Infrared studies of the interaction of carbon monoxide and dinitrogen with ferrisilicate MFI-type zeolites

Fourier transform IR spectroscopy of CO and N-2, adsorbed at liquid nitrogen temperature, was used to characterize an MFI-type H-FeZSM-5 ferrisilicate which was synthesized with a Si/Fe ratio of 50. Thermal treatment of this material at 773 and 973 K was performed in order to follow formation of extraframework species. On samples fired at 773 K Bronsted acid sites were present which gave an O-H stretching band at 3630 cm(-1). These hydroxyl groups formed adducts with both probe molecules, which were monitored by the corresponding bathochromic shift, Delta nu. Corresponding values for adsorbed CO and N-2 were Delta nu = -270 and Delta nu = -100, respectively. Increasing the firing temperature up to 973 K led to complete removal of iron from the zeolite framework, and consequent disappearance of Bronsted acidity. In this process, extraframework iron oxide species were formed which were also characterized by IR spectroscopy of the adsorbed probe molecules. Both CO and N-2 gave Lewis-type adducts with coordinatively unsaturated Fe3+ ions present in the extraframework material. A comparison is made with results of a previous study on the H-GaZSM-5 isomorph.