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

Abstract

The novel metal–organic framework [(CH 3 ) 2 NH 2 ] 2 [Ce 2 (bdc) 4 (DMF) 2 ]·2H 2 O ( Ce-MOF , H 2 bdc—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 Ce 5 (bdc) 7.5 (DMF) 4 ( Ce5 ) or its composite with nano-CeO 2 (CeO 2 @ 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-H 2 bdc-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 H 2 O and DMF along with (CH 3 ) 2 NH guest species in two distinct stages at 70 and 250 °C, respectively, yielding [Ce 2 (bdc) 3 (H 2 bdc)]. The H 2 bdc molecule is removed at 350 °C with the formation of novel modification of Ce 2 (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 (CH 3 ) 2 NH guest species, which is in line with the in situ PXRD experiment. In an oxidative atmosphere, Ce-MOF undergoes combustion to CeO 2 at a temperature as low as 390 °C. MOF-derived CeO 2 from Ce-MOF , Ce5 , and CeO 2 @ Ce5 exhibits catalytic activity in the CO oxidation reaction.