Current atmospheric models incorporate the values of vaporization enthalpies, ΔHvap, 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 (ΔHvap) as well as adsorbed on the surface of silica, graphite, and graphene particles (ΔHvap(eff)), by using simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Measurement of the corresponding activation energy values, Ea(vap) and Ea vap(eff), by TGA using a derivative method was shown to be the most reliable and practical way to assess ΔHvap and ΔHvap(eff). Enthalpies of adsorption (ΔHads) were then calculated from the differences between Ea(vap) and Ea 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 ΔHads, indicating significant interactions. For graphite particles, positive ΔHads 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 ΔHads, 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.
Abstract This study explored the production of aromatic hydrocarbons from the longer‐chain alkenes produced by the pyrolysis/cracking of crop oils. 1‐Tetradecene, serving as a model compound for these alkenes, was reformed in a batch reactor with a HZSM‐5 catalyst to produce a liquid hydrocarbon mixture with a high‐aromatic content. These reactions resulted in a >99% conversion of the 1‐tetradecene feedstock with a yield of up to 22 wt% of aromatic hydrocarbons. Surprisingly, isomers of C 3 ‐substituted benzenes along with xylenes and diaromatics (lower homologs of alkyl‐substituted indanes and naphthalenes) were the main aromatic products rather than their lower‐molecular‐weight (MW) homologs, benzene, toluene, ethylbenzene and xylenes, which are commonly formed with high selectivity during zeolite‐catalyzed reforming. The recovery of higher‐MW aromatics, and particularly bicyclic naphthalenes and indanes, provides mechanistic insights for zeolite‐catalyzed alkene reforming reactions suggesting that these higher‐MW aromatics are likely formed near the catalyst surface at pore openings. Furthermore, the production of acyclic diene intermediates in the size range of C 7 –C 10 provides insight into the overall reaction pathway. The results suggest that this reaction pathway may be a commercially viable option for the production of renewable C 3 ‐substituted aromatic chemicals/chemical intermediates as coproducts to complement the kerosene and diesel fuel blendstocks that are the primary products from crop oil cracking.
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Due to the complexity and recalcitrance of lignin, its chemical characterization is a key factor preventing the valorization of this abundant material. Multi-angle light scattering (MALS) is becoming a sought-after technique for absolute molecular weight (MW) determination of polymers and proteins. Lignin is a suitable candidate for MW determination via MALS, yet further investigation is required to confirm its absolute MW values and molecular size. Studies aiming to break down lignin into a variety of renewable products will benefit greatly from a simple and reliable determination method like MALS. Recent pioneering studies, discussed in this review, addressed several key challenges in lignin’s MW characterization. Nevertheless, some lignin-specific issues still need to be considered for in-depth characterization. This study explores how MALS instrumentation manages the complexities of determining lignin’s MW, e.g., with simultaneous fractionation and fluorescence interference mitigation. Additionally, we rationalize the importance of a more detailed light scattering analysis for lignin characterization, including aspects like the second virial coefficient and radius of gyration.
