Effective depolymerization of alkali lignin into phenolic monomers over ZrP catalysts promoted by Ni and W

A large amount of alkali lignin by-product is produced by the pulp and paper industry which can be further processed and transformed into valuable chemicals. In this study, the catalyst Ni-W/ZrP was prepared via chemical reduction for catalyzing alkali lignin depolymerization. Effective conversion of alkali lignin to phenolic monomers with assistance of formic acid acted as an internal hydrogen donor was performed in isopropanol system from 180 to 240 °C. Effects of reaction temperature, catalyst loading, and formic acid addition on lignin conversion were all investigated. Results showed that the highest oil yield was obtained (74.66 wt%) at 240 °C. The presence of formic acid could enhance alkali lignin conversion (the yield of bio-oil ranged from 17 to 23 wt% at 220 °C when the reaction conditions were changed from no added formic acid to only added formic acid). According to the GC/MS analysis, when metal loading (the mass ratio of Ni and W is 1:1) was 20 wt% (based on ZrP input), the prepared catalyst (Ni-W)20/ZrP had best selectivity for the phenolic products (88.25%, relative content). Furthermore, the obtained catalyst had different catalytic selectivities to G-type phenols and vanillin at different temperatures (from 180 to 240 °C). The quantitative analysis of bio-oil showed that the four products with high content in bio-oil were phenol, 2-methoxy (1.21 wt%), phenol, 4-ethyl-2-methoxy (0.45 wt%), vanillin (4.09 wt%), and ethenone,1-(4-hydroxy-3-methox-henyl) (0.86 wt%). FTIR analysis showed that peaks at 1267 cm−1, 1213 cm−1, 1130 cm−1, and 1046 cm−1(C-O bonds in guaiac-based and clove-based units) gradually decrease with the increase of temperature which indicates that lignin is depolymerized to monoaromatic units effectively. Finally, the catalyst (Ni-W)20/ZrP can be recycled five times without significant activity loss, indicating that it was a promising catalyst for lignin depolymerization.