Acid strength variation due to spatial location of organosulfonic acid groups on mesoporous silica

Abstract

Evaluation of acid strength was used to investigate the location of organosulfonic acid groups within mesoporous silica materials. Mesoporous silica functionalized with either propylsulfonic or arenesulfonic acid groups at different surface concentrations were synthesized via the co-condensation technique, and their resulting acid strength was determined in a methanol solution using potentiometric titration with the Gran Plot technique. A decrease in spatial distance between the sulfonic acid groups on the pore surface of the mesostructured materials was found to increase the acid strength of the resulting solid acid catalyst as result of acid site cooperativity. Comparison of these results with the catalytic performance of the organosulfonic acid-functionalized mesoporous silica materials in the methanol esterification of palmitic acid suggested that the reaction performance of these materials is not simply related to the effects induced by spatial location of the acidic sites on the acid solid catalyst. The results demonstrate the potential of intentionally designing the catalytic environments of the organic–inorganic hybrid mesoporous materials at the molecular level.