Intercalated hydrotalcites, diffraction and modelling

M. Pospíšil, P. Kovář

Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, Prague 2, CZ

pospisil@karlov.mff.cuni.cz

Molecular simulations in combination with powder X-ray diffraction (XRD) provided useful information on the arrangement of the Mg-Al layered double hydroxides (LDHs) intercalated with porphyrin molecules in the interlayer space (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin - TPPS) [1]. Molecular modelling was carried out in the Cerius²/Material Studio modelling environment [2]. Models with loading 75% anion exchange capacity (AEC) are modelled because of models are close to real samples. TPPS loading in experimentally determined samples was estimated to 80% of AEC and moreover, the coexistence of the two phases in the diffraction pattern could be possible. It was also shown that the increasing loading of anions in the interlayer space leads to the changes in the positions and in the intensities of diffraction peaks [3-4]. With respect to this information we constructed three different types of structure models.

1. Model with 100% TPPS loading in the interlayer space was built (Type 1). A set of models with different arrangement in the interlayer space was investigated. The composition of the calculated supercell was Mg₉₆Al₄₈(OH)₂₈₈(TPPS)₁₂×96H₂O.

2. Models with a contemporary coexistence of two individual phases in the interlayer space and with total TPPS loading 75% AEC were built (Type 2). We constructed a 12 layered supercell where in 9 interlayer regions the NO₃^- anions were fully exchanged for TPPS anions and other three interlammelar regions remained with the original LDH-NO₃ precursor. The interlayer distance with respect to the original LDH precursor was set to the value of 8.8 Ǻ. Structural formula of the investigated cell was Mg₃₈₄Al₁₉₂(OH)₁₁₅₂ (TPPS)₃₆ (NO₃)₄₈ 384 H₂O and cell parameters were A = 18.276 Ǻ, B = 24.368 Ǻ, and c = 225.3 Ǻ = (9 x 22.1 + 3 x 8.8) Ǻ. A set of models with different alternating of the phases was also calculated.

3. Models with 75% TPPS loading means, that in each interlayer space are 3 TPPS anions and the rest of charge was compensated by the 4 NO₃^- groups (Type 3). The composition of the supercell was Mg₉₆Al₄₈(OH)₂₈₈(TPPS)₉(NO₃)₁₂×96 H₂O, with the lattice supercell parameters A = 18.276 Ǻ, B = 24.368 Ǻ, and c = 3 × d₀₀₃ = 66.3 Å.

The aim of the molecular calculation was to obtain the optimized geometry of these three types of initial models and compare the experimental and calculated X-ray diffraction patterns of the models to solve which type of the above mentioned models exhibit the best agreement with the prepared sample.

We started calculations of the sequences with TPPS and NO₃- in random alternation of individual interlayers (e.g.,...,TPPS, TPPS, NO₃-, TPPS, NO₃-, TPPS, NO₃-, TPPS, ...). Calculated XRD patterns of these alternation exhibit considerable differences from the measured XRD patterns. Therefore, we have focused on consecutive sequences with a different number of individual interlayers TPPS and NO₃- (e.g.,...TPPS, TPPS, NO3 -, NO3 -,...). We found, that the calculated XRD patterns belong to the models containing a fully saturated part, with TPPS segregated from the non-exchanged part containing NO3 − anions, better agree with experimental one.

In all cases, the calculated powder XRD patterns are characterized by a sharp and intensive (003) diffraction line at 2θ ranging from 3.9 to 4.1° depending on the calculated model. These 2θ values are related to the basal spacing values ranging from 21.7 to 22.8 Å, and are in good agreement with the experimental value of 22.1 Å. The presence of LDH interlayers with NO₃- causes a splitting of the calculated diffraction lines due to a mixing of two kinds of the interlayers (LDH-TPPS, LDH- NO₃-). This is characterized by the diffraction lines around 8° and 12°, which are labeled as (0 0 6) and (0 0 9) in the single-phase models, respectively. In this case of Type 2 (75%) model, the supercell with 12 interlayers containing 9 LDH-TPPS and 3 LDH-NO₃- interlayers, which leads to the splitting of the diffraction line (0 0 6) into two at 2θ of 7.9 and 8.3°. Similarly, the line (0 0 9) splits into two at 11.8 and 12.2°. The splitting of (0 0 3) line (2θ=4.3°) also occurs. The models Type 2 (83% and 92 %) confirm splitting of diffraction lines.

TPPS anions in resultant model adopt a tilted orientation with an angle of 70° towards the LDH layers and are mutual horizontally shifted with respect to each other about a 1/3 to 1/2 of the TPPS diameter. We obtained good agreement between the calculated and measured powder XRD patterns with a slight difference between the calculated basal spacing and the basal spacing determined from the XRD data (22.0 Å and 22.1 Å, respectively). We can conclude, that the presence of two individual phases can contribute to the broadening of the experimental diffraction lines. If we compare the calculated XRD patterns, it is clear that both single-phase (i.e., Type 1 and Type 3) and two-phase (i.e., Type 2) models are possible. When we compared total crystal energy we can assume, that model Type 2 is much more probable, see Figure 1. The observed broadening in the experimental XRD lines is determined by the low crystallinity of the samples that is a typical feature of co precipitated LDHs composed of Mg and Al, thus overlapping cause unseeable of the split of the diffraction lines. Since the structural calculated models are perfectly periodic, the broadening of the peaks is not observed in the calculated patterns and splitting can be observed.

Figure 1. The most probable model (Type 2)

[1] P. Kovář, M. Pospíšil, E. Káfuňková, K. Lang, F. Kovanda, J Mol Model, 16, (2010), 223

[2] Accelrys Software Inc., Cerius² Modeling Environment, Release 4.5 documentation, (2003), Accelrys Software Inc., San Diego.

[3] U. Costantino, N. Coletti, M. Nochcetti, Langmuir, 15, (1999), 4454

[4] S. Miyata, Clays Clay miner, 31, (1983), 305

This work was supported by the Czech Science Foundation (207/10/1447).