Breaking the Si/Al ratio–solid yield limitation in hierarchical nanosized beta zeolite synthesis enables one-step catalytic conversion of lactic acid to lactide

Abstract

Efforts in zeolite crystal engineering have largely focused on nanosized materials, aiming to enhance mass transport and accessible active sites. For beta zeolite, however, this is often hindered by the intrinsic Si/Al ratio (SAR)-solid yield trade-off. Here, we report the synthesis of nanosized beta zeolites, with uniform dimensions of 10–20 nm, a broad SAR of 10–70, and excellent solid yields of > 90% under alkali- and fluorine-free conditions. These are achieved by using a family of spirocyclic quaternary ammonium salts as organic structure-directing agents (OSDAs), readily prepared via a simple nucleophilic reaction with commercial precursors. The resulting beta zeolites feature not only hierarchical porosity with a high surface area of ∼829 m² g⁻¹ but also abundant external active sites, which enable superior catalytic performance over commercial beta zeolites in the one-step conversion of lactic acid to lactide, affording 83.6% yield with excellent L-isomer selectivity. These results demonstrate the unique capability of spirocyclic ammonium OSDAs, opening a new strategy for synthesizing nanosized beta zeolites with simultaneously designed SAR and high yields.