Mechanisms of Furfural Reduction on Metal Electrodes: Distinguishing Pathways for Selective Hydrogenation of Bioderived Oxygenates.

Electrochemical reduction of biomass-derived platform molecules is an emerging route for the sustainable production of fuels and chemicals. However, understanding gaps between reaction conditions, underlying mechanisms, and product selectivity have limited the rational design of active, stable, and selective catalyst systems. In this work, the mechanisms of electrochemical reduction of furfural, an important biobased platform molecule and model for aldehyde reduction, are explored through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important biobased polymer precursors and fuels.

Combining Metabolic Engineering and Electrocatalysis: Application to the Production of Polyamides from Sugar.

Biorefineries aim to convert biomass into a spectrum of products ranging from biofuels to specialty chemicals. To achieve economically sustainable conversion, it is crucial to streamline the catalytic and downstream processing steps. In this work, a route that combines bio- and electrocatalysis to convert glucose into bio-based unsaturated nylon-6,6 is reported. An engineered strain of Saccharomyces cerevisiae was used as the initial biocatalyst for the conversion of glucose into muconic acid, with the highest reported muconic acid titer of 559.5 mg L(-1) in yeast. Without any separation, muconic acid was further electrocatalytically hydrogenated to 3-hexenedioic acid in 94 % yield despite the presence of biogenic impurities. Bio-based unsaturated nylon-6,6 (unsaturated polyamide-6,6) was finally obtained by polymerization of 3-hexenedioic acid with hexamethylenediamine.

UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation.

Transition metal complexes (NH3)5CoX(2+) (X = CH3, Cl) and L(H2O)MX(2+), where M = Rh or Co, X = CH3, NO, or Cl, and L is a macrocyclic N4 ligand are examined by both experiment and computation to better understand their electronic spectra and associated photochemistry. Specifically, irradiation into weak visible bands of nitrosyl and alkyl complexes (NH3)5CoCH3(2+) and L(H2O)M(III)X(2+) (X = CH3 or NO) leads to photohomolysis that generates the divalent metal complex and ˙CH3 or ˙NO, respectively. On the other hand, when X = halide or NO2, visible light photolysis leads to dissociation of X(-) and/or cis/trans isomerization. Computations show that visible bands for alkyl and nitrosyl complexes involve transitions from M-X bonding orbitals and/or metal d orbitals to M-X antibonding orbitals. In contrast, complexes with X = Cl or NO2 exhibit only d-d bands in the visible, so that homolytic cleavage of the M-X bond requires UV photolysis. UV-Vis spectra are not significantly dependent on the structure of the equatorial ligands, as shown by similar spectral features for (NH3)5CoCH3(2+) and L(1)(H2O)CoCH3(2+).

Generation of free oxygen atoms O((3)P) in solution by photolysis of 4-benzoylpyridine N-oxide.

Laser flash photolysis of 4-benzoylpyridine N-oxide (BPyO) at 308 nm in aqueous solutions generates a triplet excited state (3)BPyO* that absorbs strongly in the visible, λmax 490 and 380 nm. (3)BPyO* decays with the rate law kdecay/s(-1) = (3.3 ± 0.9) × 10(4) + (1.5 ± 0.2) × 10(9) [BPyO] to generate a mixture of isomeric hydroxylated benzoylpyridines, BPy(OH), in addition to small amounts of oxygen atoms, O((3)P). Molecular oxygen quenches (3)BPyO*, kQ = 1.4 × 10(9) M(-1) s(-1), but the yields of O((3)P) increase in O2-saturated solutions to 36%. Other triplet quenchers have a similar effect, which rules out the observed (3)BPyO* as a source of O((3)P). It is concluded that O((3)P) is produced from either (1)BPyO* or a short-lived, unobserved, higher energy triplet generated directly from (1)BPyO*. (3)BPyO* is reduced by Fe(2+) and by ABTS(2-) to the radical anion BPyO˙(-) which exhibits a maximum at 510 nm, ε = 2200 M(-1) cm(-1). The anion engages in back electron transfer with ABTS˙(-) with k = 1.7 × 10(9) M(-1) s(-1). The same species can be generated by reducing ground state BPyO with ˙C(CH3)2OH. The photochemistry of BPyO in acetonitrile is similar to that in aqueous solutions.

