Theoretical Study of 4,4′-Bipyridine Adsorption on the Bronsted Acid Sites of H-ZSM-5 Zeolite

The adsorption of 4X-bipyridine (44BPY) on the Bronsted acid sites of H-ZSM-5 zeolite is investigated by density functional calculations, at the B3LYP level, using two basis sets. The H-ZSM-5 straight channel is simulated by two 10-membered ring clusters (18Si, 2Al), saturated by hydrogen atoms (2-10T). Two different aluminum distributions were employed to study this adsorption. Because of the size of the systems considered, this study represents a significant challenge to quantum chemical methods if enough accuracy is required. The energetic and vibrational results for the species formed upon concerted interaction of bidentate 44BPY ligand via its both nitrogen lone pairs with the Bronsted acid sites in a straight channel of H-ZSM-5 zeolite are presented. The potential energy surface for double proton transfer from the zeolite (Z) to 44BPY shows only one minimum corresponding to the formation of a bidentate ion-pair complex (44BPYH(2)(2+)/Z(2-)). The other bidentate complexes having either neutral (44BPY/Z) or monoprotonated (44BPYH(+)/Z(-)) hydrogen-bonded structures are not stable, in agreement with a recent Raman spectrometry study reported for 44BPY occluded in acidic H-ZSM-5 zeolite in which the diprotonated dication 44BPYH(2)(2+) appears as the unique species observed. The calculated frequencies and frequency shifts of the adsorbed 44BPY are in good agreement with the available experimental data. The interactions responsible for the vibrational properties are essentially local. The symmetric ring stretching mode 8a of 44BPY in the 1400-1600 cm(-1) region is the most affected one upon adsorption as observed in Raman spectra.