An FT-IR study of the internal and external surfaces of HZSM5 zeolite

Abstract The interaction of HZSM5 zeolite with n-butane, iso-butane, n-heptane, 1-butene, cis-2-butene, trans-2-butene, iso-butene, benzene, toluene, ortho-xylene, para-xylene, acetonitrile and pivalonitrile has been investigated using FT-IR spectroscopy. These experiments allowed to characterize the hydrogen-bonded species of all adsorbates both with the internal OHs of HZSM5 and with the external OHs. ortho-Xylene and pivalonitrile are unable to enter the zeolite cavities in the given conditions, so that they only interact with the external sites. The other molecules interact with both internal and external sites. However, it is clear that the hindered aromatic molecules do not fully reach the internal sites, part of them being unperturbed. H-bonds apparently involve C–C σ-type orbitals of alkanes and CC π-type orbitals of alkenes and aromatics. The external OHs of ZSM5 are clearly associated with a band near 3746 cm−1, so are certainly terminal. In spite of being terminal, at least part of them are significantly acidic too, nearly like those of silica/aluminas and definitely more than the silanols of pure silica. The internal OHs (3618 cm−1) are more acidic and bridging. Internal defects are characterized by terminal silanols absorbing at 3730 cm−1.