Catalytic Upgrading of Lignite Pyrolysis Volatiles to Aromatics over Urea-Modified HZSM-5: Study on Desilication of Preserved Micropores

Abstract

This work aimed to investigate the effect of the physical properties of urea-desilicated HZSM-5 on the catalytic performance of the lignite pyrolysis volatiles upgrading to aromatics. A mild urea treatment can create more secondary mesoporous and Brønsted acid sites while well-preserving micropores, of which the first two promoted the diffusion and active cleavage of macromolecules into more small fragments, while the latter ensured the shape selectivity to form aromatics. Consequently, the urea-desilicated catalysts exhibited better performance for the production of aromatics, especially benzene and toluene in monocyclic aromatic hydrocarbons (MAHs), compared to the parent HZSM-5. Among the catalysts, HZSM-5 treated with 5 wt % urea solution for 5 h gave the highest light aromatic (LA) yield of 25.8 mg/g, in which the increased MAH yield accounted for 77% of the total increased LAs. However, excessive desilication with a higher urea concentration or longer treatment time significantly reduced the aromatic yield as a result of a considerable loss of micropores and mesoporous and Brønsted acid sites. The strategy of preserving micropores and optimizing mesoporous and Brønsted acid sites provides a new way to enhance the formation of aromatics.