Continuous flow technology-A tool for safer oxidation chemistry

The oxidation reaction is one of the most important transformations in synthetic chemistry, allowing for the introduction and modification of various functional groups. Continuous flow chemistry involving the use of...

Kinetic Probes of the Origin of Activity in MOF-Based C–H Oxidation Catalysts

The development of homogeneous catalysis is enabled by the availability of a rich toolkit of kinetics experiments, such as the Hg-drop test, that differentiate catalytic activity at ligand-supported metal complexes from potential heterogeneous catalysts derived from decomposition of molecular species. Metal-organic frameworks (MOFs) have garnered significant attention as platforms for catalysis at site-isolated, interstitial catalyst sights. Unlike homogeneous catalysis, relatively few strategies have been advanced to evaluate the origin of catalytic activity in MOF-promoted reactions. Many of the MOFs that have been examined as potential catalysts are comprised of molecular constituents that represent viable catalysts in the absence of the extended MOF lattice, and thus interfacial sites and leached homogeneous species represent potential sources of catalyst activity. Here, we demonstrate that analysis of deuterium kinetic isotope effects (KIEs) and olefin epoxidation diastereoselectivity provides direct probes of the origin of catalytic activity in MOF-promoted oxidation reactions. These analyses support direct involvement of lattice-based Fe sites in the turnover-limiting step of C–H activation with Fe-MOF-74-based materials (i.e., the MOF functions as a bona fide catalyst) and evidence that Cu2-based MOF MIL-125-Cu2O2 functions as a solid-state initiator for solution-phase oxidation chemistry and is not involved in the turnover limiting step (i.e., the MOF does not function as a catalyst). We anticipate that the simple experiments described here will provide a valuable tool for clarifying the role of MOFs in C–H oxidation reactions.

Iron-TAML Complexes: A Computational Approach to Improving Stability

<p>Researchers at the Institute for Green Oxidation Chemistry of the Carnegie Mellon University developed a group of catalysts called tetra amido macrocyclic ligand (TAML) activators. The purpose of that research was that TAML activators would breakdown pollutants in the presence of a sacrificial oxidant. Furthermore, the catalyst was designed to decompose on a required timescale, as to not damage the environment by prolonged exposure. Since the initial designs from the 1980’s, the TAML structure has undergone significant changes to increase efficiency or selectivity. Other uses of this group of catalysts have been explored, namely, the oxidation of water to molecular oxygen. This work presents a computational study using Density Functional Theory (DFT) which addresses the issue regarding the stability of certain iron-TAML intermediates in the water oxidation mechanism. Hence, the work seeks to explore how changing certain groups on the TAML ring can affect the stability of the reactive intermediates and the activation energy of the nucleophilic attack within the mechanism. The work highlights the importance of the fluorinated tail of the TAML structure in the accessibility of the desired transition state.</p>

Iron-TAML Complexes: A Computational Approach to Improving Stability

<p>Researchers at the Institute for Green Oxidation Chemistry of the Carnegie Mellon University developed a group of catalysts called tetra amido macrocyclic ligand (TAML) activators. The purpose of that research was that TAML activators would breakdown pollutants in the presence of a sacrificial oxidant. Furthermore, the catalyst was designed to decompose on a required timescale, as to not damage the environment by prolonged exposure. Since the initial designs from the 1980’s, the TAML structure has undergone significant changes to increase efficiency or selectivity. Other uses of this group of catalysts have been explored, namely, the oxidation of water to molecular oxygen. This work presents a computational study using Density Functional Theory (DFT) which addresses the issue regarding the stability of certain iron-TAML intermediates in the water oxidation mechanism. Hence, the work seeks to explore how changing certain groups on the TAML ring can affect the stability of the reactive intermediates and the activation energy of the nucleophilic attack within the mechanism. The work highlights the importance of the fluorinated tail of the TAML structure in the accessibility of the desired transition state.</p>

Nighttime and daytime dark oxidation chemistry in wildfire plumes: an observation and model analysis of FIREX-AQ aircraft data

Wildfires are increasing in size across the western US, leading to increases in human smoke exposure and associated negative health impacts. The impact of biomass burning (BB) smoke, including wildfires, on regional air quality depends on emissions, transport, and chemistry, including oxidation of emitted BB volatile organic compounds (BBVOCs) by the hydroxyl radical (OH), nitrate radical (NO3), and ozone (O3). During the daytime, when light penetrates the plumes, BBVOCs are oxidized mainly by O3 and OH. In contrast, at night or in optically dense plumes, BBVOCs are oxidized mainly by O3 and NO3. This work focuses on the transition between daytime and nighttime oxidation, which has significant implications for the formation of secondary pollutants and loss of nitrogen oxides (NOx=NO+NO2) and has been understudied. We present wildfire plume observations made during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality), a field campaign involving multiple aircraft, ground, satellite, and mobile platforms that took place in the United States in the summer of 2019 to study both wildfire and agricultural burning emissions and atmospheric chemistry. We use observations from two research aircraft, the NASA DC-8 and the NOAA Twin Otter, with a detailed chemical box model, including updated phenolic mechanisms, to analyze smoke sampled during midday, sunset, and nighttime. Aircraft observations suggest a range of NO3 production rates (0.1–1.5 ppbv h−1) in plumes transported during both midday and after dark. Modeled initial instantaneous reactivity toward BBVOCs for NO3, OH, and O3 is 80.1 %, 87.7 %, and 99.6 %, respectively. Initial NO3 reactivity is 10–104 times greater than typical values in forested or urban environments, and reactions with BBVOCs account for >97 % of NO3 loss in sunlit plumes (jNO2 up to 4×10-3s-1), while conventional photochemical NO3 loss through reaction with NO and photolysis are minor pathways. Alkenes and furans are mostly oxidized by OH and O3 (11 %–43 %, 54 %–88 % for alkenes; 18 %–55 %, 39 %–76 %, for furans, respectively), but phenolic oxidation is split between NO3, O3, and OH (26 %–52 %, 22 %–43 %, 16 %–33 %, respectively). Nitrate radical oxidation accounts for 26 %–52 % of phenolic chemical loss in sunset plumes and in an optically thick plume. Nitrocatechol yields varied between 33 % and 45 %, and NO3 chemistry in BB plumes emitted late in the day is responsible for 72 %–92 % (84 % in an optically thick midday plume) of nitrocatechol formation and controls nitrophenolic formation overall. As a result, overnight nitrophenolic formation pathways account for 56 %±2 % of NOx loss by sunrise the following day. In all but one overnight plume we modeled, there was remaining NOx (13 %–57 %) and BBVOCs (8 %–72 %) at sunrise.