Catalysts For Selective Oxidation Research Articles

Molecular oxygen-promoted reconstruction of molybdenum phosphide enables selective electrochemical oxidation of p-chlorotoluene

It is highly challenging to develop efficient catalysts for selective oxidation of p-chlorotoluene (PCT) into p-chlorobenzaldehyde (PCB) as PCB easily undergoes over-oxidation. Herein, a new electrochemical strategy was developed for selective oxidation of PCT with high selectivity toward PCB. A self-standing molybdenum phosphide (MoP) anode was directly synthesized on carbon cloth with the assistance of polyvinyl pyrrolidone. The as-synthesized MoP anode with 3D network structure, exhibits the selectivity of 91.4% toward PCB at the conversion of 75.6% in acetonitrile solution containing 0.1 M Bu4NClO4. During electrochemical oxidation, molecular oxygen-induced transformation of MoP into MoO2 initiates the activation of PCT into ClC6H5CH2+. The ClC6H5CH2+reacts with molecular O2 over electrode surface, producing the product of PCB. This work offers a promising strategy to achieve selective oxidation of CH bond based on electrochemical environment.

Mesoporous titanium-aluminosilicate as an efficient catalyst for selective oxidation of cyclohexene at mild reaction conditions

Mesoporous titanium containing alumino-silicate materials with various titanium/silicon (Ti/Si) ratio (AlSi-Ti(n); n = Ti/Si mole ratio) have been successfully synthesized by a novel single-step sodium (Na)-free method, for the first time. The obtained characterization results of the prepared materials reveal that in-situ addition of Ti into AlSi shows ordered mesoporous structure along with uniformly dispersed Ti species in +4 and +3 oxidation states suitable for selective oxidation of allylic C—H bond. The prepared mesoporouse Ti-AlSi(n) samples exhibited excellent activity in the oxidation of cyclohexene with 100% conversion and 100% selectivity to ketone-alcohol (KA) oil (cyclohex-2-en-1-ol and 2-cyclohexen-1-one) at low temperature and reaction time (35 °C and 30 min reaction time). This study suggests that AlSi-Ti(0.05) material can be a promising catalyst for the selective oxidation of cyclohexene under mild reaction conditions.

Fe3C confined in N-doped carbons derived from Fe-N bearing ionic liquids for selective oxidation of styrene into benzaldehyde with molecular oxygen

BackgroundIt is of great significance but still a challenge in the chemical industry to develop high-efficient heterogeneous catalysts for selective oxidation of styrene into benzaldehyde with molecular oxygen. MethodsFe3C nanoparticles confined in N-doped carbons (Fe3C/NC) were prepared using Fe3+ coordinated N-bearing ionic liquids as precursors and melamine as nitrogen source and template, followed by pyrolysis at 900 °C under N2 atmosphere. The physicochemical properties of the as-prepared Fe3C/NC catalyst were analyzed by FT-IR, TG, BET, TEM, XRD, Raman and XPS, respectively, and further employed as novel heterogeneous catalyst for selective oxidation of styrene into benzaldehyde with molecular oxygen. Significant findingsThe results showed that the Fe3C nanoparticles were formed and uniformly embedded into the N-doped carbon matrix, which were confirmed to be the active sites for styrene oxidation. Moreover, the 5 % Fe3C/NC catalyst possessed high surface area and dominant mesoporous structure, which could expose more accessible active sites and facilitate mass transfer. As a result, the 5 % Fe3C/NC catalyst exhibited superior catalytic performance for styrene oxidation into benzaldehyde with the appropriate styrene conversion of 58.8 % and selectivity to benzaldehyde of 64.1 % as well as no significant activity and selectivity loss after reused at least five times.

Heterogenization of Phosphotungstate Clusters into Magnetic Microspheres: Catalyst for Selective Oxidation of Alcohol in Water

Heterogenization of Phosphotungstate Clusters into Magnetic Microspheres: Catalyst for Selective Oxidation of Alcohol in Water

Energetic Order of Nb 2 O 5 and Ta 2 O 5 Polymorphs

Nb2O5 and Ta2O5 are materials with important technical applications, e.g., in electrochemistry as anode materials for Li or Na storage, or as selective oxidation catalysts. The properties of both oxides largely depend on their various polymorphs with different crystal structures. So far, the literature does not agree on which polymorphs of Nb2O5 and Ta2O5 are most stable under standard conditions. Herein, the relative stability of all relevant polymorphs of Nb2O5 and Ta2O5 is calculated using density-functional theory and the recently developed rev2-POB basis sets. First, a benchmark of the atomization-free enthalpies of Nb2O5 is performed with selected generalized gradient approximation and hybrid functionals. The effect of London dispersion on structural and energetic properties is explicitly investigated. A large scattering of atomization energies is observed for the various functionals. The range-separated hybrid functional RSHXLDA without dispersion correction proves to be the method of choice for the systems under investigation. For this reason, the stability order obtained with RSHXLDA is consisdered as most reliable.