Alkyl group versus hydrogen atom transfer from metal alkyls to macrocyclic rhodium complexes.

Macrocyclic rhodium(II) complexes LRh(H2O)(2+) react with (dmgH)2(H2O)CoR and with (H2O)5CrR(2+) by alkyl transfer for R = CH3 to generate L(H2O)RhCH3(2+). When R = C2H5, C3H7 or C4H9, the reaction takes place by hydrogen atom abstraction from the coordinated alkyl and produces L(H2O)RhH(2+) and an α-olefin.

Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids.

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe(3+) in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe(2+). The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe(2+). The photochemistry in the presence of halide ions (X(-) = Cl(-), Br(-)) generates ethyl halides via halogen atom abstraction from FeX(n)(3-n) by ethyl radicals. Near-quantitative yields of C(2)H(5)X are obtained at ≥0.05 M X(-). Competition experiments with Co(NH(3))(5)Br(2+) provided kinetic data for the reaction of ethyl radicals with FeCl(2+) (k = (4.0 ± 0.5) × 10(6) M(-1) s(-1)) and with FeBr(2+) (k = (3.0 ± 0.5) × 10(7) M(-1) s(-1)). Photochemical decarboxylation of propionic acid in the presence of Cu(2+) generates ethylene and Cu(+). Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu(+) to Cu(2+), and removes gaseous olefins to prevent accumulation of Cu(+)(olefin) complexes and depletion of Cu(2+). The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.

Intramolecular conversion of pentaaquahydroperoxidochromium(III) ion to aqueous chromium(V): potential source of carcinogenic forms of chromium in aerobic organisms.

The decomposition of the title compound (H(2)O)(5)CrOOH(2+) (hereafter Cr(aq)OOH(2+)) in acidic aqueous solutions is kinetically complex and generates mixtures of products (Cr(aq)(3+), HCrO(4)(-), H(2)O(2), and O(2)). Relative yields of individual products vary greatly with reaction conditions and initial concentrations of Cr(aq)OOH(2+). At pH 5.5 in the presence of O(2), the reaction was complete in less than a minute and generated chromate in about 70% yield. These findings, in addition to poor reproducibility of kinetic data, are indicative of the involvement of one or more reactive intermediates that consume additional amounts of Cr(aq)OOH(2+) in post-rate-determining steps. The kinetics were simplified in the presence of H(2)O(2) or ABTS(2-), both of which are capable of scavenging strongly oxidizing intermediates. The measured rate constant in 0.10 M HClO(4) at low O(2) concentrations (≤0.03 mM) was independent of the concentration of the scavengers and was, within error, the same for ABTS(2-), k = 4.9 (±0.2) × 10(-4) s(-1), and H(2)O(2), k = 5.3 (±0.7) × 10(-4) s(-1). At a constant ionic strength of 1.0 M, the reaction in the presence of either H(2)O(2) or ABTS(2-) obeyed a two-term rate law, k(obs)/s(-1) = 6.7 (±0.7) × 10(-4) + 7.6 (±1.1) × 10(-4) [H(+)]. Both in the presence and absence of ABTS(2-) as the scavenger, the yields of H(2)O(2) increased with increasing [H(+)]. These results are discussed in terms of a dual-pathway mechanism for the decay of Cr(aq)OOH(2+). The H(+)-catalyzed path leads to the dissociation of H(2)O(2) from Cr(III), while in the H(+)-independent reaction, Cr(aq)OOH(2+) is transformed to Cr(V). Both scavengers rapidly remove Cr(V) and simplify both the kinetics and products.