Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

Abstract

Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO2. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu3O3]2+ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.

Copper-Oxo Active Sites for Methane C–H Activation in Zeolites: Molecular Understanding of Impact of Methane Hydroxylation on UV–Vis Spectra

“…We again repeat that recent experimental results have shown that the separated methyl group is stabilized at BASs. ,, Quantum-mechanical computations have shown that exchange of the acidic proton of a BAS with the methyl is indeed energetically feasible . Stabilization of the acidic proton at the active-site μ-oxo atom and the methyl at a basic framework aluminate has been shown to be a crucial mechanism for avoiding overoxidation. , All of these point toward 5h .…”

“…Interestingly, recent experimental and theoretical reports have suggested that the methyl group is actually not stabilized at the active site. Several workers have shown that the methoxy is formed at Brønsted acid sites. − We have also shown that stabilization of the methyl at the active site is a recipe for overoxidation. , Considering these, we optimized the geometries of other possible structures. First, the methyl group could be stabilized away from the active site.…”

“…We repeat that in 5b – 5d , the methyl rebounds to the active site, whereas in 5h , the separated methyl is exchanged with a Brønsted acid proton (Figure ). These two possibilities have emerged as candidates for the stabilized strongly bound methoxy. ,,,,− Examination of the calculated spectra shows that 5b – 5c predict a substantial decrease in the intensities at 23 000–25 000 and 30 000–36 000 cm –1 . They, however, have higher intensities near 29 000 cm –1 .…”

“…75−77 We have also shown that stabilization of the methyl at the active site is a recipe for overoxidation. 78,79 Considering these, we optimized the geometries of other possible structures. First, the methyl group could be stabilized away from the active site.…”

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

“…The high ability of the Cu and Ni dimers to activate CH 4 with low activation barriers (<16 kcal/mol), , however, comes with a challenging trade-off, in which the produced CH 3 OH tends to be overoxidized to competitive byproducts such as formaldehyde (CH 2 O), formic acid (CH 2 O 2 ), dimethyl ether (C 2 H 6 O), and CO, ,, whose amount increases with the metal loading and is dependent on the zeolite counterion (H + or Na + ). , Arvidsson et al computationally investigated the CH 3 OH oxidation to C 2 H 6 O in the absence of metal centers, where the zeolite framework O atom (Al-bound O F ) acts as the proton-accepting site for the CH 3 O–H activation with a rather high activation barrier. A recent work by Odoh and Adeyiga, on the other hand, showed how sequential H-atom abstractions of CH 3 OH facilely occur on the [Cu 3 (μ-O) 3 ] 2+ active site to form a final product of CO.…”