Catalysts For Selective Oxidation Research Articles

Oxygen vacancies engineering of Fe doped LaCoO3 perovskite catalysts for efficient H2S selective oxidation

Fe-doped LaCoO3 perovskite catalysts synthesized via a facile citric acid-assisted sol-gel route have been applied for the H2S selective oxidation. The as-synthesized LaFexCo1−xO3 perovskites exhibit a randomly macroporous structure formed by the accumulation of nanoparticles. Characterization studies reveal that appropriate Fe substitution effectively enhances the formation of oxygen vacancies, and thus improves the lattice oxygen mobility and H2S adsorbability, respectively. The obtained LaFe0.4Co0.6O3 exhibited remarkable catalytic activity, with the H2S conversion and sulfur yield of 100% at 190 °C. A combined approach of in-situ FT-IR and in-situ Raman experiments disclosed that the plausible reaction pathway of H2S selective oxidation over LaFexCo1−xO3 perovskites follows the Mars-van Krevelen mechanism. The density functional theory calculations revealed that Fe-doping improves the formation of oxygen vacancy and enhances the adsorption of H2S. This study offers new insights for the design of highly efficient perovskites for the selective oxidation of H2S.

CeO2-Supported TiO2-Pt Nanorod Composites as Efficient Catalysts for CO Oxidation.

Supported Pt-based catalysts have been identified as highly selective catalysts for CO oxidation, but their potential for applications has been hampered by the high cost and scarcity of Pt metals as well as aggregation problems at relatively high temperatures. In this work, nanorod structured (TiO2-Pt)/CeO2 catalysts with the addition of 0.3 at% Pt and different atomic ratios of Ti were prepared through a combined dealloying and calcination method. XRD, XPS, SEM, TEM, and STEM measurements were used to confirm the phase composition, surface morphology, and structure of synthesized samples. After calcination treatment, Pt nanoparticles were semi-inlayed on the surface of the CeO2 nanorod, and TiO2 was highly dispersed into the catalyst system, resulting in the formation of (TiO2-Pt)/CeO2 with high specific surface area and large pore volume. The unique structure can provide more reaction path and active sites for catalytic CO oxidation, thus contributing to the generation of catalysts with high catalytic activity. The outstanding catalytic performance is ascribed to the stable structure and proper TiO2 doping as well as the combined effect of Pt, TiO2, and CeO2. The research results are of importance for further development of high catalytic performance nanoporous catalytic materials.

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Sustainable design of co-doped ordered mesoporous carbons as efficient and long-lived catalysts for H2S reutilization

Selective catalytic oxidation of H2S into elemental sulfur has been recognized as the optimal route for H2S reutilization, but traditional metal catalysts show poor sulfur resistance. Herein, we report a solvent-free route for designing nitrogen and defects co-doped, ordered mesoporous carbons (ND-OMCs) from co-assembly of block copolymer template with nitrogen-containing oligomers and silica defects directing agent. Through thermal curing, carbonization and etching of silica, the microporosity tandem ordered mesopores, abundant nitrogen sites (6.13 wt%) and structural defects were introduced into ND-OMCs. The ND-OMCs were successfully applied as efficient and long-lived metal-free catalysts for H2S selective oxidation to sulfur, giving nearly 100 % H2S conversion and sulfur selectivity at only 150 °C. Their remarkable activities of ND-OMCs can be well-maintained over 157 h of continuous reaction, exhibiting the sulfur capacity as high as 3295 mg/g, which were much better than mostly desulfurizes such as commercial Fe2O3, g-C3N4, and nitrogen-containing, ordered mesoporous carbons.

Evolution of Carbonate-Intercalated γ-NiOOH from a Molecularly Derived Nickel Sulfide (Pre)Catalyst for Efficient Water and Selective Organic Oxidation.

The development of a competent (pre)catalyst for the oxygen evolution reaction (OER) to produce green hydrogen is critical for a carbon-neutral economy. In this aspect, the low-temperature, single-source precursor (SSP) method allows the formation of highly efficient OER electrocatalysts, with better control over their structural and electronic properties. Herein, a transition metal (TM) based chalcogenide material, nickel sulfide (NiS), is prepared from a novel molecular complex [NiII (PyHS)4 ][OTf]2 (1) and utilized as a (pre)catalyst for OER. The NiS (pre)catalyst requires an overpotential of only 255mV to reach the benchmark current density of 10mAcm-2 and shows 63h of chronopotentiometry (CP) stability along with over 95% Faradaic efficiency in 1m KOH. Several ex situ measurements and quasi in situ Raman spectroscopy uncover that NiS irreversibly transformed to a carbonate-intercalated γ-NiOOH phase under the alkaline OER conditions, which serves as the actual active structure for the OER. Additionally, this in situ formed active phase successfully catalyzes the selective oxidation of alcohol, aldehyde, and amine-based organic substrates to value-added chemicals, with high efficiencies.

