Mechanistic Insights on the Direct Conversion of Methane into Methanol over Cu/Na–ZSM-5 Zeolite: Evidence from EPR and Solid-State NMR

Abstract

The selectively direct conversion of methane to methanol on Cu-exchanged zeolites has attracted a lot of interest because of the abundant availability of methane. A detailed reaction mechanism of the transformation of methane to methanol aimed to improve the catalyst design has been long expected. On the basis of EPR and solid-state NMR studies, we demonstrate that CH4 activates on the [Cu2O]2+ core of the Cu/Na–ZSM-5 zeolite, producing a ·CH3 radical and an ·OH radical. The generated methanol molecule, which formed from the combination of ·CH3 and the ·OH radicals, adsorbs on the Cu+ species and forms the copper methoxy species (−Cu–O–CH3) and the −Al–OH group. Through the process of hydration, the copper methoxy species and the adsorbed methanol transform into free methanol. We expect that our mechanistic understanding of the methane to methanol process could pave the way for catalyst design.

The accuracy challenge of the DFT-based molecular assignment of13C MAS NMR characterization of surface intermediates in zeolite catalysis

“…26,28,29,[32][33][34] After the cluster relaxation in OPT scheme, predicted shifts decrease the values to 49-51 ppm, which are close to the chemical shift of ''free'' methanol. 62 For BAS adsorbed DME, SP calculations predict a difference of about 4 ppm between the chemical shifts of the two carbon atoms caused by the differences in O-C interatomic distances in the optimized periodic zeolite models. The mean chemical shift values for each C atom are 68-69 ppm and 64-65 ppm.…”

Conversion of Methane into Liquid Fuels—Bridging Thermal Catalysis with Electrocatalysis

“…Two mechanisms of methane activation for homogenous [ 9 ] and heterogenous catalysis. [ 55 ] The dehydrogenation mechanism corresponds to a) the CH oxidation in homogenous catalysis and b) radical pathway in heterogenous catalysis. The deprotonation mechanism corresponds to c) CH activation in homogenous catalysis and d) surface‐stabilized M−CH 3 pathway in heterogenous catalysis.…”

“…The reaction mechanism of CuO‐based zeolites involves 1) CH 4 activation by hydrogen abstraction to form •CH 3 and •OH radicals, 2) radical rebound to form methanol, and 3) methanol desorption and catalyst regeneration by oxidants ( Figure 6 a). [ 55 ] Conventionally, the catalytic conversion of methane to methanol by Cu‐based catalysts uses N 2 O or H 2 O 2 as oxidants. To improve selectivity, a two‐step methane oxidation scheme was developed where Cu‐zeolites were activated by O 2 oxidation at a high temperature (450 °C), and subsequently reacted with methane at a milder temperature (200 °C) without O 2 to protect overoxidation of methanol.…”

“…Another recent mechanistic study using Cu‐ZSM‐5 zeolite clarified the reaction mechanism by electron paramagnetic resonance (EPR) and solid‐state NMR. [ 55 ] The authors illustrated evidence that methane is activated at the [CuOCu] 2+ site to form •CH 3 and •OH radicals. This led to the conclusion that the generated methanol was stabilized by the closed AlO moiety, which prevented the overoxidation of methanol.…”

“…Schematic representation of methane partial oxidation to methanol on Cu‐zeolites [ 55 ] and [(2,20‐bipyrimidine)PtCl 2 ] [ 42,83 ] with dehydrogenation and deprotonation mechanism, respectively. a) Cu‐zeolite catalyzed methane oxidation involving 1) CH activation and •CH 3 radical initiation, 2) radical rebound, and 3) methanol desorption and catalyst regeneration.…”