Role of Distant Al Atoms in Alkaline Earth Zeolites for Stabilization of Hydroxyl Groups

The traditional point of view about water cleavage at divalent cations in alkaline earth (AE) mordenite (MOR) and faujasite (FAU) zeolites was not supported by energy analysis at both the isolated cluster level and the computations with periodic conditions if the cation is located in the vicinity of both Al atoms. A new hypothesis about the kinetic (and not thermodynamic) stabilization of hydroxyl groups near one noncompensated distant Al atom is verified theoretically to explain the acidic nature of the AE cationic MOR and FAU forms. This model allows the development of qualitative interpretation of the temperature variations in the IR spectra upon hydration, dehydration, and rehydration

Role of Distant Al Atoms in Alkaline Earth Zeolites for Stabilization of Hydroxyl Groups

The traditional point of view about water cleavage at divalent cations in alkaline earth (AE) mordenite (MOR) and faujasite (FAU) zeolites was not supported by energy analysis at both the isolated cluster level and the computations with periodic conditions if the cation is located in the vicinity of both Al atoms. A new hypothesis about the kinetic (and not thermodynamic) stabilization of hydroxyl groups near one noncompensated distant Al atom is verified theoretically to explain the acidic nature of the AE cationic MOR and FAU forms. This model allows the development of qualitative interpretation of the temperature variations in the IR spectra upon hydration, dehydration, and rehydration

Role of Distant Al Atoms in Alkaline Earth Zeolites for Stabilization of Hydroxyl Groups

The traditional point of view about water cleavage at divalent cations in alkaline earth (AE) mordenite (MOR) and faujasite (FAU) zeolites was not supported by energy analysis at both the isolated cluster level and the computations with periodic conditions if the cation is located in the vicinity of both Al atoms. A new hypothesis about the kinetic (and not thermodynamic) stabilization of hydroxyl groups near one noncompensated distant Al atom is verified theoretically to explain the acidic nature of the AE cationic MOR and FAU forms. This model allows the development of qualitative interpretation of the temperature variations in the IR spectra upon hydration, dehydration, and rehydration

Computational Differentiation of Brønsted Acidity Induced by Alkaline Earth or Rare Earth Cations in Zeolites

For bi- and trivalent Me^<i>q</i>+ (Me = metal) cations of alkaline earth (AE) and rare earth (RE) metals, respectively, the formation of the nonacid MeOH^{(<i>q</i>‑1)+} species and acid H–O_zeo group, where O_zeo is the framework atom, from water adsorbed at the multivalent Me^<i>q</i>+(H₂O) cation in cationic form zeolites was checked at both isolated cluster (8R or 6R + 4R) and periodic (the mordenite framework) levels. Both approaches demonstrate qualitative differences for the stability of the dissociated water between the two classes of industrial cationic forms if two Al atoms are closely located. The RE forms split water while the AE ones do not, that can be a basis of different proton transfer in the RE zeolites (thermodynamic control) than in the AE forms (kinetic control). The cluster models allow quantitatively explaining nearly equal intensities <i>I</i>_HF ∼ <i>I</i>_LF of the high frequency (HF) and low frequency (LF) OH vibrations in the RE forms and lowered <i>I</i>_HF ≪ <i>I</i>_LF in the AE forms, where HF bands are assigned to the Me–OH groups in the RE and AE forms, respectively, while LF bands are assigned to the Si–O­(H)–Al groups. The role of electrostatic terms for water dissociation in the RE and AE forms is discussed

Computational Differentiation of Brønsted Acidity Induced by Alkaline Earth or Rare Earth Cations in Zeolites

For bi- and trivalent Me^<i>q</i>+ (Me = metal) cations of alkaline earth (AE) and rare earth (RE) metals, respectively, the formation of the nonacid MeOH^{(<i>q</i>‑1)+} species and acid H–O_zeo group, where O_zeo is the framework atom, from water adsorbed at the multivalent Me^<i>q</i>+(H₂O) cation in cationic form zeolites was checked at both isolated cluster (8R or 6R + 4R) and periodic (the mordenite framework) levels. Both approaches demonstrate qualitative differences for the stability of the dissociated water between the two classes of industrial cationic forms if two Al atoms are closely located. The RE forms split water while the AE ones do not, that can be a basis of different proton transfer in the RE zeolites (thermodynamic control) than in the AE forms (kinetic control). The cluster models allow quantitatively explaining nearly equal intensities <i>I</i>_HF ∼ <i>I</i>_LF of the high frequency (HF) and low frequency (LF) OH vibrations in the RE forms and lowered <i>I</i>_HF ≪ <i>I</i>_LF in the AE forms, where HF bands are assigned to the Me–OH groups in the RE and AE forms, respectively, while LF bands are assigned to the Si–O­(H)–Al groups. The role of electrostatic terms for water dissociation in the RE and AE forms is discussed