Fine-tuning the surface acidity of hierarchical zeolite composites for methanol-to-olefins (MTO) reaction

Abstract

The zeolite nanosheet composites, containing a high acidic density ZSM-5 and weak-acidic Silicalite-1 nanosheets (Si-NS) acting as a core and a shell, respectively, have been successfully fabricated by a one-pot hydrothermal synthesis. The shell thickness, textural properties, and acidity of synthesized composites can be easily adjusted by hydrothermal synthesis time and temperature. The uniform thickness of 9.4 ± 1.1 nm of Silicalite-1 shells can be observed when using the crystallization temperature of 110 °C, whereas the non-uniform distribution of zeolite shells becomes predominant with increasing the crystallization temperature due to its rapid growing rate. Interestingly, due to the different acidity of core and shell materials, the acid site distribution of the composites has been modified with respect to their isolated zeolites, and therefore the highly selective production of propylene in MTO process is greatly improved (>90% and 50% for the hierarchical composite and unmodified ZSM-5, respectively). In addition, a significantly lower coke formation could be achieved due to the suppression of further side reactions over the stacking-layered siliceous covering on high acidic density ZSM-5 surfaces. This example opens up perspectives for the development of hierarchical zeolite composites with the modified acid surfaces for highly selective production of light olefins.