Probing distance effect between Cu and acid sites in zeolite catalysts for oxidation of methane

Abstract

Abstract Cu-exchanged aluminosilicate zeolite catalysts have widely been applied in direct oxidation of methane to methanol (DMTM). The Bronsted acid sites (BAS) may cause the tandem reaction of methanol to hydrocarbons (MTH). Unraveling the distance effect of bifunctional sites in tandem catalysis was particularly significant and challenging. However, it has not been well explored in direct oxidation of methane so far. Here, we probed the distance effect between Cu and BAS in Cu/AEI zeolite catalysis on the reaction performance of direct oxidation of methane. The distance between Cu and BAS was regulated by varying the concentrations of Cu(NO3)2 solution in the Cu loading process by ion-exchange method. It was revealed that the close distance between Cu and BAS was obtained in the low Cu density, which was beneficial for the formation of hydrocarbons. The close distance between Cu and Cu sites was achieved in the high Cu density, which was prone to generate CO and CO2. The equilibrium distance between Cu and BAS was favorable for the stable production of methanol. Moreover, a physical mixture of Cu/AEI zeolite and acidic zeolites in different integration manners also revealed that the close distance between Cu and BAS was advantageous for the formation of hydrocarbons. Noteworthy, the product distribution and formation rate were highly dependent on the acidic properties and structure of acidic zeolite. This work would provide guidance for designing efficient catalysts to prevent methanol from reacting and thus maintain high methanol yield, as well as open up a new avenue for direct oxidation of methane to hydrocarbons (DMTH) via methanol intermediate at low temperatures.