From the catalytic partial oxidation of methane to Lewis acid sites in zeolites

The direct partial oxidation of methane has been a long sought-after approach to utilize natural gas available in decentralized oil fields, instead of its flaring to carbon dioxide. The low density of methane precludes transportation and storage of natural gas at remote locations, and difficulties in its on-site conversion to products of value leave behind flaring as the solitary option. Despite having a high C-H bond dissociation enthalpy, an assortment of homogeneous, heterogeneous and enzymatic approaches to methane activation have been reported. This thesis starts with a careful analysis of these systems and identifies ‘product protection’ as an indispensable concept to engineer a high-yield methane-oxidation process. The direct conversion of methane to methanol is plagued by an unfavorable selectivity-conversion paradigm that cannot be overcome unless steps to prevent the over-oxidation of the primary oxygenate to deep oxidation products, such as carbon dioxide, are incorporated. One of the strategies that is identified to surpass the selectivity-conversion limit is the in situ derivatization of methanol to more stable methyl esters. A homogeneous copper-based catalytic system that enables converting methane in high yield to methyl trifluoroacetate is studied. The lack of rigorous catalyst synthesis procedures and the possibility to recover and re-use the homogeneous catalyst are its added incentives. Notwithstanding the impressive yield-based performance of such homogeneous catalytic systems, heterogeneous approaches to methane conversion over solid catalysts and materials are more desired. Therefore, three different heterogeneous methane-oxidation catalytic systems are examined in the scope of this thesis. The first of these is the oxidation of methane over iron-containing zeolites in an aqueous solution of hydrogen peroxide. We uncover the role of the charge-compensating cation in such iron-containing zeolites on catalytic activity. Furthermore, the importance of aluminum in the zeolite to retrieve catalytically active extra-framework iron species upon sample modification is highlighted. The second heterogeneous methane-oxidation system is a modified version of the above-mentioned hydrogen peroxide-based process. The reaction medium was changed from water to trifluoroacetic acid with an objective to form methyl trifluoroacetate as the product. By doing so, we explicate the possibility to catalytically convert methane over aluminum-containing zeolites with hydrogen peroxide in the presence of the acid. More precisely, the active aluminum species adopt an octahedral coordination in the proton form of the zeolite under ambient conditions. These octahedral aluminum species, which are partially dislodged from the framework of the zeolite, can be forced back to adopt a framework-like tetrahedral coordination upon appropriate sample treatment. We progress to develop structure-property relationships for the catalytically active aluminum species, uncovering the Lewis acidic nature of octahedral framework-associated aluminum in zeolites. Subsequently, this thesis documents a novel conceptual advance in the relationship between aluminum coordination and acidity in zeolites by illustrating the ability to toggle between Lewis and Brønsted acidity, which is founded on the property of framework-associated aluminum to undergo reversible changes in geometry from tetrahedral to octahedral. While the use of hydrogen peroxide as the oxidant allows the functionalization of methane at relatively low temperatures, the higher economic cost of hydrogen peroxide compared to methanol dismisses the possibility of a methane-to-methanol process that explicitly uses hydrogen peroxide to be commercially viable. Therefore, there is a pressing need to move to molecular oxygen as the ultimate oxidant. In this context, the third heterogeneous methane-oxidation system covered in this thesis is an improved methane-to-methyl ester process for the aerobic conversion of methane to methyl trifluoroacetate. With an objective to ameliorate the issues in the conventional homogeneous systems reported for this chemistry, we develop a cobalt-catalyzed process that shows impressive performance. Over the years, numerous direct methane-oxidation processes have been developed, and a number of these continue to be improved. However, for any technology to be commercially viable, methanol needs to be produced at productivities that surpass the industrial threshold. Furthermore, such a performance must be achieved using molecular oxygen as the oxidant and under industrially-relevant process conditions