Thermal and catalytic fast hydropyrolysis of lignin: Optimization for selective production of aromatics using high-pressure tandem μ-reactor – gas chromatography/mass spectrometry

Abstract

Hydropyrolysis is a promising route for converting biomass into valuable chemicals. A high-pressure tandem μ-reactor – gas chromatography/mass spectrometry was used to study the pyrolysis of lignin and probe the effects of reaction atmosphere (H2 vs He), temperature, catalyst (HZSM-5) presence/absence, and pressure on product yields and distributions. Hydropyrolysis favored the demethoxylation and demethylation of guaiacyl-derived intermediates, generating catechol- and phenol-type compounds. The yields of guaiacyl- and catechol-type compounds decreased with increasing temperature and H2 pressure. The products formed at the highest examined temperature (700 °C) and H2 pressure (3.0 MPa) were dominated by phenol-type compounds, monoaromatic hydrocarbons (MAHs), and polycyclic aromatic hydrocarbons (PAHs). The substantial decrease in char yield with increasing H2 pressure at 700 °C indicated that active hydrogen radicals enhanced the cracking of lignin macromolecules and suppressed the condensation reactions to form char at high temperatures. HZSM-5 exhibited considerable hydrocracking activity for converting lignin pyrolysis vapor into aromatics, with an increase in H2 pressure further improving the formation of benzene, toluene, and xylenes (BTX) at the expense of phenolics. However, high catalytic upgrading temperatures favored the coupling reactions in the hydrocarbon pool, promoting the formation of PAHs. Evolved gas analysis showed that lignin hydropyrolysis involved two stages (at 250–500 and 500–800 °C), suggesting that the labile fraction of the nascent char underwent further hydrocracking at high pressure and temperature. The high production of BTX and naphthalenes through catalytic hydropyrolysis of lignin provides a promising route for the valorization of lignin-rich resources.