Design and synthesis of earth abundant transition metal catalysts for selective oxidation of light alkanes and olefins

Production of value-added chemicals by way of selective oxidation of hydrocarbons presents a prospect of establishing a new process that is sustainable and capable of providing an optimum product yield at a lower cost. Selective oxidation reactions involve hydrogen abstraction from appropriate sites from the hydrocarbons and insertion of oxygen while ensuring a stop at partial oxidation product without further oxidation and thus maximizing the production of the desired products. Earth-abundant transition metals, such as Fe and Ni, can be selective and active in such reactions when synthesized with specific structures. In Chapter 3, we primarily focus on identifying the activity and selectivity of Fe species in silicalite-1 support for oxidative dehydrogenation of ethane (ODHE) in molecular oxygen. The knowledge obtained from Chapter 3 is in turn used in Chapter 4 to design iron/silicalite-1 supported nickel oxide catalysts for ethane oxidative dehydrogenation with carbon dioxide, a much weaker oxidant but a positive environmental impact and interesting scientific challenge. In Chapter 6, nickel oxide catalysts supported on silicalite-1 were further studied for oxidative dehydrogenation of propane (ODHP) with molecular oxygen to produce propylene, one of the most desired chemicals for polymer manufacturing. In comparison to ODHE in Chapter 4, ODHP presents much more challenges in turns of reactivity and selectivity, even though propane just has one more carbon than ethane. To use the propylene produced as a result of Chapter 6, in Chapter 5, we studied the iron/silicalite-1 catalyst for propylene epoxidation in the presence of oxygen and hydrogen to produce propylene oxide. To synthesize well-defined iron catalysts supported by silicalite-1, we have developed a one-pot synthesis technique and demonstrated the effects of using an inorganic Fe source such as iron nitrate, Fe(NO3)3, and an organometallic source such as Ferrocene or bis-cyclopentadienyl iron, Fe(Cp)2 in the incorporation of framework Fe inside an MFI support silicalite-1. The increased dispersion of iron cation caused by the cyclopentadienyl ligands leads to better incorporation of the Fe inside the isolated tetrahedral sites on the samples prepared from Fe(Cp)2 making it more selective towards ODH product ethylene compared to the samples prepared for Fe(NO3)3 which mostly had extra framework iron oxide species. The nature and activity of the framework and extra framework Fe species were explained in the selective oxidation of propylene to propylene oxide. We also demonstrated the importance of a bi-functional structure for selective ODH of ethane in CO2 where a specific assembly of relevant metal oxides (i.e., NiO-FeOx) is needed. Methods to develop a secondary metal-promoted metal oxide assembly on NiO were ventured by utilizing atomic layer deposition (ALD) to observe the promotional effect on oxidative dehydrogenation of propane (ODHP). In summary, this dissertation represents a study to establish a catalyst synthesis-structure-performance relationship for selective oxidation of light alkanes to value-added chemicals using earth-abundant iron- and nickel-oxide catalysts.