Characterization of acidity in [B], [Al], and [Ga] isomorphously substituted ZSM‐5: Embedded DFT/UFF approach

Abstract

ABSTRACT:The structure and electronic properties of the Brønsted acid site in B, Al or Ga isomorphously substituted ZSM-5 zeolites were studied by using quantum cluster and embedded ONIOM approaches. In the former approach, zeolites are modeled by 5T and 12T quantum clusters, where T represents a Si or Al atom. In the latter model, called ''Embedded ONIOM'', the long-range interactions of the zeolite lattice beyond the 12T quantum cluster is included via optimized point charges added to the ONIOM(B3LYP/6-31G(d,p):UFF). Inclusion of the extended zeolitic framework covering the nanocavity has an effect on the structure and adsorption properties. We found that the OH distances and v OH of the acidic proton in zeolite obtained from both models can predict the trend of acid strength as: B-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5, which is in very good agreement with the experimental sequence. Furthermore, the PA data calculated from E-ONIOM is also consistent with the experimental trend: B-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5. It has, therefore, been demonstrated that our embedded ONIOM model provides accurate performance and can be one of the useful and affordable methods for future mechanistic studies involving petrochemical reactions.

A computational exploration into isomorphously substituted effects on hydrogen electric field gradient and chemical shielding tensors in the H‐ZSM‐5 zeolite

“…It can be compared that the calculated geometric parameters in this letter are in general agreement with previous computational chemical studies concerning on the isomorphism substitution H‐ZSM‐5 zeolite . From the comparison of the calculated bond lengths on the M‐8TH cluster models, it is observed that O–H distance of the H‐ZSM‐5 framework increases in the order B‐ZSM‐5 < Ga‐ZSM‐5 < Al‐ZSM‐5.…”

“…T represents a general tetrahedral unit including one silicon atom or M (M = B, Al, and Ga) bound to four other atoms (oxygen and/or hydrogen). M atom is an occupied T 12 site in the ZSM‐5 zeolite framework because it has been found at the intersection of the straight and sinusoidal channels, accessible to the interaction between bridging hydroxyl group and guest molecules as a catalytic active site . The proton attached to oxygen atom in the center of cluster models represents the Brönsted acid site.…”

“…In the earlier work, comparison of acidity of the isomorphously substituted ZSM‐5 zeolites has been investigated by absorption with a probe molecule such as methanol or ammonia (see Yuan et al . and Jungsuttiwong et al …”

Origin of Hofmeister Effects for Complex Systems

“…The excessive charges of kaolinite are mainly due to the deprotonation/protonation of the hydroxyl groups, which are dependent on the pH values of aqueous solutions [ 45 – 47 ]. In accord with the previous studies of aluminosilicates [ 48 – 51 ], the kaolinite cluster models were divided into two regions and simulated at different theoretical levels. The hexagonal ring of silica surface may interact directly with cations [ 44 ] and relating O and Si atoms were selected as the high-level region.…”

New trends in tailoring active sites in zeolite-based catalysts

“…20 The order of acidity is reproduced for isomorphously substituted MFI zeolites independently on the zeolite model size, while the absolute values of DPE depend on structural models, functionals, and basis sets used. 21 The DPE values for B-, Ga-, and Al-containing extra-large pore UTL zeolites calculated at the density functional theory (DFT) level using the local density approximation (LDA) and the generalized gradient approximation (GGA) were recently shown to decrease in the following sequence: B-UTL > Ga-UTL > Al-UTL. 22 Noticeably, based on the adsorption energies of NH3, the acid strength of Al-UTL was concluded to be comparable with that of Al-MOR.…”