Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV–Vis Spectroscopies

Abstract

Catalytic reaction systems for the direct conversion of methane to methanol have been previously developed using Cu zeolites. Among these materials, the Cu-CHA has been reported to show relatively high catalytic performance during the reaction of CH 4 −O 2 −H 2 O mixtures, although this catalytic activity varies with composition. In the present study, four Cu-CHA catalysts having different compositions and catalytic activity levels were prepared, and the redox properties and local structures of these specimens were analyzed using in situ X-ray absorption fine structure and UV−vis diffuse reflectance spectroscopies in conjunction with a CH 4 −O 2 −H 2 O reaction mixture. The relationships between the redox rates of the materials and catalytic activities (as reflected in the turnover frequency (TOF) during CH 4 oxidation) were assessed, and this analysis showed that the Cu-CHA reduction rate was highly correlated with activity. The data also suggested that the Cu 2+ reduction is associated with the activation of C−H bonds in CH 4 , which is the rate-determining step for the overall reaction. The effects of the local structure of the Cu-CHA on selectivity for CH 3 OH, the TOF value, and the Cu 2+ reduction rate were studied. Those Cu-CHA samples having high proportions of Cu 2+ ions coordinated to six-membered rings (Z 2 Cu) in the CHA framework exhibited higher selectivity than specimens having high proportions of [CuOH] + ions coordinated to eight-membered rings (ZCuOH), although the TOF values of the former Cu-CHA specimens were not higher than those of the latter because of the slower reduction rates of Cu 2+ species. The different catalyses can be attributed to the difference in Cu 2 O x active structures formed in the Z 2 Cu-rich samples and the ZCuOH-rich samples.

Bayesian-Optimization-Based Improvement of Cu-CHA Catalysts for Direct Partial Oxidation of CH4

“…The reason behind the composition-dependent catalytic performance of Cu-CHA is variation in the catalyst structure, including the porous structure of zeolites and the local structures of Cu active sites. − The specific surface area of all Cu-CHA catalysts is listed in Table S2. Cu-CHA catalysts showed a lower surface area than H-CHA due to Cu-ion exchange.…”

“…This is considered to be the reason for their low catalytic activity (Table S1). Meanwhile, the catalytic performance is also affected by the local structures of Cu species, which depends on the catalyst compositions. − Figure shows the Cu-IER-dependence of TON and selectivity for Cu-CHA with a Si/Al 2 ratio of 19.5. In this case, four kinds of Cu species are considered to be formed in CHA depending on Cu IER: Cu species with low TONs are mainly formed at Cu IER <10%; those with high TONs and selectivities are formed in the Cu IER range 10–20%; those having moderate TONs and low selectivities are formed in the Cu IER range 20–40%; and those having high TONs but low selectivities are formed in the Cu IER range >40%.…”

“…A TON of 2.9 represents a turnover frequency of 0.5 h −1 , which is lower than those reported previously. 9,10 This is probably due to the different pressures of the reaction gases. More importantly, Cu-CHA having a Cu IER of 14.3% and a Si/Al 2 ratio of 19.5 is superior in terms of TON and/or selectivity to the previously reported Cu-CHA catalysts (Cu-CHA having Cu IER of 39.8 and 48.4% at 10 of Si/Al 2 and those having Cu IER of 52 and 58.5% at 19.5 of Si/Al 2 in Table 1) 9,10 in the present reaction conditions.…”

“…3,7−10 Among such zeolites, Cu-CHA has been found to be particularly active; however, its CH 3 OH production is still very low, for example, a turnover frequency of 0.9 h −1 in terms of Cu active sites at 300 °C. 9,10 Therefore, improvement of catalytic activity is required for the practical application of direct partial oxidation of CH 4 .…”

“…To find highly active Cu-CHA catalysts, Cu-CHA catalysts with different compositions have been evaluated in previous studies. 7,9,10 However, identifying optimal compositions from this literature is difficult because catalytic activity and selectivity vary in a complex manner with the catalyst composition. 22 Nevertheless, if optimal Cu-CHA compositions could be identified, their active structures could be investigated, facilitating the design of more highly active catalysts for the direct partial oxidation of CH 4 .…”