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.