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.

Cotton fiber-anchored Nb single-site catalyst for selective oxidation of anilines to azoxybenzenes

This work demonstrated a facile methodology to construct single Nb site catalysts with merely 0.63 wt% Nb by employing α-hydroxy carboxylic acid-functionalized cotton fiber as green and sustainable supporting material. The as-synthesized catalysts have been characterized thoroughly and exhibited a superior catalytic activity for selective oxidation of aniline into azoxybenzene with nearly 100% utilization efficiency of H2O2 under mild and green conditions. Notably, it demonstrated the highest TOF (4806 h−1) among the currently reported heterogeneous catalysts. Moreover, the catalyst has steadily been operated under continuous-flow conditions, which afforded significant benefits compared to batch processes as reported in these transformations. The activity test and catalyst characterization indicated that the atomically dispersed Nb-peroxo coordination centers were actually catalytic sites. Especially, the α-hydroxy carboxylic acid groups on cotton fiber played an important role in coordinating/stabilizing single Nb site. Moreover, 1H MAS NMR spectra verified that there exists an acid-base interaction between arylamines and free α-hydroxy carboxylic acid groups covalently grafted on cotton fiber, which was favorable for adsorption and activation of the substrates. Mechanism studies indicated that the azoxy derivatives were produced through condensation step between N-phenylhydroxylamine and nitrosobenzene rather than direct oxidation of azobenzene. The work provides an economical and environmentally friendly approach to construct single–site catalysts for the oxidation catalysis.

Construction and Enhanced Efficiency of Bi2MoO6/ZnO Compo-Sites for Visible-Light-Driven Photocatalytic Performance.

Bi2MoO6 was one of the important bismuth-based semiconductors with a narrow bandgap, and has been widely used in selective oxidation catalysts, supercapacitors, and energy-storage devices. A series of Bi2MoO6/ZnO composite photocatalysts with different mass ratios were synthesized by the hydrothermal method. The synthesized samples were characterized by XRD, PL, UV-Vis, SEM, TEM, XPS, and BET analysis techniques. Under visible light conditions, Methylene blue (MB) was used as the target degradation product to evaluate its photocatalytic performance. The results showed that the degradation rate constant of Bi2MoO6/ZnO (0.4-BZO) was about twice that of the traditional photocatalysis of ZnO. The Bi2MoO6/ZnO composite catalyst maintained stable performance after four consecutive runs. The high photocatalytic activity of Bi2MoO6/ZnO was attributed to the efficient electron transport of the heterojunction, which accelerates the separation of electron-hole pairs and reduces the probability of carrier recombination near the Bi2MoO6/ZnO heterojunction. Bi2MoO6/ZnO nanocomposites have potential applications in the field of photodegradation.

An Fe·MOF–Mo·POM hybrid material: a novel and efficient catalyst for selective benzyl alcohol oxidation to benzaldehyde

Designing and synthesis of an efficient catalyst for the selective oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzCHO) is a significant method in green chemistry and catalysis.

Additive-free selective oxidation of aromatic alcohols with molecular oxygen catalyzed by a mixed-valence polyoxovanadate-based metal-organic framework.

Selective oxidation of alcohols to aldehydes is an industrially significant chemical transformation. Herein, we report a mixed-valence polyoxovanadate-based metal-organic framework (MOF), (H2bix)5{[Cd(bix)2][VIV8VV7O36Cl]2}·3H2O (V-Cd-MOF), for catalyzing the additive-free oxidation of a series of aromatic alcohols with high selectivity and in nearly quantitative yield to the corresponding aldehydes with O2 as the oxidant. Experimental results, corroborated with density functional theory calculations, indicate that it is the synergistic operation of the dual active sites of the VIV-O-VV building units in the polyoxovanadate cluster that is responsible for the excellent catalytic performance observed: on the one hand, the exposed and readily accessible reduced VIV site is believed to activate O2, resulting in a reactive oxygen species for the subsequent activation and breaking of the substrate's Cα-H bond. On the other hand, the VV site coordinates with the alcoholic O atom to facilitate the cleavage of the O-H bond. The catalyst can be recycled by centrifugation and re-used at least five times with uncompromised performance. To our knowledge, V-Cd-MOF represents the first example of a polyoxometalate-based MOF catalyst for additive-free selective oxidation of alcohol to aldehyde with O2 as an oxidant.

Upgrading the reflux method as novel route for competitive catalysts in alkane selective oxidation

Optimized reflux synthesis of MoVTeNbo catalysts.

