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

Ultrastable Y (USY) zeolites (Faujasite-type, <b>FAU</b>) 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 <b>FAU</b>-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 <b>FAU</b> structure's long-range order; and (2) TMA⁺ cations were tightly confined within the sodalite (<i>sod</i>) cages, while TBA⁺ cations occupied the faujasite (<i>fau</i>) 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.