Influence of Trimethylphosphine Oxide Loading on the Measurement of Zeolite Acidity by Solid-State NMR Spectroscopy
Influence of Trimethylphosphine Oxide Loading on the Measurement of Zeolite Acidity by Solid-State NMR Spectroscopy
复制标题
三甲基氧化膦负载量对固态核磁共振波谱测定沸石酸度的影响
DOI：
10.1021/acs.jpcc.1c01789
复制
发表时间：
2021
影响因子：
3.7
通讯作者：
Deng Feng
中科院分区：
文献类型：
--
作者：
Wang Yongxiang;Xin Shaohui;Chu Yueying;Xu Jun;Qi Guodong;Wang Qiang;Xia Qinghua;Deng Feng
准确表征沸石的酸强度对了解其催化性能和合理设计高效沸石催化剂具有重要意义。吸附氧化三甲基膦（TMPO）技术的31p MAS NMR谱已被广泛应用于各种固体酸催化剂的酸强度测定。本文通过二维（2D）异核和同核相关NMR实验，结合密度泛函理论计算，探讨了负载TMPO对沸石酸性表征的影响。结果表明，TMPO的负载对沸石酸强度的准确测定起着至关重要的作用。TMPO分子（P/Al = 1.0）的饱和吸附可以在一个Brønsted酸位（BAS）上形成TMPO二聚体，如果仅基于1D光谱中的31p化学位移，则会掩盖特定酸位的酸强度。通过负载tmpo的ZSM-5沸石的2D31P{1H}异核相关（hetor）和31p - 31p双量子（DQ）同核相关实验进一步证实了这一点。通过仔细控制TMPO的吸附量，例如低或中等TMPO负载（即P/Al = 0.2或P/Al = 0.4）， 31p的化学位移不仅可以准确反映沸石的BAS强度，而且由于2D31P{1H} hetor NMR分辨率高，可以区分Brønsted和Lewis酸位点。本研究结果为利用TMPO探针分子更精确地评价沸石的酸性提供了一种策略，有助于优化沸石催化剂的催化性能。
The accurate characterization of the acid strength of zeolites is of great importance to understand their catalytic performance and the rational design of highly efficient zeolite catalysts. The31P MAS NMR spectroscopy of adsorbed trimethylphosphine oxide (TMPO) technique has been widely employed to measure the acid strength of various solid acid catalysts. Here, the influence of TMPO loading on characterizing the zeolite acidity is explored by using two-dimensional (2D) hetero- and homonuclear correlation NMR experiments combined with density functional theory calculations. It is found that the TMPO loading plays a crucial role in the accurate measurement of zeolite acid strength. Saturated adsorption of TMPO molecules (P/Al = 1.0) can result in the formation of a TMPO dimer on one Brønsted acid site (BAS), which will conceal the acid strength of the specific acid site if it is merely based on31P chemical shifts in the 1D spectrum. This is further confirmed by 2D31P{1H} heteronuclear correlation (HETCOR) and31P–31P double-quantum (DQ) homonuclear correlation experiments on TMPO-loaded ZSM-5 zeolites. By carefully controlling the amount of TMPO adsorption, such as low or medium TMPO loadings (i.e., P/Al = 0.2 or P/Al = 0.4), the31P chemical shift can not only accurately reflect the BAS strength of the zeolite, but also can discriminate the Brønsted and Lewis acid sites due to well-resolved 2D31P{1H} HETCOR NMR. The results presented herein provide a strategy to assess the acidity of zeolites more precisely by a TMPO probe molecule, which is helpful for the optimization of catalytic performances of zeolite catalysts.