In-situ catalytic upgrading of lignin pyrolysis volatiles over red mud

Abstract

Red mud (RM) is a solid waste of Bayer's alumina industry production process, which comprises a mixture of diverse minerals. After calcination, the minerals in RM are present in the form of oxides, exhibiting remarkable catalytic performance for biomass pyrolysis. It is thought that pyrolysis of lignin generates renewable aromatic chemicals. In this paper, the effect of in-situ catalysis of calcinated RM on the distribution of lignin pyrolysis volatile products is investigated. In addition, upgrading of tar by components in RM, including SiO2, Al2O3, Fe2O3, and Na2O is also discussed. The results show that the addition of RM in lignin pyrolysis leads to lower char yield and higher gas yield. RM catalyzes the breaking and rearrangement of methoxy groups and induces the generation of hydroxyl groups, which leads to an increase from 1.96 to 3.96 wt% in monophenol content of the tar. Furthermore, the activation of C-O bonds by Fe2O3 and the redox properties of Fe2O3 (FeSi) combine to stimulate the production of monophenols and aldehydes by 30.8 % and 148.2 %, while Al2O3 (AlSi) boosts the production of guaiacols by 32.0 % comparing with SiO2 (Si). It is found that synergistic effect between multiple species improves the catalytic performance, the yield of tar reaches 24.5 wt% and the selectivity of oxygenated compounds achieves 94.4 % (FeAlSiNa). The catalytic performances of RM for product yield and tar component distribution are similar to that of FeSi, indicating that Fe2O3 and SiO2 play a main catalytic role in RM.