2.1 Catalyst preparation
The Cu-based catalysts were synthesized using both ion exchange and wetness impregnation methods, as illustrated in Supporting Information (SI, section 1, Scheme S1). Commercial MOR zeolite (purchased from Nankai University Catalyst Co., Ltd.) was calcined in a muffle furnace at 400 °C for 5 hours. Cu(CH3COO)2·H2O and Cu(NO3)2·3H2O (provided by Shanghai Aladdin Biotechnology Co.) were used as the copper sources for ion exchange and wetness impregnation, respectively. For the ion exchange method, 10 g MOR zeolite was added to 28 ml Cu(CH3COO)2·H2O solution (0.4molCu/l), stirred, and placed in a 90 °C water bath for 2 hours. Then, the samples were filtered, washed and dried overnight at 100 °C. Subsequently, the dried samples were calcined in a muffle furnace at 540 °C for 5 hours, yielding the ion-exchanged Cu-MOR, designated as Cu-MOR IE-1. To further enhance the Cu loading, various cycles of ion exchange (2, 3, 4, and 5) were used to synthesize Cu-MOR catalysts based on the Cu-MOR IE-1 sample. These samples, prepared through multiple ion exchanges, are denoted as Cu-MOR IE-2, Cu-MOR IE-3, Cu-MOR IE-4 and Cu-MOR IE-5. The wetness impregnation method involved dissolving Cu(NO3)2·3H2O in deionized water, adding MOR zeolite to the solution with calculated loading, and stirring intensively. After aging at room temperature for 12 hours, the samples were dried at 120 °C overnight and calcined in a muffle furnace at 540 °C for 5 hours. Finally, the obtained samples are denoted as X wt.% Cu-MOR (X = 2, 5, 10, 15, and 20) based on different Cu loadings.