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The pyrolysis of isobutene was investigated using a tubular flow reactor at an absolute pressure of ∼0.82 atm over a temperature range of 610–860 °C with residence times ranging from ∼0.5 to ∼2.4 s. The initial concentration of the fuel ranged from 5 to 50 mol%. These data were compared to the predictions of a fundamentally based detailed kinetic model. The model accurately predicted the observed fuel conversion, production of light products, and the formation of several important molecular weight growth species. The primary pathways that lead to the fuel decay and the formation of major products are discussed. In particular, H-atom abstraction from isobutene results in the formation of the 2-methyl allyl radical, which undergoes a β-scission reaction to form allene plus methyl. The subsequent addition reaction of 2-methyl allyl to allene is energetically favored and provides a route to the formation of stable molecular weight growth products that are readily converted to benzene and toluene. The potential energy surface for this reaction was derived from CBS-QB3 calculations and the corresponding temperature and pressure dependent rate constants are obtained. The model predictions were also compared and generally are in good agreement with multiple published isobutene pyrolysis data sets that were measured under significantly different conditions.

The primary objective of this work is to develop an improved fundamentally-based mechanism that describes the molecular weight growth kinetics observed during propene pyrolysis. Earlier attempts to describe the kinetics in terms of theoretically plausible reactions generally under-predicted the low temperature reactivity. To address this issue, propene pyrolysis experiments were performed at 575–875 °C with nominal residence times of ∼2.4, 1.2 and 0.5 s at ∼0.83 atm. These data were compared to a kinetic model that includes several reactions that involve allyl radicals. Specifically, electronic structure calculations at the CBS–QB3 level were performed for various allyl radical reactions, including addition, recombination, and hydrogen abstraction. This updated model is able to capture the observed fuel conversion, production of major products, and formation of molecular weight growth species. The sensitivity and rate of production analyses show that allyl reactions play important roles for both fuel conversion and product formation. In particular, allyl addition to propene leads to production of CH3 and H. The H-atoms can add to propene to form CH3 radicals, and both CH3 and H can abstract from propene to regenerate allyl, completing the reaction chain. This model also successfully predicts the fuel conversion and major products for selected literature propene pyrolysis data.

A systematic comparison of sulfur poisoning on Ni/YSZ and Ni/K2O-CaAl2O4, a commercially available reforming catalyst, demonstrated the heightened and rapid degradation of Ni/YSZ methane reforming activity. Ni/K2O-CaAl2O4 has nearly 15 times the hydrogen uptake capacity of Ni/YSZ which implies a difference in active nickel area. Because of this difference in active nickel surface area, Ni/K2O-CaAl2O4 was diluted in pure α-Al2O3 to achieve the same active nickel surface area as Ni/YSZ. The turnover frequencies (TOF) for steam methane reforming without sulfur on Ni/YSZ and Ni/K2O-CaAl2O4 were similar, although there was some deactivation on Ni/K2O-CaAl2O4 possibly as a result of coking which was observed visually. Sulfur deactivation on both catalysts was examined for H2S, SO2, and COS at concentrations of 1, 3, and 5 ppm. Ni/YSZ deactivated rapidly to an activity close to zero. Ni/K2O-CaAl2O4 deactivated quickly in the first 20 min, but then reached a non-zero steady state activity. The relative deactivation rates for the sulfur species examined were COS > SO2 ≥ H2S. Reaction temperatures of 650 °C, 750 °C, and 800 °C were evaluated, but temperature did not strongly affect deactivation rates for either catalyst. The overarching result of this study is that Ni/YSZ methane reforming activity is more sensitive to sulfur deactivation than a commercial reforming catalyst. The effect is so strong, that the use of Ni/YSZ with any hydrocarbon fuel may require removal of sulfur to sub-ppm levels.

The competition between substitution (S N 2) and elimination (E2) has been studied for the reactions of methyl, ethyl, isopropyl, and tert- butyl iodide with Cl − , CN − , and HS − in the gas phase. Previous studies have shown a dominance of the S N 2 mechanism for sulfur anions and for some cyanide–alkyl iodide reactions. Although our results support this conclusion for the reactions studied, they reveal that competition between the S N 2 and E2 pathways exists for the isopropyl reactions. Steric and electronic effects, upon alkyl group substitution, produce looser and less stable S N 2 transition states, however, they can favor the E2 process. These opposing effects on barrier heights produce E2/S N 2 competition as steric hindrance increases around the α-carbon, however the relative differences in intrinsic barrier heights lead to significantly different branching ratios. This interpretation is discussed in terms of reaction efficiencies, kinetic isotope effects, linear basicity–reactivity relationships, electrostatic models, and transition state looseness parameters.

We have measured the gas-phase acidities of nine compounds: formic acid, acetic acid, 1,3-propanedithiol, 2-methyl-2-propanethiol, 3-methyl-1-butanethiol, 2-propanethiol, 1-propanethiol, ethanethiol, and methanethiol, with acidities ranging from 338.6 to 351.1 kcal mol −1 using proton transfer kinetics and the resulting equilibrium constants. These acids were anchored to the well-known acidity of hydrogen sulfide; the measured acidities are in good agreement with previous experimental values, but error bars are significantly reduced. The gas-phase acidity of 3-methyl-1-butanethiol was determined to be 347.1 (5) kcal mol −1 ; there were no previous measurements of this value. Entropies of deprotonation were calculated and enthalpies of deprotonation were determined.

The reactivities of mono- and dihalocarbene anions (CHCl − , CHBr − , CF 2 − , CCl 2 − , and CBrCl − ) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS 2 , COS, CO 2 , O 2 , CO, and N 2 O). These anions were found to demonstrate diverse chemistry as illustrated by formation of multiple product ions and by the observed reaction trends. The reactions of CHCl − and CHBr − occur with similar efficiencies and reactivity patterns. Substitution of a Cl atom for an H atom to form CCl 2 − and CBrCl − decreases the rate constants; these two anions react with similar efficiencies and reactivity trends. The CF 2 − anion displays remarkably different reactivity; these differences are discussed in terms of its lower electron binding energy and the effect of the electronegative fluorine substituents. The results presented here are compared to the reactivity of the CH 2 − anion, which has previously been reported.