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

Abstract

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 structuredirecting 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 structuredirecting 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 Co 2+ 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 Co 2+ -titratable Al−Al pairs that increased (2 × Co 2+ /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 Co 2+ -titratable Al−Al pairs that decreased (2 × Co 2+ /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.

Synthesis Route to Single-Walled Zeolite Nanotubes Enabled by Tetrabutylammonium Hydroxide

“…For MFI and MEL, this means that Al–Al pairs tend to form in the organic-rich (low Na + ) gels by using TPAOH and TBAOH, respectively. Conversely, single-site Al formed in CHA (SSZ-13) in the organic-rich gels using TMAda + rich gels. − Table shows a summary of the effect of Na + on the resulting zeolite product during the ZNT synthesis. ZNT was obtained using the originally reported NaOH route and the new TBAOH route described here.…”

“…Previous work showed no such EFAl peak in NaH-ZNT, meaning that the EFAl could be arising from the difference in Na + -content between NaH-ZNT and TBA-Al-ZNT, where the more H + -rich zeolite is more prone to framework-Al removal. Another possibility is the potential difference in Al distribution associated with the organic/inorganic cation ratio in the gel, where the TBA synthesis could be giving more Al–Al pairs, − which could be prone to framework removal (dealumination) upon calcination, as the local Al-content is higher than single site Al. Similar results have been observed for conventional MEL elsewhere .…”

“…Similar results have been observed for conventional MEL elsewhere. 25 To test this hypothesis, two ion exchanged samples were prepared. TBA-SDA-ZNT was ion exchanged with Na , where the proton is H-bonding with either adsorbed H 2 O or framework oxygen.…”

Paired Site Characterization in Aluminosilicate Zeolites through Limited Alkylammonium Stability

“…With the potential for the proximity of Al sites to tune the function of their charge balancing cations, the ability to quantify the fraction of Al sites paired in a zeolite framework is needed. Early efforts by Dědeček et al, , built upon by Janda and Bell, Di Iorio and Gounder, ,, and others, , relied on divalent metal cations to probe framework-incorporated aluminum atoms neighboring each other. Unlike monovalent cations (Na + , Li + ) that only balance the charge on a single negatively charged framework oxygen, divalent cations (e.g., Cu 2+ , Co 2+ ) balance the charge of up to two negatively charged bridging framework oxygens.…”

Paired Site Characterization in Aluminosilicate Zeolites through Limited Alkylammonium Stability

“…73 With the potential for the proximity of Al sites to tune the function of their charge balancing cations, the ability to quantify the fraction of Al sites paired in a zeolite framework was needed. Early efforts by Dědeček, [74][75] built upon by Bell, 76 Gounder, 73,[77][78] and others, 53,79 two negatively charged bridging framework oxygens. If two negatively charged framework oxygens are sufficiently proximal to be accessed by a single unique divalent cation, both negatively charged oxygens can be simultaneously balanced.…”