Biomass Pyrolysis: Thermal Decomposition Mechanisms of Furfural and Benzaldehyde

The thermal decompositions of furfural and benzaldehyde have been studied in a heated microtubular flow reactor. The pyrolysis experiments were carried out by passing a dilute mixture of the aromatic aldehydes (roughly 0.1%-1%) entrained in a stream of buffer gas (either He or Ar) through a pulsed, heated SiC reactor that is 2-3 cm long and 1 mm in diameter. Typical pressures in the reactor are 75-150 Torr with the SiC tube wall temperature in the range of 1200-1800 K. Characteristic residence times in the reactor are 100-200 μsec after which the gas mixture emerges as a skimmed molecular beam at a pressure of approximately 10 μTorr. Products were detected using matrix infrared absorption spectroscopy, 118.2 nm (10.487 eV) photoionization mass spectroscopy and resonance enhanced multiphoton ionization. The initial steps in the thermal decomposition of furfural and benzaldehyde have been identified. Furfural undergoes unimolecular decomposition to furan + CO: C₄H ₃O-CHO (+ M) → CO + C₄H₄O. Sequential decomposition of furan leads to the production of HC≡CH, CH₂CO, CH₃C≡CH, CO, HCCCH₂, and H atoms. In contrast, benzaldehyde resists decomposition until higher temperatures when it fragments to phenyl radical plus H atoms and CO: C₆H₅CHO (+ M) → C₆H₅CO + H → C₆H₅ + CO + H. The H atoms trigger a chain reaction by attacking C₆H ₅CHO: H + C₆H₅CHO → C₆H ₆CHO* → C₆H₆ + CO + H. The net result is the decomposition of benzaldehyde to produce benzene and CO.