Direct crystallization of CHA-type zeolite from amorphous aluminosilicate gel by seed-assisted method in the absence of organic-structure-directing agents

Abstract

Highly crystalline CHA-type aluminosilicate zeolites with the Si/Al ratio of around 4–5 were successfully synthesized by a seed-assisted method in the absence of organic-structure-directing agents (OSDAs); a method for so-called “OSDA-free synthesis” of the CHA zeolite has been newly developed. In this OSDA-free system, in addition to the seeds with the CHA topology, a judicious use of potassium or cesium cations together with sodium cations is a crucial key to the crystallization of the amorphous aluminosilicate gel into the CHA structure. The Si/Al ratio of the CHA zeolite was increased by decreasing the total amount of alkali cations added and/or the use of cesium cations in place of potassium ones. Furthermore, the employment of the amorphous aluminosilicate gel containing boron species led to the formation of the CHA zeolite with a high Si/Al ratio of 6.1 compared with the synthetic chabazite crystallized through the interzeolite conversion of the FAU zeolite (Si/Al: 2.0). The catalytic performance of the prepared CHA zeolites was also evaluated. In the methanol-to-olefins reaction, the catalyst life was improved by increasing the Si/Al ratio of the CHA zeolite.