Framework Bond Chemistry Governs Seed Activity through Molecular Precursor Regulation in SSZ-39 Crystallization

Abstract

Seed-assisted synthesis is widely used to accelerate zeolite crystallization, yet the molecular origin of differences in seed activity remains unclear. Here, we show that seed activity in SSZ-39 crystallization is governed by framework bond chemistry and its control over the dissolved molecular precursor ensemble. By combining Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS), magic-angle-spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy, crystallization kinetics, and density functional theory (DFT) calculations, we compare SAPO-18, NH4A, homologous SSZ-39, and seed-free systems. All systems yield phase-pure AEI zeolite, but their crystallization rates differ sharply: SAPO-18 completes crystallization within 12 h, approximately six times faster than the seed-free system. FT-ICR MS reveals that active seeds shift aluminosilicate precursors toward low-condensation oligomers (<6 T atoms) and enrich the corresponding OSDA-associated complexes. DFT calculations show that these low-condensation aluminosilicates interact more strongly with DMDMP cations than more highly condensed counterparts, linking precursor speciation to OSDA utilization and nucleation. The exceptional activity of SAPO-18 arises from facile alkaline hydrolysis of weak P–O–Al linkages, which promotes extensive seed dissolution, while the released phosphate remains outside the SSZ-39 framework. In contrast, stronger Si–O–Al and Si–O–Si linkages in aluminosilicate seeds limit dissolution and generate less favorable precursor ensembles. These results identify framework bond chemistry as a molecular descriptor of seed activity and recast zeolite seeds as chemical regulators rather than passive structural templates, providing a basis for rational and lower-OSDA zeolite synthesis.