Xin Shaohui, Wang Qiang, Xu Jun, Chu Yueying, Wang Pengfei, Feng Ningdong, Qi Guodong, Trébosc Julien, Lafon Olivier, Fan Weibin, Deng Feng
National Centre for Magnetic Resonance in Wuhan , State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics , CAS Key Laboratory of Magnetic Resonance in Biological Systems , Wuhan Institute of Physics and Mathematics , Chinese Academy of Sciences , Wuhan 430071 , China . Email:
University of Chinese Academy of Sciences , Beijing 100049 , China.
Chem Sci. 2019 Sep 12;10(43):10159-10169. doi: 10.1039/c9sc02634g. eCollection 2019 Nov 21.
The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is "NMR-invisible" owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% HSO) in the zeolite.
明确表征沸石中不同的酸性位点对于理解其催化性能以及合理设计高效沸石催化剂至关重要。除了各种已得到充分表征的骨架外铝物种，三配位骨架铝物种也可作为沸石中的路易斯酸位点，由于其极其扭曲的局部环境，该位点是“核磁共振不可见的”。在此，我们提供了一种可行且可靠的方法，通过灵敏度增强的二维（2D）多核相关核磁共振实验结合三甲基氧化膦（TMPO）探针分子来阐明脱水H-ZSM-5沸石中三配位骨架铝的酸性本质。已明确鉴定出两种类型的三配位骨架铝位点，其占总布朗斯特酸和路易斯酸位点的11.6%。此外，还发现三配位骨架铝位点与布朗斯特酸位点之间紧密的空间接近性产生的协同效应导致沸石中产生超强酸性（酸强度强于100% HSO）。
