Highly selective production of light aromatics from lignin through integrated approach of the torrefaction deoxygenation pretreatment and the dual-catalyst catalytic fast pyrolysis using Nb modified γ-Al2O3 and HZSM-5

Abstract

Light aromatics are extremely important building blocks for manufacturing of numerous industrial products. The integrated approach of the torrefaction deoxygenation pretreatment (TDP) and the dual-catalyst catalytic fast pyrolysis (DC-CFP) using the HZSM-5 and metal (Nb, Zn, Ga, Ni, Ce) modified γ-Al2O3 as catalyst was employed to enhance the production of light armatics from the hardwood lignin. The torrefaction deoxygenation mechanism was established based on the evolution of chemical structure of lignin. The maximum removal rate of oxygen element was 22.82%, achieved at the TDP temperature of 300 °C. The oxygen element was removed through the cleavage of β–O–4 bond, the aliphatic–OH and aliphatic–HC═O on the side-chain, and the aromatic–OH and aromatic–OCH3 on the aromatic nucleus, releasing as the oxygenated gaseous compounds (CO2, H2O, and CO). The synergistic effect between HZSM-5 and metal modified γ-Al2O3 in DC-CFP was observed, remarkably improving the yield of light aromatics. Metal modified γ-Al2O3 aimed to first convert lignin into macromolecular phenols, and HZSM-5 was complementary used to convert the macromolecular phenols into light aromatics by the deoxygenation reactions. The maximum selective yield of bio-BTX were 74.39%, obtained at TDP temperature of 250 °C, lignin-to-catalyst ratio of 1:3, and HZSM-5-to-15%Nb/Al2O3 of 2:2.