Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

Abstract

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/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 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 CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.

Autothermal Oxidative Coupling of Methane over Pt/Al2O3 Catalysts Doped with Rare Earth Oxides

“…In summary, REO addition to a Pt/Al 2 O 3 OCM catalyst was found to be beneficial, as the C 2 selectivity was at least equal to the reference sample 100Al at high dilutions and promoted at lower dilutions. As for both Pt-OCM [30] and REO-driven OCM [28] a dependency on gas phase reactions and thus methyl radical formation was reported previously, one can deduce that for our catalyst systems these effects occur simultaneously, hereby improving C 2 formation. Zhou et al [28] found Sm 2 O 3 as the most effective compound for CH 3 • generation, which outperformed other REOs by far; however, the same study also revealed La 2 O 3 as the dopant yielding the highest C 2 selectivity, followed by Sm 2 O 3 .…”

“…2C). Since sufficiently high temperatures were reported as prerequisite for methyl radical formation, subsequent gas phase recombination to form C 2 H 6 , and subsequent dehydrogenation [27,[29][30][31], this more pronounced heat evolution is, in addition to methyl radical production from heterogeneous reaction pathways, an important factor that enables the higher C 2 selectivities observed over the REO-doped catalyst samples (Fig. 1A).…”

“…Please note that the temperature shown in Fig. 2C is the downstream temperature monitored with a distance of about 5 mm from the catalyst and is therefore not representing the temperature inside the catalyst itself, which is likely even higher [29][30][31]. La 2 O 3 is known to stabilize Al 2 O 3 by either inhibiting a phase transition or by forming mixed oxides [44][45][46].…”

“…Due to high temperatures resulting from the simultaneously ongoing CPOX at very short contact times, the reaction runs autothermally and consequently an additional energy input becomes obsolete. Recent studies by Chawla et al [30,31] combining experiments and simulations underscore that methyl radicals play a key role during Pt-catalyzed OCM (Pt-OCM). These CH 3 • species mainly originate from homogeneous gas phase chemistry that is enabled by the high reaction temperatures, and their formation typically involves the attack of an H radical, an OH radical, or an O radical on the CH 4 molecule.…”

“…Recent studies by Chawla et al. [30, 31] combining experiments and simulations underscore that methyl radicals play a key role during Pt‐catalyzed OCM (Pt‐OCM). These CH 3 • species mainly originate from homogeneous gas phase chemistry that is enabled by the high reaction temperatures, and their formation typically involves the attack of an H radical, an OH radical, or an O radical on the CH 4 molecule.…”