Synthesis of a molecularly defined single-active site heterogeneous catalyst for selective oxidation of N-heterocycles

Detection of CO2 Locally Generated by Formate Dehydrogenase Using Carbonate Ion-Selective Micropipette Electrodes

Many technologies involve immobilizing catalysts such as enzymes on surfaces, and the catalytic activities or functional efficiencies of these surface-bound catalysts can vary depending on orientations, localized binding sites, active sites, and intrinsic molecular nature. Accurate and rapid quantification of reaction products from surface-immobilized catalysts is crucial for understanding the selectivity, mechanisms, and reaction dynamics of catalytic systems and for revealing heterogeneous catalytic activities and reaction sites for applications such as biosensors and energy conversion/generation systems. Here, we demonstrate the feasibility of localized enzymatic activity measurements using microscale carbon dioxide (CO2)-sensitive ion-selective electrode (ISE) pipettes (0.5-2.5 μm tip radius) as a probe, with in situ potentiometric scanning electrochemical microscopy (SECM). We develop carbonate (CO32-) ionophore-incorporated ISEs exhibiting a Nernstian response (26.7 mV/decade) with a detection limit of 1.72 μM and explore surface-immobilized formate dehydrogenase (FDH) activity by detecting CO2 generated by the enzymatic reaction via potentiometric measurements. SECM is used for real-time spatial/temporal investigation of FDH immobilized onto the surface at a micrometer-scale resolution. Moreover, unlike voltammetric techniques based on faradaic reactions, the potentiometric measurements using ISEs allow highly sensitive and selective detection of CO32-, rendering efficient quantification of CO2 without interference from solution composition changes arising from faradaic processes. The total amount of CO2 generated at an FDH-immobilized Au ultramicroelectrode is quantified as a function of coenzyme, i.e., NAD+, and substrate, i.e., formate, concentrations both in constant tip-sample distance mode and variable depth mode. Finally, we demonstrate the use of the ISE to quantify CO2 levels in blood serum.

Selective Adsorbent Design with Multifunctional Surfaces: Innovating Solutions for Heterogeneous Catalysis in Water

Heterogeneous catalytic systems with water as the solvent often have the disadvantage of cross-contamination, while concerns about the purification and workup of the aqueous phase after reactions are rare in the lab or industry. In this context, designing and developing the functional selective solid adsorbent and revealing the adsorption mechanism can provide a new strategy and guidelines for constructing supported heterogeneous catalysts to address these issues. Herein, we report the stable composite adsorbent (Fe/ATP@PPy: magnetic Fe3O4/attapulgite with the polypyrrole shell) that features an integrated multifunctional surface, which can effectively tune the selective adsorption processes for Cu2+, Co2+, and Ni2+ ions and nitrobenzene via the cooperative chemisorption/physisorption in an aqueous system. The adsorption experiments showed that Fe/ATP@PPy displayed significantly higher adsorption selectivity for Ni2+ than Cu2+ and Co2+ ions, especially which exhibited an approximate 100.00% removal for both Ni2+ ions and nitrobenzene in the mixture system with a low concentration. Furthermore, combined tracking adsorption of Ni2+ ions and X-ray photoelectron spectroscopy characterization confirmed that the effective adsorption occurs via ion transfer coordination; the pathway was further validated at the molecular level through theoretical modeling. In addition, the selective adsorption mechanism was proposed based on the adsorption experiment, characterization, and the corresponding density functional theory calculation.

Selective electrochemical acceptorless dehydrogenation reactions of tetrahydroisoquinoline derivatives

Selective dehydrogenation reactions of tetrahydroisoquinoline derivatives through electrochemical oxidation are disclosed. In the presence of nitric acid, the selective partial dehydrogenation of tetrahydroisoquinolines to form 3,4-dihydroisoquinolines was achieved via anodic oxidation. The results of CV (Cyclic Voltammograms) experiments and DFT calculations showed the 3,4-dihydroisoquinolines protonated by an external Brønsted acid to be less prone than their unprotonated counterparts to oxidation under electrochemical conditions, thus avoiding their further dehydrogenation. Moreover, a TEMPO-mediated electrochemical oxidation enabled a complete dehydrogenation to yield fully aromatized isoquinolines. Thus, tunable processes involving electrochemical dehydrogenation of tetrahydroisoquinolines could be used to selectively produce various 3,4-dihydroisoquinolines and isoquinoline derivatives.

