Chemistry−Structure−Simulation or Chemistry−Simulation−Structure Sequences? The Case of MIL-34, a New Porous Aluminophosphate

A new aluminophosphate, MIL-34, is investigated from its as-synthesized structure to its calcined microporous form. Single-crystal X-ray diffraction measurements on the as-synthesized MIL-34 (Al4(PO4)4OH·C4H10N, space group P-1, a = 8.701(3) Å, b = 9.210(3) Å, c = 12.385(3) Å, α = 111.11(2)°, β = 101.42(2)°, γ = 102.08(2)°, V = 863.8(4) Å3, Z = 2, R = 3.8%) reveal a 3-D open framework where Al atoms are in both tetrahedral and trigonal bipyramidal coordinations. It contains a 2-D pore system defined by eight rings where channels along [100] cross channels running along [010] and [110]. CBuA molecules are trapped at their intersection. 27Al, 31P, and 1H MAS NMR spectroscopies corroborate these structural features. Calcination treatments of a powder sample of the as-synthesized MIL-34 indicate its transformation into the related template-free structure that is stable up to 1000 °C. Lattice energy minimizations are then used in order to anticipate the crystal structure of the calcined MIL-34, starting with the knowledge of the as-synthesized structure exclusively. Energy minimizations predict a new regular zeotype structure (AlPO4, space group P-1, a = 8.706 Å, b = 8.749 Å, c = 12.768 Å, α = 111.17°, β = 97.70°, γ = 105.14°, V = 846.75 Å3, Z = 2) together with a thermodynamic stability similar to that of existing zeotype AlPOs. Excellent agreement is observed between the diffraction pattern calculated from the predicted calcined MIL-34 and the experimental X-ray powder diffraction pattern of the calcined sample. Finally, the atomic coordinates and cell parameters of the calcined MIL-34 predicted from the simulations are used to perform the Rietveld refinement of the calcined sample powder pattern, further corroborated by 27Al and 31P NMR measurements. This unique combination of experiment and simulation approaches is an interesting and innovative strategy in materials sciences, where simulations articulate the prediction of a possible template-free framework from its as-synthesized templated form. This is especially valuable when straightforward characterizations of the solid of interest with conventional techniques are not easy to carry out.