Hydrothermal treatment on ZSM-5 extrudates catalyst for methanol to propylene reaction: Finely tuning the acidic property

•HZSM-5 powder and extrudates are treated with steam at varying temperature and time.•More Lewis acidity is obtained on the extrudates due to the γ-Al2O3 binder.•The acidity can be finely tailored through the optimization of steaming parameters.•Optimized steaming significantly increases C3= selectivity in the MTP initial stage.•Extrudates have high selectivity to C3= and longevity by steaming at 500 °C for 12 h.

ZSM-5 powder and extrudates are hydrothermally treated by varying temperature and time. Before and after treatment, the materials are characterized and evaluated in the methanol to propylene (MTP) process. The changes in textural properties are compared and particular attention is paid to the nature, number, and ratio of Brønsted (B) and Lewis (L) acid sites. The extrudates catalyst contains substantially a large number of acid sites, especially the L acid sites and hierarchical porous structure due to the addition of the alumina binder. The number of acid sites and the B/L ratio could be finely adjusted by proper hydrothermal treatment. And the hydrothermal treatment parameter on the extrudates plays an important effect on its MTP catalytic performance and product distribution. After steaming at 500 °C for 12 h, both the zeolite and extrudates catalyst show a high initial activity, achieving ca. 12 percentage point rise in the propylene selectivity during the MTP reaction. However, with time on stream, the treated extrudates perform the steady propylene selectivity (around 43%) and in particular the longest was 89 h of catalytic lifetime, compared to the non-treated extrudates (45 h) and treated zeolite (57 h). The enhanced catalytic performance could be contributed by the appropriate B/L ratio and number of acid sites, in combination with the mesoporous structure, which are crucial for decreasing the hydrogen transfer reaction and inhibiting the coke deposition.

Graphical abstractExtrudates exhibit more Lewis (L) acid sites than HZSM-5, due to the addition of binder (γ-Al2O3). Well reservation of L acidity and elimination of Brønsted acidity could occur through hydrothermal treatment on extrudates, which results in an enhanced selectivity to propylene and prolonged catalytic lifetime during methanol to propylene (MTP) conversion.Figure optionsDownload full-size imageDownload as PowerPoint slide