(758c) Rational Design of Single-Site-Based Catalysts for Selective Oxidation of Methane to Methanol
Effective partial oxidation of methane to methanol in a selective manner has long been a holy grail of catalysis. Nowadays, a two-step syngas process via high temperatures and centralized production is used for converting methane into methanol. Despite over a century of research no direct, selective and industrially viable process exists for direct conversion of methane to methanol. Recently, trying to mimic the enzymatic conversion of methane to methanol (Methane monooxygenase, or MMO), a great deal of theoretical and experimental research has been dedicated to study single metallic sites embedded in zeolites or MOFs. Nevertheless, these processes are either continuous but hindered by the selectivity-conversion limit or are non-continuous and have turnovers close to unity. This originates from inherently more reactive C-H bonds in methanol compared to methane. Recently, few strategies have been identified that can alleviate the dire consequences of the selectivity-conversion limit (1). These include the âcollectorâ strategy and diffusion limited systems with very low methane activation barriers. Herein, using a large database of embedded single metallic sites including doped 2D materials (e.g., graphene or boron nitride) and doped oxides (e.g., ZnO, TiO2, GeO2, Alumina) we develop theoretical insights into both oxidation of the single sites (using different oxidants including N2O, O3 and O2) and methane activation by the oxygenated single sites. We develop a scaling-relation-based approach that connects the transition states for activation of the oxidants, methane and methanol to few adsorbatesâ binding energies. Next, we use these relations to build kinetic models and screen candidates for effective methane conversion. We also investigate and identify systems in which (a) the methane activator and methanol collector can be integrated in the same framework of material, and (b) due to very low methane activation barriers the system becomes diffusion limited and thus goes beyond the selectivity-conversion limit.
(1) Latimer, A. A., Kakekhani, A., Kulkarni, A. R., & NÃ¸rskov, J. K. (2018). Direct Methane to Methanol: The SelectivityâConversion Limit and Design Strategies. ACS Catalysis, 8(8), 6894-6907.