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+/Cu2+ 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 Cu2+ loading, it is also suggested that Cu2+ 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 Cu2+ and Cu+ ions, respectively. Cu ions in Cu-ZSM-5 samples can be reduced to Cu0 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 Cu0, Cu+, and Cu2+ sites, respectively. Methanol formation from syngas over Cu-ZSM-5 is observed at 250 °C and 2 MPa.