Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents
Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents
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DOI：
10.1021/acs.chemmater.2c01083
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发表时间：
2022-07
影响因子：
8.6
通讯作者：
Elizabeth E. Bickel;Alexander E. J. Hoffman;S. Lee;Hannah E. Snider;Claire T. Nimlos;Natalie K. Zamiechowski-Natalie-K.-Z
中科院分区：
文献类型：
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作者：
Elizabeth E. Bickel;Alexander E. J. Hoffman;S. Lee;Hannah E. Snider;Claire T. Nimlos;Natalie K. Zamiechowski-Natalie-K.-Z
沸石骨架中Al中心的排列影响着Brnsted酸性羟基的结构和形态，以及在酸和氧化还原催化中作为活性中心的金属阳离子和络合物的结构和形态，但对于许多沸石拓扑结构，还没有开发出系统地改变Al排列的合成方法。本文报道了以四丁基铵为有机结构导向剂，用不同电荷密度的无机(Na+)和有机(Tba+)混合分子筛[(Na+/Tba+)−=0∼5，(Na++Tba+)/Si=0−5，(Na++Tba+)/Si=0]合成了Al含量不同的MEL沸石分子筛。用TBA+作为唯一的结构导向剂(SDA)结晶的MEL分子筛，每个通道交叉点含有一个TBA+[每96个T位晶胞(U.C.)有4个TBA+]，具有不同的主体组成(Si/Al和GT；23)，符合电荷密度失配理论。利用Co-MEL分子筛上的阳离子中心平衡，确定了Co2+作为MEL中Al近位选择性滴定剂的水相离子交换条件。由TbA+单独结晶的MEL含有少量Co2+可滴定的Al−Al对，随着Al总含量的增加(2×Co2+/Al=0.2−0.4)增加(Si/Al=35−118)，单独用四丙基铵结晶的MEL也是如此。MEL是由含Co2+的可滴定Al−Al对的Na+和TbA+的混合物结晶而成的，随着闭塞Na+含量的增加(0.0−2.4Na+/U.C.)而减小(2×Co2+/Al=0.2 2/U.C.)。对MEL样品中封闭的OSDA和无机SDA含量的分析揭示了Na+和TBA+竞争封闭的证据。密度泛函理论估计的能量表明，与MFI相比，MEL中Na+与OSDA共封闭的可能性较小。这些发现，连同我们之前对MFI和CHA骨架的结果表明，当单价无机SDA和OSDA竞争分子筛骨架的空隙和环空间时，往往会形成位置隔离的Al排列
: 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 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