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

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropio nitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile an d benzene over different H-ZSM5 zeolite samples was investigated by FTIR sp ectroscopy. PN does not enter the cavities of MFI structures at room temper ature 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 iso butene produced by their decomposition, easily enters and polymerizes in th e internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavi ties. 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 acces s of single molecules can be influenced by other molecules in a mixture. Th e experiments also confirmed that two kinds of terminal silanol groups, ind istinguishable from the point of view of the OH stretching band but differi ng in their Bronsted acidity, are located at the external surface of the ZS M5 zeolite crystals. Additionally, it was established that the external ZSM 5 zeolite surface, even for low Al content samples, contains two types of L ewis acid sites. Conversely, no evidence was found for the presence of brid ging Si-(OH)-Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging "zeolitic'' Bronst ed 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 sil icon in a silica framework.