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

Abstract

The post-activation reactions of methane (CH4) to methanol (CH3OH), formaldehyde (CH2O), and dimethyl ether (C2H6O) are crucial issues in the CH4 selective oxidation to CH3OH over metal-exchanged zeolites. In the present work, we utilize density functional theory calculations to investigate several possible reactions following the CH4 activation on the mono­(μ-O)­Cu2 II, bis­(μ-O)­Cu2 III, and bis­(μ-O)­Ni2 III active sites anchored in the ZSM-5 zeolite framework. In the mono­(μ-O)­Cu2 case, we found that a CH3 ligand formed during the CH4 activation is favorably oxidized to CH3OH or C2H6O when H2O or CH3OH are, respectively, present on the reduced (CH3)­OF–CuI–OH–CuI site. Nonetheless, the reaction rates are predicted to be lower than the CH4 activation, confirming the fact that the CH3OH extraction step using steam requires a longer time. Similarly, although the bis­(μ-O)­Cu2 active site is reported to easily form and desorb CH3OH, the reduced CuII–O–CuII center is active to oxidize the formed CH3OH to CH2O with high exothermicity and reaction rate. The bis­(μ-O)­Ni2 active site, on the other hand, not only is reported to facilely form and desorb CH3OH but also is resistant to the overoxidation reaction forming CH2O, due to an early occupancy of the Ni δ* acceptor orbital at the H–CH2OH activation stage, resulting in a product-like (late) transition structure, where one of the Ni2+ centers is already reduced to a highly unstable Ni+. This work provides insights into the reaction mechanisms and elaborates the importance of the CH3O formation to achieve high-selectivity CH3OH.

Molecular insight into the role of zeolite lattice constraints on methane activation over the Cu–O–Cu active site

“…5–7 This Holy-grail reaction, however, is only economical and thermodynamically preferred if performed at low temperature. 8 The copper-exchanged zeolite has been successful in catalyzing the low-temperature oxidation of CH 4 directly to CH 3 OH through a so-called stepwise process. 9–13 In this approach, the Cu-zeolite catalyst is first activated with O 2 or N 2 O to form highly active Cu-oxo sites 14–16 before proceeding to the main reaction of CH 4 oxidation and CH 3 OH extraction.…”

“…9–13 In this approach, the Cu-zeolite catalyst is first activated with O 2 or N 2 O to form highly active Cu-oxo sites 14–16 before proceeding to the main reaction of CH 4 oxidation and CH 3 OH extraction. 8,17 The reduced Cu–zeolite catalyst then must be reactivated to continue the second cycle of the reaction. Although this process is industrially unfavorable, the knowledge gained from it has led researchers to the development of the continuous catalytic approach.…”

Density Functional Theory Studies of the Direct Conversion of Methane to Methanol Using O2 on Graphitic MN4G-BN (M = Fe, Co, Cu) and CuN4G-PN Single-Atom Catalysts

“…17,70 Alternatively, the formed CH 3 OH might transform to CH 3 O−M and B/PO−H (FS3) or CH 2 OH−M and B/ PO−H (FS4) species through CH 3 O−H and H−CH 2 OH bond cleavages, respectively, which are regarded as the initial stage of CH 3 OH oxidation. 25 As shown in Figure 3, the formation of FS3 on the FeN 4 G-BN and CoN 4 G-BN systems is energetically more stable than the formation of FS4 and the direct CH 3 OH desorption, suggesting that the overoxidation reaction via the CH 3 O−H bond cleavage is highly possible. However, in contrast, the formation of FS3 on CuN 4 G-PN is predicted not to occur as the calculation of FS3 on this system cannot reach a minimum and eventually reverts to the structure of FS2.…”

Conditions to meet for the [CuOH]+ site to be favorable and reactive toward the conversion of methane to methanol over Cu-MOR zeolite

“…24,29 This Cu dimer active site, however, has been reported to be prone to the formation of methoxy species that may lead to overoxidation reactions. 30,31 In more recent studies, the formation of [CuOH] + and [Cu 3 O 3 ] 2+ sites have also been reported in SSZ-13 and MOR zeolites, [32][33][34] respectively, although the former, in particular, was argued whether or not to have a direct involvement in the CH 4 conversion due to its self-reduction nature that makes it suitable only as a precursor for the Cu dimer formation. 35,36 Density functional theory (DFT) calculations have also shown that the CuOH site anchored on the SSZ-13 zeolite kinetically hinders the CH 4 activation given a high C-H activation barrier of about 70 kcal mol −1 .…”

“…24,29 This Cu dimer active site, however, has been reported to be prone to the formation of methoxy species that may lead to overoxidation reactions. 30,31…”