Building N-hydroxyphthalimide organocatalytic sites into a covalent organic framework for metal-free and selective oxidation of silanes.

A novel crystalline covalent organic framework COF-NHPI was built by a bottom-up strategy to guarantee highly ordered embedment of the N-hydroxyphthalimide (NHPI) units as nitroxyl radical organocatalytic sites. The COF-NHPI was demonstrated to be a metal-free, highly selective and heterogeneous catalyst for the efficient oxidation of various silanes to the corresponding silanols. Mechanistic studies revealed that the critical phthalimido N-oxyl radical was generated in situ to govern the catalysis.

Enhanced catalysis of a vanadium-substituted Keggin-type polyoxomolybdate supported on the M3O4/C (M = Fe or Co) surface enables efficient and recyclable oxidation of HMF to DFF.

In the reaction of oxidizing 5-hydroxymethylfurfural (HMF), attaining high efficiency and selectivity in the conversion of HMF into DFF presents a challenge due to the possibility of forming multiple products. Polyoxometalates are considered highly active catalysts for HMF oxidation. However, the over-oxidation of products poses a challenge, leading to decreased purity and yield. In this work, metal-organic framework-derived Fe3O4/C and Co3O4/C were designed as carriers for the vanadium-substituted Keggin-type polyoxomolybdate H5PMo10V2O40·35H2O (PMo10V2). In this complex system, spinel oxides can effectively adsorb HMF molecules and cooperate with PMo10V2 to catalyze the aerobic oxidation of HMF. As a result, the as-prepared PMo10V2@Fe3O4/C and PMo10V2@Co3O4/C catalysts can achieve efficient conversion of HMF into DFF with almost 100% selectivity. Among them, PMo10V2@Fe3O4/C exhibits a higher conversion rate (99.1%) under milder reaction conditions (oxygen pressure of 0.8 MPa). Both catalysts exhibited exceptional stability and retained their activity and selectivity even after undergoing multiple cycles. Studies on mechanisms by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy revealed that the V5+ and Mo6+ in PMo10V2, together with the metal ions in the spinel oxides, act as active centers for the catalytic conversion of HMF. Therefore, it is proposed that PMo10V2 and M3O4/C (M = Fe, Co) cooperatively catalyze the transformation of HMF into DFF via a proton-coupled electron transfer mechanism. This study offers an innovative approach for designing highly selective and recyclable biomass oxidation catalysts.

Nitrogen and fluorine co-doped mesoporous carbon as an efficient metal-free catalyst for selective oxidation of 2-methylnaphthalene

N/F co-doped carbon materials prepared with an aniline-type benzoxazine as a carbon precursor show a better catalytic performance for the selective oxidation of 2-methylnapthalene.

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

Preparation of a Nanorod-like Mo-VOx Catalyst for Gas Phase Selective Oxidation of Methyl Lactate with Air

Selective oxidation of higher alcohols to carbonyls is one of the most important chemical transformations in research and industrial chemistry. Gas phase selective oxidation of alcohols with air as oxidant is a “green chemistry” synthesis path, although it is a great challenge to maintain high conversion with high selectivity. In this work, we synthesized nanorod-like Mo-VOx catalysts through regulation of the Mo/V ratio via a solid-state reaction method, and the group of Mo promoted VOx catalysts exhibited the combination of high catalytic activity and selectivity. The methyl pyruvate (MP) selectivity over 0.17Mo-VOx catalysts reached 79.6% at a relatively lower reaction temperature of 180 °C with 90.1% methyl lactate conversion. The characterization results revealed that the 0.17Mo-VOx catalysts showed a nanorod-like morphology with larger specific surface area and smaller crystalline size, generated active MoV2O8 crystalline phase, and could give rise to active V4+/V5+ redox couples as well as surface oxygen species. In terms of structure–activity relationship, when Mo was initially introduced into VOx, the MoV2O8 phase was gradually formed, and MP yield over yMo-VOx catalysts increased significantly together with V4+ proportion. However, excessive Mo would not form more active MoV2O8 phases, and the MP yield decreased.

Carbon-based metal-free catalysts for selective oxidation of glycerol to glycolic acid

Exploring efficient and cheap catalyst for the selective oxidation of glycerol to valuable carboxylic acids is a critical and complicated challenge. Herein, we innovatively realized the efficient oxidation of glycerol to glycolic acid over metal-free catalyst under mild conditions via carbon material defect engineering. Multiple characterizations revealed that the stepwise functionalization of active carbon caused by simple high temperature control is adopted to fine-tune the composition and electronic structure of oxygen-containing functional groups. Specifically, the abundant ester and carboxyl functional groups exhibit stronger electronegativity than the hydroxyl group, and this enhanced electronic coupling effect strengthens the adsorption of glycerol, thus improving the catalytic activity. Kinetic analysis and density functional theory calculations demonstrated that the oxygen-containing functional groups (mainly ester and carboxyl) could promote the C–H bond activation of RCH2O* intermediate and once C–C bond cleavage of glyceric acid, ensuring the high selectivity of glycolic acid (53.2% over AC-400 catalyst). We anticipate that this work could sheds some light on the application and design of more new type of carbon-based metal-free catalysts in the aqueous phase oxidation of various polyols.

