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

Abstract

Propylsulfonic acid-functionalized periodic mesoporous benzenesilica (Ph-PMO-SO3H, 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 –SO3H protons to higher 1H chemical shifts with increase of the sulfonic acid strength suggesting the formation of hydrogen bonds involving neighboring –SO3H 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 –SO3H proton, thus showing that solid 1H NMR appears to be a convenient tool to predict the catalytic activity of 1 in water.