Lignin pyrolysis under NH3 atmosphere for 4-vinylphenol product: An experimental and theoretical study

Conversion of lignin into the valuable chemicals is critical for the high-valued utilization of lignin. In this study, a simple method to produce 4-vinylphenol through lignin pyrolysis under NH3 atmosphere was proposed. Lignin pyrolysis behavior was investigated at 400?800 ?C by a fixed-bed reactor. Formation mechanism of 4-vinylphenol was explored with density functional theory calculations. Results showed that NH3 promoted the cracking of biochar to increase the yield of bio-oil and gaseous product. H2 was the main component, and its yield increased significantly at higher temperature (reaching 190 mL/g), while the yield of CH4 and CO2 decreased largely. Introduction of NH3 promoted formation of phenols without methoxy through removing ?O-CH3 branches of phenols with methoxy. The 4-vinylphenol yield increased about five times, and reached 4.89 wt% with the relative content of 23.43% at 600 ?C. For the formation mechanism of 4-vinylphenol, in initial pyrolysis stage, hydrogen abstraction of amino can promote the C?-O bond cleavage. In the branched chain crack stage, NH3 provides more H free radicals and can also promote dehydration and decarbonylation reaction to form 2methoxy-4-vinylphenol, and reduce the high energy barrier of demethoxylation significantly in the final stage. Furthermore, the possible reaction pathways of producing 4-vinylphenol from lignin pyrolysis under NH3 atmosphere were proposed.

Automated summary

The conversion of lignin into valuable chemicals, such as 4-vinylphenol, is essential for its high-value utilization. This study proposed a simple method to produce 4-vinylphenol through lignin pyrolysis under NH3 atmosphere, which increased bio-oil and gaseous product yield. NH3 promoted formation of phenols and significantly increased the yield of H2 while decreasing CH4 and CO2 yield. The formation mechanism of 4-vinylphenol involves hydrogen abstraction, crack stage, and significant reduction of energy barrier.