Single Molecule Spectroscopy for Characterization of Acid Acid Catalysts

Mesoporous silicas are promising solid acid catalysts since their pore sizes can be tailored form two to tens of nanometers and they can be modified by incorporation of metals and organic functional groups. Strong acidity can be achieved by functionalizing silica with sulfonic acid groups. Researchers have shown extensive interest to mesoporous materials for their potential to be used as heterogeneous catalysts. However, our understanding of where the acid sites are incorporated into the matrix and how their properties vary from site to site is incomplete. Conventional characterization methods provide only limited information of the distribution of the functional sites and acid strengths. Our research will use single molecule spectroscopy to study the locations and properties of the acid functionalities in silicas-based materials. In this technique, probe molecules at nanomolar concentration are introduced into a material during its preparation.  These molecules interact with acid sites, and are converted from unprotonated to protonated forms.  These forms fluoresce at different wavelengths, providing a way to probe individual sites.

Two pH sensitive probes (C.SNARF-1 and ACISA) have been employed in our laboratory to provide information of the distribution of acid site strengths, the location of acid sites relative to the materials surface, and ultimately, the catalytic rates of certain model reactions. The acidity of alumina-silica catalysts with varying compositions was studied using C.SNARF-1. In addition, aligned porous silica structures were studied, where diffusion of molecules into the pores can be followed using single molecule spectroscopy.  From these studies, we can obtain valuable information of acid sites location and acid strength variability.