Facile Preparation of Nitrogen-Doped Carbon Spheres with Wrinkled Cage-Supported Single-Atom Copper Catalysts for Selective Oxidation of Glycerol to Formic Acid

Selective conversion of the surplus glycerol (GLY) to a hydrogen storage material, formic acid (FA), in heterogeneous catalytic systems is still challenging in the catalysis community. Herein, nitrogen-doped carbon spheres with wrinkled cage-supported single-atom Cu catalysts (Cu/NCSWCs) were prepared by wet ball milling of 5,5′-diamino-3,3′-bis(1H-1,2,4-triazole) (DABT) and copper nitrate with hydrothermal carbon-encapsulated dendritic fibrous nanosilica (DFNS) as the hard template in methanol. After the removal of DFNS, structural characterization revealed that single copper atoms with four nitrogen coordination were successfully immobilized on NCSWCs (CuN4/NCSWCs), which is attributed to the fact that DABT could act as the chelating agent and nitrogen source. Furthermore, due to their unique CuN4 structures and large meso/macropore volumes derived from their wrinkled cages, CuN4/NCSWCs exhibited the highest catalytic activity and selectivity in oxidation of GLY to FA using H2O2 as the oxidant. In contrast, only C3 and C2 products were produced when O2 was used as the oxidant. Electron paramagnetic resonance measurements indicated that H2O2 and O2 can be activated to generate ·OH and ·O2– over CuN4/NCSWCs, respectively. Experimental studies and density functional theory calculations revealed that ·OH adsorbed on CuN4 structures reacted with GLY to produce FA through consecutive steps involving dehydrogenation, oxidation, and oxidative cleavage. Moreover, CuN4/NCSWCs also exhibited high catalytic stability during eight consecutive catalytic runs.

High performance BaCO3-CeO2 composite catalyst for solvent-free selective oxidation of cyclohexane with molecular oxygen

BaCO3-CeO2 composite catalysts were prepared by the hydrothermal synthesis method, and the influence of Ba/Ce on the structural and catalytic performance for selective oxidation of cyclohexane with molecule oxygen was studied. The characterization results exhibited that the doping amount of BaCO3 would influence the morphology, the content of surface Ce3+ and surface oxygen species (Oβ), and alkalinity. Meanwhile, the amount of alkali is the key to the decomposition of cyclohexyl hydrogen peroxide (CHHP). 10.5% conversion of cyclohexane and 85.0% selectivity of KA oil were achieved under the reaction conditions of 1.0 MPa O2 and 150 °C for 2 h. The excellent catalytic activity was attributed to its high atomic ratios of Ce3+/Cetotal and Oβ/Ototal on the surface and optimal redox capacity.

Decagram-scale synthesis of heterometallic Ag/Ti cluster as sustainable catalyst for selective oxidation of sulfides

Atomically precise heterometallic clusters constitute a new family of model catalysts for understanding the fundamental catalysis process widely occurred in supported nanocatalysts, however, their syntheses and structure/composition modulations are nonetheless daunting challenges. Herein, three novel heterometallic Ag/Ti clusters with different Ag-doping ratios, Ag4Ti14, Ag4Ti6 and Ag4Ti2, have been readily synthesized in decagram-scale quantities through ligand engineering strategy. Thereinto, Ag4Ti2 can be efficiently prepared within 3 h in a quite high yield > 95 %. More remarkably, Ag4Ti2 shows unprecedented catalytic activity (>90 % conversion within 1 min) and selectivity (nearly 100 %) towards oxidation of thioethers with a wide substrate scope even including notorious 2-chloroethyl ethyl sulfide (CEES) at room temperature (turnover frequency = 11,880 h−1), which is mainly attributed to its full-surface-exposure-doping Ag atoms (affinity for binding of substrate) on TiO support and their cooperativity, whereas the other two are not the same case. Moreover, the Ag4Ti2 is highly robust in catalysis process and can be readily recovered and reused at least six cycles without observable loss of activity and selectivity, enabling this cluster hold great promise in industrial applications. This work not only establishes a general method to scale-up synthesis of Ag/Ti clusters but also provides an atom-precise model catalyst to understand the structure–property correlations of real-world conventional nanocatalysts such as silver doped-TiO2 nanoparticles.