Initial Pyrolysis Reactions in Unmodified and Flame-Retardant Cotton

Abstract

Thermogravimetric data were used to make Arrhenius plots for the vacuum pyrolyses of unmodified cotton and of cotton finished with various add-ons of THPOH-NH3 and THPS-urea-Na₂HPO₄ flame retardants. These plots show that all pyrolyses occurred in consecutive stages: The first and second initial stages, associated with the less ordered regions of the cotton fibers, and the main cellulose pyrolysis reaction, associated with cellulose crystallites. Cotton decrystallized by ball milling showed only the two initial pyrolysis stages. The second stage followed first-order kinetics. The first pyrolysis stage in unmodified cotton was characterized by a moderately low activation energy and by a large negative entropy of activation; the second stage showed a larger activation energy and a less negative entropy of activation. Mechanisms involving cellulose chain scission and chain unzipping are proposed for the first and second stages, respectively. Add-on of the two flame retardants had contrasting effects on the two initial pyrolysis stages. These effects are explained in terms of the way in which the flame retardants are deposited in the less ordered regions of cotton fibers.     

NMR Characterization of Substituted Aromatic Poly (Ether Sulofone)s

Abstract

The synthesis of a variety of substituted bisphenol A polysulfones, including nitro, amino, aminomethyl, ethyl, and methyl derivatives, is described. Nuclear magnetic resonance (NMR) (both proton and carbon, and several 2-D experiments) data confirm conclusions on the substitution site based on arguments on inductive effects in the phenyl rings. The proton ortho to the oxygen in the bisphenol A (BPA) residue is replaced in electrophilic substitution reactions. The degree of substitution was also calculated from the NMR results. The ethyl and methyl derivatives were expected, from the starting reactants, to each have a BPA ring substituted. The NMR data showed that, on the average, this is true. The nitro derivative also has substitution in every BPA ring, while the amino and aminomethyl derivatives have only intermittent BPA rings substituted. Measured degrees of substitution (DS) varied from 0.11 to 2.25.     

Electron Transfer between Hydrogen‐Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex

Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal‐to‐ligand charge‐transfer excited state of the [Re(CO)₃(phen)(py)]⁺ complex (phen=1,10‐phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady‐state and time‐resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited‐state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton‐coupled electron‐transfer process.     

Valence‐to‐core X‐ray emission spectroscopy of titanium compounds using energy dispersive detectors

X‐ray emission spectroscopy (XES) of transition metal compounds is a powerful tool for investigating the spin and oxidation state of the metal centers. Valence‐to‐core (vtc) XES is of special interest, as it contains information on the ligand nature, hybridization, and protonation. To date, most vtc‐XES studies have been performed with high‐brightness sources, such as synchrotrons, due to the weak fluorescence lines from vtc transitions. Here, we present a systematic study of the vtc‐XES for different titanium compounds in a laboratory setting using an X‐ray tube source and energy dispersive microcalorimeter sensors. With a full‐width at half‐maximum energy resolution of approximately 4 eV at the Ti Kβ lines, we measure the XES features of different titanium compounds and compare our results for the vtc line shapes and energies to previously published and newly acquired synchrotron data as well as to new theoretical calculations. Finally, we report simulations of the feasibility of performing time‐resolved vtc‐XES studies with a laser‐based plasma source in a laboratory setting. Our results show that microcalorimeter sensors can already perform high‐quality measurements of vtc‐XES features in a laboratory setting under static conditions and that dynamic measurements will be possible in the future after reasonable technological developments.     

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.     

