The Low‐Pressure Pyrolysis of Butadiyne (C4H2)

Abstract

AbstractButadiyne (C4H2) mixed with helium (maximum 1:9) was pyrolyzed in a low‐pressure flow reactor at temperatures between 700 and 1000°C and total pressures of 3 to 13 mbar. The residence time (maximum 45 ms) was varied by a movable inlet lance for the C4H2. Samples were drawn through a molecular beam system and analyzed mass‐spectrometrically. The main products were C2H2, H2, polyyines (C6H2, C8H2), oligomers of the composition (C4H2)n, n = 2 to 5, polymer and coke. Most of the H2 and a smaller part of C2H2 and C6H2 were formed heterogeneously while the oligomers and C8H2 were generated in homogeneous reaction. The oligomers with n ≧ 3 are most probably aromatic compounds with C2H side groups. They are intermediates which repeatedly split off their C2H groups until the base hydrocarbons benzene, naphthalene, and phenanthrene or anthracene remain. Addition of C2H2 to the pyrolyzing C4H2 led to a mixed oligomerization while addition of H atoms had no measurable effect. A large part of the C4H2 decomposed heterogeneously with an apparent activation energy of 109 kJ mol−1. The activation energy for oligomer formation ranged from 145 to 168 kJ mol−1 increasing with higher degree of oligomerization. — A mechanism for the formation of polyynes and oligomers is discussed starting with an excited C4H2 which can add C4H2 to give either a linear or a branched dimer C8H2. The results are compared to the formation of polycyclic aromatic compounds in the burned gas of fuel‐rich flames that also contains C2H2 and C4H2.