Oxidative Coupling of Methane over Pt/Al${}_2$O${}_3$ at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al${}_2$O${}_3$-coated catalytic honeycomb monolith were conducted with varying CH${}_4$/O${}_2$ ratios, N${}_2$ dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH${}_3$ radicals in the gas phase, which are essential for the formation of C2 species. Lower CH${}_4$/O${}_2$ ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH${}_4$/O${}_2$ ratio of 1.4. ... mehrThereby, C2 species selectivity of 7% was achieved at CH${}_4$/O${}_2$ ratio of 1.6, with 35% N${}_2$ dilution.