Relationship between 1H chemical shifts of deuterated pyridinium ions and Brønsted acid strength of solid acids.

Deuterated pyridine (pyridine-d5) is one of the NMR probe molecules widely used for determination of acid strength of solid catalysts. However, the correlation between the 1H chemical shift of adsorbed pyridine-d5 and the Brønsted acid strength of solid acids has rarely been investigated. Here, an 8T zeolite model with different Si-H bond lengths is used to represent the Brønsted acid sites with different strengths (from weak, strong, to superacid) and to predict the pyridine adsorption structure as well as the 1H chemical shift. The theoretical calculation suggests that a smaller 1H chemical shift of the pyridinium ions on the solid acids indicates a stronger acid strength. On the basis of the results of theoretical calculations, a linear correlation between the pyridine-d5 1H chemical shift and the proton affinity (PA) of the Brønsted acid site has been derived. In combination with the available 1H MAS NMR experimental data, we conclude that pyridine-d5 can be used as a scale to characterize the solid acid strength.