Catalytic pyrolysis of maize cob lignin over activated red mud catalyst for value-added mono-phenol production

Lignin is a kind of natural aromatic polymer with complex three-dimensional amorphous structure. It has also gradually become an ideal raw material for value-added fine chemical production. The pyrolysis liquefaction technology can be used to achieve the depolymerization of lignin into phenolic compounds. However, the thermal depolymerization process is normally associated with many highly reactive benzene ring radicals. These radicals are not sensitive to the formation of high-value mono-phenol, due to they can further be condensed to coke. A feasible pathway, including catalytic pyrolysis of lignin, can be selected to produce value-added mono-phenol. Herein, the selection of catalyst become critical to the directional catalytic pyrolysis with monophenols as the target product. The physical-chemical characteristics of catalysts can directly determine the composition and enrichment of mono-phenol in bio-oil. At present, many low-cost catalytic materials have been synthesized to further broaden the source of the catalysts for save-costing technology of catalytic pyrolysis. In this paper, the acid digestion-alkali precipitation coupled calcination treatment was employed to activate the red mud waste derived from alumina industry. The synthetic low-cost catalyst was introduced into the catalytic pyrolysis of maize cob lignin process, to produce the value-added mono-phenol in bio-oil. Proximate analysis, ultimate analysis, and ultraviolet-visible spectroscopy (UV-Vis) were used to characterize the maize cob lignin. The synthesized low-cost catalytic materials (denoted as ACRM) were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy coupling with energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), thermogravimetry-differential thermal gravimetry (TG-DTG) and the N2 isothermal absorption-desorption analysis. The distributions of bio-oil groups and main phenolic compounds were investigated at the function of ACRM samples. The comparative analysis of ACRM with commercial catalysts was performed to evaluate the application potential of red mud as a low-cost catalyst for the catalytic pyrolysis lignin. Results showed that the maize cob lignin can serve as a renewable raw material to produce high-value aromatic chemicals, indicating abundant para-aromatic ring structure, with unique ferulic acid (FA) and para-coumaric acid (PCA). The activated process can significantly improve the surface morphology, pore structure and catalytic properties of red mud. The activated red mud can produce pore structure and abundant active metal oxides (Fe2O3, Al2O3 and TiO2), indicating a high specific surface area (72.36 m2/g). Most Na and Ca elements were efficiently removed from the red mud structure, indicating the reduction of strong alkalinity. The relative peak area of phenol and alkylphenols can reach up to 60.38% at the function of ACRM catalysts, mainly due to the enhanced dehydroxylation, demethylation, demethoxy reaction, and alkylation reaction. Compared with commercial molecular sieves, the modified red mud was a supplement to mesoporous molecular sieves with a better recycling performance. A possible reaction pathway of lignin pyrolysis vapors was proposed under the function of ACRM catalyst. Therefore, the activated red mud catalyst was used as a low-cost catalyst material for lignin catalyzed pyrolysis to produce value-added mono-phenol, indicating the energy utilization of typical waste resources with potential economic and ecological benefits. The application of ACRM can be expected to guide the utilization of solid waste. The findings can provide a sound reference for the reasonable disposal of waste resources from biorefining, pulp engineering and alumina industry. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.