Catalytic pyrolysis of lignin using low-cost materials with different acidities and textural properties as catalysts

Abstract

Catalytic fast pyrolysis of lignin was performed using low-cost materials with different acidities and textural properties as catalysts in the present work. The main focus is to understand the role of low-cost catalysts in the fast pyrolysis of lignin. The four most commonly used low-cost catalysts, ilmenite (FeTiO3), bentonite (Al-Si-OH), activated carbon (AC) and red mud (RM), were selected. The results show that bentonite, red mud and activated carbon effectively enhance the dehydration reaction, which is regarded as the dominant way to eliminate oxygen during the pyrolysis process, due to the existence of strong acidic sites. However, only activated carbon is found to be effective in promoting the production of monocyclic aromatic hydrocarbons (MAHs). Two metallic catalysts, i.e., bentonite and red mud, have strong acidities but quite low surface areas and less porous structures. Therefore, the dehydrated intermediates produced are especially easy to repolymerize to form char or coke without the restriction of obtaining a porous structure during the pyrolysis process. Activated carbon has not only a certain acidity but also a rich porous structure. Lignin fast pyrolysis-derived oxygenates can diffuse and react on the well-dispersed active sites within the pores of activated carbons. The catalytic performance of the activated carbon are supposed to be determined by the pore size. Only pores of similar size to lignin fast pyrolysis-derived oxygenates (0.6–1 nm) seems to be effective for the production of MAHs. Pores larger or smaller than lignin fast pyrolysis-derived oxygenates both tend to cause coke deposition rather than MAHs formation.