4 research outputs found
Distinguishing Enolic and Carbonyl Components in the Mechanism of Carboxylic Acid Ketonization on Monoclinic Zirconia
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
chromatographâmass 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
Current
atmospheric models incorporate the values of vaporization
enthalpies, <i>Î<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>Î<i>H</i></i><sub>vap</sub>) as well as adsorbed on the surface
of silica, graphite, and graphene particles (<i>Î<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>aâŻvap</sub><sup>eff</sup>, by TGA using a derivative
method was shown to be the most reliable and practical way to assess <i>Î<i>H</i></i><sub>vap</sub> and <i>Î<i>H</i></i><sub>vap</sub><sup>eff</sup>. Enthalpies of adsorption
(<i>Î<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>aâŻvap</sub><sup>eff</sup>, thus paving a way to modeling the solidâgas phase
partitioning in atmospheric particulate matter (PM). The PAH adsorption
on silica particle surfaces (representing nâÏ* interactions)
resulted in negative values of <i>Î<i>H</i></i><sub>ads</sub>, indicating significant interactions. For graphite
particles, positive <i>Î<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 (37.4â145.7 K),
signifying negative, more PM-relevant values of <i>Î<i>H</i></i><sub>ads</sub>, apparently reflecting ÏâÏ*
interactions and ranging between â7.6 and +32.6 kJ mol<sup>â1</sup>, i.e., even larger than for silica, â3.3 to
+8.3 kJ mol<sup>â1</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
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 5â25
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
2â5% DMSO with up to 25 g/L lignin, significant lignin modification
was observed in 1â6 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 desorptionâpyrolysisâgas
chromatographyâmass 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
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 430â440 °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 DielsâAlder 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