Lignin-enriched waste hydrothermal liquefaction with ZVMs and metal-supported Al2O3 catalyst

Abstract

Huge amount of technical lignin is produced in paper and pulp industries annually. In this article, lignin hydrothermal liquefaction (HTL) was conducted in the presence of zero valent metals (ZVMs): Zn, Fe, Ni and Co and metal-supported Al2O3 catalysts (Ni/Al2O3, Fe/Al2O3, Co/Al2O3 and Cu/Al2O3) in order to investigate the possibility to valorize this waste. The bio-crude yields of lignin HTL at 330 °C for 60 min were 32.2% Fe > 29.8% Ni > 28.6% Zn > 26.5% blank >25.4% Co. Fe and Zn act as hydrogen producer being oxidized by subcritical water; the hydrogen stabilized the lignin HTL intermediates, enhancing bio-crude yields which were further improved when Fe and Zn are combined with hydrogenation catalyst Ni/Al2O3 inducing an even higher bio-crude yield of 36.0 and 36.1%, respectively. The energy recovery of Fe + Ni/Al2O3 and Zn + Ni/Al2O3 tests reached 67.8% and 66.3%, significantly higher than 47.4% of blank test. Both ZVMs and metal-supported Al2O3 catalysts showed good demethoxylation function. Zn/Fe and water reaction environment was demonstrated to be promising substitution of high pressure gaseous hydrogen for lignin waste liquefaction process.