Understanding the Impact of Reaction Conditions on the Deactivation of HZSM-5 during the Methanol-To-Hydrocarbons Process: On the Critical Role of HCHO

The methanol-to-hydrocarbon (MTH) reaction offers a promising non-fossil fuel pathway for the production of olefins and aromatics. A formaldehyde (HCHO) intermediate has attracted great attention due to its critical role in the aromatization and coking process of the MTH reaction. However, because of the difficulty in the direct detection of HCHO, details on the impact of reaction condition on its formation and conversion were seldom studied. Here, synchrotron radiation photoionization mass spectrometry (SR-PIMS) was applied to thoroughly study the changes in HCHO and other relevant species under different reaction conditions, including contact time, temperature, zeolite acidity, and methanol concentration. It is revealed that higher temperature and acidity promoted the formation of HCHO and its transformation to aromatics and coke, whereas higher methanol concentration mainly enhanced hydrogen transfer reactions, resulting in a higher yield of polyunsaturated species and, consequently, more aromatics. Combining the results from SR-PIMS and soft X-ray absorption spectroscopy of the spent catalyst, it is verified that HCHO formation promoted coke formation, but the presence of olefinic species can shift the methanol conversion pathway toward methylation reactions and then limit the level of HCHO yield. Therefore, we proposed a butanol cofeeding strategy that can limit the contribution of HCHO-induced coke formation and extend the catalyst lifetime by up to ∼60%.