Synthesis of phenol from degraded lignin using synergistic effect of iron-oxide based catalysts: Oxidative cracking ability and acid-base properties

The effects of ZrO2 and TiO2 incorporated into Fe2O3 matrix on oxidative cracking of degraded lignin and on the acid-base properties were investigated. After lignin degradation, cracking into lower-molecular-weight products was greatest using ZrO2-FeOX. Reactivity of the lattice oxygen was evaluated using H2-TPR, which revealed that the reactivity was improved. Thus, ZrO2-FeOX promoted oxidative decomposition of lignin to oligomers. In the cracking of 2-methoxyphenol, TiO2-FeOX and ZrO2-FeOX resulted in a 5- to 6-fold greater yield of phenol than the yield over Fe2O3. According to Mulliken population analysis, the charge density difference between Fe-O increased by ca. 12% in TiO2-FeOX and ZrO2-FeOX as compared with Fe2O3. This result suggests that addition of TiO2 and ZrO2 improved the acid-base properties of the catalyst, which promoted demethoxylation of 2-methoxyphenol. Thus, ZrO2-FeOX enhanced oxidative decomposition using its lattice oxygen that converted degraded lignin into lower molecule oligomers, followed by demethoxylation to produce phenol.