Metal-Organic Frameworks Decorated Cu2O Heterogeneous Catalysts for Selective Oxidation of Styrene

The selective oxidation of styrene with highly efficient, environmentally benign, and cost-effective catalysts are of great importance for sustainable chemical processes. Here, we develop an in situ self-assembly strategy to decorate Cu-based metal-organic framework (MOF) Cu-BDC-NH2 nanocrystals on Cu2O octahedra to construct a series of Cu2O@Cu-BDC-NH2 catalysts for selective oxidation of styrene. Using H2O2 as green oxidants, the optimized sample of Cu2O@Cu-BDC-NH2-8h could achieve 85% styrene conversion with 76% selectivity of benzaldehyde under a mild condition of 40 °C. The high performance of the as-prepared heterogeneous catalysts was attributed to the well-designed Cu+/Cu2+ interface between Cu2O and Cu-BDC-NH2 as well as the porous MOF shells composed of the uniformly dispersed Cu-BDC-NH2 nanocrystals. The alkaline properties of Cu2O and the –NH2 modification of MOFs enable the reaction to be carried out in a base-free condition, which simplifies the separation process and makes the catalytic system more environmentally friendly. Besides the Cu2O octahedra (od-Cu2O), the Cu2O cuboctahedrons (cod-Cu2O) were synthesized by adjusting the added polyvinyl pyrrolidone, and the obtained cod-Cu2O@Cu-BDC-NH2 composite also showed good catalytic performance. This work provides a useful strategy for developing highly efficient and environmentally benign heterogeneous catalysts for the selective oxidation of styrene.

High N2 selectivity of Pt-V-W/TiO2 oxidation catalyst for simultaneous control of NH3 and CO emissions

We report a superior N2 selectivity in the catalytic emission control of NH3 and CO over PtVW/TiO2 catalysts synthesized by the co-impregnation method. The PtVW/TiO2 selective oxidation catalyst (SCO) enabled the following surface reactions over bimetallic Pt-V sites: (1) selective catalytic reduction of internally generated NOx species by NH3 (i-SCR) and (2) selective catalytic reduction of NOx by the CO (CO-SCR) present in the feed gas. The enhanced N2 selectivity was characterized in terms of adsorbent behavior and catalyst surface properties as a function of the catalyst formulations. It was found that the strong interaction between the Pt and V metal species increased both the reducibility and the surface acidity of the catalyst, which could be a crucial factor in the enhancement of N2 selectivity. Surface IR measurement and DFT calculations relates these enhanced catalytic properties to the removal of the generated NOx via reaction with stored imide, amide (–NHx) and isocyanate (–NCO) species on the catalyst surface. These stored species enhance the N2 selectivity through the i-SCR and CO-SCR mechanisms. A dual-zoned bench-scale monolith reactor has been tested, with a commercial SCR (front) and the developed SCO (rear) in series. This dual-zoned system showed high N2 selectivity and outstanding CO and NH3 oxidation.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

A review on sensitivity of operating parameters on biogas catalysts for selective oxidation of Hydrogen Sulfide to elemental sulfur

Hydrogen sulfide (H2S) is a critical problem for biogas applications, such as electricity and heat generation, or the production of different chemical compounds, due to corrosion and toxic effluent gases. The selective catalytic oxidation of H2S to S is the most promising way to eliminate H2S from biogas due to the lack of effluents, therefore can be considered a green technology. The most extensively used catalysts for H2S selective oxidation can be classified in two groups: metal oxide-based catalysts, including vanadium and iron oxides, and carbon-based catalysts. Numerous studies have been devoted to studying their different catalytic performances. For industrial applications, the most suitable catalysts should be less sensitive to the operating parameters like the temperature, O2/H2S ratio, and H2O content. More specifically, for metal oxides and carbon-based catalysts, the temperature and O2/H2S ratio have a similar effect on the conversion and selectivity, but carbon-based catalysts are less sensitive to water in all operating conditions.

