Aluminum sulphate-assisted stepwise dealumination of OSDA-free low-silica chabazite for methanol-to-olefin reaction

Abstract

The synthesis of chabazite (CHA) aluminosilicate without organic structure-directing agent (OSDA) is a green process, but producing the CHA zeolite with excess acid sites, structural instability and limited applications due to a high aluminum content. Therefore, it is meaningful to develop an effective dealumination method to partially remove the framework aluminum atoms from the OSDA-free low-silica CHA zeolite. A novel stepwise dealumination and stabilization strategy through the subsequent treatment of Al2(SO4)3, H2O, HNO3 and NH4F was developed, which not only increased the Si/Al ratio from 2.4 to 5.0, but also stabilized the framework structure. Simultaneously, the removal of Al atoms optimized the acidity, introduced mesopores into the CHA framework and enhanced the diffusion property. Compared with low-silica CHA, the obtained high-silica CHA zeolite displayed attractive catalytic performance in methanol-to-olefin (MTO) reaction, showing a methanol conversion above 80% for more than 360 min with a maximum selectivity of 87% for low olefins (C2= - C4=).