UC Riverside

Heterogenous catalysts often contain complex mixtures of dormant and active sites. Selective deposition of reactive organometallic or inorganic fragments using surface organometallic methods can be used to study structure-property relationships in these materials. The most common reaction pathway using surface organometallic methods involves the reaction of a surface hydroxyl on a metal oxide with an organometallic. A less common pathway involves reaction with strong Lewis acid sites, which is critically important to form ion-pairs for olefin polymerization catalysis. These Lewis sites are usually present in low concentrations on metal oxides, making the study of these ion-pairs a challenge. Surface organometallic methods can be used to develop model systems that mirror the activity of these highly reactive Lewis sites with higher surface coverage.

This work describes the reactivity of dehydroxylated silica with homogeneous Lewis acids to develop novel derivatized supports that maintain well-defined Lewis acidic properties. The first part of this work discusses the reactivity of a Lewis superacid, Al(OC(CF3)3)(PhF), with the silanol -OH sites on silica to form the well-defined support ≡SiOAl(OC(CF3)3)2(O(Si≡)2). Fluoride ion affinity (FIA) calculations, along with experimental trends observed through Guttman-Beckett studies, show that this well-defined aluminum site is a stronger Lewis acid than B(C6F5)3 and Al(OC(CF3)3)(PhF). This catalyst exhibits characteristic reactivity patterns of Lewis acids, such as methide abstraction of Cp2Zr(CH3)2 to generate organometallic ion-pairs that are active in olefin polymerization. Characterization of these supports by solid state NMR further confirms structural insights. The next part of this thesis examines the reaction of Cp2Hf(CH3)2 with the aluminum site to form cationic Cp2Hf(CH3)+ sites. These sites react with polypropylene and H2 to form oils with moderate molecular weights. These results demonstrate that Ziegler-Natta-type active sites are compatible with polyolefin upcycling. The final part of this thesis examines the reactivity of a Lewis acidic carboranyl borane with isolated silanols to generate a well-defined Lewis site (MeoCb2B)(OSi≡). The three-coordinate boron site isolated on the support maintains strong Lewis acidity, and reactions of this support with Cp2Hf(CH3)2 show that the Lewis sites also demonstrate methide abstraction, but with low efficiency, likely due to the steric environment of the carborane.