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Cellulose-derived 1,2-propanediol undergoes C–C coupling with methanol to produce higher diols, which then undergo C–O and C–C scission to produce fuel-range aliphatic alcohols.

Ethylene oligomerization over carbon-supported cobalt catalysts at 200 °C selectively produces linear olefins (>80% linearity up to C12 olefins at 30% LAO isomer distribution up to C8 olefins at

Pyrolysis oil (PO) is an inexpensive biofuel produced via the fast pyrolysis of lignocellulosic biomass. However, PO is unstable and has low energy density, making it unsuitable as a fuel in conven...

Here, we show that C4–C12 linear olefins, including linear alpha olefins, can be selectively produced from ethylene over a stable cobalt oxide on carbon catalyst. Both bulk and surface cobalt phases are CoO when the catalyst is stable, suggesting CoO is the stable cobalt phase for oligomerization. During the reaction, polyethylene forms in the catalyst pores which influences the product selectivity. The catalyst is more stable at higher temperatures (∼200 °C) likely due to reduction of Co3O4 to CoO while rapid deactivation is observed at lower temperatures (e.g., 80–140 °C). The product selectivity can be fit to two different Schulz Flory distributions, one from C4 to C10 olefins and one above C10 olefins, suggesting that transport restrictions influence product selectivity. At 48.3% conversion, product linearities up to C12 olefins are above 90%, making it the most selective heterogeneous catalyst to linear olefins to date in the absence of activators and/or solvents.

Guerbet coupling to oligomerize ethanol to heavy alcohols over a hydroxyapatite (HAP) catalyst is explored through the development and application of a reaction kinetics model that describes coupli...

Biomass conversion to alcohols using supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHO) with CuMgAl mixed metal oxide is a promising process for biofuel production. We demonstrate how maple wood can be converted at high weight loadings and product concentrations in a batch and a semi-continuous reactor to a mixture of C2–C10 linear and cyclic alcohols. Maple wood was solubilized semi-continuously in supercritical methanol and then converted to a mixture of C2–C9 alcohols and aromatics over a packed bed of CuMgAlOx catalyst. Up to 95 wt% of maple wood can be solubilized in the methanol by using four temperature holds at 190, 230, 300, and 330 °C. Lignin was solubilized at 190 and 230 °C to a mixture of monomers, dimers, and trimers while hemicellulose and cellulose solubilized at 300 and 330 °C to a mixture of oligomeric sugars and liquefaction products. The hemicellulose, cellulose, and lignin were converted to C2–C10 alcohol fuel precursors over a packed bed of CuMgAlOx catalyst with 70–80% carbon yield of the entire maple wood. The methanol reforming activity of the catalyst decreased by 25% over four beds of biomass, which corresponds to 5 turnovers for the catalyst, but was regenerable after calcination and reduction. In batch reactions, maple wood was converted at 10 wt% in methanol with 93% carbon yield to liquid products. The product concentration can be increased to 20 wt% by partially replacing the methanol with liquid products. The yield of alcohols in the semi-continuous reactor was approximately 30% lower than in batch reactions likely due to degradation of lignin and cellulose during solubilization. These results show that solubilization of whole biomass can be separated from catalytic conversion of the intermediates while still achieving a high yield of products. However, close contact of the catalyst and the biomass during solubilization is critical to achieve the highest yields and concentration of products.

Catalytic C–H bond activation of methane and ethane on a series of silica supported platinum catalysts (Pt/SiO2) was studied by using hydrogen/deuterium (H/D) exchange. Kinetic experiments demonstrate that under the reaction conditions studied, the rate of C–H bond activation shows approximate first order dependence in alkane and inverse first order dependence in D2. The rate of C–H activation is affected by the presence of sodium on the silica support, where sodium-free supports have the fastest rates of C–H activation, as assessed by H/D exchange. CO adsorption and FTIR studies indicate that the Pt particles on the sodium-free support are more electron-deficient, having the most blue-shifted linear CO stretch, while sodium-containing supports are more electron-donating, having the most red-shifted linear CO stretch. It is proposed, based on the results described in this article and previous work in the literature, that more electron-donating supports cause the Pt particles to be more electron-rich and t...

Supercritical methanol depolymerization and hydrodeoxygenation (SCM-DHDO) of biomass is a technology to produce C2–C9 alcohols in a single reaction step. Previous research has shown that this techn...

A series of calcined MgAl mixed metal oxide catalysts with low Cu loadings (0.1 – 1.5 wt%) were tested for ethanol oligomerization to distillate-range molecules. The low Cu loading catalysts (0.1 – 0.6 wt%) had high selectivity to linear chain C4+ alcohols (49 – 63% selectivity) and C6+ esters (45 – 66% of total esters). More specifically, the diesel fuel precursor selectivities were over 75% for low Cu loading catalysts (0.1 – 0.6 wt%), with a decrease to 49% for a higher Cu loading catalyst (>1.2 wt% Cu) due to increased ethyl acetate and acetone formation. Alcohol and ester product selectivities follow a Schultz-Flory distribution. Alcohol selectivity was found to vary inversely with BET surface areas of the catalysts, whereas ester selectivity was found to increase with higher base site counts. However, Cu wt% loading was found to have a stronger impact in overall catalyst activity. The catalysts deactivated proportionally to the number of turnovers mainly due to coking, but could be regenerated by calcination.

Synthesis of distillate-range fuels from biomass-derived alcohols has recently received considerable attention due to projected increases in the demands of these fuels and the extensive commercialization of alcohol production. Here we present a two-stage process by which an alcohol such as ethanol or 1-butanol can be converted with high yields into distillate-range ethers and olefins by combining Guerbet coupling and intermolecular dehydration. The ethers can be used as cetane-improvers in diesel fuel, while the olefins can be hydrogenated and blended with gasoline or oligomerized and hydrogenated to jet-range paraffins. The first stage was executed using calcium hydroxyapatite to produce higher linear and branched alcohols at above 80% selectivity at up to 40% conversion with high stability for over 400 h time-on-stream operation. Increasing conversion decreases selectivity, producing predominantly mono-ene and diene byproducts. Etherification was performed using the acidic resin Amberlyst™ 70 at around 65% conversion. Linear alcohols were converted at above 90% selectivity while branched alcohols were far more selective to olefins (65–75%). Etherification occurs via two mechanisms: a direct mechanism involving the reaction of two alcohols and an indirect mechanism between an alcohol and equilibrated pool of olefins. Cross-etherification was observed between linear and branched alcohols, improving the selectivity to ethers in conversion of the latter. A mixture of C4+ alcohols produced from ethanol condensation at 40% conversion was effectively utilized in etherification at selectivities comparable to the model mixtures. An overall process is presented for the conversion of ethanol to diesel-range ethers and olefins with yields of approximately 80%.

We report a comparative study of isolated FeII, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated FeII species on silica prepared by grafting the open cyclopentadienide iron complex, bis(2,4-dimethyl-1,3-pentadienide) iron(II) or Fe(oCp)2. The grafting and evolution of the surface species was elucidated by 1H NMR, diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopies. The oxidation state and local structure of surface Fe were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure. The initial grafting of iron proceeds by one surface hydroxyl Si–OH reacting with Fe(oCp)2 to release one diene ligand (oCpH), generating a SiO2-bound FeII(oCp) species, 1-FeoCp. Subsequent treatment with H2 at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, 1-C. Dispersion ...

Correction for ‘Catalytic synthesis of distillate-range ethers and olefins from ethanol through Guerbet coupling and etherification’ by Nathaniel M. Eagan et al., Green Chem., 2019, 21, 3300–3318.

