrate, which do not only facilitate the heterolytic scission of C-H bond, but also promotes H2O2 and O2 activation in thermo- and photocatalysis, respectively.

Mechanistic insights into the oxidation of catalytically relevant AgCu near‐surface alloy interfaces

AbstractRecent heterogeneous catalysis studies have demonstrated that synergy between Ag and Cu can lead to more selective partial oxidation chemistries. We performed a series of scanning tunneling microscope experiments to gain a better understanding of the AgCu system under oxidative conditions. These experiments were carried out by exposing sub‐monolayer coverages of Ag on Cu(111), in the form of a near‐surface alloy (NSA), to range of oxygen exposures and temperatures. This enabled us to study the initial stages of oxidation of well‐defined Ag/Cu interfaces with atomic resolution and thereby understand the dynamic response of the AgCu NSA to oxygen environments. At low oxygen exposures, oxidation was observed on exposed Cu terraces and at the interface between the AgCu NSA and Cu(111). Higher oxygen exposure led to the segregation of Cu atoms up through the Ag layer and the appearance of surface adsorbed oxygen. Significant phase segregation of Cu was then observed at higher oxygen exposures at elevated temperatures, evidenced by the formation of Cu oxide patches within and on the top of the Ag layer. These results provide a more detailed picture of how AgCu NSAs interact with, and restructure in response to, oxygen.