Vibrational spectroscopy of H2, N2, CO and NO adsorbed on H, Li, Na, K-exchanged ferrierite

The vibrational spectroscopy of H-2, N-2, CO and NO adsorbed on the zeolite ferrierite is described. The spectral modifications induced by the adsorption process on the protonic form are compared with those observed on the alkaline-exchanged forms. Different environments of Bronsted sites and of alkaline counterions in ferrierite are evidenced, which differ in the local electric field associated with the cationic species. These different environments are associated with the cation locations in the channels and cavities. In particular, ions located in the 10-membered ring channels are more available for the interaction with CO, N-2 and H-2, while ions in the cages form weaker adducts. On passing from lithium-, to sodium- and to potassium-exchanged samples; the local fields probed by H-2, N-2 and CO are increasingly dependent upon the distribution of the ions in the framework. In K-ferrierite the interaction cannot be solely described in terms of 1:1 K+/B (B=N-2, CO) adducts. As far as H-ferrierite is concerned, it is inferred from the observed shift of the v(OH) mode upon interaction with H-2, N-2 and CO that the bridged strong acid groups show an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta. (C) 2000 Elsevier Science B.V. All rights reserved.