4 research outputs found

    Distinguishing Enolic and Carbonyl Components in the Mechanism of Carboxylic Acid Ketonization on Monoclinic Zirconia

    No full text
    This study contributes toward understanding the mechanism of catalytic formation of mixed ketones in an attempt to improve their selectivity vs symmetrical ketones. A pulsed microreactor placed inside a gas chromatograph–mass spectrometer instrument was used to identify the source of carbonyl group and quantify its distribution among products of zirconia-catalyzed cross-ketonization reaction of a mixture of carboxylic acids, with the carbonyl group of one of the acids selectively labeled by <sup>13</sup>C. A concept of enolic and carbonyl components in the ketonization mechanism was introduced to distinguish the sources of alkyl and acyl groups, respectively. The least branched acid was found to be the predominant source of CO<sub>2</sub>, the essential byproduct of ketonization. Thus the least branched acid is the preferred source of the alkyl group of the cross-ketone product, while the most branched acid provides the acyl group. Increased branching at the α carbon next to the carbonyl group decreased the reactivity of both the enolic and the carbonyl components. Following a pseudo first order kinetic analysis, the relative reaction rates for a common enolic component with a pair of different carbonyl components were measured by the method of competing reactions to obtain mechanistic insights. The distinction between two possible paths in the cross-ketonization mechanism was characterized quantitatively by assessing the difference in activation energies; the results obtained were explained by the steric effect of substituents. On the basis of detailed kinetic analysis, the rate-limiting step most likely occurs after the enolic component activation

    An Approach to the Estimation of Adsorption Enthalpies of Polycyclic Aromatic Hydrocarbons on Particle Surfaces

    No full text
    Current atmospheric models incorporate the values of vaporization enthalpies, <i>Δ<i>H</i></i><sub>vap</sub>, obtained for neat standards, thus disregarding the matrix effects on volatilization. To test the adequacy of this approximation, this study measured enthalpies of vaporization for five polycyclic aromatic hydrocarbons (PAHs) in the form of neat standards (<i>Δ<i>H</i></i><sub>vap</sub>) as well as adsorbed on the surface of silica, graphite, and graphene particles (<i>Δ<i>H</i></i><sub>vap</sub><sup>eff</sup>), by using simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Measurement of the corresponding activation energy values, <i>E</i><sub>a</sub><sup>vap</sup> and <i>E</i><sub>a vap</sub><sup>eff</sup>, by TGA using a derivative method was shown to be the most reliable and practical way to assess <i>Δ<i>H</i></i><sub>vap</sub> and <i>Δ<i>H</i></i><sub>vap</sub><sup>eff</sup>. Enthalpies of adsorption (<i>Δ<i>H</i></i><sub>ads</sub>) were then calculated from the differences between <i>E</i><sub>a</sub><sup>vap</sup> and <i>E</i><sub>a vap</sub><sup>eff</sup>, thus paving a way to modeling the solid–gas phase partitioning in atmospheric particulate matter (PM). The PAH adsorption on silica particle surfaces (representing n−π* interactions) resulted in negative values of <i>Δ<i>H</i></i><sub>ads</sub>, indicating significant interactions. For graphite particles, positive <i>Δ<i>H</i></i><sub>ads</sub> values were obtained; i.e., PAHs did not interact with the particle surface as strongly as observed for PM. PAHs on the surface of graphene particles evaporated in two stages, with the bulk of the mass loss occurring at temperatures lower than those with the neat standard, just as on graphite. Yet, unlike graphite, a small PAH fraction did not evaporate until higher temperatures compared to case of the neat standards and other particle surfaces (37.4–145.7 K), signifying negative, more PM-relevant values of <i>Δ<i>H</i></i><sub>ads</sub>, apparently reflecting π–π* interactions and ranging between −7.6 and +32.6 kJ mol<sup>–1</sup>, i.e., even larger than for silica, −3.3 to +8.3 kJ mol<sup>–1</sup>. Thus, current atmospheric models may underestimate the partitioning of organic species in the particle phase unless matrix adsorption is taken into account

    Fungal Biotransformation of Insoluble Kraft Lignin into a Water Soluble Polymer

    No full text
    Low substrate solubility and slow decomposition/biotransformation rate are among the main impediments for industrial scale lignin biotreatment. The outcome and dynamics of kraft lignin biomodification by basidiomycetous fungi, <i>Coriolus versicolor</i>, were investigated in the presence of dimethyl sulfoxide (DMSO). The addition of 2 vol % DMSO to aqueous media increased the lignin solubility up to 70%, while the quasi-immobilized fungi (pregrown on agar containing kenaf biomass) maintained their ability to produce lignolytic enzymes. Basidiomycetous fungi were able to grow on solid media containing both 5–25 g/L lignin and up to 5 vol % DMSO, in contrast to no growth in liquid media as a free suspended culture. When a fungal culture pregrown on agar was used for lignin treatment in an aqueous medium containing 2–5% DMSO with up to 25 g/L lignin, significant lignin modification was observed in 1–6 days. The product analysis suggests that lignin was biotransformed, rather than biodegraded, into an oxygenated and cross-linked phenolic polymer. The resulting product showed the removal of phenolic monomers and/or their immediate precursors based on gas chromatography and thermal desorption–pyrolysis–gas chromatography–mass spectrometry analyses. Significant intramolecular cross-linking among the reaction products was shown by thermal carbon analysis and <sup>1</sup>H NMR spectroscopy. An increase in polarity, presumably due to oxygenation, and a decrease in polydispersity of the lignin treatment product compared to untreated lignin were observed while using liquid chromatography

    Triacylglyceride Thermal Cracking: Pathways to Cyclic Hydrocarbons

    No full text
    Thermal cracking of triacylglyceride (TG) oils results in complex mixtures, containing nearly 20% cyclic hydrocarbons, which can be further processed into middle-distillate transportation fuels and byproduct chemicals. The occurrence patterns of cyclic products obtained via the thermal cracking of several TG feedstocks, such as canola and soybean oils, as well as triolein and tristearin (conducted at 430–440 °C in the absence of catalysts under vacuum), were investigated to probe possible formation mechanisms. Detailed gas chromatographic characterization furnished full molar homology/molecular size and partial isomeric profiles for cyclopentanes, cyclopentenes, cyclohexanes, cyclohexenes, aromatics, and polycyclic aromatic hydrocarbons (PAHs). It was found that the data were inconsistent with previously proposed mechanisms involving the Diels–Alder reaction as a single pathway. An alternate mechanism was proposed and supported with experimental evidence based on the intramolecular cyclization of alkenyl and alkadienyl radicals formed as a result of TG cracking. The product homology profiles corroborate the proposed mechanism and show the depletion of medium-size alkenes coupled with the accumulation of corresponding monocyclic hydrocarbons (those with the matching number of carbon atoms). Similarly, the product mixtures were depleted of long-chain alkyl-substituted monocyclic hydrocarbons because of the formation of the corresponding PAHs as long as sufficient time is available. Entropy appears to determine the type and size of cyclic hydrocarbons formed
    corecore