Efficient Depolymerization of Alkaline Lignin to Phenolic Compounds at Low Temperatures with Formic Acid over Inexpensive Fe-Zn/Al2O3 Catalyst

In this study, an inexpensive catalyst (Fe-Zn/Al2O3) was investigated to efficiently convert alkaline lignin into phenolic monomers with formic acid at low temperatures. The catalyst was successfully prepared by a co-impregnation method and showed high selectivity for production of phenolic monomers. With the optimal reaction condition, i.e., a mass ratio of formic acid to lignin of 4:1 and a reaction temperature of 180 degrees C for 6 h, the highest yield of bio-oil of 28.31 wt % was obtained with formic acid as the in situ hydrogen source in the presence of Fe-Zn/Al2O3 catalyst. It was indicated from the composition analysis of bio-oil that the main phenolic monomers were 2-methoxy-phenol (2.86 wt %, based on alkaline lignin), vanillin (2.83 wt %, based on alkaline lignin), and apocynin (2.06 wt %, based on alkaline lignin). Compared with fresh catalyst, the spent one was also investigated for physical and chemical properties, and results confirmed by SEM (scanning electron microscopy) and TG/DTG (thermogravimetry/derivate thermogravimetry) analyses showed that solid products formed by lignin depolymerization were adsorbed on the surface of the Fe-Zn/Al2O3 catalyst. Finally, recyclability tests demonstrated that the catalytic performance of Fe-Zn/Al2O3 was still active and stable after five consecutive recycling runs.