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

The interaction of HZSM5 zeolite with n-butane, iso-butane, n-heptane, 1-bu tene, cis-2-butene, trans-2-butene, iso-butene, benzene, toluene, ortho-xyl ene, para-xylene, acetonitrile and pivalonitrile has been investigated usin g FT-IR spectroscopy. These experiments allowed to characterize the hydroge n-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 t he zeolite cavities in the given conditions, so that they only interact wit h the external sites. The other molecules interact with both internal and e xternal sites. However, it is clear that the hindered aromatic molecules do not fully reach the internal sites, part of them being unperturbed. H-bond s apparently involve C-C sigma-type orbitals of alkanes and C=C pi-type orb itals of alkenes and aromatics. The external OHs of ZSM5 are clearly associ ated with a band near 3746 cm(-1), so are certainly terminal. In spite of b eing terminal, at least part of them are significantly acidic too, nearly l ike those of silica/aluminas and definitely more than the silanols of pure silica. The internal OHs (3618 cm(-1)) are more acidic and bridging. Intern al defects are characterized by terminal silanols absorbing at 3730 cm(-1). (C) 2000 Elsevier Science B.V. All rights reserved.