In-situ Diffraction Crystallization Study of a porous Ti(IV) oxo-cluster carboxylate MOF and its Water-Induced Ti(III) Formate

The in-situ crystallization kinetics of the highly porous Ti(IV)12 oxo-cluster based metal-organic framework (MOF) MIP-177(Ti)-LT (MIP: Materials from Institute of porous solids of Paris, LT: Low Temperature) were studied by synchrotron time-resolved powder X-ray diffraction. When dry formic acid (FA) was used as solvent the crystallization does follow the Johnson-Mehl-Avrami-Kolmogorov kinetics with an energy barrier of 74 kJ·mol-1 without any observed intermediate crystalline phases. However, when small quantities of water (≥ 2.5 vol%) were introduced in the reaction solution, it led to the formation of the novel clathrate Ti(HCOO)3·¾CO2·¼H2O, a microporous MOF belonging to the ReO3 net-like CO2 clathrates in analogy to the previously described M(III)-formates (M = Al, Mn, Fe, Ga, In). Although the formation mechanism in the presence of water remains unclear, one possible explanation is that MIP-177(Ti⁴⁺)-LT decomposes formic acid under solvothermal conditions, resulting in the production of CO₂ and H₂, which may promote the crystallization of Ti³⁺-formate clathrate.