Aerobic Oxidative Synthesis of Formamides from Amines and Bioderived Formyl Surrogates.

Herein we present a new strategy for the oxidative synthesis of formamides from various types of amines and bioderived formyl sources (DHA, GLA and GLCA) and molecular oxygen (O2) as oxidant on g-C3N4 supported Cu catalysts. Combined characterization data from EPR, XAFS, XRD and XPS revealed the formation of single CuN4 sites on supported Cuphen/C3N4 catalysts. EPR spin trapping experiments disclosed ⋅OOH radicals as reactive oxygen species and ⋅NR1R2 radicals being responsible for the initial C-C bond cleavage. Control experiments and DFT calculations showed that the successive C-C bond cleavage in DHA proceeds via a reaction mechanism co-mediated by ⋅NR1R2 and ⋅OOH radicals based on the well-equilibrated CuII and CuI cycle. Our catalyst has much higher activity (TOF) than those based on noble metals.

Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal-Organic Framework.

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported by a UiO-66 metal-organic framework (Cu/UiO-66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density-functional-theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O2 , by reaction of O2,ad with COad , leading to the formation of an O atom connecting the Cu center with a neighboring Zr4+ ion as the rate limiting step. This is removed in a second activated step.

Photoexcited cobalt catalysed endo-selective alkyl Heck reaction.

Herein, we report an intramolecular endo-selective Heck reaction of iodomethylsilyl ethers of phenols and alkenols. The reaction leads to the formation of seven- and eight-membered siloxycycles in excellent yields, which could be further converted into the corresponding allylic alcohols upon oxidation. Thus, this method could be used for the selective (Z)-hydroxymethylation of o-hydroxystyrenes and alkenols. Rapid scan EPR experiments and DFT calculations suggest a concerted ß-hydrogen elimination event to take place in the triplet state.

Tailoring Active Cu2 O/Copper Interface Sites for N-Formylation of Aliphatic Primary Amines with CO2 /H2.

Heterogeneously catalyzed N-formylation of amines to formamide with CO2 /H2 is highly attractive for the valorization of CO2 . However, the relationship of the catalytic performance with the catalyst structure is still elusive. Herein, mixed valence catalysts containing Cu2 O/Cu interface sites were constructed for this transformation. Both aliphatic primary and secondary amines with diverse structures were efficiently converted into the desired formamides with good to excellent yields. Combined ex and in situ catalyst characterization revealed that the presence of Cu2 O/Cu interface sites was vital for the excellent catalytic activity. Density functional theory (DFT) calculations demonstrated that better catalytic activity of Cu2 O/Cu(111) than Cu(111) is attributed to the assistance of oxygen at the Cu2 O/Cu interface (Ointer ) in formation of Ointer -H moieties, which not only reduce the apparent barrier of HCOOH formation but also benefit the desorption of the desired N-formylated amine, leading to high activity and selectivity.

Glycolaldehyde as a Bio-Based C1 Building Block for Selective N-Formylation of Secondary Amines.

Biomass derived glycolaldehyde was employed as C1 building block for the N-formylation of secondary amines using air as oxidant. The reaction is atom economic, highly selective and proceeds under catalyst free conditions. This strategy can be used for the synthesis of cyclic and acyclic formylamines, including DMF. Mechanistic studies suggest a radical oxidation pathway.

Oxygen vacancies in Ru/TiO2 - drivers of low-temperature CO2 methanation assessed by multimodal operando spectroscopy.

Hydrogenation of CO2 is very attractive for transforming this greenhouse gas into valuable high energy density compounds. In this work, we developed a highly active and stable Ru/TiO2 catalyst for CO2 methanation prepared by a solgel method that revealed much higher activity in methanation of CO2 (ca. 4-14 times higher turnover frequencies at 140-210°C) than state-of-the-art Ru/TiO2 catalysts and a control sample prepared by wetness impregnation. This is attributed to a high concentration of O-vacancies, inherent to the solgel methodology, which play a dual role for 1) activation of CO2 and 2) transfer of electrons to interfacial Ru sites as evident from operando DRIFTS and in situ EPR investigations. These results suggest that charge transfer from O-vacancies to interfacial Ru sites and subsequent electron donation from filled metal d-orbitals to antibonding orbitals of adsorbed CO are decisive factors in boosting the CO2 methanation activity.

