A Mechanistic Study of HZSM-5-Catalyzed Guaiacol Amination Using Photoionization Time-of-Flight Mass Spectrometry

Thermal-catalytic conversion and amination (TCCA) of lignin and lignin derivates over zeolites is a promising and renewable method to produce aromatic amines, but suffers from product diversity. Currently, no unambiguous mechanism could fully describe the chemistry of this process. In this work, the TCCA mechanism of guaiacol, a typical lignin model compound, with ammonia over HZSM-5 was investigated by online photo ionization time-of-flight mass spectrometry combined with density functional theory. Various products including amines, pyrroles, and pyridines were identified. The formation of methylamine and aminophenol below 400 degrees C via nucleophilic substitutions is attributed to the strong adsorption of ammonia on the active site of HZSM-5. Aniline is the major product above 400 degrees C coproduced with pyrroles and pyridines. It is suggested that the reactions among radical intermediates (center dot CH3 and center dot NH2) and molecules (guaiacol and catechol) lead to poor aniline selectivity via transmethylation, amination, and partial deoxygenation reactions. Hydroamination is proposed as the main formation mechanism of pyrroles and pyridines. The maximum yield of aniline can be achieved at 650 degrees C owing to the enhancement of amination and deoxygenation and the suppression of transmethylation reactions.

Automated summary

The thermal-catalytic conversion and amination of lignin over zeolites is a promising method to produce aromatic amines, but the exact mechanism of this process is still unclear. In this study, the TCCA mechanism of guaiacol with ammonia over HZSM-5 was investigated, and various products including amines, pyrroles, and pyridines were identified. The formation of methylamine and aminophenol was attributed to the strong adsorption of ammonia on the active site of HZSM-5, while aniline was the major product above 400 degrees C, produced through reactions among radical intermediates and molecules. The formation of pyrroles and pyridines was proposed to be mainly through hydroamination. The maximum yield of aniline was achieved at 650 degrees C.