(233b) Probing Individual Acidic Catalytic Sites with Single Molecule Spectroscopy

Conventional methods for characterizing the acids sites in heterogeneous catalysts provide information on the average acidity in the catalyst, but provide limited information on the location of different acids sites.  Single molecule spectroscopy, where individual fluorescent molecules interact with a catalyst surface, emitting light if they encounter specific sites, offers potential for probing where individual acid sites are located.  In this paper, we apply single molecule spectroscopy to characterize the acids sites in acid-functionalized mesoporous silica. Mesoporous silica, in which hexagonally packed tubular mesochannels are aligned, is prepared through self-assembly of silica precursors with surfactants.  Sulfuric acid groups are attached to the silanol groups both on outside and inside surfaces of the porous material.  This highly ordered structure provides an avenue to study the distribution of the acid sites with respect to locations in the mesoporous matrix.  A coumarin-based dye is synthesized and used as a pH indicator since its florescence property is sensitive to pH down to 2.5.  With this dye, the acid sites and the acidity of the sites can be visualized by using confocal or wide field microscope.  The same silica material is utilized to catalyze the polymerization reaction of thiophene monomers. The obtained poly-thiophene molecules become fluorescent with the excitation of laser. By applying thiophene to silica film and recoding the fluorescence of poly-thiophene, in-situ characterization of the solid matrix is operated.