Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N-2, increased as Cu loading increased. N-2 selectivities as high as 90-95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H-2 (H-2-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu 21 ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H2-TPR results suggested that in Cu-ZSM-5 with a Cut loading of 2.9 wt% and below, Cu+ existed even after oxidizing pretreatment. The activity of AN decomposition over CU/SiO2 suggested that CuO could form N2, but, independent of the CuO dispersion, nitrogen oxides (NO,) were formed above 350 degrees C. Cu' and the square-pyramidal and distorted square-pyramidal forms of Cu2+ showed low activity for AN decomposition. Temperature-programmed desorption of NH3 suggested that N2 formation from NH3 proceeded on Cu2+, resulting in the formation of Cu+. The Cu+ ions were oxidized to Cu2+ at around 300 degrees C. Thus, high N-2 selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu, which can be reversibly reduced and oxidized. (c) 2005 Elsevier B.V. All rights reserved.