Probing Slow Chemical Exchange of Pyridine Molecules at Acid Magnesium Hydroxide Fluoride Surfaces by 15N NMR

15N NMR of pyridine has been used to study Lewis and Brønsted sites at the acid surface of magnesium hydroxide fluoride at two different pyridine loadings with ratios of excess pyridine compared to pyridine molecules bound to Lewis and Brønsted sites of about 1:1 (highly loaded) and 0.15:1 (low loaded), respectively. The Lewis and Brønsted sites can be distinguished by the 15N chemical shift of pyridine at about −100 ppm (LPy) and −175 ppm (BPy). In both samples a very broad 15N resonance is found around −88 ppm assigned to weakly adsorbed (excess) pyridine molecules via hydrogen bridges (HPy). Additionally, another signal at about −49 ppm is observed in the highly loaded sample only representing physisorbed pyridine (PPy). A slow chemical exchange process of strongly bound LPy and BPy molecules with excess HPy and PPy molecules can be monitored using 15N exchange NMR. It takes place on a time scale of about 50–100 ms at room temperature. All pyridine molecules have the same 15N T1 and its value increases from 800 ms (highly loaded sample) to 5.3 s (low loading) hinting on changed pyridine mobility depending on the loading level.