A mechanistic study of the synthesis of zeolite SSZ-24

X-ray diffraction, elemental analysis, scanning electron microscopy, and ²⁹Si magic angle spinning (MAS), ¹H-¹³C cross-polarization (CP) MAS and ¹H-₂₉Si CP MAS NMR spectroscopies are used to study SSZ-24 synthesis from aqueous silicate mixtures containing alkali and N,N,N-trimethyl-1-adamantammonium (TMAA⁺) cations. SSZ-24 only forms from mixtures prepared using fumed silica, KOH, and TMAAOH, with TMAA/Si≥0.15. SSZ-24 nuclei form through Van der Waals and coulombic interactions between the anionic silicate species and the TMAA⁺ cations in the gelatinous synthesis mixture. The results cannot distinguish whether SSZ-24 nuclei form in the liquid or solid phase of the synthesis mixture. No evidence was found for occlusion of the TMAA⁺ cations in the solid phase of the gel prior to SSZ-24 crystallization, suggesting that the rates of zeolite nucleation and crystallization may be comparable. Computer simulations reveal that the non-bonded interactions between TMAA⁺ and the SSZ-24 lattice are weaker than the non-bonded interactions between tetrapropylammonium (TPA⁺) cations and the silicalite-1 framework, which may explain why silicalite forms much faster than SSZ-24 when TPAOH is substituted for TMAAOH in the synthesis mixture.