Molecular Models and Precursors for Catalytic Single Sites Incorporating Group 13 and 14 Elements
The work contained within this dissertation describes the synthesis and study of a series of molecular complexes that mimic the connectivity and geometry of active sites in heterogeneous catalysis. The molecular complexes serve two purposes; they serve as models of surface sites for spectroscopic comparison to metals sites on silica described in the literature and as thermolytic molecular precursors to new materials and isolated metal centers on silica supports by the thermolytic molecular precursor (TMP) method. This dissertation focuses on the development of molecular precursors and model complexes based on group 13 and 14 elements which can be used to model or generate metal sites with relevance to catalysis. In chapter one, methods are explored and developed to control the nuclearity of gallium metal centers in silica materials formed by direct decomposition of molecular precursors which influences their acidity and coordination environments. In chapter two, novel molecular models of catalytic sites reported in the literature are synthesized, characterized and used to investigate proposed initiation steps in heterogeneous catalysis. Finally, chapter three explores new molecular precursors for doping metal centers on silica with germanium, which has previously been shown to influence catalysis.
Chapter one describes the use of a new molecular precursor, Ga[OSi(OtBu)3]3•THF, to generate gallium-containing silica materials by direct thermolysis of the molecular precursor dissolved in a hydrocarbon solvent. The use of a block co-polymer, P-123 Pluronic, as templating agent during thermolysis was investigated in conjunction with the inclusion of tris(tert-butoxy)silanol as a co-thermolysis partner. Co-thermolysis was pursued to tune the Ga:Si ratio of the resulting materials, denoted UCB1-GaSiX. When P-123 pluronic was added as a template in the direct thermolysis of Ga[OSi(OtBu)3]3•THF, the resulting material displayed uniform vermicular pores. However, a decrease in the uniformity of these vermicular pores and eventual generation of primarily textural mesoporosity was found as the gallium content of the final material was decreased through co-thermolysis with tris(tert-butoxy)silanol. It was observed by powder X-ray diffraction and 71Ga MAS-NMR that the inclusion of tris(tert-butoxy)silanol in the precursor mixture also affected the geometry and state of gallium; elimination of observable gallium oxide domains and formation of proposed isolated sites as the gallium content of the material decreased is reported.
Chapter two describes the synthesis and characterization of a series of gallate and aluminate complexes containing M–O–SiO3 linkages that mimic two important sites in zeolite chemistry; framework sites with the structure M[–O–SiO3]4–, and partially-hydrolyzed framework sites with the structure HOM[–O–SiO3]3–. These anionic complexes were paired with Li(12-Crown-4)x cations to form charge-separated or weakly-associated cation-anion pairs. Single-crystal X-ray diffraction studies, vibrational spectroscopy, NMR spectroscopy and EXAFS predictions were performed in order to place the synthesized complexes within the context of zeolite literature and molecular models of group 13 metals in silica previously reported. An investigation of the reactivity of the models for partially-hydrolyzed framework sites, HOM[–O–SiO3]3–, with alcohols was performed and showed formation of ROM[–O–SiO3]3– structures in analogy with proposed initiation steps in the formation of active species for Meerwin-Ponndorf-Verley (MPV) transfer hydrogenation catalysts.
Chapter three describes the generation of a new ligand precursor for the synthesis of thermolytic molecular precursors to tertiary metal oxides containing first-row transition metals, germanium and silicon. The synthesis of iron-, chromium- and manganese-based precursors is described and their thermolytic behavior is investigated. These precursors undergo thermolysis at moderate temperatures and were investigated for the deposition of germanium and transition metal atoms onto a silica support. The grafting of these precursors onto silica was explored and the resulting surface species were probed by EPR and NMR spectroscopies. The grafted materials were calcined to remove organic residues and the changes in the resulting surface species from the grafted precursors was investigated by UV-vis and EPR spectroscopies. The resulting calcined materials were compared to Fe, Mn, and Cr centers deposited in the absence of germanium for their catalytic activity in alcohol oxidation and alkene epoxidation, however, in cases where catalytic activity was observed, the activity was indistinguishable between germanium-doped and un-doped materials for a given metal center. Possible explanations for this are provided and related to prior work on the TMP method in order to be illustrative of the successes and potential limitations of molecular precursor design for generation of new materials.