Kinetic Modeling of the Synergistic Role of Water and Pd in Ethane Oxidation to Acetic Acid over Pd–MoV Oxides

The oxidative dehydrogenation of ethane provides a low-temperature alternative for ethene production that contrasts with more energy-intensive incumbent endothermic alternatives. Although the ability to produce, on demand, higher value oxygenate products such as acetic acid could materially impact the viability of such an oxidative process, reaction pathways for the formation of C2 oxygenates remain obscure. In this work, we assess the role of water and trace Pd on rates and pathways for acetic acid production over unsupported Mo–V oxides at 300 °C and ambient pressure and identify roles of Pd distinct from those proposed previously in the literature. The highly complex role of water in this chemistry─inhibiting rates through reversible hydroxyl recombinative desorption steps on the one hand and facilitating ethene and ethane activation on the other─necessitates the use of integral reactor models even at exceedingly low contact times that would otherwise be considered differential. Pd appears to not only participate in the secondary conversion of ethene to acetic acid but also increase acetic acid yields by providing an alternative, OH*-assisted pathway for ethane activation, and by relieving product inhibition through the hydrogenation of acetate species abundant on the extended oxide surface. This work illustrates how the local water concentration in the reactor and the Pd content in the catalyst are nonindependent catalyst and reactor design variables─design variables that can be tuned synergistically in an effort to coproduce ethene and acetic acid at substantive yields from ethane through exothermic oxidative processes.