INFRARED SPECTROSCOPIC STUDY OF THE ISOCYANATE SURFACE COMPLEX OVER CU-ZSM-5 CATALYSTS

The adsorption of HNCO on H-ZSM-5 and on Cu-containing ZSM-5, SiO2, and Al2O3 catalysts was investigated by means of Fourier transform infrared spectroscopy in the temperature range 200-673 K. HNCO readily adsorbs on ZSM-5 at 200-300 K to give an intense absorption band at 2260 cm(-1). This band is the asymmetric stretch of NCO bonded to Al3+. The formation of Si4+-NCO absorbing at 2300 cm(-1) requires a higher temperature, at least 373-473 K. The migration of NCO from Al3+ to Si4+ was observed. The location of absorption bands due to NCO bonded to Cu depended on the state of the Cu; it was at 2230-2240 cm(-1) for Cu-0-NCO, 2200-2210 cm(-1) for Cu+-NCO and 2180-2185 cm(-1) for Cu2+-NCO. The isocyanate on Cu metal was found to be more stable than that on Pt metals. The spillover of NCO from Cu to the support also occurred, but its extent was less than on the supported Pt metals. (C) 1995 Academic Press, Inc.