Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents

The arrangement of Al sites in zeolite frameworks influences the structure and speciation of Brønsted acidic hydroxyl groups and of metal cations and complexes that behave as active sites in acid and redox catalysis, but synthetic approaches to systematically alter Al arrangement have yet to be developed for many zeolite topologies. Herein, we report the synthesis of MEL zeolites with varied Al contents (Si/Al = 35–118) using tetrabutylammonium (TBA+; TBA+/Si = 0.3) as the organic structure-directing agent (OSDA) and with fixed Al content (Si/Al ∼ 50) using mixtures of inorganic (Na+) and organic (TBA+) SDAs of different charge density [(Na+/TBA+)gel = 0–5, (Na++TBA+)/Si = 0.3]. MEL zeolites crystallized using TBA+ as the sole structure-directing agent (SDA) contained one TBA+ per channel intersection [4 TBA+ per 96 T-site unit cell (u.c.)], with varying bulk compositions (Si/Al > 23) consistent with charge density mismatch theory. Aqueous-phase ion exchange conditions to use Co2+ as a selective titrant of proximal Al sites in MEL were determined and validated by a cation site balance on Co-MEL zeolites. MEL crystallized from TBA+ alone contained finite fractions of Co2+-titratable Al–Al pairs that increased (2 × Co2+/Al = 0.2–0.4) with total Al content (Si/Al = 35–118), as also observed for MFI crystallized with tetrapropylammonium (TPA+) alone. MEL crystallized from mixtures of Na+ and TBA+ contained fractions of Co2+-titratable Al–Al pairs that decreased (2 × Co2+/Al = 0.22–0.10) with increasing occluded Na+ content (0.0–2.4 Na+/u.c.). Analysis of occluded OSDA and inorganic SDA content in MEL samples reveal evidence for competitive occlusion of Na+ and TBA+. Density functional theory-estimated energies reveal that Na+ co-occlusion with OSDAs is less likely in MEL than MFI frameworks. These findings, together with our prior results on MFI and CHA frameworks, indicate that site-isolated Al arrangements tend to form when monovalent inorganic SDAs and OSDAs compete for occupancy within void and ring spaces of zeolite frameworks.