Understanding the high catalytic activity of propylsulfonic acid-functionalized periodic mesoporous benzenesilicas by high-resolution ¹H solid-state NMR spectroscopy

Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO ₃H, 1) has been shown to be exceptional solid catalysts in the acid-catalyzed condensation of indole on benzaldehyde. The reasons for this distinct behavior are so far not completely understood. Here, we present a study involving the combination of advanced high-resolution solid state magic-angle spinning (MAS) NMR characterization with the results of the application of hydrated and dehydrated 1 with different acid loadings in the acid-catalyzed condensation of indole on benzaldehyde attempting an explanation of the higher performance of these materials when compared with the conventional solid catalysts. ¹H MAS NMR investigations show the displacement of the propylsulfonic -SO ₃H protons to higher ¹H chemical shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring -SO ₃H groups. The acid strength of 1 is lowered by the presence of water. At low sulfonic acid loading the catalytic activity is surprisingly high and independent of the water presence. The 2D ¹H- ¹H recoupling MAS NMR experiments indicate that the phenyl rings may protect the acidic sites against water solvation, thus affording a plausible explanation for the negligible effect of water on the catalytic activity of 1 with low acid loading. For a proton loading higher than 0.40 mmol g ^-1, we observed a linear relationship between the catalyst TOF and the chemical shift value of the -SO ₃H proton, thus showing that solid ¹H NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.