On the real role of hydroxide in zeolite crystallization: electrochemically-generated OH synthesis

Hydrothermal synthesis of zeolites is typically conducted in basic media, where the hydroxide anion (OH - ) acts as a mineralizing agent, promoting the transformation of precursors into crystalline frameworks. OH - is introduced together with (in)organic structure-directing agents ((in)OSDAs), thus always accompanied by a counter-cation (e.g., Tetrapropylammonium + , Na + ). Consequently, for systems containing only monovalent ions, the OH - /SDA + ratio generally remains at 1 (or below). Contrary to this common approach, we employed a dual electrochemical cell as a novel, unconventional reactor for the in situ cathodic generation of OH-, termed Electrochemically-generated OH Synthesis (EOS). Demonstrated for the first time, expanding the OH - /OSDA + ratio from 0.8 to 1.2 at a constant OSDA content during Silicalite-1 crystallization enabled us to decouple the role of OH - from that of the OSDA. Crystal size, solid yield, and OSDA uptake strongly depended on the OH - content, confirming a broader role of OH - than that of a mineralizer. The evolution of silicate precursors, crystallization kinetics, and particularly the nucleation step were witnessed as the most affected parameters. This decoupling further shows that OSDA uptake by pure-silica frameworks is not solely governed by templating, but also depends on the charge state of the precursors, which is directly influenced by OH - content. EOS reveals an alternative gel-mediated precursor pathway prior to Silicalite‑1 crystallization, highlighting the influence of precursor state and crystallization pathway on the final material properties.