Zeolites at the Molecular Level: What Can Be Learned from Molecular Modeling

Abstract

This review puts the development of molecular modeling methods in the context of their applications to zeolitic active sites. We attempt to highlight the utmost necessity of close cooperation between theory and experiment, resulting both in advances in computational methods and in progress in experimental techniques.

Keywords: Brønsted and Lewis acid sites; DFT; electron density distribution; molecular modeling; multiple bond activation; transition metal sites; wave function methods; zeolite.

Figure 1

The basics of the structural chemistry of zeolites explained: zeolites are crystalline, microporous tectoaluminosilicates, wherein Si and Al atoms, collectively referred to as ‘T atoms’, are tetrahedrally coordinated by O atoms. These TO₄ units, connected by their apices (i.e., O atoms), form a three-dimensional network (left panel). Viewing the lattice as an ionic solid, the replacement of Si⁴⁺ ions by Al³⁺ ones creates an excess of negative charge, which is compensated by extraframework cations M⁺ (e.g., H⁺, NH₄⁺, metal cations).

Figure 2

View of the lattices of zeolites (a) MFI and (b) FAU, with the main Cu(I) sites depicted (terminology for Cu(I) sites in MFI taken from Nachtigallová et al. 1999 [16]). The colors used for the atoms are: Cu—orange, Al—grey, Si—blue, O—red.

Figure 3

The optimized structures of the QMPot models used for the modeling of CO adsorption on Cu(I) exchanged FAU, the embedded cluster (QM level) depicted with balls and sticks, the remaining unit cell contents (MM level) with lines: (a) the models of site II (12T cluster), the equilibrium 3(3) Cu(I)–lattice coordination is switched to 2(1) upon CO adsorption; (b) site III model (8T cluster), here, the Cu(I) binding mode is 2(2) before and after CO adsorption. m(n) coordination denotes the Cu(I) ion bound to the m lattice O atoms belonging to n distinct tetrahedra. The following colors were used for the atoms: Cu—orange, Al—grey, Si—blue, O—red, H—white, C—black.

Figure 4

The Cu(I) siting and its change upon adsorption of one and two NO molecules, according to QMPot modeling (only the nearest neighborhood of Cu(I) ions shown, all distances in Å): (a–c) site II in FAU with 1, 2 and 3 Al/6T rings, (d) site III in FAU and (e) Z6 site in MFI. The colors used for the atoms are: Cu—orange, Al—grey, Si—blue, O—red, H—white, N—dark blue.

Figure 5

CO adsorption on Brønsted site In MAZ zeolites, as predicted by periodic DFT modeling. TxOy denotes Brønsted acid sites with Al occupying the Tx crystallographic position and the protonated O atom situated in y crystallographic sites (the MAZ lattice with distinct T and O crystallographic positions is shown in the upper insert). The formation of stable carbonyls is likely due to Brønsted sites compromising their intrinsic stability and the strength of CO binding. The proposed assignment of experimental OH bands is as follows: (1) the more red-shifted band is due to the more stable carbonyls in the large 12T cavity (solid red frame), and (2) the less red-shifted band is due to the less stable carbonyls in the more confined 8T cavity (dotted red frame). The colors for the atoms are: Al—grey, Si—blue, O—red, H—white, C—black.

Figure 6

The results of ETS-NOCV/PBE-D3(BJ)/ZORA/TZP energy decomposition describing [C₂H₄]–[Cu/AgM7] bonding. Additionally, the most relevant NOCV-based deformation density channels Δρ_orb(i), together with the corresponding energies, ΔE_orb(i) are presented.

Figure 7

The results of ETS-NOCV/PBE-D3(BJ)/ZORA/TZP energy decomposition describing [M7]–[Cu/AgC₂H₄] bonding. Additionally, the most relevant NOCV-based deformation density channels Δρ_orb(1+2), together with the corresponding energies ΔE_orb(1+2), are presented.

Figure 8

The results of ETS-NOCV/PBE-D3(BJ)/ZORA/TZP energy decomposition describing C₂H₄–Cu/Ag bonding. Additionally, the most relevant NOCV-based deformation density channels Δρ_orb(i), together with their corresponding energies ΔE_orb(i), are presented.

Figure 9

Typical results of CASSCF-VB analysis on the example of [(T1)Cu(NH₃)₃(NO)]⁺, singlet state (see Ref. [129] for details): (a) selected CASSCF(16,14) natural orbitals (delocalized), being linear combinations of the Co 3d and NO π*,π with fractional occupation numbers annotated below; (b) selected localized orbitals obtained by unitary transformation of the CASSCF(16,14) natural orbitals along with their qualitative designation as Co d_xz, d_z2, d_yz and NO π*_x,y, π_x,y fragment orbitals; (c) percentage contributions of the participating resonance structures; (d) schematic representation of the most relevant individual electronic configurations in terms of the Co d_xz, d_z2, d_yz and NO π*_x, π*_y fragment orbitals (colors correspond to colors used in the pie chart in (c)).