A Comparison of Monomeric Phenols Produced from Lignin by Fast Pyrolysis and Hydrothermal Conversions

Abstract

Abstract Lignin, a by-product in biorefining industries such as lignocellulose-to-ethanol and paper-making processes, is a promising renewable source for value-added phenolic chemicals. Hydrothermal conversion and fast pyrolysis are two main thermo-chemical approaches for depolymerization of lignin. Hydrothermal conversion is a low temperature (250–500°C) and high pressure (5–30 MPa) process particularly suited for high moisture materials, whereas fast pyrolysis takes place at atmospheric pressure and moderate temperatures in the absence of oxygen and requires drying of the feedstock. In this paper, we present experimental results that provide a comparison of distributions of monomeric phenols produced by fast pyrolysis and hydrothermal conversion of lignin. Hydrothermal conversion experiments were performed in a 150 ml autoclave at 300°C for 30 minutes with different alkaline concentrations. Pyrolysis was carried out in a spouted bed reactor at 500°C. Hydrothermal conversion of lignin under 1 M alkaline solutions resulted in lower char yields (12.1%) compared to fast pyrolysis, which resulted in 54.5% solid yields. Monomer phenols obtained from hydrothermal conversion of lignin were found to have a narrower distribution dominated by catechol and its methyl derivative. In contrast, fast pyrolysis produced a wider distribution of monomeric phenols dominated by guaiacol and its derivatives with various chains substituted para to the hydroxyl group. Mechanisms of lignin fast pyrolysis and the methoxy group conversion under hydrothermal conditions were proposed respectively. In conclusion, hydrothermal technology is better suited for production of value added monomer phenols from lignin.