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Shannon S Stahl - One of the best experts on this subject based on the ideXlab platform.
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cooperative electrocatalytic Alcohol Oxidation with electron proton transfer mediators
Nature, 2016Co-Authors: Artavazd Badalyan, Shannon S StahlAbstract:A co-catalyst system for electrochemical Alcohol Oxidation composed of a bipyridine copper catalyst and an electron-proton-transfer mediator called TEMPO operates at much lower potential and is faster than TEMPO alone. Organic nitroxyls such as TEMPO (2,2,6,6-tetramethyl-1-piperidine N-oxyl) are effective electrocatalysts for Alcohol Oxidation across a broad range of substrates, making them promising for electro-organic synthesis applications. Their use in energy-conversion applications, however, is limited by the high electrode potentials required to generate the reactive oxoammonium species. Now Artavazd Badalyan and Shannon Stahl have developed a co-catalyst system for electrochemical Alcohol Oxidation that uses both TEMPO and a bipyridine copper catalyst. The dual catalyst requires a much lower potential than does TEMPO alone and the Alcohol Oxidation proceeds faster. This work has implications for the development of electrocatalysts composed of non-precious metals, with applications extending beyond Alcohol Oxidation. The electrochemical Oxidation of Alcohols is a major focus of energy and chemical conversion efforts, with potential applications ranging from fuel cells to biomass utilization and fine-chemical synthesis1,2,3,4,5,6,7. Small-molecule electrocatalysts for processes of this type are promising targets for further development8, as demonstrated by recent advances in nickel catalysts for electrochemical production and Oxidation of hydrogen9,10,11. Complexes with tethered amines that resemble the active site of hydrogenases12 have been shown both to catalyse hydrogen production (from protons and electrons) with rates far exceeding those of such enzymes11,13 and to mediate reversible electrocatalytic hydrogen production and Oxidation with enzyme-like performance14. Progress in electrocatalytic Alcohol Oxidation has been more modest. Nickel complexes similar to those used for hydrogen Oxidation have been shown to mediate efficient electrochemical Oxidation of benzyl Alcohol, with a turnover frequency of 2.1 per second. These compounds exhibit poor reactivity with ethanol and methanol, however15. Organic nitroxyls, such as TEMPO (2,2,6,6-tetramethyl-1-piperidine N-oxyl), are the most widely studied electrocatalysts for Alcohol Oxidation5,6,7,16,17,18,19. These catalysts exhibit good activity (1–2 turnovers per second) with a wide range of Alcohols18 and have great promise for electro-organic synthesis7. Their use in energy-conversion applications, however, is limited by the high electrode potentials required to generate the reactive oxoammonium species. Here we report (2,2′-bipyridine)Cu/nitroxyl co-catalyst systems for electrochemical Alcohol Oxidation that proceed with much faster rates, while operating at an electrode potential a half-volt lower than that used for the TEMPO-only process. The (2,2′-bipyridine)Cu(II) and TEMPO redox partners exhibit cooperative reactivity and exploit the low-potential, proton-coupled TEMPO/TEMPOH redox process rather than the high-potential TEMPO/TEMPO+ process. The results show how electron-proton-transfer mediators, such as TEMPO, may be used in combination with first-row transition metals, such as copper, to achieve efficient two-electron electrochemical processes, thereby introducing a new concept for the development of non-precious-metal electrocatalysts.
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Mechanism of Copper/Azodicarboxylate-Catalyzed Aerobic Alcohol Oxidation: Evidence for Uncooperative Catalysis
Journal of the American Chemical Society, 2015Co-Authors: Scott D. Mccann, Shannon S StahlAbstract:Cooperative catalysis between CuII and redox-active organic cocatalysts is a key feature of important chemical and enzymatic aerobic Oxidation reactions, such as Alcohol Oxidation mediated by Cu/TEMPO and galactose oxidase. Nearly 20 years ago, Marko and co-workers reported that azodicarboxylates, such as di-tert-butyl azodicarboxylate (DBAD), are effective redox-active cocatalysts in Cu-catalyzed aerobic Alcohol Oxidation reactions [Marko, I. E., et al. Science 1996, 274, 2044], but the nature of the cooperativity between Cu and azodicarboxylates is not well understood. Here, we report a mechanistic study of Cu/DBAD-catalyzed aerobic Alcohol Oxidation. In situ infrared spectroscopic studies reveal a burst of product formation prior to steady-state catalysis, and gas-uptake measurements show that no O2 is consumed during the burst. Kinetic studies reveal that the anaerobic burst and steady-state turnover have different rate laws. The steady-state rate does not depend on [O2] or [DBAD]. These results, toge...
