Research progress on methane conversion coupling photocatalysis and thermocatalysis

Identifying the crucial role of water and chloride for efficient mild oxidation of methane to methanol over a [Cu2(μ-O)]2+-ZSM-5 catalyst

Mechanistic Insights into Direct Methane Oxidation to Methanol on Single-Atom Transition-Metal-Modified Graphyne

Distinct Behaviors of Cu- And Ni-ZSM-5 Zeolites toward the Post-activation Reactions of Methane

The methane partial oxidation and methanol formation over ZSM–5 with different oxygenated active centers have been explored by the density functional theory (DFT) method, aiming at finding the catalyst with high activity and high selectivity to methanol. The activation energies of methane dehydrogenation on different (M–O–M)2+ active centers at different spin states are calculated, which are also compared with the corresponding values of the reported (Cu–O–Cu)2+ active center. Results show that (Cu–O–Pd)2+/ZSM–5(S = 0), (Cu–O–Co)2+/ZSM–5(S = 0), (Cu–O–Co)2+/ZSM–5(S = 1) and (Zn–O–Zn)2+/ZSM–5(S = 1) are the preferred catalysts for CH bond cleavage of methane. Moreover, electronic properties analysis exhibits that the more charges accumulated on the oxygen atom of active center, the lower the energy required for CH bond cleavage. And the greater H atom affinity energy (EH) of active center always corresponds to the lower activation energy (Ea) of methane dehydrogenation. Furthermore, the results for the formation of methanol show that (Zn–O–Zn)2+/ZSM–5(S = 1) can be used to catalyze the formation of methanol with lower activation energy of 74.3 kJ·mol−1. Additionally, the formation of methanol is easier than the possible side products (ethane and ethylene). Therefore, it can be concluded that (Zn–O–Zn)2+/ZSM–5(S = 1) exhibits excellent high activity and selectivity to methanol during methane partial oxidation to methanol. 

Effect of ZSM–5 with different active centers on methane partial oxidation

Density functional theory (DFT) calculations were performed to investigate the catalytic cycle of methane conversion to methanol over both [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites in the Cu-ZSM-5 catalyst. The [Cu2(O2)]2+ site is found to be active for the partial oxidation of methane to methanol, and although it has a higher energy barrier in the methane activation step, it involves a very low energy barrier in the methanol formation step (36.3 kJ mol-1) as well as a lower methanol desorption energy (52.5 kJ mol-1). As the [Cu2(O2)]2+ active site is also thermodynamically stable, it may play an important role during methane conversion to methanol. Furthermore, the methane activation step follows the homolytic route and the heterolytic route for the [Cu2(O2)]2+ and [Cu2(μ-O)]2+ active sites, respectively, whereas the methanol formation step follows the direct radical rebound mechanism and the indirect rebound mechanism, respectively. Our calculations further indicate that the electronic properties of the reactive O atoms in the active site influence their reactivity toward methane oxidation. More specifically, the higher the spin density and the more negative the charge of the reactive O atom at the active site are, the lower the energy barrier for methane activation will be; and the more negative the charge of the hydroxyl group in the reaction intermediate during the partial oxidation of methane to methanol is, the higher energy barrier of the methanol formation step will be in the triplet state. Furthermore, we used a larger cluster model to predict the mechanism of the methane activation step and the effect of atomic charge of the O atom at the [Cu2(μ-O)]2+ and [Cu2(O2)]2+ active sites on the energy barriers of partial oxidation of methane to methanol, and the conclusions drawn employing the larger cluster model are consistent with those drawn using the smaller double-5T-ring cluster model. In addition, different from the traditional mechanism for methane activation at [Cu2(O2)]2+, which consists of two transition states, we find that the partial oxidation of methane at [Cu2(O2)]2+ can also occur via a single step by direct insertion of one of the O atoms at the active site into the C-H bond of methane.

Xi Yu

Papers

Catalytic cycle of the partial oxidation of methane to methanol over Cu-ZSM-5 revealed using DFT calculations.