Scalable and selective deuteration of (hetero)arenes.

Isotope labelling, particularly deuteration, is an important tool for the development of new drugs, specifically for identification and quantification of metabolites. For this purpose, many efficient methodologies have been developed that allow for the small-scale synthesis of selectively deuterated compounds. Due to the development of deuterated compounds as active drug ingredients, there is a growing interest in scalable methods for deuteration. The development of methodologies for large-scale deuterium labelling in industrial settings requires technologies that are reliable, robust and scalable. Here we show that a nanostructured iron catalyst, prepared by combining cellulose with abundant iron salts, permits the selective deuteration of (hetero)arenes including anilines, phenols, indoles and other heterocycles, using inexpensive D2O under hydrogen pressure. This methodology represents an easily scalable deuteration (demonstrated by the synthesis of deuterium-containing products on the kilogram scale) and the air- and water-stable catalyst enables efficient labelling in a straightforward manner with high quality control.

Dihydroxyacetone valorization with high atom efficiency via controlling radical oxidation pathways over natural mineral-inspired catalyst.

Diminishing fossil fuel resources and calls for sustainability are driving the urgent need for efficient valorization of renewable resources with high atom efficiency. Inspired from the natural goethite mineral with Mn paragenesis, we develop cost-effective MnO2/goethite catalysts for the efficient valorization of dihydroxyacetone, an important biomass-based platform molecule, into value-added glycolic acid and formic acid with 83.2% and 93.4% yields. The DHA substrates first undergo C-C cleavage to selectively form glycolic acid and hydroxymethyl (·CH2OH) radicals, which are further oxidized into formic acid. The kinetic and isotopic labeling experiments reveal that the catalase-like activity of MnO2 turns the oxidative radicals into oxygen, which then switches towards a hydroxymethyl peroxide (HMOO) pathway for formic acid generation and prevents formic acid over-oxidation. This nature-inspired catalyst design not only significantly improves the carbon efficiency to 86.6%, but also enhances the oxygen atom utilization efficiency from 11.2% to 46.6%, indicating a promising biomass valorization process.

Ni-In Synergy in CO2 Hydrogenation to Methanol.

Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2 but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3 using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In2O3. NiO-In2O3 was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3 and NiO end members have similar high specific surface areas and morphology. The main products of CO2 hydrogenation are CH3OH and CO with CH4 being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcat h), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3 at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2 hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2 is mainly due to low-barrier H2 dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2 on Ov.

Supported CuII Single-Ion Catalyst for Total Carbon Utilization of C2 and C3 Biomass-Based Platform Molecules in the N-Formylation of Amines.

The shift from fossil carbon sources to renewable ones is vital for developing sustainable chemical processes to produce valuable chemicals. In this work, value-added formamides were synthesized in good yields by the reaction of amines with C2 and C3 biomass-based platform molecules such as glycolic acid, 1,3-dihydroxyacetone and glyceraldehyde. These feedstocks were selectively converted by catalysts based on Cu-containing zeolite 5A through the in situ formation of carbonyl-containing intermediates. To the best of our knowledge, this is the first example in which all the carbon atoms in biomass-based feedstocks could be amidated to produce formamide. Combined catalyst characterization results revealed preferably single CuII sites on the surface of Cu/5A, some of which form small clusters, but without direct linking via oxygen bridges. By combining the results of electron paramagnetic resonance (EPR) spin-trapping, operando attenuated total reflection (ATR) IR spectroscopy and control experiments, it was found that the formation of formamides might involve a HCOOH-like intermediate and . NHPh radicals, in which the selective formation of . OOH radicals might play a key role.

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution.

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta3N5 (Ta3N5:Mg+Zr) nanoparticles by a brief NH3 nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta3N5 under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Insight into the properties of MnO2-Co3O4-CeO2 catalyst series for the selective catalytic reduction of NOx by C3H6 and NH3.

