Zheng Mingji, Wang Qiang, Chu Yueying, Tan Xuechao, Huang Weidong, Xi Yujie, Wang Yongxiang, Qi Guodong, Xu Jun, Hong Suk Bong, Deng Feng
National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
J Am Chem Soc. 2024 Oct 30;146(43):29417-29428. doi: 10.1021/jacs.4c08408. Epub 2024 Sep 19.
The inevitable dealumination process of zeolite Y is closely related to ultrastabilization, enhanced Bro̷nsted acidity, and deactivation throughout its life cycle, producing complex aluminum and acidic hydroxyl species. Most investigations on dehydrated zeolites have focused on the Bro̷nsted 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 Bro̷nsted acidic penta-coordinated Al species (Al(V)-BAS) in the dealumination process. Two-dimensional (2D) through-bond and multiquantum H-{Al} 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. H 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.
Y型沸石不可避免的脱铝过程与其超稳化、增强的布朗斯特酸度以及整个生命周期中的失活密切相关，会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铝（Al(IV)）的布朗斯特酸度和与三配位铝（Al(III)）位点相关的路易斯酸度上，而脱铝沸石中的五配位铝（Al(V)）几乎未被讨论。这主要是因为将可检测到的Al(V)过度简化地视为仅具有路易斯酸度的骨架外物种。在此，我们报道了在脱铝过程中布朗斯特酸性五配位铝物种（Al(V)-BAS）的形成。二维（2D）通过键和多量子H-{Al}相关固体核磁共振实验表明，在脱铝的Y型沸石中存在桥连的Si-OH-Al(V)结构。与传统观念不同，传统观念认为在沸石脱铝初期水的攻击会导致沸石骨架Al-O键断裂，而观察到的作为脱铝中间体的Al(V)由于水在骨架四面体铝（Al(IV)）上的直接解离而与一个布朗斯特酸性对直接相关，从而绕过了Al-O键的断裂。H双量子固体核磁共振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)-BASs的酸度比与Al(IV)相关的传统桥连羟基更强。我们还能够从具有CHA拓扑结构的脱铝SSZ-13沸石中检测到Al(V)-BAS位点，这表明其形成并非特定于沸石的骨架结构。
