Structure and Catalytic Characterization of a Second Framework Al(IV) Site in Zeolite Catalysts Revealed by NMR at 35.2 T
Structure and Catalytic Characterization of a Second Framework Al(IV) Site in Zeolite Catalysts Revealed by NMR at 35.2 T
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DOI：
10.1021/jacs.0c00590
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发表时间：
2020-04-22
影响因子：
15
通讯作者：
White, Jeffery L.
中科院分区：
文献类型：
--
作者：
Chen, Kuizhi;Horstmeier, Sarah;White, Jeffery L.
超高场Al-27{H-1} 2D相关NMR实验表明，酸性沸石催化剂在脱水和催化活性状态下至少存在两个含羟基的骨架Al（IV）位。除了已知的铝（IV）在框架桥酸位点（BAS），一个新的网站创建的第二个四面体铝原子和它的羟基质子在沸石HZSM-5的明确解决在35.2 T场强，使最近开发的串联混合（SCH）磁铁技术。结合计算建模、在多个场强下的广泛Al-27 MQMAS实验和H-1 MAS NMR实验，这些数据表明，相对于处于脱水和水合状态的BAS，该第二四面体配位Al位点（表示为Al（IV）-2）经历增加的化学位移和独特的四极参数。这些新的实验数据，支持的计算和催化反应工作，表明第二个网站来自部分键合的框架（SiO）（4-）n-Al（OH）n物种，显着提高催化剂在苯的脱烃转移和正己烷裂解反应的反应活性。Al（IV）-2位点由骨架结晶缺陷或由骨架Al在形成骨架外Al之前的不完全合成后水解产生。由Al（IV）-2物质产生的该第二酸性质子位点的群体显示出通过合成后催化剂处理来控制，对于不同的催化剂结构应该是通用的，并且当它们存在时显著增强所引用的探针反应中的催化剂反应性。本文的结果传达了迄今为止关于活性沸石催化剂结构的最高磁场强度数据，并且首次能够检测干燥HZSM-5催化剂上的Al和H缔合，即，在典型的最终用途工艺的代表性条件下。
Ultrahigh field Al-27{H-1} 2D correlation NMR experiments demonstrate that at least two framework Al(IV) sites with hydroxyl groups can exist in acidic zeolite catalysts in their dehydrated and catalytically active states. In addition to the known Al(IV) at the framework bridging acid site (BAS), a new site created by a second tetrahedral Al atom and its hydroxyl group protons in zeolite HZSM-5 is clearly resolved at 35.2 T field strengths, enabled by recently developed series-connected hybrid (SCH) magnet technology. Coupled with computational modeling, extensive Al-27 MQMAS experiments at multiple field strengths, and H-1 MAS NMR experiments, these data indicate that this second tetrahedrally coordinated Al site (denoted Al(IV)-2) experiences an increased chemical shift and unique quadrupolar parameters relative to the BAS in both dehydrated and hydrated states. These new experimental data, supported by computational and catalytic reaction work, indicate that the second site arises from partially bonded framework (SiO)(4-)n-Al(OH) n species that significantly increase catalyst reactivity in benzene hydride-transfer and n-hexane cracking reactions. Al(IV)-2 sites result either from framework crystallization defects or from incomplete postsynthetic hydrolysis of a framework Al, prior to the formation of extraframework Al. Populations of this second acidic proton site created by the Al(IV)-2 species are shown to be controlled via postsynthetic catalyst treatments, should be general to different catalyst structures, and significantly enhance catalyst reactivity in the cited probe reactions when they are present. The results herein communicate the highest magnetic field strength data on active zeolite catalyst structures to date and enable for the first time the detection of Al and H association on a dry HZSM-5 catalyst, i.e., under conditions representative of typical end-use processes.