Evaluated Kinetics of Terminal and Non-Terminal Addition of Hydrogen Atoms to 1-Alkenes: A Shock Tube Study of H + 1-Butene

Abstract

Single-pulse shock tube methods have been used to thermally generate hydrogen atoms and investigate the kinetics of their addition reactions with 1-butene at temperatures of 880 to 1120 K and pressures of 145 to 245 kPa. Rate parameters for the unimolecular decomposition of 1-butene are also reported. Addition of H atoms to the π bond of 1-butene results in displacement of either methyl or ethyl depending on whether addition occurs at the terminal or nonterminal position. Postshock monitoring of the initial alkene products has been used to determine the relative and absolute reaction rates. Absolute rate constants have been derived relative to the reference reaction of displacement of methyl from 1,3,5-trimethylbenzene (135TMB). With k(H + 135TMB → m-xylene + CH3) = 6.7 × 10(13) exp(-3255/T) cm(3) mol(-1) s(-1), we find the following: k(H + 1-butene → propene + CH3) = k10 = 3.93 × 10(13) exp(-1152 K/T) cm(3) mol(-1) s(-1), [880-1120 K; 145-245 kPa]; k(H + 1-butene → ethene + C2H5) = k11 = 3.44 × 10(13) exp(-1971 K/T) cm(3) mol(-1) s(-1), [971-1120 K; 145-245 kPa]; k10/k11 = 10((0.058±0.059)) exp [(818 ± 141) K/T), 971-1120 K. Uncertainties (2σ) in the absolute rate constants are about a factor of 1.5, while the relative rate constants should be accurate to within ±15%. The displacement rate constants are shown to be very close to the high pressure limiting rate constants for addition of H, and the present measurements are the first direct determination of the branching ratio for 1-olefins at high temperatures. At 1000 K, addition to the terminal site is favored over the nonterminal position by a factor of 2.59 ± 0.39, where the uncertainty is 2σ and includes possible systematic errors. Combining the present results with evaluated data from the literature pertaining to temperatures of <440 K leads us to recommend the following: k∞(H + 1-butene → 2-butyl) = 1.05 × 10(9)T(1.40) exp(-366/T) cm(3) mol(-1) s(-1), [220-2000 K]; k∞(H + 1-butene → 1-butyl) = 9.02 × 10(8)T(1.40) exp(-1162/T) cm(3) mol(-1) s(-1) [220-2000 K]. Analogous rate constants for other unbranched 1-olefins should be very similar. Despite this, a factor of three discrepancy in the branching ratio for terminal and nonterminal addition is noted when comparing the present values with recommendations from a recent model of the important H + propene reaction. This difference is suggested to be well outside of the possible experimental errors of the present study or the expected differences with 1-butene. There thus appear to be inconsistencies in the current model for propene. In particular the addition branching ratio from that model should not be used as a reference value in extrapolations to other systems via rate rules or automated mechanism generation techniques.

A Shock Tube Study of H Atom Addition to Cyclopentene

“…At the temperatures of our experiments, values of γ less than 1.1 would require, on a per site basis, that the high-pressure rate constant for H atom addition to cyclopentene exceed that for terminal addition of H to 1-butene. Such a result would be strongly at odds with trends in rates of H atom addition to olefins observed in our work on 1-butene [19] and 2-pentene [38], and in numerous measurements at temperatures in the range of 250-400 K [15,[35][36][37]39,40], all of which show that addition to the terminal site of an alkene is significantly faster than to an alkyl-substituted site. To maintain selfconsistency in H atom addition rates, we suggest the reasonable lower limit of γ is about 1.5.…”

“…The dotted and solid lines indicate the respective empirical and TST fits to the experimental CPE data (filled data points with 2σ uncertainties). Results for butenes are as indicated; values for 1‐butene are multiplied by 2 to normalize the number of addition sites relative to CPE. References: 2000CDK = Clarke et al ; 2017AM = Awan and Manion (unpublished work).…”

“…Near 1000 K, these rate constants are about 10% larger than those later derived by Sheen et al [30] in a reanalysis of the H + propene/135TMB system using a detailed chemical kinetic model and spectral uncertainty propagation and minimization techniques. At our temperatures, the uncertainty in k 13 is about a factor of 1.5 [19,29]. The determined relative rates are typically good to about 10% and the existing database of rate constants measured relative to 135TMB provides valuable comparisons and a consistent scaling of absolute values.…”

“…The only other kinetic study of H + cyclopentene appears to be that of Clarke et al [15], who investigated the reaction at 298-370 K and 2.7-8.7 kPa using a flow reactor equipped with a microwave plasma to generate hydrogen atoms and a resonance fluorescence detector to monitor Hatom decays. Abstraction of H is negligible at these temperatures [19,[34][35][36][37], and their results should pertain to the high-pressure rate constant for addition of H. They also report H addition rates for several other alkenes that have been studied with direct methods by other investigators. In cases where multiple studies exist, the works show excellent agreement in the relative rate constants and very good agreement in the absolute values.…”

The Importance of Relative Reaction Rates in the Optimization of Detailed Kinetic Models

“…The present analysis assumes that relative rates and branching ratios can be experimentally measured with much higher accuracy than one can determine the absolute rate constants. While measurements must be assessed on a case-by-case basis, this is the usual situation for experiments that establish direct competitions and follow distinct products with an accurate analytical technique: In work from this laboratory, for example, uncertainties of around 10% (2σ ) in relative values are common [44,[51][52][53][54][55][56], and such results are not dissimilar to measurements reported elsewhere in the literature [41,57]. In contrast.…”