Determination and properties of acid sites in H-ferrierite - A comparison of ferrierite and MFI structures

FTIR spectra of OH groups in the stretching region and adsorbed d(3)-acetonitrile and pyridine on H-ferrierite and for comparison on H-ZSM-5 were investigated. A lower stability of bridging hydroxyls of H-ferrierite (3604 cm(-1)) with Si/Al 8.4 and its framework structure compared to those of H-ZSM-5 (3612 cm(-1)) with Si/Al 12.6 was found. IR spectra of OH groups and of adsorbed d(3)-acetonitrile and pyridine on H-ferrierite and H-ZSM-5 treated in vacuum at 670 K showed that pyridine adsorption in ferrierite takes place only on a part of the OH groups, while the smaller d(3)-acetonitrile molecule is suitable for quantitative determination of the number of Bronsted and Lewis sites in both zeolite structures, d(3)-Acetonitrile adsorption on H-ferrierite yielded a band at 2297 cm(-1) corresponding to the interaction with Bronsted sites, and two types of Lewis sites, strong and weak, were reflected in bands of adsorbed acetonitrile at 2325 and 2310 cm(-1), respectively, both sites originating from dehydroxylation of protonic sites. For quantitative analysis of the acid sites, integral intensities of the IR bands were used in corroboration with the aluminium content in the zeolites, assuming that during dehydroxylation two hydroxyls yield one Lewis site. Using this approach, extinction coefficients were calculated for bridging OH groups, nu(OH) at 3604 cm(-1), epsilon(OH) = 4.05 +/- 0.2 cm mu mol(-1), d(3)-acetonitrile adsorbed on Bronsted sites, nu(C=N)-B at 2297 cm(-1), epsilon(B) = 2.05 +/- 0.1 cm mu mol(-1), and on strong Lewis sites nu(C=N)-L-1 at 2325 cm(-1) or weak Lewis sites nu(C=N)-L-2 at 2310 cm(-1), epsilon(L) = 3.6 +/- 0.2 cm mu mol(-1). (C) 1998 Elsevier Science B.V. All rights reserved.