A series of MnO2-Co3O4-CeO2 catalysts with different ceria loading (0.75, 1.26 and 1.88 Ce/Mn molar ratio) were synthesized by a co-precipitation technique and the catalytic activity was tested for selective catalytic reduction of NOx by C3H6 or NH3. The catalysts were characterized by various physicochemical techniques to examine the effect of ceria loading on the properties of catalysts, such as crystallinity of metal species, surface area, porosity, and acidity using physical adsorption analysis, SEM-EDX, H2-TPR, XRD, NH3-TPD and in-situ FTIR spectroscopy. Ceria loading had a significant effect on the reduction of NOx, with the catalyst having low amount of ceria loading (Ce/Mn = 0.75) showing excellent performance at low-temperature conditions, but the activity declined at higher temperature. The high ceria loading (Ce/Mn = 1.88) catalyst showed poor activity compared to the counterparts owing to the lower number of acid sites and the resulting lower adsorption capacity.

Avoiding Pitfalls in Comparison of Activity and Selectivity of Solid Catalysts for Electrochemical HMF Oxidation.

Electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) offers a renewable approach to produce the value-added platform chemical 2,5-furandicarboxylic acid (FDCA). The key for the economic viability of this approach is to develop active and selective electrocatalysts. Nevertheless, a reliable catalyst evaluation protocol is still missing, leading to elusive conclusions on criteria for a high-performing catalyst. Herein, we demonstrate that besides the catalyst identity, secondary parameters such as materials of conductive substrates for the working electrode, concentration of the supporting electrolyte, and electrolyzer configurations have profound impact on the catalyst performance and thus need to be optimized before assessing the true activity of a catalyst. Moreover, we highlight the importance of those secondary parameters in suppressing side reactions, which has long been overlooked. The protocol is validated by evaluating the performance of free-standing Cu-foam, and CuCoO modified with NaPO2 H2 and Ni, which were immobilized on boron-doped diamond (BDD) electrodes. Recommended practices and figure of merits in carefully evaluating the catalyst performance are proposed.

Visible-Light-Induced Palladium-Catalyzed Dehydrogenative Carbonylation of Amines to Oxalamides.

The palladium-catalyzed oxidative carbonylation of amines toward the synthesis of oxalamides has been established around 30 years ago and it usually needs the presence of (over)stoichiometric amounts of oxidant. In this work, the first transformation of this type in which the oxidant was replaced by visible light is described. The new approach uses a simple robust Pd complex, which can even be partially recycled. A mechanistic reason is provided and supported by control experiments and EPR studies, showing that PdI was formed and Pd0 was the active species. Both nitrogen- and the intermediate acyl radical can be detected. Moreover, the formation of hydrogen was confirmed by gas GC.

Rhodium-catalyzed carbonylative coupling of alkyl halides with thiols: a radical process faster than easier nucleophilic substitution.

How to make a carbonylative coupling faster than the easier nucleophilic substitution? In this communication, a rhodium-catalyzed radical-based carbonylative coupling of alkyl halides with thiolphenols has been realized. Thioesters were isolated in good yields in general.

Promoting Photocatalytic Hydrogen Evolution Activity of Graphitic Carbon Nitride with Hole-Transfer Agents.

Visible light-driven photocatalytic reduction of protons to H2 is considered a promising way of solar-to-chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photocatalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydrogen (H2 ) evolution process. Here, for the first time, the hole-transfer dynamics from Pt-loaded sol-gel-prepared graphitic carbon nitride (Pt-sg-CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H2 production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H2 production for 3 days. A detailed charge-transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time-resolved photoluminescence and in situ electron paramagnetic resonance studies.

Selective nickel-catalyzed fluoroalkylations of olefins.

Mild and selective nickel-catalyzed trifluoromethylation and perfluoroalkylation reactions of alkenes were developed to provide fluorinated olefins, including natural products, pharmaceuticals, and variety of synthetic building blocks in good to excellent yields (38 examples). Control experiments, kinetic measurements and in situ EPR studies reveal the importance of radical species and the formation of 1,2-adducts as intermediates.

Effect of Formaldehyde in Selective Catalytic Reduction of NOx by Ammonia (NH3-SCR) on a Commercial V2O5-WO3/TiO2 Catalyst under Model Conditions.

