Synthesis of a mixed-linker Ce-UiO-67 metal-organic framework

Abstract

Ce-based metal-organic frameworks (MOFs) have recently gained scientific interest, since Ce is the most abundant rare-earth element in the Earth's crust and since their synthesis has some advantages, including first of all their redox activity, the high porosity of these crystalline materials, and Ce availability. In particular, Ce(iv)-based MOFs, such as Ce-UiO-66 and Ce-UiO-67, are synthesised under mild conditions. For most applications, the presence of functional groups in the frameworks is needed; in this context, linkers containing N-functionalities have been highlighted, as they allow for the incorporation of a large variety of metal cations. In order to insert N-functionalities for the sake of successive metal-functionalization of materials as Ce-UiO-67, we have successfully synthesised a mixed-linker version of this MOF, by incorporating 2,2'-bipyridine-5,5'-dicarboxylic acid together with the conventional biphenyl-4,4'-dicarboxylic acid linker; we have worked on a reproducible and upscalable procedure using benzoic acid as the modulator, without altering the original framework topology. Mixed-linker Ce-UiO-67 MOFs exhibit good thermal stability, high Brunauer-Emmett-Teller (BET) SSAs and microporosity. The pristine samples are highly stable if stored in a desiccator, as demonstrated by the preservation of their high crystallinity for at least 18 months.

Fig. 1. A) PXRD pattern of #1 Ce-UiO-67-5% BPyDC (blue line) compared to those of the linkers H₂BPDC (dashed pink line) and H₂ByPDC (dashed violet line), showing no unreacted linkers residues. B) and C) show magnifications of the same PXRD patterns. For the MOF, intensities normalized to the 5.58° peak.

Fig. 2. PXRD patterns of the three batches of Ce-UiO-67-5% BPyDC, #1 (blue line), #2 (red line) and #3 (green line), compared to that of Ce-UiO-67 (black line). Intensities normalized to the 5.58° peak.

Fig. 3. TGA profiles of the three batches of Ce-UiO-67-5% BPyDC, #1 (blue line), #2 (red line) and #3 (green line), compared to that of Ce-UiO-67 (black line), reported as weight loss (%) related to CeO₂, set as 100%.

Fig. 4. N₂ adsorption–desorption isotherms at 77 K for the three batches of Ce-UiO-67-5% BPyDC: #1 in blue, #2 in red and #3 in green, respectively. For the sake of clarity, we have selected one of each three adsorption points up to 150 cm³ g⁻¹ STP and then one every other.

Fig. 5. Raman spectra of the H₂BPDC linker (dashed pink curve) and H₂BPyDC linker (dashed purple curve), Ce-UiO-67 (black) and #3 Ce-UiO-67-5% BPyDC (green) in powder form. The spectra are reported in arbitrary units. Asterisks underline the band similarities at 1602 cm⁻¹, 1494 cm⁻¹ and 1240 cm⁻¹ between #3 Ce-UiO-67-5% BPyDC (green) and H₂BPyDC linkers (dashed purple curve). The right panel reports the data magnified by a factor of 2 in respect to the left panel.