Molecular simulations of Zn-Al Layered Double Hydroxide intercalated with porphyrin anions.

P. Kovář¹, M. Pospíšil¹, K. Lang²

¹Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Prague, Czech Rebublic

²Institute of Inorganic Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Řež 250 68, Czech Republic

Kovar@karlov.mff.cuni.cz

Layered Double Hydroxides (LDHs) belong to the inorganic layered solids consisted of the rigid layers containing two kinds of metallic atoms and the interlayers containing exchangeable compensating anions and water molecules. These materials are attractive due to their simple modification and they are used in many branches like catalysis, precursors for chemical reactions, drug delivery, decontamination of water, soils, etc. The intercalation of porphyrins into Layered Double Hydroxides based on ion exchange plays an important role in designing of new materials with optical properties, which can be used as photofunctional units [1]. Advanced methods of preparation of the sample allow us to obtain well crystallized samples and their structure analysis can provide deeper structural details. Zn(II)-5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin – (ZnTPPS) shown in Fig. 1 was intercalated into Zn₂-Al LDH and a high crystallinity was achieved by the coprecipitation procedures followed by a post-synthesis hydrothermal treatment.

Molecular simulations and quantum chemistry calculations combined with X-ray diffraction, thermogravimetry and electron density measurements were used in the structure analysis. The geometry and dimensions of ZnTPPS were optimized by the quantum-chemistry computational program Turbomole v5.9 using the RI-DFT method with B-P86 functional [2]. The optimized models of porphyrins were subsequently used in the molecular simulations. The cell parameters were determined from experimental XRD patterns: a = b = 3.064 Å. 96 cells were linked to obtain the layer [Zn₆₄Al₃₂(OH)₁₉₂]³²⁺ with the lattice parameters: A = 49.024 Ǻ and B = 18.384 Ǻ. The basal spacing in initial models was equal to the value of the experimental basal spacing (23.05 Å) obtained from the experimental XRD data. The estimated loading of ZnTPPS anions in the interlayer space was over 90% of anion exchange capacity (AEC). It was approximated by structural models with the 100% loading of AEC. The initial models contained 4 water molecules per [Zn₄Al₂(OH)₁₂]²⁺. The composition of initial structure models was [Zn₁₉₂Al₉₆(OH)₅₇₆][(ZnTPPS)₂₄·192 H₂O] and a set of models with various orientations of guest anions with respect to the host layers and with respect to each other was created. The minimization of the initial models was carried out in the Universal force field [3], the electrostatic energy was calculated by Ewald summation method [4] and the van der Waals energy was calculated by Lennard-Jones potential [5]. The space group was P1 and the porphyrin pyrroles were kept in their geometry as it was obtained by ab-initio calculations. The models were minimized iteratively in two steps, with fixed and variable cell parameters to obtain a good estimation of the orientation of the guest with respect to the host layers and a good agreement with the experimental basal spacing. The minimized models were refined by quench dynamics in an NVT (constant number of atoms, constant volume and constant temperature) statistical ensemble at a temperature of 300 K. One dynamics step was 0.001 ps and 200 ps of dynamics were carried out.

The results are summarized in Figs. 2 and 3. Fig. 2 shows the calculated and experimental powder XRD patterns with basal diffraction lines characterizing the interlayer arrangement and are compared for 2θ from 3 to 25°. The arrangement of the guests in the interlayer space corresponding to the calculated powder XRD pattern is shown in Fig. 3. The porphyrin anions are horizontally shifted, the horizontal shift ranges from one third to one half of the porphyrin diameter and the guests nearly homogeneously occupy the interlayer space. The low intensity peaks in the calculated XRD especially between 5 and 6° and between 16 and 18° are caused by the forced periodicity of central zinc atoms of ZnTPPS. It indicates that in the real sample one can expect no order of the guests in the interlayer space. The ZnTPPS porphyrin planes exhibit a tilted orientation with respect to the normal. The average calculated value of tilted angle is of 14 °.

Figure 1. Molecular structure of ZnTPPS and its dimensions.

Figure 2. Experimental (a) and calculated (b) XRD basal diffractions of Zn₂Al/ZnTPPS intercalate.

Figure 3. Top view of the linked supercells on the arrangement of the guests in the interlayer space.

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Acknowledgements.

The work was supported by GAČR 205/08/0869 and by MSM 0021620835.