Crystallization of metal oxides with well-defined morphologies for converting methane to high-value chemicals

In metal oxide catalysis, the performance of the catalyst often depends on the exposed crystal facets. For challenging industrial processes such as oxidative coupling of methane (OCM), it has been reported that MgO particles exposing a high ratio of {111} surface areas yield higher C2 selectivity than reactions over MgO(100) cubic facets.1 Thus, suitable strategies for maximizing the exposure of specific facets and attaining controlled morphologies of metal oxide crystals for OCM studies are needed. Moreover, high-index facets are often desirable for a broad range of chemical reactions given the large number of undercoordinated metal sites that are available as active sites for catalysis. Here, we will discuss the crystallization of structured metal oxide materials via molten salt syntheses (MSS), where the main driving force(s) and factors that determine particle morphology have largely remained elusive. We show that the formation of polar facets is facilitated in salt media when the metal oxide is generated via a liquid-to-solid reaction (with intermediates in the molten state).2 The presence of residual water and ions impact oxide crystallization in ways that still remain unknown, but are not necessarily governed by adsorption- stabilization processes. Correlations between metal oxide catalyst faceting and corresponding OCM performance will be presented.

(1) Hargreaves, J. S. J.; Hutchings, G.J.; Joyner, R.W.; Catal. Today 1990, 6, 481−488.

(2) Susman, M. D.; Pham, H. N.; Datye, A. K.; Chinta, S.; Rimer, J. D.; Chem. Mater. 2018, 30, 2641−2650.