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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 °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.

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In order to realize the high value-added utilization of alkali lignin, the catalytic hydrogenolysis of alkali lignin was performed in this study with assistance of formic acid acting as an internal hydrogen donor over the catalyst Ni-ZrO2/γ-Al2O3 prepared by the chemical reduction method. Effects of temperature, formic acid, and catalyst on the catalytic performance were investigated, and the recyclability for catalyst was also explored. Results showed that increasing the temperature (from 180 to 240 °C) and adding catalyst (0.5 g catalyst/g lignin) were beneficial for bio-oil yield, and the presence of FA was advantageous for lignin conversion. Catalytic abilities of catalyst were weakened due to the recycle reflected on a decrease in bio-oil yield. GC/MS analysis showed that the relative content of vanillin was relatively high (65.65%) at 180 °C, while that of alkyl G-type phenols was dominant (around 70%) at 240 °C. It was interpreted that the prepared catalyst exhibited excellently catalytic selectivity for specific phenolic products, especially for G-type phenols, due to the highly catalytic performance for the cleavage of C–O–C/C–C bonds in alkali lignin.