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>Δ<i>H</i></i>_vap, 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>_vap) as well as adsorbed on the surface of silica, graphite, and graphene particles (<i>Δ<i>H</i></i>_vap^eff), by using simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Measurement of the corresponding activation energy values, <i>E</i>_a^vap and <i>E</i>_a vap^eff, by TGA using a derivative method was shown to be the most reliable and practical way to assess <i>Δ<i>H</i></i>_vap and <i>Δ<i>H</i></i>_vap^eff. Enthalpies of adsorption (<i>Δ<i>H</i></i>_ads) were then calculated from the differences between <i>E</i>_a^vap and <i>E</i>_a vap^eff, 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>_ads, indicating significant interactions. For graphite particles, positive <i>Δ<i>H</i></i>_ads 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>_ads, apparently reflecting π–π* interactions and ranging between −7.6 and +32.6 kJ mol^–1, i.e., even larger than for silica, −3.3 to +8.3 kJ mol^–1. Thus, current atmospheric models may underestimate the partitioning of organic species in the particle phase unless matrix adsorption is taken into account

Novel Two-Step Process for the Production of Renewable Aromatic Hydrocarbons from Triacylglycerides

A two-step process was developed for the production of aromatic hydrocarbons from triglyceride (TG) oils. In the first reaction step, TG (soybean) oil was noncatalytically cracked and purified by distillation to produce an organic liquid product (OLP). The resulting OLP was then converted into aromatic compounds in a second reaction using a zeolite catalyst, HZSM-5. In this second reaction, three main factors were found to influence the yield of aromatic hydrocarbons: the SiO₂:Al₂O₃ ratio in the HZSM-5, the reaction temperature and the OLP-to-catalyst ratio. Upon cursory optimization, up to 58 w/w% aromatics were obtained. Detailed analyses revealed that most of the alkenes and carboxylic acids, and even many of the unidentified/unresolved compounds, which are characteristic products of noncatalytic TG cracking, were reformed into aromatic hydrocarbons. Instead of BTEX compounds, which are the common products of C₂–C₈ alkene and other feedstock reforming with HZSM-5, longer-chain alkylbenzenes dominated the reformate (along with medium-size <i>n-</i>alkanes). Another novel feature of the two-step process was a sizable (up to 13 w/w%) concentration of alicyclic hydrocarbons, both cyclohexanes and cyclopentanes. Thus, this novel two-step process may provide a new route for the production of renewable aromatic hydrocarbons as an important coproduct with transportation fuel products

Thermal Carbon Analysis Enabling Comprehensive Characterization of Lignin and Its Degradation Products

We have developed a novel thermal carbon analysis (TCA) method that provides both carbon mass balance and thermal fractionation profiles. Though not providing chemical structural information, this method enables a comprehensive characterization of both lignin and its degradation products, potential renewable and sustainable feedstocks. TCA is essential as a complement to a qualitative chemical speciation by thermal desorption–pyrolysis gas chromatography–mass spectrometry (TD–Py–GC–MS). Mono- and diaromatic oxygenated compounds were used as model compounds to optimize the method. The influence of various parameters such as solvents, amounts of sample loaded, and temperature ramp configuration, were investigated. A multistep temperature program with TD and pyrolytic temperatures with and without oxygen was employed for analysis of untreated lignin, where up to 55 wt % evolved in the presence of oxygen only, this fraction being unaccounted for by currently used methods. The TCA results were supported by thermogravimetric analysis with a matching heating ramp resulting in a similar mass distribution; however, TCA has the advantage of being selective for carbon. For lignin degradation products, the TD steps of TCA yielded similar recoveries as a solvent extraction followed by GC–MS. Thus, TCA may be used for screening significant product fractions to quantify the previously uncharacterized oligomer/polymer and char fractions

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 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 ¹H 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 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