Methane-to-methanol conversion over zeolite Cu-SSZ-13, and its comparison with the selective catalytic reduction of NOx with NH3†
The direct conversion of methane into methanol is considered as one of the holy grails in hydrocarbon chemistry and recently it was found that small pore zeolites, such as Cu-SSZ-13, Cu-SSZ-16 and Cu-SSZ-39, are active for this process. Here, we propose a reaction mechanism based on spectroscopic evidence for the methane-to-methanol reaction over Cu-SSZ-13 (Si/Al = 20). Using in situ FT-IR and operando UV-vis-NIR DRS, performed on a series of different Cu–ion-exchanged SSZ-13 zeolites, both a mono-nuclear site or a dimeric copper active site are consistent with the observations of this study. These proposed active site(s) are characterized by a νOH at ∼3654 cm−1 and a charge transfer (CT) transition at ∼29 000 cm−1. We have further evidence to complete the full catalytic cycle to methanol, including the formation of the reaction intermediate Cu(CH3)(H2O), which is characterized by overtone transitions, i.e., a 2νCH at ∼4200 cm−1 and a 2νOH at ∼5248 cm−1. We found that increasing the pre-oxidation temperature from 450 °C to 550 °C resulted in a 15% increase in methanol production, as well as a concomitant increase of the 29 000 cm−1 CT transition. Furthermore, Cu-exchanged SSZ-13 zeolites, which perform well in the NH3-SCR reaction at 200 °C (the low temperature regime), also show a high activity in the methane-to-methanol reaction and vice versa, leading us to believe that this material has a similar if not the same active site for both the catalytic reduction of NO and the stepwise reaction towards methanol.