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

Abstract

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-PM0 material (Ph-PMOPrS 03 H) 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-PrS 03H compared to the less active propylsulfonic acid-functionalized periodic mesoporous silica catalysts (PMS-PrS03H). 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-PrS03H) and Ph-PMO-PrS03H are compared. The causes of the different activities are investigated. The strength of the Bronsted acidity of the sulfonic acid groups S03H 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-PrS03H exhibit the higher catalytic activity. The TOF depends on the S03H loadings. The aromatic phenyl rings that composed the mesoporous hybrid organosilica walls of the PhPMO-PrS03H materials does not confer to the sulfonic groups a stronger acidity since a very low difference was observed between the v8a vibration wavenumbers of the DMPH' molecule of SBA-15-PrS03H and Ph-PMO-PrS03H catalysts. The strength of the acid sites is influenced by the close SiOH or S03H environment. The HI values are slightly superior for Ph-PMO-PrS03H solid than for SBA-15-PrS03H 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 S03H groups. (C) 2013 Elsevier B.V. All rights reserved.