Direct Synthesis of Highly Siliceous Faujasite-type Zeolite Enabled by Low Charge Density Organic Structure-directing Agents.

Ultrastable Y (USY) zeolites (Faujasite-type, FAU) with high SiO2/Al2O3 ratios (SARs) have been widely applied in fluidized catalytic cracking and hydrocracking processes. However, their preparation typically involves labor-intensive post-treatments that inevitably introduce defects, extra-framework species, and compositional gradients. Herein, we report the direct synthesis of FAU-type zeolite with a record-high SAR up to 21.28, which shows superior catalytic performance in the catalytic cracking of cumene and straight-run diesel. This was achieved by using a highly siliceous initial gel and seed, together with low charge-density organic structure-directing agents (OSDAs) of tetramethylammonium (TMA+) and tetrabutylammonium (TBA+) cations, while minimizing the use of high charge-density inorganic Na+ cations. Comprehensive NMR analyses, including two-dimensional (2D) heteronuclear correlation (HETCOR) experiments (1H-13C, 1H-27Al, 1H-29Si), and synchrotron radiation X-ray diffraction-based Rietveld refinements, revealed that (1) TMA+ and TBA+ preferentially interact with Si species over Al species in the initial gel, promoting Si incorporation into the FAU structure's long-range order; and (2) TMA+ cations were tightly confined within the sodalite (sod) cages, while TBA+ cations occupied the faujasite (fau) supercages, adopting folded butyl chain configurations. The encapsulation of low charge-density OSDA cations, coupled with the exclusion of high charge-density Na+ cations, resulted in a highly siliceous Y zeolite with a SAR of 21.28. The synthesized Y zeolite exhibited improved durability and enhanced activity in cumene cracking and comparable selectivity for the target products [liquefied petroleum gas (LPG) and gasoline] while reducing coke formation by 28% in straight-run diesel catalytic cracking compared to conventional USY zeolites.