Investigation of the Active-Site Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Using In Situ UV–Vis Spectroscopy

Abstract

Among Cu-containing zeolites, Cu-CHA exhibits relatively high catalytic performance in direct oxidation of CH 4 to CH 3 OH in a continuous-flow reaction of a CH 4 −O 2 −H 2 O gas mixture. The catalytic activity of Cu-CHA varies with its composition, e.g., Cu loading and Si/Al ratio, suggesting formation of different Cu active-site structures depending on the composition. In the present study, to identify catalytically active structures of Cu species, 18 different Cu-CHA catalysts having three Si/Al ratios and six Cu loadings at each Si/Al ratio were prepared, and their catalytic performance was evaluated. Based on the catalytic performance, the Cu species in Cu-CHA catalysts were categorized into three groups: those having high selectivity but low catalytic activity were formed at a low Cu/Al ratio (group A); those having high selectivity and high catalytic activity for CH 3 OH production were formed at a medium Cu/Al ratio (group B); and those having high activity for the complete oxidation of CH 4 to CO 2 were formed at a high Cu/Al ratio (group C). Thus, three representative catalysts corresponding to the three groups were selected at each Si/Al ratio and analyzed using in situ UV−vis spectroscopy, from which the spectral features of Cu species involved in the catalytic reaction were extracted from the difference between the spectra under reduction conditions and those under oxidation conditions. Based on the comparison of the spectral features with the simulated spectra of various Cu active-site structures in the literature, the main species of the three groups were suggested: Cu 2+ species in the six-membered ring of SSZ-13 zeolites and water-coordinated CuOH + in the eight-membered ring of SSZ-13 zeolite for group A; mono-(μ-oxo) di-Cu species for group B; and μ-η2:η2 peroxide di-Cu, trans-μ-1,2-peroxo di-Cu and/or mono-/bis-(μ-hydroxo) di-Cu species for group C. Therefore, the mono-(μ-oxo) di-Cu species in Cu-CHA is proposed as the active species for the catalytic direct oxidation of CH 4 to CH 3 OH.

Structure and Reactivity of Binuclear Cu Active Sites in Cu-CHA Zeolites for Stoichiometric Partial Methane Oxidation to Methanol

“…Cu-exchanged zeolites facilitate partial methane oxidation (PMO) to methanol in stoichiometric − and continuous − cycles. After high-temperature oxidative treatments (typically with O 2 ), a variety of Cu structures have been proposed to form on zeolite supports, including mononuclear sites (Cu 2+ , [CuOH] + ), , binuclear sites (bis­(μ-oxo)­dicopper ([Cu­(O) 2 Cu] 2+ ), , mono­(μ-oxo)­dicopper ([CuOCu] 2+ ), ,− μ-(η 2 :η 2 )­peroxo dicopper ([CuO 2 Cu] 2+ ), , cis- and trans -μ-1,2-peroxo dicopper ([CuOOCu] 2+ ), ,, bis­(μ-hydroxyl)­dicopper ([Cu­(OH) 2 Cu] 2+ ), − trinuclear sites (mono­(μ-oxo)­tricopper), ,, and even higher-nuclearity sites (Scheme ). − …”

“…Brezicki et al concluded that mono-(μ-oxo)­dicopper sites were responsible for selective methane oxidation to methanol while trans -μ-1,2-peroxo dicopper sites led to overoxidation products (CO, CO 2 ) on Cu-MFI and Cu-MOR, based on evidence from UV–visible spectra, reaction stoichiometries consistent with the total oxygen content among products formed (per active Cu), and DFT calculations of the relative stabilities of various binuclear Cu sites in MOR . Tsuchimura and co-workers studied Cu-CHA of varying Cu/Al ratio, observed mononuclear and various binuclear species in UV–visible spectra, and concluded that a mono­(μ-oxo)­dicopper species present at intermediate Cu loadings had the highest rates for catalytic methane oxidation . Lercher and co-workers reported evidence for tris­(μ-oxo)­tricopper sites in Cu-MFI and Cu-MOR , based on CH 3 OH yields (per Cu) that correlated linearly with one-third of the total Cu content, supported with in situ EXAFS spectra.…”

Continuous selective conversion of methane to methanol over a Cu-KFI zeolite catalyst using a water–O2 mixture as the oxygen source

“…8,25 To date, researchers have made great efforts to solve the challenging issues in continuous MTM conversion, namely, how to improve the conversion rate of methane and the selectivity toward methanol? Following a report on the evidence for water as the oxygen source in MTM conversion by Sushkevich et al in 2017, 2 continuous MTM conversion using water 26,27 or a water–O 2 mixture 20,28,29 as the oxidants has garnered significant interest due to the large potential to improve methanol yield and selectivity. Recently, Sun et al 20 reported a continuous catalytic MTM oxidation reaction using Cu-CHA, Cu-MOR and Cu-MFI as the catalysts in the presence of 2% water and 400 ppm O 2 .…”

“…It was revealed that a rather low O 2 concentration (O 2 /CH 4 < 1/2000) is the key to improving the methanol selectivity and avoiding the overoxidation to CO 2 . Soon after that report, Tsuchimura et al 29 also studied the catalytic continuous MTM reaction over Cu-CHA using a CH 4 –H 2 O–O 2 mixture as the reaction gas and a high methanol yield of ∼0.9 mol mol Cu −1 h −1 with a selectivity of 88% was achieved. Researchers have made considerable progress in continuous MTM conversion using heterogeneous catalysis.…”

“…, [Cu 2 O] 2+ , [Cu 2 O 2 ] 2+ , [Cu 3 O 3 ] 2+ ) hosted in the zeolites are active in activating the methane C–H bonds, which have been widely reported as the dominant active species for MTM conversion. 19,29,39–41 Moreover, based on in situ experiments and DFT calculations, recent studies reveal that the Cu( ii )–OH sites confined on the 8MR and the isolated Cu 2+ sites on the 6MR of CHA are also potential active sites for the MTM reaction. 34,42,43 In the present work, increasing the Cu/Al ratio of Cu/KFI leads to an increase in the quantity of Cu x O y clusters and isolated Cu 2+ sites, and thereby improved MTM conversion activity was obtained.…”