Dimethyl palladium(II) complexes with various bi- and tri-dentate ligands, (L2)PdII(CH3)2, react readily with the one-electron oxidant ferrocenium hexafluorophosphate (Fc+) in acetone-d6. These oxidations typically proceed by one of two pathways. Oxidatively-induced methyl transfer forms trimethyl palladium(IV) intermediates which usually undergo reductive C−C coupling and form ethane. Alternatively, oxidation can cause Pd–C bond homolysis, which yields predominantly methane and methylferrocene, a product of methyl radical trapping. The reaction selectivity is dependent on ligand identity and likely due to the ability of different classes of ligands to stabilize intermediates in the PdIV oxidation state. Complexes containing the bidentate diimine ligands 1,10-phenanthroline (phen) or 2,3-dimethyl-1,4-di-4-tolyl-1,4-diazabuta-1,3-diene (Tol2DAB) adhere to the ethane producing pathway previously reported for (tBu2bpy)Pd(CH3)2 (tBu2bpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine), as does the complex of the potentially tridentate 6-methyl-N,N′-bis-2-pyridinyl-2-pyridinamine ligand. In contrast, Fc+ oxidations of complexes with bis-carbene (1,1′-methylene-3,3′-tert-butyldiimidazole-2,2′-diylidene, tBuCCtBu), bis-phosphine (tert-butyl-bis(diphenylphosphinomethyl)amine (tBuN(CH2PPh2)2, PCNCP), or tris(pyrazolyl)methane ligands resulted in little or no ethane production. These reactions likely involve either methyl radicals or trimethyl PdIV complexes that are stable towards C−C reductive elimination. Oxidation of (TMEDA)Pd(CH3)2 (TMEDA = (N,N,N′,N′-tetramethylethylenediamine) yields both methane and ethane, suggesting that both Pd–C bond homolysis and oxidative methyl transfer are competitive in this case.

Control over the oxidation state and crystalline phase of thin-film iron oxides was achieved by low-temperature atomic layer deposition (ALD), utilizing a novel iron precursor, bis(2,4-methylpentadienyl)iron. This low-temperature (T = 120 °C) route to conformal deposition of crystalline Fe3O4 or α-Fe2O3 thin films is determined by the choice of oxygen source selected for the second surface half-reaction. The approach employs ozone to produce fully oxidized α-Fe2O3 or a milder oxidant, H2O2, to generate the Fe(2+)/Fe(3+) spinel, Fe3O4. Both processes show self-limiting surface reactions and deposition rates of at least 0.6 Å/cycle, a significantly high growth rate at such mild conditions. We utilized this process to prepare conformal iron oxide thin films on a porous framework, for which α-Fe2O3 is active for photocatalytic water splitting.

Tuning the aryl substituents of N,N′-diaryl-2,3-dimethyl-1,4-diaza-1,3-butadiene (DAB) ligands promotes the challenging C–C bond-forming reductive elimination from PtIV diimine complexes [(DAB)Pt(CH3)3(solvent)]+ (2) under mild conditions. Experimental results and density functional calculations indicate that 2,6-aryl substitution promotes reductive elimination by facilitating dissociation of the coordinating solvent by close to 10 kcal mol–1, but too much steric bulk inhibits the formation of 2 in the one-electron outersphere oxidation of (DAB)Pt(CH3)2 (1).

Copper-dioxygen (CuO2) adducts are frequently proposed as intermediates in enzymes, yet their electronic and vibrational structures have not always been understood. [Cu(eta1-O2)TMG3tren]+ (TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) features end-on (eta1) O2 coordination in the solid state. Described here is an investigation of the compound's solution properties by nuclear magnetic resonance spectroscopy, density functional calculations, and oxygen isotope effects. The study yields two major findings. First, [Cu(eta1-O2)TMG3tren]+ is paramagnetic due to a triplet electronic structure; this is in contrast to other copper compounds where O2 is bound in a side-on manner. Second, the oxygen equilibrium isotope effect upon O2 binding to copper(I) (18O EIE [triple bond] K(16O16O)/K(16O18O) = 1.0148 +/- 0.0012) is significantly larger than those determined for iron and cobalt eta1-O2 adducts. This result is suggested to reflect greater ionic (CuII-O2-I) character within the valence bond description. A revised interpretation of the physical origins of the 18O EIEs upon O2 binding to redox metals is also advanced along with experimental data that should be used as benchmarks for interpreting 18O kinetic isotope effects upon enzyme reactions.

