Molecular Compositions, Structures, and Acid Properties of Silica-alumina, Silicate, and Aluminosilicate Catalysts and Supports

Silica-aluminas, silicates, and aluminosilites (zeolites) have been widely used as heterogeneous catalysts and supports, because compared to homogeneous catalysts, their solid-acid properties are often desirable for selectivity and separation reasons. However, despite decades of use, much remains unknown with regard to the molecular origins of their acidities and how they may be controlled. For zeolites, although the molecular origins of their acidities are known, the molecular interactions that direct the formation of crystalline frameworks are poorly understood, limiting the structures and compositions, and thus acid and/or adsorption properties, which can be obtained. The molecular origins of acidity of silica-aluminas, on the other hand, remain heavily debated, prohibiting predictive control of their acid properties. A greater understanding of the local compositions and structures of solid acid sites is required to develop predictive syntheses of solid acid catalysts or supports with tunable catalytic and/or adsorption properties.;Advanced nuclear magnetic resonance (NMR) techniques, in conjunction with X-ray diffraction (XRD) and infrared (IR) spectroscopy, provide insights on the local compositions, structures, and interactions among the various components of silicates, aluminosilicates, and silica-aluminas. For example, the roles of structure-directing and mineralizing agents and their interactions with silicate species during zeolite synthesis can be determined using one- and two-dimensional NMR techniques. Such interactions provide insight on zeolite nucleation and subsequent crystallization that are expected to aid in the development of new predictive synthesis protocols for zeolites with tunable surface properties. For amorphous silica-aluminas, the combination of probe molecule adsorption, catalytic testing, and NMR and IR characterization is vital for determining the molecular origins of their surface acidities. Adsorption of basic probe molecules, in conjunction with NMR and IR spectroscopy, allow for the compositions, structures, strengths, and distributions of surface acid sites on silica-aluminas to be determined. Such detailed understanding of acid sites is used to establish correlations among the Bronsted and Lewis acidities as well as correlations between the bulk and chemical properties of silica-aluminas. Ultimately, the used of advanced spectroscopic techniques has allowed for greater understanding of solid acidity, which can be used to aid in the development of new solid acid catalysts and supports.