Study of Higher Hydrocarbon Production during Ethylacetylene Pyrolysis Using Laser-Generated Vacuum-Ultraviolet Photoionization Detection

Article abstract of DOI:10.1021/j100371a026

Higher hydrocarbon formation during the pyrolysis of ethylacetylene in a microjet reactor was studied by vacuum-ultraviolet photoionization time-of-flight mass spectrometry.At the wavelength employed, this ionization technique allows for the simultaneous detection of both stable and intermediate polyatomic species with ionization potentials below 10.49 eV, including most hydrocarbons with two or more carbon atoms.Minimal fragmentation simplifies the determination of parent species and allows identification of probable reaction pathways involving hydrocarbon radicals as well as stable species.The pyrolysis of ethylacetylene was carried out in the fast-flow microjet reactor (residence times 1-2 ms) at temperatures from 300 to 1600 K.At temperatures below 1500 K, products are predominantly linear conjugated compounds that are either primary pyrolysis products such as C3H3 or products of C1 and C2 addition and abstraction reactions.The first greater than four carbon hydrocarbons to be detected at 2-ms residence time were mass 78, at 1450 K, and mass 92, at 1280 K (likely predominantly benzene and toluene, respectively).At higher reactor temperatures, a progression of polymerization products was observed including likely aromatic species.Accompanying this increase, particularly above temperatures of 1600 K, are dramatic decreases in the concentration of species with fewer than 10 carbon atoms, due to their combination to form large polyaromatic hydrocarbon species.Analysis of the data shows that production rates of masses 78 and 92 amu are consistent with a low activation energy reaction of C4H5 with acetylene and methylacetylene (propyne).A mechanism involving the recombination of C3H3 radicals followed by isomerization can also account for mass 78 production, assuming it proceeded at high-pressure-limit recombination rate which has been estimated by previous investigators.