中国科学院精密测量科学与技术创新研究院邓风团队，使用固体核磁共振光谱法揭示了Y型脱铝沸石中五配位铝的Bronsted酸性。相关研究成果于2024年9月19日发表在《美国化学会杂志》。

Y型沸石不可避免的脱铝过程与超稳定、Bronsted酸度增强和整个生命周期失活密切相关，产生复杂的铝和酸性羟基物种。大多数关于脱水沸石的研究都集中在四配位Al（Al（IV））的Bronsted酸度和与三配位Al的路易斯酸度上，这使得脱铝沸石中的五配位Al几乎没有讨论。这主要是由于将可检测的Al（V）视为具有路易斯酸度的唯一额外骨架物种的观点过于简单。

该文报道了脱铝过程中Bronsted酸性五配位铝物种（Al（V）-BAS）的形成。二维（2D）通键和多量子¹H-{27Al}相关固态NMR实验证明，脱铝Y型沸石中存在桥接Si-OH-Al（V）结构。与传统观点不同，即在沸石脱铝的初始阶段，水侵蚀会导致沸石骨架Al-O键断裂，观察到的作为脱铝中间体的Al（V）与BAS对直接相关，因为水在骨架四面体铝（Al（IV））上直接解离，从而绕过了Al-O键的断裂。

¹H双量子固态NMR实验和理论计算为这一机制提供了进一步的证据，而吡啶吸附实验证实了Al（V）-BAS的酸性比与Al（IV）相关的传统桥接羟基更强。研究人员还从具有CHA拓扑结构的脱铝SSZ-13沸石中检测到Al（V）-BAS位点，这表明其产生并非特定于沸石的骨架结构。

附：英文原文

Title: Revealing the Bronsted Acidic Nature of Penta-Coordinated Aluminum Species in Dealuminated Zeolite Y with Solid-State NMR Spectroscopy

Author: Mingji Zheng, Qiang Wang, Yueying Chu, Xuechao Tan, Weidong Huang, Yujie Xi, Yongxiang Wang, Guodong Qi, Jun Xu, Suk Bong Hong, Feng Deng

Issue&Volume: September 19, 2024

Abstract: The inevitable dealumination process of zeolite Y is closely related to ultrastabilization, enhanced Bronsted acidity, and deactivation throughout its life cycle, producing complex aluminum and acidic hydroxyl species. Most investigations on dehydrated zeolites have focused on the Bronsted acidity of tetra-coordinated Al (Al(IV)) and Lewis acidity associated with tricoordinated Al (Al(III)) sites, which has left the penta-coordinated Al (Al(V)) in dealuminated zeolites scarcely discussed. This is largely due to the oversimplified view of detectable Al(V) as an exclusively extra-framework species with Lewis acidity. Here we report the formation of Bronsted acidic penta-coordinated Al species (Al(V)-BAS) in the dealumination process. Two-dimensional (2D) through-bond and multiquantum 1H-{27Al} correlation solid-state NMR experiments demonstrate the presence of a bridging Si–OH–Al(V) structure in dealuminated Y zeolites. Different from the conventional belief that water attack leads to the breaking of zeolite framework Al–O bonds in the initial stage of zeolite dealumination, the observed Al(V) as a dealumination intermediate is directly correlated with a BAS pair because of the direct dissociation of water on the framework tetrahedral aluminum (Al(IV)), thus bypassing the cleavage of Al–O bonds. 1H double-quantum solid-state NMR experiments and theoretical calculations provide further evidence for this mechanism, whereas pyridine adsorption experiments confirm stronger acidity of Al(V)-BASs than the traditional bridging hydroxyl groups associated with Al(IV). We were also able to detect the Al(V)-BAS site from dealuminated SSZ-13 zeolite with CHA topology, suggesting that its creation is not specific to the framework structure of zeolites.

DOI: 10.1021/jacs.4c08408

Source: https://pubs.acs.org/doi/abs/10.1021/jacs.4c08408