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial V2O5-WO3/TiO2 catalyst in NH3-SCR of NOx under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH3 to a formamide (HCONH2) surface intermediate. This deprives NH3 partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N2 selectivity. Between 250 and 400 °C, HCONH2 decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO2 and NO, which enters the NH3-SCR reaction.

Cobalt Single-Atom Catalysts with High Stability for Selective Dehydrogenation of Formic Acid.

Metal-organic framework (MOF)-derived Co-N-C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co-N-C catalyst achieves superior activity, better acid resistance, and improved long-term stability compared with nanoparticles synthesized by a similar route. High-angle annular dark-field-scanning transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and X-ray absorption fine structure characterizations reveal the formation of CoII Nx centers as active sites. The optimal low-cost catalyst is a promising candidate for liquid H2 generation.

Visible-Light Photocatalytic Ozonation Using Graphitic C3N4 Catalysts: A Hydroxyl Radical Manufacturer for Wastewater Treatment.

Photocatalytic ozonation (light/O3/photocatalyst), an independent advanced oxidation process (AOP) proposed in 1996, has demonstrated over the past two decades its robust oxidation capacity and potential for practical wastewater treatment using sunlight and air (source of ozone). However, its development is restricted by two main issues: (i) a lack of breakthrough catalysts working under visible light (42-43% of sunlight in energy) as well as ambiguous property-activity relationships and (ii) unclear fundamental reasons underlying its high yield of hydroxyl radicals (•OH). In this Account, we summarize our substantial contributions to solving these issues, including (i) new-generation graphitic carbon nitride (g-C3N4) catalysts with excellent performance for photocatalytic ozonation under visible light, (ii) mechanisms of charge carrier transfer and reactive oxygen species (ROS) evolution, (iii) property-activity relationships, and (iv) chemical and working stabilities of g-C3N4 catalysts. On this basis, the principles/directions for future catalyst design/optimization are discussed, and a new concept of integrating solar photocatalytic ozonation with catalytic ozonation in one plant for continuous treatment of wastewater regardless of sunlight availability is proposed.The story starts from our finding that bulk/nanosheet/nanoporous g-C3N4 triggers a strong synergy between visible light (vis) and ozone, causing efficient mineralization of a wide variety of organic pollutants. Taking bulk g-C3N4 as an example, photocatalytic ozonation (vis/O3/g-C3N4) causes the mineralization of oxalic acid (a model pollutant) at a rate 95.8 times higher than the sum of photocatalytic oxidation (vis/O2/g-C3N4) and ozonation. To unravel this synergism, we developed a method based on in situ electron paramagnetic resonance (EPR) spectroscopy coupled with an online spin trapping technique for monitoring under realistic aqueous conditions the generation and transfer of photoinduced charge carriers and their reaction with dissolved O3/O2 to form ROS. The presence of only 2.1 mol % O3 in the inlet O2 gas stream can trap 1-2 times more conduction band electrons than pure O2 and shifts the reaction pathway from inefficient three-electron reduction of O2 (O2 → •O2- → HO2• → H2O2 → •OH) to more efficient one-electron reduction of O3 (O3 → •O3- → HO3• → •OH), thereby increasing the yield of •OH by a factor of 17. Next, we confirmed band structure as a decisive factor for catalytic performance and established a new concept for resolving this relationship, involving "the number of reactive charge carriers". An optimum balance between the number and reducing ability of photoinduced electrons, which depends on the interplay between the band gap and the conduction band edge potential, is a key property for highly active g-C3N4 catalysts. Furthermore, we demonstrated that g-C3N4 is chemically stable toward O3 and •O2- but that •OH can tear and oxidize its heptazine units to form cyameluric acid and further release nitrates into the aqueous environment. Fortunately, •OH usually attacks organic pollutants in wastewater in preference to g-C3N4, thus preserving the working stability of g-C3N4 and the steady operation of photocatalytic ozonation. This AOP, which serves as an in situ •OH manufacturer, would be of interest to a broad chemistry world since •OH radicals are active species not only for environmental applications but also for organic synthesis, polymerization, zeolite synthesis, and protein footprinting.