An improved detailed chemical kinetic model for C3-C4 linear and iso-alcohols and their blends with gasoline at engine-relevant conditions

Propanol and butanol isomers have received significant research attention as promising fuel additives or neat biofuels. Robust chemical kinetic models are needed that can provide accurate and efficient predictions of combustion performance across a wide range of engine relevant conditions. This study seeks to improve the understanding of ignition and combustion behavior of pure C3-C4 linear and iso-alcohols, and their blends with gasoline at engine-relevant conditions. In this work, a kinetic model with improved thermochemistry and reaction kinetics was developed based on recent theoretical calculations of H-atom abstraction and peroxy radical reaction rates. Kinetic model validations are reported, and the current model reproduces the ignition delay times of the C3 and C4 alcohols well. Variations in reactivity over a wide range of temperatures and other operating conditions are also well predicted by the current model. Recent ignition delay time measurements from a rapid compression machine of neat iso-propanol and iso-butanol [Cheng et al., Proc. Combust Inst. (2020)] and blends with a research grade gasoline [Goldsborough et al., Proc. Combust Inst. (2020)] at elevated pressure (20–40 bar) and intermediate temperatures (780–950 K) were used to demonstrate the accuracy of the current kinetic model at conditions relevant to boosted spark-ignition engines. The effects of alcohol blending with gasoline on the autoignition behavior are discussed. The current model captures the suppression of reactivity in the low-temperature and negative-temperature-coefficient (NTC) region when either isopropanol and isobutanol are added to a research grade gasoline. Sensitivity and reaction flux analysis were performed to provide insights into the relevant fuel chemistry of the C3-C4 alcohols.     

A comprehensive experimental and improved kinetic modeling study on the pyrolysis and oxidation of propyne

To improve our understanding of the combustion characteristics of propyne, new experimental data for ignition delay times (IDTs), pyrolysis speciation profiles and flame speed measurements are presented in this study. IDTs for propyne ignition were obtained at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10 and 30 bar, over a wide range of temperatures (690–1460 K) using a rapid compression machine and a high-pressure shock tube. Moreover, experiments were performed in a single-pulse shock tube to study propyne pyrolysis at 2 bar pressure and in the temperature range 1000–1600 K. In addition, laminar flame speeds of propyne were studied at an unburned gas temperature of 373 K and at 1 and 2 bar for a range of equivalence ratios. A detailed chemical kinetic model is provided to describe the pyrolytic and combustion characteristics of propyne across this wide-ranging set of experimental data. This new mechanism shows significant improvements in the predictions for the IDTs, fuel pyrolysis and flame speeds for propyne compared to AramcoMech3.0. The improvement in fuel reactivity predictions in the new mechanism is due to the inclusion of the propyne + H?₂ reaction system along with ?H radical addition to the triple bonds of propyne and subsequent reactions.     

Experimental and kinetic modeling study of tetralin: A naphtheno-aromatic fuel for gasoline,jet and diesel surrogates

Distillate fuels contain significant proportions of naphtheno-aromatic components and tetralin is a suitable surrogate component to represent this molecular moiety. The presence of aromatic and naphthyl rings makes kinetic modeling of tetralin very challenging. Primary radicals formed during the oxidation of tetralin can be aryl, benzylic or paraffinic in nature. Using available information on reaction paths and rate constants of naphthenes and alkyl-aromatics, a kinetic model of tetralin has been developed in the current study with emphasis on low-temperature chemistry and high-pressure conditions. Due to the lack of high-level quantum chemical calculations on reaction pathways of tetralin, analogous rates from ab-initio studies on benzylic and paraffinic radicals have been adopted here. Some modifications to the reaction rate rules are incorporated to account for the unique characteristics of tetralin's molecular structure. Important reaction channels have been identified using reaction path and brute force sensitivity analyses. In order to investigate the model performance at low temperatures, new experiments are carried out in a rapid compression machine on blends of tetralin and 3-methylpentane. Blending of low-reactivity tetralin with a high-reactivity alkane allowed the investigation of tetralin ignition at very low temperatures (665 – 856 K). The kinetic model developed in the current study is found to predict the current experiments and literature data adequately. The new model will aid in high-fidelity surrogate predictions at engine-relevant conditions.