Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents
Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents
复制标题
DOI：
10.1021/acs.chemmater.0c03154
复制
发表时间：
2020-11-10
影响因子：
8.6
通讯作者：
Gounder, Rajamani
中科院分区：
文献类型：
--
作者：
Nimlos, Claire T.;Hoffman, Alexander J.;Gounder, Rajamani
有机和无机结构导向剂（SDAs）的分子结构和阳离子电荷密度影响Al取代物在沸石骨架中的位置和排列。然而，由于其低对称性框架（12个独特的四面体位点）固有的复杂性，MFI沸石很难建立强大的合成-结构关系，这产生了一个巨大的Al-Al位点对组合空间，需要通过密度泛函数理论（DFT）进行详尽的建模，并通过实验进行量化。在这里，我们开发了一种实验方案，可重复量化MFI中Co2+可滴定的Al-Al位点对，并通过确证光谱和阳离子位平衡数据验证了饱和吸收。以四丙基铵（TPA(+)）作为唯一的SDA， MFI分子筛在不同Al含量（Si/Al = 37-185; 0.52-2.52 Al / cell）下结晶，其组成范围与电荷密度失配理论一致，并且每个通道相交的一个TPA（+）与配对Al的分数（0.0-0.34）随着Al含量的增加而增加。DFT计算使用96个t位点MFI单元细胞进行，其中包含一个孤立的Al位点（全部96个构型）或各种Al-Al位点对（13680个总构型中的1773个），电荷分别由一个或两个TPA（+）平衡，揭示了TPA（+）的阳离子N和阴离子晶格电荷之间的静电相互作用对Al定位能的主要影响。结合Al-Al位点对Co2+交换能的DFT计算，理论预测两个TPA（+）阳离子被限制在相邻的通道交叉点内，可以形成许多Co2+可滴定的Al-Al位点对集合，合理地解释了仅使用TPA（+）结晶的MFI样品中大量成对Al位点的存在。在合成凝胶中使用TPA（+）和Na+作为辅助sdas，在恒定的SDA/AI比（TPA(+) + Na+/Al = 30）下改变Na+/TPA（+）比（0-5），结晶的MFI具有相似的体积Al含量（Si/AI接近50），但Al的成对分数不同（0.12-0.44）。使用电荷中性有机SDAs（季戊四醇或1,4-重氮杂二醇[2.2.2]辛烷和甲胺的混合物，以及Na+来补偿框架Al）进行单独结晶实验，结晶出的MFI具有相似的体积Al含量（Si/AI接近50），但成对的Al含量较低(
The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, developing robust synthesis-structure relations for MFI zeolites is difficult because of the complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space of Al-Al site pairs to exhaustively model by density functional theory (DFT) and quantify by experiment. Here, we develop an experimental protocol to reproducibly quantify Co2+-titratable Al-Al site pairs in MFI with saturation uptakes validated by corroborating spectroscopic and cation site balance data. Using tetrapropylammonium (TPA(+)) as the sole SDA, MFI zeolites were crystallized with varying Al contents (Si/Al = 37-185; 0.52-2.52 Al per unit cell) within a composition range consistent with charge density mismatch theory and the ocdusion of one TPA(+) per channel intersection with fractions of paired Al (0.0-0.34) that increased with bulk Al content. DFT calculations performed using a 96 T-site MFI unit cell containing either an isolated Al site (all 96 configurations) or various Al-Al site pairs (1773 out of 13 680 total configurations), charge balanced by one or two TPA(+), respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA(+) and the anionic lattice charge on Al siting energies. Together with DFT calculations of Co2+ exchange energies at Al-Al site pairs, theory predicts that two TPA(+) cations confined within adjacent channel intersections can form many Al-Al site pair ensembles that are Co2+-titratable, rationalizing the considerable presence of paired Al sites in MFI samples crystallized using only TPA(+). The use of TPA(+) and Na+ as co-SDAs in the synthesis gel, while varying the Na+/TPA(+) ratio (0-5) at a constant SDA/AI ratio ((TPA(+) + Na+/Al = 30), crystallized MFI with a similar bulk Al content (Si/AI approximate to 50) but varying fractions of Al in pairs (0.12-0.44). Separate crystallization experiments performed using charge-neutral organic SDAs, either pentaerythritol or a mixture of 1,4-diazabicydo[2.2.2]octane and methylamine, together with Na+ to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/AI approximate to 50) but with lower fractions of Al in pairs (