A study of the external and internal sites of MFI-type zeolitic materials through the FT-IR investigation of the adsorption of nitriles Original Research Article

The adsorption of acetonitrile (AN) and pivalonitrile (2,2-dimethylpropionitrile, PN) has been investigated on pure silicalite S1, on a ZSM5 zeolite apparently free from extraframework material and another ZSM5 zeolite apparently rich in extraframework material, on a titanium silicalite TS1 and on a boralite sample. AN enters the cavities of MFI type structures and interacts with both internal and external acid sites while PN does not enter such cavities and only interacts with the external sites. Terminal silanols, whose acidity can vary from weak to medium, are present in all cases mostly at the external surface, while very acidic bridging Si–OH–Al sites of ZSM5 zeolite are apparently exclusively located at the internal channel surface. Lewis acidic Ti cations are evident both in the internal and on the external surface of TS1. Weakly acidic BOH sites of boralite are distributed among internal and external surface. Extraframework alumina-like species of ZSM5 zeolite are apparently located in the internal cavities. Terminal silanols and hydrolized defects are located mostly at the external surface of silicalite. The mechanism of incorporation and charge balance of boron atoms as well as the structure of the hydroxy groups of boralite are still not well established.