Thermal Decomposition Kinetics of Propylcyclohexane Jason A. Widegren and Thomas J. Bruno* Physical and Chemical Properties DiVision, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305-3328 As part of a large-scale thermophysical property measurement project, the decomposition kinetics of propylcyclohexane was investigated. Decomposition reactions were performed at 375, 400, 425, and 450 °C in stainless steel ampule reactors. At each temperature, the extent of decomposition was determined as a function of time by gas chromatography. These data were used to derive first-order rate constants for the decomposition of propylcyclohexane. Decomposition rate constants ranged from 3.66 × 10 -7 s -1 at 375 °C to 8.63 × 10 -5 s -1 at 450 °C. Among other things, these rate constants are useful for planning property measurements at high temperatures. Based on the amount of time required for 1% of the sample to decompose (t 0.01 ), we found that allowable instrument residence times ranged from about 8 h at 375 °C to about 2 min at 450 °C. The kinetic data were also used to determine Arrhenius parameters of A ) 2.56 × 10 16 s -1 and E a ) 283 kJ · mol -1 . In addition to the decomposition kinetics, we have also done a GC-MS analysis in order to identify the most abundant decomposition products. Introduction Kerosene-based fuels are of great importance for military and aerospace applications. 1-4 Such fuels are complex mixtures of hydrocarbons with hundreds or even thousands of constitu- ents. 2,5-7 Each type of kerosene fuel must meet certain physical property specifications, but substantial compositional variation is possible. 2,5-7 Because of their complexity, the compositions of kerosene-based fuels are often reported in terms of chemical classes (e.g., linear alkanes, aromatics) instead of individual compounds (e.g., dodecane, hexylbenzene). Cycloalkanes, like propylcyclohexane, which are also called naphthenes or cyclo- paraffins, are an important class of chemicals in many kerosene fuels. 2,5-8 A large-scale project involving the thermophysical properties of kerosene-based fuels is in progress at the National Institute of Standards and Technology (NIST). One aspect of this project involves the measurement of thermophysical properties of kerosene-based fuels. 9-14 This is followed by the development of equations of state to correlate the property data. 9,11,14 Such work enhances design and operational specifications of these fluids and facilitates new applications. Another part of the project on kerosene-based fuels involves the development of fuel models based on “surrogate” mixtures. The complexity and compositional variability of “real” kerosene fuels can be an obstacle to fundamental research and modeling. 5,6 For this reason, the development of simple mixtures (surrogates) that closely approximate the behavior of real fuels is critical. 6,13,15 Such surrogate mixtures might contain only a dozen or fewer components. Mixture components are typically chosen to represent classes of chemicals in the fuel being modeled. Following this line of thought, propylcyclohexane has been chosen for several reasons as a representative cycloalkane in the surrogate fuel mixtures being developed at NIST. 15,16 The inclusion of propylcyclohexane in surrogate fuel mixtures required a complete fluid model for this compound, thus requiring that its thermophysical properties be measured over a wide range of temperatures and pressures. This includes temperatures greater than 300 °C and pressures greater than 10 MPa, areas in which data are scarce. 17 Under these conditions decomposition is a serious concern because it can affect the validity of the data that are obtained and the performance, lifetime, and safety of the instruments used to collect the data. Obviously, the extent of decomposition that occurs during thermal equilibration and property measurement has a direct impact on data quality. Additionally, decomposition of hydro- carbons can lead to the formation of solid deposits 18-21 that may affect instrument performance and that are often difficult to remove. Changes in composition can also result in cata- strophic increases in pressure. Recent work on kerosene-based fuels clearly validates such concerns for property measurements at high temperature and pressure. 22-26 In order to avoid problems with decomposition during property measurements, one must understand the kinetics of decomposition. Herein we report the results of a study of the kinetics of thermal decomposition for propylcyclohexane. First-order rate constants for the thermal decomposition of propylcyclohexane were determined from 375 to 450 °C by use of a method that we previously developed for the kerosene-based fuels and a series of organic Rankine cycle fluids. 22,26,27 That is, the fluid was thermally stressed in ampule reactors made of 316L stainless steel, and the extent of decomposition was determined as a function of time by gas chromatography. The Arrhenius parameters for thermal decomposition were determined from a plot of the rate constants as a function of the temperature. 22,27-31 The Arrhenius parameters are especially useful because they can be used to predict decomposition rates at temperatures other than those determined experimentally. The initial motivation for this work was to establish operating ranges for thermo- physical property measurements on propylcyclohexane, but the results clearly have value for modeling and engineering studies as well. Experimental Chemicals. Reagent-grade acetone, toluene, and dodecane were obtained from commercial sources. All had purities of no less than 99% and were used as received. Propylcyclohexane was obtained from Aldrich and used as received. It had a stated purity of 99%, which is consistent with our own GC analyses of unheated samples of the propylcycohexane (e.g., see the top * To whom correspondence should be addressed. Tel.: (303) 497- 5158. Fax: (303) 497-5927. E-mail: bruno@boulder.nist.gov. Ind. Eng. Chem. Res. 2009, 48, 654–659 654 10.1021/ie8008988 This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society Published on Web 12/11/2008