Alcohol imination catalyzed by carbon nanostructures synthesized by C(sp2)-C(sp3) free radical coupling

Imines are important intermediates for synthesizing various fine chemicals, with the disadvantage of requiring the use of expensive metal-containing catalysts. We report that the dehydrogenative cross-coupling of phenylmethanol and benzylamine (or aniline) directly forms the corresponding imine with a yield of up to 98%, and water as the sole by-product, in the presence of a stoichiometric base, using carbon nanostructures as the "green" metal-free carbon catalysts with high spin concentrations, which is synthesized by C(sp2)-C(sp3) free radical coupling reactions. The catalytic mechanism is attributed to the unpaired electrons of carbon catalysts to reduce O2 to O2·-, which triggers the oxidative coupling reaction to form imines, whereas the holes in the carbon catalysts receive electrons from the amine to restore the spin states. This is supported by density functional theory calculations. This work will open up an avenue for synthesizing carbon catalysts and offer great potential for industrial applications.

Biocatalytic and Chemo-Enzymatic Synthesis of Quinolines and 2-Quinolones by Monoamine Oxidase (MAO-N) and Horseradish Peroxidase (HRP) Biocatalysts

The oxidative aromatization of aliphatic N-heterocycles is a fundamental organic transformation for the preparation of a diverse array of heteroaromatic compounds. Despite many attempts to improve the efficiency and practicality of this transformation, most synthetic methodologies still require toxic and expensive reagents as well as harsh conditions. Herein, we describe two enzymatic strategies for the oxidation of 1,2,3,4-tetrahydroquinolines (THQs) and N-cyclopropyl-N-alkylanilines into quinolines and 2-quinolones, respectively. Whole cells and purified monoamine oxidase (MAO-N) enzymes were used to effectively catalyze the biotransformation of THQs into the corresponding aromatic quinoline derivatives, while N-cyclopropyl-N-alkylanilines were converted into 2-quinolone compounds through a horseradish peroxidase (HRP)-catalyzed annulation/aromatization reaction followed by Fe-mediated oxidation.

Photocatalytic Synthesis of Quinolines via Povarov Reaction under Oxidant-Free Conditions

We describe here an approach for synthesizing quinolines either from N-alkyl anilines or from anilines and aldehydes. A dual-catalyst system consisting of a photocatalyst and a proton reduction cocatalyst is employed. Without the use of any sacrificial oxidant and under extremely mild conditions, the reactions afford quinolines in excellent yields and produce H₂ as a byproduct.

Catalytic oxidative dehydrogenation of N-heterocycles with nitrogen/phosphorus co-doped porous carbon materials

A metal-free oxidative dehydrogenation of N-heterocycles utilizing a nitrogen/phosphorus co-doped porous carbon (NPCH) catalyst is reported. The optimal material is robust against traditional poisoning agents and shows high antioxidant resistance. It exhibits good catalytic performance for the synthesis of various quinoline, indole, isoquinoline, and quinoxaline ‘on-water’ under air atmosphere. The active sites in the NPCH catalyst are proposed to be phosphorus and nitrogen centers within the porous carbon network.

Green oxidations made easy. Metal-free dehydrogenation of N-heterocycles are possible in using N,P-co-doped porous carbon materials “on” water using air.

Recent advances in the synthesis of N-heteroarenes via catalytic dehydrogenation of N-heterocycles

Bio-active molecules having N-heteroarene core are widely used for numerous medicinal applications and as lifesaving drugs. In this direction, dehydrogenation of partially saturated aromatic N-heterocycles shows utmost importance for the synthesis of heterocycles. This feature article highlights the recent advances, from 2009 to April 2021, on the dehydrogenation of N-heteroaromatics. Notable features considering the development of newer catalysis for dehydrogenations are: (i) approaches based on precious metal catalysis, (ii) newer strategies and catalyst development technology using non-precious metal-catalysts for N-heterocycles having one or more heteroatoms, (iii) Synthesis of five or six-membered N-heterocycles using photocatalysis, electrocatalytic, and organo-catalytic approaches using different homogeneous and heterogeneous conditions' (iv) metal free (base and acid-promoted) dehydrogenation along with I₂, N-hydroxyphthalimide (NHPI) and bio catalyzed miscellaneous examples have also been discussed, (v) mechanistic studies for various dehydrogenation reactions and (vi) synthetic applications of various bio-active molecules including post-drug derivatization are discussed.

