Investigation on the optimal oxidation agent for a maximum yield of sulfonic acid groups in MCM-41

Abstract

For proton conduction e.g. in PEM fuel cells as well as acid catalysis materials with high and uniformly distributed loading with highly acidic moieties like SO 3H groups are required. Thus, mesoporous Si-MCM-41 was functionalized with bis(3-triethoxysilylpropyl)-disulfide (TESPD) by the co-condensation method. In order to find an optimum oxidation agent for transferring sulfur containing compounds into SO 3H groups with a maximum yield without using certain catalysts or solvents, the TESPD modified MCM-41 materials were oxidized with different oxidation agents, i.e. (i) Br 2, (ii) H 2O 2 and (iii) HNO 3. The amount of sulfonic acid groups which are anchored by organic linkages on the mesoporous surface were determined by ion exchange capacity. A maximum yield of sulfonic acid was found for samples oxidized by Br 2 (98%), followed by H 2O 2 (80%) and HNO 3 (66%). The surpassing yield of SO 3H groups achieved with Br 2 as a new oxidation agent was demonstrated by proton conductivity measurements since it increases with increasing amount of SO 3H groups.