Influence of Trimethylphosphine Oxide Loading on the Measurement of Zeolite Acidity by Solid-State NMR Spectroscopy

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. The 31 P 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 on 31 P chemical shifts in the 1D spectrum. This is further confirmed by 2D 31 P{ 1 H} heteronuclear correlation (HETCOR) and 31 P– 31 P 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), the 31 P 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 2D 31 P{ 1 H} 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.