Tuning Ligand-Coordinated Single Metal Atoms on TiO2 and their Dynamic Response during Hydrogenation Catalysis

Ligand-coordinated supported catalysts (LCSCs) are of growing interest for heterogeneous single-atom catalysis. Here, the effect of the choice of organic ligand on the activity and stability of TiO₂ -supported single-atom Pt-ligand catalysts was investigated for ethylene hydrogenation. The activity of these catalysts showed a significant dependence on the choice of ligand and also correlated with coordination number for Pt-ligand and Pt-Cl^- . Of the three ligands examined in this study, the one with the lowest Pt coordination number, 1,10-phenanthroline-5,6-dione (PDO), showed the lowest reaction temperature and highest reaction rate, likely due to those metal sites being more accessible to reactant adsorption. In-situ X-ray absorption spectroscopy (XAS) experiments showed that the activity also correlated with good heterolytic dissociation of hydrogen, which was supported by OH/OD exchange experiments and was the rate-determining step of the hydrogenation reaction. In these in-situ XAS experiments up to 190 °C, the supported Pt-ligand catalyst showed excellent stability against structural and chemical change. Instead of Pt, the PDO ligand could be coordinated with Ir on TiO₂ to form Ir LCSCs that showed slow activation by loss of Ir-Cl bonds, then excellent stability in the hydrogenation of ethylene. These results provide the chance to engineer ligand-coordinated supported catalysts at the single-atom catalyst level by the choice of ligand and enable new applications at relatively high temperature.

Air oxidized activated carbon catalyst for aerobic oxidative aromatizations of N-heterocycles

Air oxidized activated carbon offers a robust, efficient, metal-free and recyclable catalyst for aromatizations of N-heterocycles, O2 being the terminal oxidant.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Gel Electrocatalysts: An Emerging Material Platform for Electrochemical Energy Conversion

Seeking sustainable and cost-effective energy sources is one of the significant challenges for the sustainable development of modern society. To date, considerable expectations have been held for technologies, such as fuel cells and electrolyzers, where the performance strongly depends on electrochemical conversion processes that can generate and store chemical energy through the breaking or formation of chemical bonds. However, those advanced technologies are severely limited by the efficiency, selectivity, and durability of electrocatalysis. Thanks to their hierarchically porous architecture, compositional and structural tunability, and ease of functionalization, the family of gel materials opens exciting opportunities for advanced energy technologies. Unique advances in gel materials based on controllable compositions and functions enable gel electrocatalysts to potentially break the limitations of current materials, enhancing the device performance of electrochemical energy. Here, recent developments and challenges for nanostructured gel-based materials for electrocatalysis applications are summarized. Future possibilities and challenges for gel electrocatalysts in terms of synthesis and applications are also discussed.

A General Strategy to Atomically Dispersed Precious Metal Catalysts for Unravelling Their Catalytic Trends for Oxygen Reduction Reaction

Atomically dispersed precious metal catalysts have emerged as a frontier in catalysis. However, a robust, generic synthetic strategy toward atomically dispersed catalysts is still lacking, which has limited systematic studies revealing their general catalytic trends distinct from those of conventional nanoparticle (NP)-based catalysts. Herein, we report a general synthetic strategy toward atomically dispersed precious metal catalysts, which consists of "trapping" precious metal precursors on a heteroatom-doped carbonaceous layer coated on a carbon support and "immobilizing" them with a SiO₂ layer during thermal activation. Through the "trapping-and-immobilizing" method, five atomically dispersed precious metal catalysts (Os, Ru, Rh, Ir, and Pt) could be obtained and served as model catalysts for unravelling catalytic trends for the oxygen reduction reaction (ORR). Owing to their isolated geometry, the atomically dispersed precious metal catalysts generally showed higher selectivity for H₂O₂ production than their NP counterparts for the ORR. Among the atomically dispersed catalysts, the H₂O₂ selectivity was changed by the types of metals, with atomically dispersed Pt catalyst showing the highest selectivity. A combination of experimental results and density functional theory calculations revealed that the selectivity trend of atomically dispersed catalysts could be correlated to the binding energy difference between *OOH and *O species. In terms of 2 e^- ORR activity, the atomically dispersed Rh catalyst showed the best activity. Our general approach to atomically dispersed precious metal catalysts may help in understanding their unique catalytic behaviors for the ORR.

