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

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 SO3H 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 SO3H 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) Br2, (ii) H2O2 and (iii) HNO3. 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 Br2 (98%), followed by H2O2 (80%) and HNO3 (66%). The surpassing yield of SO3H groups achieved with Br2 as a new oxidation agent was demonstrated by proton conductivity measurements since it increases with increasing amount of SO3H groups.

Different oxidation agents, Br2 (route A), H2O2 (routes B and C) and HNO3 (route D), have been tested for an optimum transfer of sulfur containing groups in Si-MCM-41 to highly acidic sulfonic acid groups.Figure optionsDownload as PowerPoint slideHighlights
► Disulfide functionalized MCM-41 was oxidized in the absence of catalyst or solvent.
► A maximum yield of SO3H groups was found for oxidation with Br2 (98%).
► Proton conductivity of the SO3H-MCM-41 was best after oxidation with Br2.