Thermal Decomposition of CH3CHO Studied by Matrix Infrared Spectroscopy and Photoionization Mass Spectroscopy

A heated SiC microtubular reactor has been used to decompose acetaldehyde and its isotopomers (CH ₃CDO, CD ₃CHO, and CD ₃CDO). The pyrolysis experiments are carried out by passing a dilute mixture of acetaldehyde (roughly 0.1-1) entrained in a stream of a buffer gas (either He or Ar) through a heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 50-200 Torr with the SiC tube wall temperature in the range 1200-1900 K. Characteristic residence times in the reactor are 50-200 μs after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. The reactor has been modified so that both pulsed and continuous modes can be studied, and results from both flow regimes are presented. Using various detection methods (Fourier transform infrared spectroscopy and both fixed wavelength and tunable synchrotron radiation photoionization mass spectrometry), a number of products formed at early pyrolysis times (roughly 100-200 μs) are identified: H, H ₂, CH ₃, CO, CH ₂CH=OH, HC ≡CH, H ₂O, and CH ₂=C=O; trace quantities of other species are also observed in some of the experiments. Pyrolysis of rare isotopomers of acetaldehyde produces characteristic isotopic signatures in the reaction products, which offers insight into reaction mechanisms that occur in the reactor. In particular, while the principal unimolecular processes appear to be radical decomposition CH ₃CHO (+M) → CH ₃+H+CO and isomerization of acetaldehyde to vinyl alcohol, it appears that the CH ₂CO and HCCH are formed (perhaps exclusively) by bimolecular reactions, especially those involving hydrogen atom attacks.