Oxidase-Inspired Selective 2e/4e Reduction of Oxygen on Electron-Deficient Cu

Development of low-cost and efficient (electro)catalysts with tunable 2e/4e oxygen reduction reaction (ORR) selectivity toward energy conversion, biomimetic catalysis, and biosensing has attracted growing interest. Herein, we reported that carbon nanohybrids with O- or N-coordinated Cu (Cu-OC or Cu-NC) showed superior activity for 2e and 4e electrocatalytic ORR with selectivities of 84.0% and 97.2%, respectively. Experimental evidence demonstrated that the strong electron-rich O-doped carbon in Cu-OC donated electrons to Cu²⁺, weakening the binding strength of H₂O₂ at Cu-O centers and facilitating the 2e ORR pathway for selective production of H₂O₂. However, the poor electron-donor ability of the N-doped carbon in Cu-NC made Cu-N sites more electron deficient due to the reduced electron transfer from N-doped carbon to Cu²⁺, promoting 4e ORR by enhancing adsorption of O₂ and the ORR intermediates. The high 4e ORR activity of Cu-NC rendered its potential for application in a Zn-air battery and oxidase-mimicking activity for 3,3',5,5'-tetramethylbenzidine (TMB) and ascorbic acid (AA) oxidation. The maximal velocity (V_max) of TMB and AA oxidation over Cu-NC was higher than some natural oxidases and noble-metal-based artificial enzymes. The lower activation energy for AA oxidation over Cu-NC resulted in a 263-fold higher oxidative rate than TMB, further prompting nonenzymatic sensing of AA by the competitive oxidation strategy.

Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports

Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.

Co3O4-CuCoO2 Nanomesh: An Interface-Enhanced Substrate that Simultaneously Promotes CO Adsorption and O2 Activation in H2 Purification

Nanomaterials are widely used as redox-type reaction catalysts, while reactant adsorption and O₂ activation are hardly to be promoted simultaneously, restricting their applications in many important catalytic fields such as preferential CO oxidation (CO-PROX) in H₂-rich stream. In this work, an interface-enhanced Co₃O₄-CuCoO₂ nanomesh was initially synthesized by a hydrothermal process using aluminum powder as a sacrificial agent. This nanomesh is systematically characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption, X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, Raman spectroscopy, X-ray absorption near-edge spectroscopy, hydrogen temperature-programmed reduction, and oxygen temperature-programmed desorption. It is demonstrated that the nanomesh possesses high-density nanopores, enabling a large number of CO adsorption sites exposed to the surface. Meanwhile, electron transfer from O^2- to Co³⁺/Co²⁺ and the weakened bonding strength of Co-O bond at surfaces promoted the oxygen activation and redox ability of Co₃O₄. When tested as a catalyst for CO-PROX, this nanomesh with an optimized pore structure and a surface electronic structure, exhibits a strikingly high catalytic oxidation activity at low temperatures as well as a broader operation temperature window (i.e., CO conversion >99.0%, 100-200 °C) in the CO selective oxidation reaction. The present finding should be highly useful in promoting the quest for better CO-PROX catalysts, a hot topic for proton exchange membrane fuel cells and automotive vehicles.

Deconstructive di-functionalization of unstrained, benzo cyclic amines by C–N bond cleavage using a recyclable tungsten catalyst

With H₂WO₄ as the catalyst and H₂O₂ as the oxidant, we herein report a deconstructive difunctionalization of the C–N bond in unstrained, benzo cyclic amines to generate an ester group and nitro group simultaneously.