Iron-based catalysts and zeolites were coupled in different ways to form a tandem catalyst. The symbol '|' indicated that the catalysts before and after the symbol were made into particles and placed in the upper and lower layers, and the two layers were separated by 0.5 cm quartz sand (Fe|3Z5). The symbol '/' indicated that the catalysts before and after the symbol were made into particles, and then were alternately filled in 6 layers (Fe/3Z5). The symbol '&' indicated that the two catalyst components were separately made into particles, then mixed evenly and placed in the same layer (Fe&3Z5). The symbol '*' represented that the two catalyst components were mixed in powder form and granulated into particles (Fe*3Z5).