Incorporation of various heterometal atoms in CHA zeolites by hydrothermal conversion of FAU zeolite and their performance for selective catalytic reduction of NOx with ammonia

Abstract

Heterometal atom incorporation in CHA aluminosilicate zeolites ([Al, M]-CHA, M = Fe, Ga, Sn) was successfully achieved by hydrothermal conversion of heterometal-incorporated FAU aluminosilicates ([Al, M]-FAU). X-ray powder diffraction (XRD), scanning electron microscopy, diffuse reflectance UV–vis spectroscopy, magic angle spinning NMR, and nitrogen adsorption measurements confirmed the formation of [Al, M]-CHA zeolites with heterometal atoms occupying homogeneously distributed tetrahedral coordination sites. The choice of hydrothermal conversion using [Al, M]-FAU was prompted by the inability to produce [Al, M]-CHA zeolites from amorphous hydrogels. The hydrothermal conversion of [Al, Fe]-FAU to [Al, Fe]-CHA was characterized by XRD, electrospray ionization mass spectrometry, and UV–vis spectroscopy. Analyses of liquid and solid phases during synthesis indicated that metal species present in the solid phase play an important role in the transformation process. We also investigated the effectiveness of Cu-loaded CHA zeolites for the selective catalytic reduction (SCR) of NOx by ammonia (NH3-SCR). Catalytic performance depended strongly on the kind and/or amount of heterometal atom in the [Al, M]-CHA zeolite. Although all fresh catalysts exhibited similar NO conversion efficiencies, there was a difference in NH3 conversion effectiveness at low reaction temperatures. Cu-loaded [Al, Ga]-CHA provided almost 100% NH3 conversion at 150 °C. The [Al, Sn]-CHA catalyst exhibited high stability even after hydrothermal treatment at 900 °C for 4 h. These results confirm hydrothermal conversion as an effective method for synthesizing heterometal-incorporated zeolite catalysts with high NH3-SCR performance.