Heterogeneous Droplet Catalyst for Selective Oxidation of Methane

Abstract Solid catalysts play vital roles as heterogeneous catalysts in numerous applications in the chemical, petrochemical, pharmaceutical, energy, food and automobile industries, facilitating large-scale production and facile product separation. However, conventional heterogeneous catalysts possess three major shortcomings that hinder their wide-spread application, including i) inferior catalytic activity and selectivity caused by a diversity of active sites, ii) tendency towards deactivation caused by agglomeration of catalyst particles, and iii) undesired carbon deposition in transformation of hydrocarbons. 1-4 In this research, we present a new kind of heterogeneous catalyst that consists of a liquid metal with dissolved catalytically active metal atoms, denoted as a heterogeneous droplet catalyst (HDC). Taking galinstan as the liquid metal with Cu as catalytically active metal solute (denoted as Cu-HDC) as an example, we demonstrate that the dissolved Cu atoms promote selective oxidation of methane into primary oxygenates at room temperature with a superb activity of 1940 mmol·gCu-1·h-1 and a selectivity of over 90%, outperforming state-of-the-art industrial catalysts. The liquid metal solvent ensures a high, entropically driven dispersion of the Cu atoms and prevents carbon deposition. The highly promising Cu-HDC catalyst shows a stable operation over 240 hours. By employing in-situ X-ray absorption spectroscopy (XAS), near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT), we show that the narrow d-orbital of the dissolved Cu atoms in HDC facilitates formation of adsorbed methyl radicals *CH3 and impedes their transformation into *CH2 (where the * notation is used to denote an adsorption site) 5,6. These features enhance the selectivity towards primary oxygenates and prevent over-oxidization and formation of C2+ products. As the phase of the HDC is different from those of gas, aqueous and organic solution phases its separation is as facile as that of solid catalysts. Unlike conventional supported catalysts that are designed with well-defined crystal structures, crystal facets and defects, HDCs are characterized by weak interactions between the liquid metal support and the active metal solute, by having mobile surfaces and by dynamic catalytically active centers. These unprecedented features endow HDCs unique adsorption behavior and adaptive activation of molecules. For this reason, HDCs have the potential to become a major category of catalysts in the field of heterogeneous catalysis.