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process development of cui abno nmi catalyzed aerobic Alcohol Oxidation
Organic Process Research & Development, 2015Co-Authors: Janelle E Steves, Yuliya Preger, Joseph R Martinelli, Christopher J Welch, Thatcher W Root, Joel M Hawkins, Shannon S StahlAbstract:An improved Cu/nitroxyl catalyst system for aerobic Alcohol Oxidation has been developed for the Oxidation of functionalized primary and secondary Alcohols to aldehydes and ketones, suitable for implementation in batch and flow processes. This catalyst, which has been demonstrated in a >50 g scale batch reaction, addresses a number of process limitations associated with a previously reported (MeObpy)CuI/ABNO/NMI catalyst system (MeObpy = 4,4′-dimethoxy-2,2′-bipyridine, ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl, NMI = N-methylimidazole). Important catalyst modifications include the replacement of [Cu(MeCN)4]OTf with a lower-cost Cu source, CuI, reduction of the ABNO loading to 0.05–0.3 mol%, and use of NMI as the only ligand/additive (i.e., without a need for MeObpy). Use of a high flash point solvent, N-methylpyrrolidone, enables safe operation in batch reactions with air as the oxidant. For continuous-flow applications compatible with elevated gas pressures, better performance is observed with acetonitrile...
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mechanism of Alcohol Oxidation mediated by copper ii and nitroxyl radicals
Journal of the American Chemical Society, 2014Co-Authors: Bradford L Ryland, Scott D. Mccann, Thomas C Brunold, Shannon S StahlAbstract:2,2′-Bipyridine-ligated copper complexes, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are highly effective catalysts for aerobic Alcohol Oxidation. Considerable uncertainty and debate exist over the mechanism of Alcohol Oxidation mediated by CuII and TEMPO. Here, we report experimental and density functional theory (DFT) computational studies that distinguish among numerous previously proposed mechanistic pathways. Oxidation of various classes of radical-probe substrates shows that long-lived radicals are not formed in the reaction. DFT computational studies support this conclusion. A bimolecular pathway involving hydrogen-atom-transfer from a CuII–alkoxide to a nitroxyl radical is higher in energy than hydrogen transfer from a CuII–alkoxide to a coordinated nitroxyl species. The data presented here reconcile a collection of diverse and seemingly contradictory experimental and computational data reported previously in the literature. The resulting Oppenauer-like reaction pathway furt...
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mechanism of copper i tempo catalyzed aerobic Alcohol Oxidation
Journal of the American Chemical Society, 2013Co-Authors: Jessica M Hoover, Bradford L Ryland, Shannon S StahlAbstract:Homogeneous Cu/TEMPO catalyst systems (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) have emerged as some of the most versatile and practical catalysts for aerobic Alcohol Oxidation. Recently, we disclosed a (bpy)CuI/TEMPO/NMI catalyst system (NMI = N-methylimidazole) that exhibits fast rates and high selectivities, even with unactivated aliphatic Alcohols. Here, we present a mechanistic investigation of this catalyst system, in which we compare the reactivity of benzylic and aliphatic Alcohols. This work includes analysis of catalytic rates by gas-uptake and in situ IR kinetic methods and characterization of the catalyst speciation during the reaction by EPR and UV–visible spectroscopic methods. The data support a two-stage catalytic mechanism consisting of (1) “catalyst Oxidation” in which CuI and TEMPO–H are oxidized by O2 via a binuclear Cu2O2 intermediate and (2) “substrate Oxidation” mediated by CuII and the nitroxyl radical of TEMPO via a CuII-alkoxide intermediate. Catalytic rate laws, kinetic iso...
Steven L Suib - One of the best experts on this subject based on the ideXlab platform.
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oms 2 for aerobic catalytic one pot Alcohol Oxidation wittig reactions efficient access to α β unsaturated esters
Chemcatchem, 2014Co-Authors: Jagadeswara R Kona, Cecil K Kingondu, Amy R Howell, Steven L SuibAbstract:Manganese oxide octahedral molecular sieve (OMS) materials with well-defined pores have been extensively studied over two decades due to their intriguing chemical and physical properties. OMS-2, the synthetic cryptomelane form of manganese oxide, was synthesized by a modified reflux method and was found to be highly active for obtaining α,β-unsaturated esters (up to 95 % yield and with high diastereoselectivities) from a variety of benzyl, heteroaryl, allyl and alkyl Alcohols via one-pot Alcohol Oxidation-Wittig reaction. The transformation utilizes air as the stoichiometric oxidant, and the inexpensive catalyst can be recovered and reused.
Xinquan Hu - One of the best experts on this subject based on the ideXlab platform.
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efficient no equivalent for activation of molecular oxygen and its applications in transition metal free catalytic aerobic Alcohol Oxidation
Journal of Organic Chemistry, 2007Co-Authors: Weimin Mo, Dong Xu, Zhenlu Shen, Baoxiang Hu, Xinquan HuAbstract:tert-Butyl nitrite (TBN) was identified as an efficient NO equivalent for the activation of molecular oxygen. The unique property of TBN enabled TEMPO-catalyzed aerobic Alcohol Oxidation to be performed in high-volume efficiency. Up to a 16 000 turnover number was achieved in this transition-metal-free aerobic catalytic system. Under the optimal reaction conditions, various Alcohols were converted into their corresponding carbonyl compounds with TEMPO/HBr/TBN as catalyst. The newly developed method was suitable for the Oxidation of solid substrate Alcohols with high melting point and/or low solubility under the help of minimum solvent to form a slurry.
