Tuning the Cerium-Based Metal-Organic Framework Formation by Template Effect and Precursor Selection

The novel metal-organic framework [(CH₃)₂NH₂]₂[Ce₂(bdc)₄(DMF)₂]·2H₂O (Ce-MOF, H₂bdc-terephthalic acid, DMF-N,N-dimethylformamide) was synthesized by a simple solvothermal method. Ce-MOF has 3D connectivity of bcu type with a dinuclear fragment connected with eight neighbors, while three types of guest species are residing in its pores: water, DMF, and dimethylammonium cations. Dimethylamine was demonstrated to have a decisive templating effect on the formation of Ce-MOF, as its deliberate addition to the solvothermal reaction allows the reproducible synthesis of the new framework. Otherwise, the previously reported MOF Ce₅(bdc)_7.5(DMF)₄ (Ce5) or its composite with nano-CeO₂ (CeO₂@Ce5) was obtained. Various Ce carboxylate precursors and synthetic conditions were explored to evidence the major stability of Ce-MOF and Ce5 within the Ce carboxylate-H₂bdc-DMF system. The choice of precursor impacts the surface area of Ce-MOF and thus its reactivity in an oxidative atmosphere. The in situ PXRD and TG-DTA-MS study of Ce-MOF in a nonoxidative atmosphere demonstrates that it eliminates H₂O and DMF along with (CH₃)₂NH guest species in two distinct stages at 70 and 250 °C, respectively, yielding [Ce₂(bdc)₃(H₂bdc)]. The H₂bdc molecule is removed at 350 °C with the formation of novel modification of Ce₂(bdc)₃, which is stable at least up to 450 °C. According to the total X-ray scattering study with pair distribution function analysis, the most pronounced local structure transformation occurs upon departure of DMF and (CH₃)₂NH guest species, which is in line with the in situ PXRD experiment. In an oxidative atmosphere, Ce-MOF undergoes combustion to CeO₂ at a temperature as low as 390 °C. MOF-derived CeO₂ from Ce-MOF, Ce5, and CeO₂@Ce5 exhibits catalytic activity in the CO oxidation reaction.