The use of oxygen ( 18 O) isotope fractionation as a mechanistic probe of chemical and biological oxidation reactions, particularly those which involve metal–O2 adducts, is currently being explored. Summarized here are reactions of enzymes and inorganic compounds for which competitive isotope effect measurements have been performed using natural abundance molecular oxygen and isotope ratio mass spectrometry. The derived 18 O equilibrium isotope effects (EIEs) and kinetic isotope effects (KIEs) reflect the ground state and transition state structures, respectively, for reactions of 16 O– 16 O and 18 O– 16 O. Normal isotope effects (>1) characterize the binding of O2 to transition metal centers. The magnitudes, which are primarily determined by the decrease in the O–O force constant accompanying formal electron transfer from the metal to O2, suggest that metal superoxo complexes can be distinguished from metal peroxo complexes. Because 18 O isotope effects can be measured during catalytic turnover, they complement existing approaches to elucidating the structures of activated oxygen intermediates based on low-temperature spectroscopy and crystallographic analysis of inorganic model compounds. © 2006 Elsevier B.V. All rights reserved.

Oxygen equilibrium isotope effects ((18)O EIEs) upon the formation of metal superoxide and peroxide structures from natural abundance O(2) are reported. The (18)O EIEs determined over a range of temperatures are compared to those calculated on the basis of vibrational frequencies. Considering all vibrational modes in a "full frequency model" is found to reproduce the empirical results better than "cut-off" models which consider only the most isotopically sensitive modes. Theoretically, the full frequency model predicts that (18)O EIEs arise from competing enthalpic and entropic influences resulting in nonlinear variations with temperature. Experimental evidence is provided for an increase in the magnitude of the EIE, in some instances implicating a change from inverse to normal values, as the temperature is raised. This finding is not easily reconciled with the common intuition that (18)O EIEs arise from a reduction of the O-O force constant and attendant changes in zero point energy level splitting. Instead a dominant entropic effect, as described here, is expected to characterize isotope effects upon reversible binding of small molecules to metal centers in enzymes and inorganic compounds.

Copper and topaquinone (TPQ) containing amine oxidases utilize O2 for the metabolism of biogenic amines while concomitantly generating H2O2 for use by the cell. The mechanism of O2 reduction has been the subject of long-standing debate due to the obscuring influence of a proton-coupled electron transfer between the tyrosine-derived TPQ and copper, a rapidly established equilibrium precluding assignment of the enzyme in its reactive form. Here, we show that substrate-reduced pea seedling amine oxidase (PSAO) exists predominantly in the Cu(I), TPQ semiquinone state. A new mechanistic proposal for O2 reduction is advanced on the basis of thermodynamic considerations together with kinetic studies (at varying pH, temperature, and viscosity), the identification of steady-state intermediates, and the analysis of competitive oxygen kinetic isotope effects, (18)O KIEs, [kcat/KM((16,16)O2)]/[kcat/KM((16,18)O2)]. The (18)O KIE = 1.0136 +/- 0.0013 at pH 7.2 is independent of temperature from 5 degrees C to 47 degrees C and insignificantly changed to 1.0122 +/- 0.0020 upon raising the pH to 9, thus indicating the absence of kinetic complexity. Using density functional methods, the effect is found to be precisely in the range expected for reversible O2 binding to Cu(I) to afford a superoxide, [Cu(II)(eta(1)-O2)(-I)](+), intermediate. Electron transfer from the TPQ semiquinone follows in the first irreversible step to form a peroxide, Cu(II)(eta(1)-O2)(-II), intermediate driving the reduction of O2. The similar (18)O KIEs reported for copper amine oxidases from other sources raise the possibility that all enzymes react by related inner-sphere mechanisms although additional experiments are needed to test this proposal.

Identifying intermediates in catalytic oxidation reactions requires the development of new probes of structure and mechanism. Reported here are proof-of-concept studies of oxygen (18O) isotope effects upon reversible O2-binding reactions of classic inorganic compounds. It is shown that the 18O equilibrium isotope effects may be used to differentiate structures where O2 is bound as a side-on peroxide ligand versus an end-on superoxide ligand. The application of 18O equilibrium isotope effects to the interpretation of 18O kinetic isotope effects and the study of O2 activation mechanisms is also discussed.