On the Lewis acidity of protonic zeolites

Abstract

IR spectra of hydroxyl groups, adsorbed CO, pivalonitrile and pyridine on three H-MFI zeolite samples and on two H-Y faujasites are reported and discussed. Samples richer in Al (H-MFI (Si/Al2 = 30) and H-Y (Si/Al2 = 5.1)) show the presence of extraframework species and the presence of Lewis acidity together with Brønsted acidity. H-MFI with lowest Al content (Si/Al2 = 280) does not show any extraframework species (EF) and only presents Brønsted acidity. H-MFI with intermediate Al content (Si/Al2 = 50) possess very small amount of EF species and of Lewis acidity. H-Y with low Al content (Si/Al2 = 30) does not show extraframework species but shows the presence of Lewis acidity together with Brønsted acidity. The role of extraframework material as carrier of Lewis acidity is confirmed. It is proposed that Lewis acidity of low Al-content H-Y can arise from framework tetrahedral Al ions, which can enlarge their coordination to five without any previous dehydroxylation. A support for this hypothesis is given by the reversible shift of the LF OH stretching band, whose extent depends on the strength of the basic molecules: this is certainly not due to a direct interaction of the OH groups responsible for the LF band, which are located in cavities (sodalite cavity and hexagonal prisms) where the molecular probes cannot access

Ethylene production via catalytic dehydration of diluted bioethanol: A step towards an integrated biorefinery

“…2 from room temperature up to 500°C, leading to the disappearance of the bands due to H-bound pyridine (1597 cm -1 ) and better evidencing the components at 1622 cm -1 assigned to Py coordinated over Lewis acidic Al 3+ ions, quite similar to those typical of alumina and silicaalumina and also frequently reported for zeolites having EF contributions [54]. The component at 1606 cm -1 could be assigned to pyridine coordinated on weaker sites and decreases quite quickly upon outgassing at increasing temperatures.…”

“…bands above 1600 cm -1 and at 1455 cm -1 ) could still be detected in the dealuminated samples, in spite of the absence of EF Al atoms. This effect has been discussed by Phung et al [54] for HY zeolite, in which Lewis acidity arises also from framework Al ions enlarging their coordination state.…”

“…On the other side, the overall intensity of the pyridine bands, compared for all samples in the subtraction spectra recorded after outgassing at 200°C is slightly reduced for sample 8 in comparison with spectra of catalyst 1, because of the absence of EF Al sites. Moreover, the relative intensities of bands typical of pyridinium ion and molecular coordinated pyridine (for instance bands at 1635 and 1620 cm -1 [54]) which is a rough indication of the acidic character of these zeolites, is slightly higher for sample 8 than , readily disappearing after outgassing. These are assigned to weakly bound pyridine, possibly H-bound to silanol, whose stretching peaks appear as negative bands in the subtraction spectra (not reported) [46].…”

Tuning of product selectivity in the conversion of ethanol to hydrocarbons over H-ZSM-5 based zeolite catalysts

“…The features of adsorbed molecular pyridine (i.e. the 19b band at 1451-1448 cm −1 , together with 8a bands in the 1615-1595 cm −1 region) show the presence of significant amounts of Lewis acidity on the Z(700) sample, attributed to extraframework Alcontaining species [37], on Fe1PZ and, more intense, on Ni1PZ. In contrary, these bands are very weak, if at all, on 1PZ and 6PZ samples.…”

Zeolite Catalysts for Phenol Benzoylation with Benzoic Acid: Exploring the Synthesis of Hydroxybenzophenones

“…In regard to the acidity of H-Y, it is reported in the literature that Lewis acidity may also play an important role, even for samples having low Al content (see, for instance [103], and the references therein).…”