Production of 4-Ethylphenol from Lignin Depolymerization in a Novel Surfactant-Free Microemulsion Reactor

In order to meet the requirements of sustainable development, the production of aromatic compounds from renewable biomass is of great concern. Herein, a surfactant-free microemulsion (SFME) system composed of n-octane, 2-propanol, and water was explored for the depolymerization of lignin though a hydrogen transfer reaction of 2-propanol with acidic ionic liquids (ILs) as catalysts. Experimental results show that the phenol monomer yield from bagasse lignin in the SFME system is at least 4 times higher than that in its corresponding water-free binary system, together with a high selectivity for 4-ethylphenol of 67.8%. In particular, the results also reveal that the controllable polarity and large surface area of the SFME and the aggregation of lignin at the SFME surface are key factors in response to the enhanced yields of phenolic monomers through intensive characterizations. Mechanism studies imply that this system tailors mainly the esterified p-coumarate unit in lignin, and 4-ethylphenol is produced by a cascade reaction, involving hydrolysis, decarboxylation, and hydrogenation. Furthermore, this SFME system also exhibits excellent performance for the depolymerization of other herbaceous lignins, yielding 128.1 mg g–1 phenolic monomers with 59.1% 4-ethylphenol selectivity for corncob lignin. It is thus believed that the process intensification by microemulsion can significantly demonstrate unprecedented potential to accomplish highly efficient lignin conversion.