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Water dissociation and Brønsted acidity in rare earth (RE) form zeolitesтезисы доклада
- Авторы: Rybakov A.A., Larin A.V., Zhidomirov G.M.
- Сборник: Book of Abstracts of 13-th V.A. Fock Meeting on Quantum and Computational Chemistry
- Серия: V.A. Fock Meetings on Quantum and Computational Chemistry
- Тезисы
- Год издания: 2012
- Место издания: Евразийский Национальный Университет им. Л.Н. Гумилёва, Астана, Казахстан
- Первая страница: 90
- Аннотация: The Bronsted acidity of both AE and RE cationic form zeolites is usually explained by water dis-sociation at 2- or 3-valent cations, respectively: (q+)(H2O) + [Si-O-Al]- -> MeOH(q-1)+ + Si-O(H)-Al (1) However, theoretical justification of reaction (1) for any zeolite type and any cation remains uncertain. Recently, the authors have found that the AE cations cannot split water if two Al atoms are close enough one to another near the cationic site [A.V. Larin, A.A. Rybakov, G.M. Zhidomirov, J. Phys. Chem. C, DOI: 10.1021/jp205028c]. In this report, water dissociation is dis-cussed in the La cationic forms using both isolated cluster approach and the solution with imposed periodic boundary conditions. For tri-valent cations of RE metals the stabilization of products of water dissociation in cationic form zeolites was checked at both isolated cluster (8R or 6R+4R) and periodic (MOR type) levels. The isolated cluster approach was performed using GAUSSIAN03 at the B3LYP and MP2 levels using 6-31G*(Si, Al, O, H, Ca)/LANL2DZ(La) basis set. At the periodic level, projected-augmented wave (PAW) method and the PBE functional were used with VASP code. Both the 8R and 6R+4R clusters showed the higher stability of the dissociated form. The heats of water dissociation in four CaMOR and seven LaMOR models correspond to endo- and exothermic reactions, respectively. In the case of the closest Al location in model 3 of LaMOR we have succeeded to estimate the small barrier of 0.89 kcal/mol of water dissociation. The barrier is close to that (1.20 kcal/mol) for inverse reaction (1) in the model 0 of CaMOR. This distinction between the AE and RE forms allows justifying the idea of thermodynamic control for proton locations in the RE zeolites (due to deprotonation energy variations) while the proton transfer in the AE forms has to be limited kinetically (due to the barrier values between the attainable O-sites). This difference correlates with the remote location of OH groups relative to the cation in AE forms so that the vibrational OH band does not depend on the AE cation type (Mg, Ca, Sr) in MOR and Y types. The respective peaks of acid OH in LaY and CeY are shifted by 15 cm-1 being dependent on the cation type. The authors thank the FUNDP, the FNRS-FRFC, and the Loterie Nationale (convention 2.4578.02) for the use of the Namur iSCF Centre. The authors are grateful to Computer Complex SKIF "Chebyshev" of Moscow State University for computational time.
- Добавил в систему: Рыбаков Андрей Александрович