Green, solvent-free synthetic protocol for Ce-MOFs with UiO-66 topology and photocatalytic applications
Ferdinando Costantino,a Matteo Campanelli,a Tiziana del Giacco,a Filippo de Angelis,a,b Edoardo Mosconi,b Marco Taddeic
aDipartimento di Chimica Biologia e Biotecnologia, University of Perugia
bComputational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123 Perugia, Italy
cEnergy Safety Research Institute, Swansea University, United Kingdom
Metal-organic frameworks (MOFs) are a class of porous crystalline materials constituted of metallic clusters and organic linkers connected in an ordered fashion and containing accessible channels to gases and solvents. They have attracted increasing attention during the past decade due to their high porosity, stability, tunable structures and controllable surface functionalities. Ce-MOF with UiO-66 topology are constituted of hexanuclear Ce6O4(OH)412+ linked by various dicarboxylic linkers in a fcu framework. These materials display a growing interest in photocatalytic applications due to the low energy partially empty Ce4+ 4f orbitals which allow the formation of long-life charge separation states induced by ligand-to-metal charge transfer (LMCT) upon UV light irradiation. The kinetics of this process can be related with the LMCT energy, ELMCT, which is defined as the energy change upon transferring the photogenerated electron from the photoexcited linker orbital to the lowest unoccupied metal orbital. Herein we report on a novel solvent-free synthetic procedure for Ce-UiO-66 metal-organic frameworks with several substituted dicarboxyilic linkers. We used the simple BDC (BDC = 1,4-benzenedicarboxylic acid) and the substituted Br-BDC, NO2-BDC and 4F-BDC ligands to build Ce-MOFs with UiO-66 structure. Furthermore, we also used 2,5-pyridine dicarboxylic acid (PyDC) affording another UiO-66 structure. The light absorption of the studied MOFs was shifted depending on the nature of the substituting group of the carboxylic linker thus calculating BG values ranging in the 2.7 to 3.0 eV. The photocatalytic performance of these MOFs was tested in the photo-oxydation reaction of substituted benzylic alcohols to the corresponding aldheydes under 355 nm light irradiation. The conversion of 4-methoxy benzylalcohol was fully selective and complete after 75 to 90 min in the case of Ce-PyDC-UiO-66 and Ce-BDC-UiO-66 respectively. DFT simulations have been carried out to evaluate the density of states (DOS) of Ce-UiO-66-H and Ce-PyDC. 39For Ce-UiO-66-H,we calculated a band gap of 2.99 while for Ce-PyDC we obtain a value of 2.79 eV in excellent agreement with the absorption onset (λonset) Interestingly, we found that the decreasing of the band gap found for Ce-PyDC is essentially related to an upshift of VB of about 0.2 eV due to the presence of an additional band associated to the to the pyridine states
Campanelli et.al. 2019, Submitted
Wu et.al. J. Am. Chem. Soc. 2018,140, 7904–7912.