Heterogeneously-catalyzed aerobic oxidation of furfural to furancarboxylic acid with CuO-Promoted MnO2

A cost-effective and sustainable noble-metal free catalyst system based on ubiquitously available Mn–Cu bimetallic oxides was served as efficient catalysts for furfural selective oxidation to furancarboxylic acid (FA). Interestingly, Mn2Cu1Ox exhibited an excellent furfural conversion of 99% with quantitative selectivity toward FA. Especially, we demonstrate the significant weakening of the Mn–O bonds with the incorporation of CuO into the Mn–Cu oxides, resulting in an improved OL reactivity of Mn2Cu1Ox, which brings about a higher catalytic activity for furfural oxidation. More importantly, Mn2Cu1Ox could exhibit YFA>90% over 5 cycles of reusability test. Through this study, the relationship between the morphology, surface chemistry, and catalytic activity of Mn–Cu bimetallic oxides are elucidated, providing a simple and environmentally friendly catalytic strategy and scientific basis for the development of Mn–Cu bimetallic oxides bioderived molecular aerobic oxidation materials.

Templated Synthesis of Mesoporous Co3O4 Nanostructures for the Liquid-Phase Aerobic Oxidation of Benzyl Alcohol to Benzaldehyde

Mesoporous Co3O4 nanostructures, templated from different mesoporous silicas, SBA-3, SBA-5, and SBA-16, were used as catalysts for the liquid-phase selective oxidation of benzyl alcohol using molecular oxygen as the oxidant. The nanostructured Co3O4-16, templated from SBA-16, showed the best activity for the reaction, with 97.5% benzyl alcohol conversion and 100% benzaldehyde selectivity obtained after 3 h of reaction at 100 °C. This could be attributed to its large number of oxygen vacancies and low surface Co3+/Co2+ ratio that were beneficial to O2 activation, together with its three-dimensional nanostructure that facilitated the adsorption of benzyl alcohol, compared to Co3O4-3 and Co3O4-15. The material could be reused without losing activity if the organics adsorbed on the surface of the used sample were removed by calcination at 300 °C for 1 h. A mechanism for the reaction was proposed, considering that the activation of molecular oxygen was the rate-determining step of the reaction.

New progress in zeolite synthesis and catalysis.

The rational design synthesis of zeolite catalysts with effective, environmentally benign and atom-economic routes is a major topic in the field of microporous materials, as it would avoid the high labor cost and inefficiency of traditional trial-and-error methods in developing new structures and dispel environmental concerns regarding the industrial mass production of zeolites. Catalytic applications of zeolite materials have expanded from conventional single functionalities, such as solid acids or selective oxidation catalysts to bi/multifunctionalities through combination with metals or metal oxides. This is a response to new requirements from petrochemical and fine chemical industries, such as precise control of product distribution, conversion of low-carbon resources for chemical production, and solutions to increasingly severe environmental problems related to CO2 and NOx. Thus, based on the systematic knowledge of zeolite chemistry and science that researchers have acquired in the past half-century and the development requirements, remarkable progress has been made in zeolite synthesis and catalysis in the past 10 years. This includes the manipulation of zeolitic monolayers derived from layered zeolites and germanosilicates to construct novel zeolite materials and effective and green zeolite syntheses as well as the synergistic interaction of zeolites and metal/metal oxides with different space distributions in the conversion of low-carbon resources. With many zeolite catalysts and catalytic processes being developed, our understanding of the close relationship between zeolite synthesis, structure and catalytic properties has deepened. Researchers are gradually approaching the goal of rationally designing zeolite catalysts with precisely controlled activity and selectivity for particular applications.

