Selective catalytic conversion of Kraft lignin into monoaromatic hydrocarbons over niobium oxide catalysts

Abstract

Catalytic fast pyrolysis (CFP) of lignin targeting for benzene, toluene and xylenes is attractive considering lignin's chemical structures. Zeolites have been widely used for producing aromatic hydrocarbon via a well-known hydrocarbon pool mechanism from lignocellulose pyrolysis vapors because of their unique shape-selective catalytic abilities, however, the large amount of phenols existing in the product reveals the catalyst's deficiency in direct deoxygenating of phenols. Finding a catalyst with a strong deoxidization ability against various lignin pyrolysis intermediates is necessary. Here, two niobium oxide-based catalysts, niobium oxophosphate (m-NbOP) and niobium oxide (m-NbO) with uniform mesoporous structures (pore sizes of 2-5 nm), large external areas (487 m2·g−1 for m-NbOP and 244 m2·g−1 for m-NbO), and strong acidities (1.25 mmol·g−1for m-NbOP and 0.69 mmol·g−1 for m-NbO) were synthesized and used without need for hydrogen in lignin CFP at 650 °C. Compared to zeolites, both niobium oxide-based catalysts considerably increased aromatic's contents and the selectivity to benzene, toluene, and xylenes but decreased the contents of phenols to some extent, indicating the improved abilities in direct deoxygenation by aromatic CO bond cleavage of lignin. A total carbon yield of benzene, toluene and xylenes of 39.2% was achieved over m-NbOP.