Synthesis, Crystal Structure, and Photocatalytic Properties of Two Isoreticular Ce(IV)-MOFs with an Infinite Rod-Shaped Inorganic Building Unit

Abstract

The use of the V-shaped linker molecules 4,4′-oxydibenzoic acid (H2ODB) and 4,4′-carbonyldibenzoic acid (H2CDB) led to the discovery of two isoreticular Ce­(IV)-based metal–organic frameworks (MOFs) of composition [CeO­(H2O)­(L)], L = ODB2–, CDB2–, denoted CAU-58 (CAU = Christian-Albrechts-University). The recently developed Ce-MOF synthesis approach in acetonitrile as the solvent proved effective in accessing Ce­(IV)-MOF structures with infinite rod-shaped inorganic building units (IBUs) and circumventing the formation of the predominantly observed hexanuclear [Ce6O8] cluster. For the structure determination of the isoreticular MOFs, three-dimensional electron diffraction (3D ED) and powder X-ray diffraction (PXRD) data were used in combination with density functional theory (DFT) calculations. [CeO­(H2O)­(CDB)] shows reversible H2O adsorption by stirring in water and thermal treatment at 190 °C, which leads to a unit cell volume change of 11%. The MOFs feature high thermal stabilities (T > 290 °C), which exceed those of most Ce­(IV)-MOFs and can be attributed to the infinite rod-shaped IBU. Surface and bulk oxidation states of the cerium ions were analyzed via X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). While Ce­(III) ions are observed by the highly surface-sensitive XPS method, the bulk material contains predominantly Ce­(IV) ions according to XANES. Application of the MOFs as catalysts for the catalytic degradation of methyl orange in aqueous solutions was also studied. While degradation activity for both MOFs was observed, only CAU-58-ODB revealed enhanced photocatalytic activity under ultraviolet (UV) light. The photocatalytic mechanism likely involves a ligand-to-metal charge transfer (LMCT) from the linkers to the Ce­(IV) centers. Analyses by XANES and inductively coupled plasma-optical emission spectroscopy (ICP-OES) demonstrate that leaching of Cerium ions as well as partial reduction of Ce­(IV) to Ce­(III) takes place during catalysis. At the same time, PXRD data confirm the structural stability of the remaining MOF catalysts.

Discovery and In Situ Crystallization Studies of Cerium-Based Metal–Organic Frameworks with V-Shaped Linker Molecules

“…In our ongoing efforts to understand the formation of Ce-MOFs, we report the discovery and characterization of two porous Ce­(III)-MOFs containing V-shaped linker molecules that are denoted as CAU-59 ([Ce 2 (H 2 O)­(SDB) 3 ], 1 ) and Ce-RPF-4 ([Ce 2 (hfipbb) 3 ], 2 ). Synthesis conditions are based on our previous studies , using acetonitrile as the solvent and harsh solvothermal synthesis conditions. An in situ reduction of the employed Ce­(IV) salt is observed, which results in the formation of two porous Ce­(III)-MOFs.…”

“…The quasi-simultaneous measurements of XRD and XANES at the Ce K-edge give insights into the relation between oxidation states and local environments of cerium ions and the crystallization of the desired MOF. Four different compounds were studied, which can be obtained by a similar synthetic approach including the Ce­(III)-MOFs 1 and 2 presented herein as well as the recently reported Ce­(IV)-MOFs CAU-58-ODB and CAU-58-CDB . Syntheses of the CAU-58 MOFs differ only by the lack of acetic acid, which was used in the synthesis of 1 and 2 .…”

“…A pressure buildup is observed in the reactors, which remains after cooling to room temperature. This gas evolution was also previously described for this synthetic approach, where it could be identified as nitrogen . After a controlled pressure release, the products were filtered off and washed three times with N , N -dimethylformamide (DMF) to remove residual linker molecules since they either are not soluble (H 2 SDB) or have a low solubility (H 2 PFDB, max.…”

“…The quasi-simultaneous XANES/XRD in situ investigations in this work were performed at the BM23 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The syntheses of Ce­(III)-MOFs 1 and 2 as well as the two recently reported Ce­(IV)-MOFs, CAU-58-ODB and CAU-58-CDB (ODB 2– = 4,4′-oxydibenzoate, CDB 2– = 4,4′-carbonyldibenzoate), were studied in situ by quasi-simultaneous XAS and XRD measurements. The storage ring was operated at a target current of 196 + 4 mA (uniform filling + single bunch).…”

“…We recently reported the synthesis of two new Ce­(IV)-MOFs, denoted as CAU-58-ODB and CAU-58-CDB (CAU = Christian-Albrechts-University), and their application as photocatalysts for the photodegradation of methyl orange . The compounds were obtained using the V-shaped linker molecules 4,4′-oxydibenzoic acid (H 2 ODB) and 4,4′-carbonyldibenzoic acid (H 2 CDB) and a recently established synthesis approach in acetonitrile.…”