Systematic Study of Cr3+ Substitution into Octahedra-Based Microporous Aluminoborates

Single crystals of pure aluminoborate PKU-1 (Al3B6O11(OH)5·nH2O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the R3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by 27Al NMR results. This distortion of Al3O6 octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported R3̅ to the presently reported R3. We applied a pretreatment to prepare Al3+/Cr3+ aqueous solutions; as a consecquence, a very high Cr3+-to-Al3+ substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr3+ substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr3+. We also systematically investigated the thermal behaviors of PKU-1:xCr3+ (0 ≤ x ≤ 0.50) by thermogravimetric–differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr3+-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ x ≤ 0.20, PKU-1:xCr3+ transferred to the PKU-5:xCr3+ (Al4B6O15:xCr3+) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al4B2O9:xCr3+. For x > 0.20 in PKU-1:xCr3+, the heat treatment led to a composite of Cr3+-substituted PKU-5 and Cr2O3, so the doping upper limit of Cr3+ in PKU-5 structure is around 20 atom %.