Acidity Differences between Inorganic Solids Induced by Their Framework Structure. A Combined Quantum Mechanics/Molecular Mechanics ab Initio Study on Zeolites

Abstract

The influence of the zeolite framework type (FAU, CHA, MOR, MFI) and the crystallographic position on the acidity of zeolites is investigated. The most stable Brønsted acid sites of the high-silica frameworks are considered: O1-H (FAU), O1-H (CHA), Al4-O2(H)-Si (MOR), and Al7-O17(H)-Si4 (MFI, sinusoidal channel). The latter is compared with the less stable Al12-O24(H)-Si12 position (MFI, channel intersection). Both the heat of the deprotonation and the heat of ammonia adsorption are considered as measures of acid strength. A novel hybrid computational scheme is used that combines the quantum mechanical cluster description (QM) of the active site with interatomic potentials (Pot) for the periodic zeolite framework. Specifically, the Hartree-Fock method (QM) is combined with ab initio shell model potentials (Pot) for the zeolite framework and its interaction with ammonia and ammonium ions. Complete relaxation of the framework is possible within this scheme and long-range corrections to the reaction energies are obtained from the shell model potentials. The total QM-Pot reaction energies are remarkably stable with increasing cluster size. The calculated heats of deprotonation suggest the acidity sequence Y (1171 kJ/mol) > CHA (1190 kJ/mol) > MOR (1195 kJ/mol) > ZSM-5 (1200 kJ/mol), which is neither explained by local structure effects nor by crystal potential effects alone. The calculated heats of NH 3 adsorption suggest the sequence MOR > CHA ≈ Y > ZSM-5. The different order is caused by specific interactions of NH 4 + with the negatively charged catalyst surface. The predicted heats of NH 3 adsorption are -119, -114, -113, and -109 kJ/mol, respectively. Comparison is made with microcalorimetry and TPD data.

DFT Cluster Modeling of Molecular and Dissociative Hydrogen Adsorption on Zn2+Ions with Distant Placing of Aluminum in the Framework of High-Silica Zeolites

“…As known to us, investigations were performed in the finite cluster approximation similar to that used in this work (see [22,23] and references herein) or in the combined quantum mechanics-interatomic potential function approach (QM-Pot) [24,25]. It was also shown earlier [26] that consideration of whole extended zeolite lattice is necessary for the correct explanation of deprotonation energy for different zeolites. According to reference [25], the long-range correction to the activation barriers for intersite proton motion in H-ZSM-5 zeolite with one aluminum per unit cell varies from À6:4 to þ7:1 kcal=mol, while the same correction for on-site motion (proton jumps between oxygen atoms in the first coordination of an aluminum atom) was reported to be equal to þ18:8 kcal=mol.…”

Structure and function of oxide nanostructures: catalytic consequences of size and composition

“…35 ). 36 The higher DPE of Brønsted acid sites in H-BEA led to lower values of the rate constant of the elimination step (Fig. 10) than on POM clusters.…”

Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments

“…117,363 In the latter case the silica hydrogarnet in the all- The mechanical embedding scheme, in its QMPot implementation as proposed by Sauer and coworkers, 364 has reached a high popularity in the study of Brönsted sites (both acidic and cation exchanged ones) in zeolites. 365,366 More recently, thanks to the increased computing power, the method has been extended to adopt DFT to treat the real system within the PBC. In the applications, PBE functional and plane waves as implemented in the VASP code have been adopted as the low level of calculus.…”