Progress of vanadium phosphorous oxide catalyst for n-butane selective oxidation

The utilization of lighter alkanes into useful chemical products is essential for modern chemistry and reducing the CO2 emission. Particularly, n-butane has gained special attention across the globe due to the abundant production of maleic anhydride (MA). Vanadium phosphorous oxide (VPO) is the most effective catalyst for selective oxidation of n-butane to MA so far. Interestingly, the VPO complex exists in more or less fifteen different structures, each one having distinct phase composition and exclusive surface morphology and physiochemical properties such as valence state, lattice oxygen, acidity etc., which relies on precursor preparation method and the activation conditions of catalysts. The catalytic performance of VPO catalyst is improved by adding different promoters or co-catalyst such as various metals dopants, or either introducing template or structural-directing agents. Meanwhile, new preparation strategies such as electrospinning, ball milling, hydrothermal, barothermal, ultrasound, microwave irradiation, calcination, sol–gel method and solvothermal synthesis are also employed for introducing improvement in catalytic performance. Research in above-mentioned different aspects will be ascribed in current review in addition to summarizing overall catalysis activity and final yield. To analyze the performance of the catalytic precursor, the reaction mechanism and reaction kinetics both are discussed in this review to help clarify the key issues such as strong exothermic reaction, phosphorus supplement, water supplement, deactivation, and air/n-butane pretreatment etc. related to the various industrial applications of VPO.

Ultrasound-assisted pseudohomogeneous tungstate catalyst for selective oxidation of alcohols to aldehydes

The idea of applying ultrasound (US) as a green activation method in chemical transformations, especially in catalytic alcohol oxidations, technically and ecologically appeals to chemists. In the present work, as an attempt to fulfill the idea of designing an eco-friendly system to oxidize alcoholic substrates into corresponding aldehydes, we developed multifunctional tungstate-decorated CQD base catalyst, A-CQDs/W, and examined its sonooxidation performance in presence of H2O2 as a green oxidant in aqua media. By comparing the catalyst performance in oxidize benzyl alcohol as a testing model to benzaldehyde (BeOH) prior and after US irradiation—trace vs 93%- the key role of ultrasonic irradiation in achieving high yield is completely appreciated. Exceptional thermal and compression condition that is created as a result of acoustic waves is in charge of unparalleled yield results in this type of activation method. The immense degree of reagent interaction in this method, ensures the maximum yield in notably low time, which in turn leads to decrease in the number of unreacted reagents and by-products. Meanwhile, the need for using toxic organic solvents and hazardous oxidants, auxiliaries and phase transfer catalyst (PTC) is completely obviated.

Promotional catalytic activity and reaction mechanism of Ag-modified Ce0.6Zr0.4O2 catalyst for catalytic oxidation of ammonia

Ce1-xZrxO2 composite oxides (molar, x = 0-1.0, interval of 0.2) were prepared by a cetyltrimethylammonium bromide-assisted precipitation method. The enhancement of silver-species modification and catalytic mechanism of adsorption-transformation-desorption process were investigated over the Ag-impregnated catalysts for low-temperature selective catalytic oxidation of ammonia (NH3-SCO). The optimal 5 wt.% Ag/Ce0.6Zr0.4O2 catalyst presented good NH3-SCO performance with >90% NH3 conversion at temperature (T) ≥ 250°C and 89% N2 selectivity. Despite the irregular block shape and underdeveloped specific surface area (∼60 m2/g), the naked and Ag-modified Ce0.6Zr0.4O2 solid solution still obtained highly dispersed distribution of surface elements analyzed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) (mapping), N2 adsorption-desorption test and X-ray diffraction (XRD). H2 temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that Ag-modification enhanced the mobility and activation of oxygen-species leading to a promotion on CeO2 reducibility and synergistic Ag0/Ag+ and Ce4+/Ce3+ redox cycles. Besides, Ag+/Ag2O clusters could facilitate the formation of surface oxygen vacancies that was beneficial to the adsorption and activation of ammonia. NH3-temperature programmed desorption (NH3-TPD) showed more adsorption-desorption capacity to ammonia were provided by physical, weak- and medium-strong acid sites. Diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the activation of ammonia might be the control step of NH3-SCO procedure, during which NH3 dehydrogenation derived from NHx-species and also internal selective catalytic reduction (i-SCR) reactions were proposed.

Self-Assembly of Chiral Ferrocene-Functionalized Polyoxotitanium Clusters for Photocatalytic Selective Sulfide Oxidation.

