FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite

Abstract

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0–25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Brønsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si–(OH)–Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging “ zeolitic” Brønsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework.