An NMR Scale for Measuring the Base Strength of Solid Catalysts with Pyrrole Probe: A Combined Solid-State NMR Experiment and Theoretical Calculation Study

Pyrrole has been employed as an NMR probe molecule to determine the base strength of solid catalytic materials; however, the quantitative correlation between the H-1 chemical shift of adsorbed pyrrole and the intrinsic base strength of solid catalysts is still lacking in the previous work. Here, solid-state NMR experiments and density functional theory (DFT) calculations were employed to explore the adsorption structures and H-1 chemical shifts of adsorbed pyrrole molecules over the zeolites with varied base strengths. Based on a generic 8T zeolite ((SiH3)(3)-Si-X-Si-O-(SiH3)(3), X = O or NH), various calculated models with different Si H bond lengths were constructed to represent the basic sites with varied strengths and used to predict the pyrrole adsorption structures as well as the H-1 chemical shifts. The solid-state NMR experimental results demonstrated that a larger H-1 chemical shift of adsorbed pyrrole corresponds to a stronger basic site on solid catalysts. A linear correlation between the H-1 chemical shift of adsorbed pyrrole and the proton affinity (PA) value of solid bases was theoretically derived, which is independent of the basic central atoms (e.g., O or N). In combination with the available H-1 MAS NMR experimental data, it is conclusive that pyrrole could be used as a probe to quantitatively characterize the intrinsic basicity of various solid catalysts, and the H-1 chemical shift threshold for superbasicity is 10.0 ppm. In addition, the influence of confinement effect on the adsorption structures and H-1 chemical shifts of pyrrole over basic catalysts was investigated as well.