Alfons Baiker - One of the best experts on this subject based on the ideXlab platform.
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Advances in Infrared Spectroscopy of Catalytic Solid–Liquid Interfaces: The Case of Selective Alcohol Oxidation
Topics in Catalysis, 2009Co-Authors: Davide Ferri, Alfons BaikerAbstract:Progress in the use of ATR-IR spectroscopy to improve the understanding of liquid-phase heterogeneous catalytic reactions is illustrated using the example of the Oxidation of benzyl Alcohol over Pd/Al_2O_3 and Bi–Pd/Al_2O_3. The in situ studies performed in both batch and continuous reactor cells provide rich information on the reaction pathway and important facets of the mechanism, such as the nature of active Pd sites and the effect of the Bi-promoter. The combination of CO site blocking prior to reaction and isotopic labeling suggests that Alcohol dehydrogenation occurs uniformly over Pd nanoparticles, but only selected sites may allow desorption of the product benzaldehyde thus providing the required selectivity. Promotion of Pd/Al_2O_3 using bismuth produces infrared spectra free of adsorbed CO. This information demonstrates that Bi is deposited on selected adsorption sites (terraces rather than defects) and simultaneously confirms that open terraces favor product decomposition. Experiments performed in the continuous reactor cell using different catalyst film thickness show that reactions can be studied under kinetic or mass transfer limited conditions depending on catalyst film thickness. This allowed to study the Alcohol Oxidation under conditions of oxygen diffusion limitation, which are preferably applied in praxis in order to prevent catalyst deactivation by over-Oxidation.
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Probing Active Sites During Palladium-Catalyzed Alcohol Oxidation in “Supercritical” Carbon Dioxide
Catalysis Letters, 2003Co-Authors: Jan-dierk Grunwaldt, Matteo Caravati, Michael Ramin, Alfons BaikerAbstract:Structural information has been gained during aerobic benzyl Alcohol Oxidation in “supercritical” carbon dioxide at 150 bar on alumina-supported palladium by X-ray absorption spectroscopy while monitoring simultaneously the performance of the catalyst. The reduction of the catalyst by benzyl Alcohol could be monitored by the analysis of the near-edge region of the Pd K-edge. The palladium constituent was mainly in metallic state under operating conditions. Partial reOxidation was observed when only oxygen in “supercritical” carbon dioxide in the absence of Alcohol was fed. The catalytic activity of the PdO_x/Al_2O_3 catalyst during benzyl Alcohol Oxidation was comparable to that in a conventional continuous fixed-bed reactor and depended on the oxygen concentration in the feed. The rate of Alcohol conversion went through a maximum when the oxygen concentration was increased. At maximum rate, part of the palladium was in the oxidized state. Upon further increase of the oxygen concentration, the activity decreased because of the formation of surface palladium oxide. The reaction rate in “supercritical” carbon dioxide was strikingly higher than that observed for the corresponding liquid-phase Oxidation.
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Catalyst potential: a key for controlling Alcohol Oxidation in multiphase reactors
Catalysis Today, 1995Co-Authors: Tamas Mallat, Alfons BaikerAbstract:Abstract The possibilities and advantages of measuring the catalyst potential during Alcohol Oxidation with molecular oxygen and supported platinum metal catalysts is discussed. The steady-state catalyst potential is a sensitive indicator of the balance between Oxidation and reduction processes taking place on the active sites. In this short review we focus on the application of the potential measurements for adjusting the rate of oxygen supply to the rate of Alcohol Oxidation and avoiding catalyst deactivation. Other uses are the control of selectivity in the consecutive reaction sequence Alcohol → aldehyde → acid and the elucidation of the nature of catalyst deactivation. The partial Oxidation of cinnamyl Alcohol in a multi-phase reactor has been chosen as an example.
Jagadeswara R Kona - One of the best experts on this subject based on the ideXlab platform.
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oms 2 for aerobic catalytic one pot Alcohol Oxidation wittig reactions efficient access to α β unsaturated esters
Chemcatchem, 2014Co-Authors: Jagadeswara R Kona, Cecil K Kingondu, Amy R Howell, Steven L SuibAbstract:Manganese oxide octahedral molecular sieve (OMS) materials with well-defined pores have been extensively studied over two decades due to their intriguing chemical and physical properties. OMS-2, the synthetic cryptomelane form of manganese oxide, was synthesized by a modified reflux method and was found to be highly active for obtaining α,β-unsaturated esters (up to 95 % yield and with high diastereoselectivities) from a variety of benzyl, heteroaryl, allyl and alkyl Alcohols via one-pot Alcohol Oxidation-Wittig reaction. The transformation utilizes air as the stoichiometric oxidant, and the inexpensive catalyst can be recovered and reused.
