Reaction Mechanisms of Anisole Pyrolysis at Different Temperatures: Experimental and Theoretical Studies

Abstract

Pyrolysis of tar compounds plays an important role in designing optimal thermochemical processes for the conversion of biomass into globally important commodities. Therefore, it is crucial to understand the relevant reaction mechanisms and be able to predict the decomposition products of these compounds at different temperatures. In this study, the pyrolysis of anisole, which serves as an important model compound for lignin, a major component of biomass tar, was studied in a laminar-flow reactor system of N2 gas at temperatures of 300–650 °C and a residence time of 1 s. The decomposition products were analyzed using a gas chromatograph with mass spectrometric and flame ionization detectors. To gain insights into the reaction mechanisms, detailed studies of the unimolecular and bimolecular decomposition pathways were carried out using the density functional theory and high-level coupled cluster methods. Anisole was found to decompose at temperature as low as 400 °C, which is the lowest reported temperature for anisole decomposition. Formation of benzene and toluene at low temperatures (400–450 °C) is explained by the low-energy barrier ipso-addition of CH3 and H radicals at the methoxy moiety of anisole. At 500–550 °C, multiple reaction mechanisms lead to the formation of benzofuran, methylcyclopentadiene, benzaldehyde, cyclopentadiene, ethylbenzene, styrene, and o-xylene. Finally, at 600–650 °C, indene, phenol, and cresol were detected. The obtained results are expected to contribute to the development of predictive kinetic models for the decomposition of anisole and other similar compounds.