(445a) Acidic Strength Determination of Organic Acid Functionalized Mesoporous Silica by Potentiometric Titration

Organic acid functionalized mesoporous silica catalysts are promising substitutes for homogeneous acids in condensed phase reactions. A comparison of their activity to the activity of their homogeneous analogs can only be substantiated by determining their acidic strength and total acid capacity in an environment similar to the reaction phase. Previously, we addressed the role of the solvent on the acidic strength and demonstrated that the characterization studies with the probe molecules do not always provide reliable insight about the acidic properties in the condensed phase. Alternatively, potentiometric titration can in principle account for the interactions with the solvent as well. However, the conditions under which the titrations are performed have to be chosen carefully as they affect the measured values. In the work to be presented, the effect of titration parameters on the determination of acidic properties of organic acid functionalized mesoporous silica will be discussed. Unlike homogeneous titrations, heterogeneous titrations involve mass transfer limitations, which can be overcome by salt ion-exchange. This phenomenon is clearly demonstrated by the electrode response time measurements as well as by titration curves at varying salt concentrations. The presence of salt not only facilitates the proton loss of the acid, but also the proton uptake of the base. Although salts are required for accurate measurements, their extensive addition leads to activities that deviate from actual concentrations significantly. The activity coefficient also deviates from unity when the bulk catalyst concentration is increased. By exploring the effect of titration parameters, an approach for reliable potentiometric titration of organic acid functionalized mesoporous silicas having different acid strength and loadings will be presented.