Cerium-Based Coordination Network Formation: An In Situ X-ray Absorption Spectroscopy and Powder X-ray Diffraction Study

The Ce(NO3)3·6H2O/H2TDC/CH3COOH/CH3CN (H2TDC = 2,5-thiophenedicarboxylic acid) chemical system was studied under solvothermal reaction conditions. Four different phases that successively crystallized as a function of time were observed. Three coordination networks, [CeIV(TDC)(CH3COO)2] (1), [CeIII4(TDC)3(CH3COO)6] (2a), and [CeIII(TDC)(CH3COO)] (3), could be isolated as orange, beige, and white phase pure products, respectively, and their crystal structures were resolved from powder X-ray diffraction data. Another crystalline compound (2b) was observed in situ, which seems to be structurally related to compound 2a. Compound 2a is a metal–organic framework (MOF) with a pore size of ∼3 Å. The use of CeIII(NO3)3 as the starting material, the different colors of the products, and the crystal structures indicated a peculiar redox behavior with a Ce(III)–Ce(IV)–Ce(III) redox transformation during product formation of 1, 2a, and 3. The oxidation states of 1 and 3 were confirmed by ex situ X-ray absorption near-edge structure (XANES) measurements, and the crystallization process was followed using quasi-simultaneous in situ powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS) measurements. During the reaction, the consecutive crystallization in the order 1–2b–3 was clearly observed. Linear combination fitting (LCF) of the in situ XAS data also affirmed the formation of the title compounds.