Role of acidity and hydrophobicity in the remarkable catalytic activity in water of sulfonic acid-functionalized phenyl-PMO materials

• Ph-PMO-SO3H is an active catalyst for aqueous organic reactions.
• Hydrophobic index (HI) is governed by the SO3H loading.
• At same SO3H loading, catalytic activity is not correlated to the HI.
• Acid strength of SO3H groups depends on SiOH or SO3H interactions.

Periodic mesoporous benzene-silica (Ph-PMO) are interesting materials for catalytic applications due to their high surface area, meso- and molecular-scale order, high organic content in the wall and alternated hydrophobic-hydrophilic behavior. Then, propylsulfonic acid-functionalized Ph-PMO material (Ph-PMO-PrSO3H) is a very active acid solid for catalyzing aqueous organic reaction. Acid strength and hydrophobic properties are usually suggested to explain the distinct behavior of Ph-PMO-PrSO3H compared to the less active propylsulfonic acid-functionalized periodic mesoporous silica catalysts (PMS-PrSO3H). Here we present a systematic study in which, the catalytic performances in the reaction of indole with benzaldehyde of propylsulfonic acid-functionalized SBA-15 (SBA-15-PrSO3H) and Ph-PMO-PrSO3H are compared. The causes of the different activities are investigated. The strength of the Brønsted acidity of the sulfonic acid groups SO3H are characterized by adsorption of 2,6-dimethylpyridine (DMP) monitored by FTIR spectroscopy. The hydrophobic index (HI) of each solid is determined by water/toluene competitive adsorption. The calculation of the turnover frequencies (TOFs) enables to show that, among all the samples studied, Ph-PMO-PrSO3H exhibit the higher catalytic activity. The TOF depends on the SO3H loadings. The aromatic phenyl rings that composed the mesoporous hybrid organosilica walls of the Ph-PMO-PrSO3H materials does not confer to the sulfonic groups a stronger acidity since a very low difference was observed between the ν8a vibration wavenumbers of the DMPH+ molecule of SBA-15-PrSO3H and Ph-PMO-PrSO3H catalysts. The strength of the acid sites is influenced by the close SiOH or SO3H environment. The HI values are slightly superior for Ph-PMO-PrSO3H solid than for SBA-15-PrSO3H sample even when trimethylsilyl groups are incorporated in this latter catalyst. The HI values decrease a lot as a function of increasing concentration of hydrophilic SO3H groups.

Combining catalytic tests with characterizations using FTIR spectroscopy and water/toluene competitive adsorption, we describe the role of hydrophobicity, acid strength and stability in the remarkable activity of sulfonic acid-functionalized periodic mesoporous benzene-silica material in the acid-catalyzed aqueous organic reactions.Figure optionsDownload high-quality image (189 K)Download as PowerPoint slide