Catalysis to Eliminate Needless Chlorine in Industry - Direct Synthesis of H2O2 and “Green” Epoxidations
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H2O2 is an environmentally-benign and selective oxidant useful for epoxidations and bleaching, yet, its use is limited because the anthraquinone oxidation process is viable only at very large scales. Currently, many oxidation processes use chlorine-based oxidants (Cl2, HOCl) at a global rate of 63·109 kg Cl yr-1. Within the United States roughly 60% of chlorine is used as an oxidant in processes where chlorine does not appear in the final products (e.g., production of propylene oxide). The direct synthesis of H2O2 (H2 + O2 → H2O2) could enable H2O2 to be produced cost-effectively on-site, and even in situ, which would curtail needless use of chlorine, and in turn, decrease environmental chlorine pollution. This seminar will describe findings based on rate measurements, isotope effects, and in situ spectroscopy that reveal mechanisms, active intermediates, site requirements, and functional descriptors for the formation of H2O2 and its use in epoxidations. First, direct synthesis of H2O2 involves chemistry at the solid-liquid interface between the complex “broths” needed for high selectivities and supported monometallic (Pd) or bimetallic clusters. Our finding suggest that solvent molecules themselves (e.g., methanol) act as co-catalysts and likely facilitate unexpected proton-electron transfer processes that form H2O2 while surface reactions that cleave O-O bonds produce water. The rates of these two pathways differ dramatically in their sensitivity on the composition of the solvent and the catalyst surface, which provides opportunities for developing highly selective catalytic systems. Second, transition metal atoms contained in the framework of zeolites or grafted to the surface of mesoporous oxides are used to epoxidize olefins with H2O2. The identities and reactivities of the oxidizing surface species that form (i.e., –OOH or –O2) upon irreversible activation of H2O2 on these catalysts depend on the identity and intrinsic properties of the metal substituents. Among only Group 4 and 5 transition metals, measurable increases in the electrophilicity of metal centers lead to epoxidation rates that increase over five-orders of magnitude and which are accompanied by a concomitant 100-fold increase in selectivities. Within the micropores of zeolites, dispersive interactions with the surroundings of active sites can be used to preferentially stabilize reactive states that lead to epoxidations, and therefore, increase selectivities to epoxides and their rates of formation. In ongoing work, we aim to develop guiding principles and reactivity/selectivity descriptors for the design of metallic catalysts for the direct synthesis of H2O2 and subsequent selective oxidations by peroxo and hydroperoxo intermediates.