Investigation of the Kinetic Stabilization of a Ce⁴⁺‐based MOF by in‐situ Powder X‐ray Diffraction

Abstract

We herein investigated the synthesis of the Ce4+‐based metal‐organic framework [Ce6(μ3‐O)4(μ3‐OH)4(PZDC)4(OH)4(H2O)4] denoted as Ce‐DUT‐67‐PZDC (where DUT stands for Dresden University of Technology and PZDC2– stands for 3,5‐pyrazoledicarboxylate) by in‐situ powder X‐ray diffraction methods under solvothermal conditions using synchrotron radiation. It was observed that the obtained MOF is rapidly formed between 120 and 140 °C, but only as an intermediate product in this preparative system. After prolonged reaction times, Ce4+ is reduced to Ce3+ and reacting with formic acid from the reaction mixture to form the final product [Ce(O2CH)3]. During the formation of trivalent cerium formate, the Ce(IV)‐MOF completely re‐dissolves. Based on the composition of the reaction mixture and the recorded data, a plausible reaction mechanism was established. This mechanistic scheme includes a reversible precipitation of the Ce(IV)‐MOF under the given conditions and the reduction of Ce4+ by formate anions generated via hydrolysis of the solvent dimethylformamide.

Unravelling Why and to What Extent the Topology of Similar Ce‐Based MOFs Conditions their Photodynamic: Relevance to Photocatalysis and Photonics

“…Besides structural defects, it is also likely that small amounts of reduced Ce 3+ ions are present in the materials. It has been reported that the synthesis solvent can reduce Ce 4+ of the cluster, ultimately leading to Ce 3+ containing MOFs or side products . XANES analysis of a Ce 6 ‐MOF has shown that up to 1 Ce 3+ ion per cluster can be present without any loss of structural integrity .…”

“…It has been reported that the synthesis solvent can reduce Ce 4+ of the cluster, ultimately leading to Ce 3+ containing MOFs or side products. [41,42] XANES analysis of a Ce 6 -MOF has shown that up to 1 Ce 3+ ion per cluster can be present without any loss of structural integrity. [43] The following time-resolved single crystal fluorescence microscopy experiments will provide insights on their presence and impact.…”

In situ X-ray Diffraction Investigation of the Crystallisation of Perfluorinated Ce(IV)-based Metal-Organic Frameworks with UiO-66 and MIL-140 architectures

“…. 31 Therefore, future efforts will be aimed at upscaling the synthetic protocol using CAN and HNO3. The fact that the reaction temperature (60 °C) is well below the normal boiling point of water is a great advantage for upscaling, because there is no need for sophisticated, pressure resistant vessels.…”

“…30 The synthesis of these MOFs is typically carried out in N,N-dimethylformamide (DMF)/water mixtures as a reaction medium and in situ XRD studies have been key in revealing how the desired product is completely crystallised within a few minutes. 31 However, this phase is metastable and, if left in the reaction environment, undergoes dissolution to form Ce(III)-formate, having formula Ce(O2CH)3. Formation of Ce(III)-formate was attributed to the reaction between Ce(III), derived from reduction of Ce(IV) in solution, and formate generated by hydrolysis of DMF.…”

“…Dissolution of the Ce(IV)-based MOF is driven by the decrease of Ce(IV) concentration in solution due to its reduction to Ce(III), which is continuously withdrawn from the reaction environment as Ce(III)-formate. 31 A recent article by some of us reported on the water-based synthesis of two perfluorinated Ce(IV)-based analogues of the already well investigated Zr(IV)-MOFs UiO-66 and MIL-140A ( Figure S1). 32 The pure MIL-140A phase, named F4_MIL-140A(Ce), was obtained reacting stoichiometric amounts of cerium ammonium nitrate (CAN) and tetrafluoroterephthalic acid (H2TFBDC) as building blocks in water at 110 °C for 24 h, whereas the pure UiO-66 phase, named F4_UiO-66(Ce), was obtained in the same conditions, with addition of 100 equivalents of acetic acid (AcOH).…”