Systematic Study of Cr<sup>3+</sup> Substitution into Octahedra-Based Microporous Aluminoborates

Single crystals of pure aluminoborate PKU-1 (Al<sub>3</sub>B<sub>6</sub>O<sub>11</sub>(OH)<sub>5</sub>·<i>n</i>H<sub>2</sub>O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the <i>R</i>3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by <sup>27</sup>Al NMR results. This distortion of Al3O<sub>6</sub> octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported <i>R</i>3̅ to the presently reported <i>R</i>3. We applied a pretreatment to prepare Al<sup>3+</sup>/Cr<sup>3+</sup> aqueous solutions; as a consecquence, a very high Cr<sup>3+</sup>-to-Al<sup>3+</sup> substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr<sup>3+</sup> substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr<sup>3+</sup>. We also systematically investigated the thermal behaviors of PKU-1:<i>x</i>Cr<sup>3+</sup> (0 ≤ <i>x</i> ≤ 0.50) by thermogravimetric–differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr<sup>3+</sup>-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ <i>x</i> ≤ 0.20, PKU-1:<i>x</i>Cr<sup>3+</sup> transferred to the PKU-5:<i>x</i>Cr<sup>3+</sup> (Al<sub>4</sub>B<sub>6</sub>O<sub>15</sub>:<i>x</i>Cr<sup>3+</sup>) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al<sub>4</sub>B<sub>2</sub>O<sub>9</sub>:<i>x</i>Cr<sup>3+</sup>. For <i>x</i> > 0.20 in PKU-1:<i>x</i>Cr<sup>3+</sup>, the heat treatment led to a composite of Cr<sup>3+</sup>-substituted PKU-5 and Cr<sub>2</sub>O<sub>3</sub>, so the doping upper limit of Cr<sup>3+</sup> in PKU-5 structure is around 20 atom %.