Here, we systematically studied the self-assembly behavior of chiral polyoxytitanium clusters for the first time. Through the cooperative assembly of ferrocenecarboxylic acid and ketoxime ligands, we successfully incorporated the planar chirality of ferrocene (Fc) into the layered {Ti5} building blocks. The resulting {Ti5Fc} clusters can be used as structural units to assemble into large ordered structures in various ways; either a pair of {Ti5Fc} enantiomers are bridged by organic adhesive to form sandwich structures or two homochiral {Ti5Fc} units participate in the assembly to form the large clusters. Depending on the assembly modes, the chirality of {Ti5Fc} can be transferred to large nanoclusters or disappear to form mesostructures. The difference of the assembly modes between the {Ti5Fc} units can also tune the photoelectric activity of the resulting clusters, which has been verified by using {Ti10Fc-6/7} as catalysts for photocatalytic selective sulfide oxidation. This work not only is an important breakthrough in the study of the self-assembly of chiral nanoclusters but also provides an important reference for understanding of chiral transfer on the nanoscale.

Learning Design Rules for Selective Oxidation Catalysts from High-Throughput Experimentation and Artificial Intelligence.

The design of heterogeneous catalysts is challenged by the complexity of materials and processes that govern reactivity and by the fact that the number of good catalysts is very small in comparison to the number of possible materials. Here, we show how the subgroup-discovery (SGD) artificial-intelligence approach can be applied to an experimental plus theoretical data set to identify constraints on key physicochemical parameters, the so-called SG rules, which exclusively describe materials and reaction conditions with outstanding catalytic performance. By using high-throughput experimentation, 120 SiO2-supported catalysts containing ruthenium, tungsten, and phosphorus were synthesized and tested in the catalytic oxidation of propylene. As candidate descriptive parameters, the temperature and 10 parameters related to the composition and chemical nature of the catalyst materials, derived from calculated free-atom properties, were offered. The temperature, the phosphorus content, and the composition-weighted electronegativity are identified as key parameters describing high yields toward the value-added oxygenate products acrolein and acrylic acid. The SG rules not only reflect the underlying processes particularly associated with high performance but also guide the design of more complex catalysts containing up to five elements in their composition.

Atomically Defined Undercoordinated Copper Active Sites over Nitrogen-Doped Carbon for Aerobic Oxidation of Alcohols.

Selective aerobic oxidation of alcohols offers an attractive means to address challenges in the modern chemical industry, but the development of non-noble metal catalysts with superior efficacy for this reaction remains a grand challenge. Here, this study reports on such a catalyst based on atomically defined undercoordinated copper atoms over nitrogen-doped carbon support as an efficient, durable, and scalable heterogeneous catalyst for selective aerobic oxidation of alcohols. This catalyst exhibits extremely high intrinsic catalytic activity (TOF of 7692 h-1 ) in the oxidation of cinnamyl alcohol to afford cinnamaldehyde, along with exceptional recyclability (at least eight cycles), scalability, and broad substrate scope. DFT calculations suggest that the high activity derives from the low oxidation state and the unique coordination environment of the copper sites in the catalyst. These findings pave the way for the design of highly active and stable single atom catalysts to potentially address challenges in synthetic chemistry.

Synthesis of TixSn1-xO2 mixed metal oxide for copper catalysts as high-efficiency NH3 selective catalytic oxidation

A series of y Cu/TixSn1-xO2 catalysts for selective catalytic oxidation (NH3-SCO) were synthesized using TixSn1-xO2 as catalyst support and CuO as active species. Compared with pure TiO2 support, the strong metal-support interaction between Cu active species and TixSn1-xO2 was significantly promoted by the Sn-doping, thereby improving the catalytic activity and N2 selectivity of the catalyst, which meets the demand of diesel vehicles to completely convert NH3. The results of XRD and HRTEM showed that the particle size of 20 Cu/TiSnO2 sample is much more highly dispersed. The results of NH3-TPD and in-situ DRIFTS indicated that the copper species in 20 Cu/TixSn1-xO2 can provide more Lewis acid sites than 20 Cu/TiO2, which is conducive to the adsorption and reaction of NH3. Meanwhile, XPS results proved that adsorbed oxygen on the catalyst surface is the main active oxygen species. Finally, in-situ DRIFTs and kinetic measurements found that the NH3-SCO reaction over 20 Cu/TiSnO2 and 20 Cu/TiO2 follow the i-SCR and imide (–NH) mechanisms, respectively. This catalyst will provide solutions for the future design and application of ammonia slip from mobile and stationary sources.

Rational ensemble design of alloy catalysts for selective ammonia oxidation based on machine learning

High-throughput computation and machine learning studies are conducted for the rational design of ensembles of alloy catalysts for selective catalytic oxidation of NH3 (NH3-SCO).