Copper Sites in Copper-Exchanged ZSM-5 for CO Activation and Methanol Synthesis:  XPS and FTIR Studies

Abstract

Cu-ZSM-5 samples with various Cu(+)/Cu(2+) ratios have been successfully prepared in three different methods and studied by XPS and FTIR. Cu(+) ions are found, by XPS, to mainly exist in the samples prepared by solution ion exchange and microwave solid-state reaction exchange. On the basis of the chemical shift of the Al 2p core level to a higher binding energy [Al(species II)], with the atomic ratio of Cu(+)/Al(species II) being approximately one, and the vanishing of the IR band at 3640 cm(-)(1) due to copper exchange, it is suggested that Cu(+) ions locate in the open channels of the ZSM-5 framework and bind to mixed (Si- and Al-) bridged oxide ion. From the ZSM-5 pore structure consideration, as well as from the observed decrease in the IR intensity ratio of bands 550/450 cm(-)(1) with enhanced Cu(2+) loading, it is also suggested that Cu(2+) ions are a predominant Cu species in solid-state reaction exchanged Cu-ZSM-5 samples and exist as a CuO cluster in small cages as well as in open channels. The IR bands at 907 and 964 cm(-)(1) are identified as zeolite asymmetric internal vibration perturbed by exchanged Cu(2+) and Cu(+) ions, respectively. Cu ions in Cu-ZSM-5 samples can be reduced to Cu(0) by CO adsorption at room temperature. The IR bands at 2128, 2158, and 2176 cm(-)(1) are observable and ascribed to the CO adsorbed on Cu(0), Cu(+), and Cu(2+) sites, respectively. Methanol formation from syngas over Cu-ZSM-5 is observed at 250 degrees C and 2 MPa.