In Situ X‐ray Diffraction Investigation of the Crystallisation of Perfluorinated Ce^IV‐Based Metal–Organic Frameworks with UiO‐66 and MIL‐140 Architectures**

Abstract

We report on the results of an in situ synchrotron powder X‐ray diffraction study of the crystallisation in aqueous medium of two recently discovered perfluorinated CeIV‐based metal–organic frameworks (MOFs), analogues of the already well investigated ZrIV‐based UiO‐66 and MIL‐140A, namely, F4_UiO‐66(Ce) and F4_MIL‐140A(Ce). The two MOFs were originally obtained in pure form in similar conditions, using ammonium cerium nitrate and tetrafluoroterephthalic acid as reagents, and small variations of the reaction parameters were found to yield mixed phases. Here, we investigate the crystallisation of these compounds, varying parameters such as temperature, amount of the protonation modulator nitric acid and amount of the coordination modulator acetic acid. When only HNO3 is present in the reaction environment, only F4_MIL‐140A(Ce) is obtained. Heating preferentially accelerates nucleation, which becomes rate determining below 57 °C. Upon addition of AcOH to the system, alongside HNO3, mixed‐phased products are obtained. F4_UiO‐66(Ce) is always formed faster, and no interconversion between the two phases occurs. In the case of F4_UiO‐66(Ce), crystal growth is always the rate‐determining step. A higher amount of HNO3 favours the formation of F4_MIL‐140A(Ce), whereas increasing the amount of AcOH favours the formation of F4_UiO‐66(Ce). Based on the in situ results, a new optimised route to achieving a pure, high‐quality F4_MIL‐140A(Ce) phase in mild conditions (60 °C, 1 h) is also identified.

A Novel Cerium(IV)‐Based Metal‐Organic Framework for CO2 Chemical Fixation and Photocatalytic Overall Water Splitting

“…Furthermore, among these occurred Ce IV ‐MOFs, most of them have UiO‐66‐type architecture and thus their IBUs are dominated by the hexanuclear Ce−O clusters [7,8] . Very recently, several Ce IV ‐MOFs with MIL‐140 topology have been reported, in these frameworks the three‐dimensional (3D) network are constructed through the interconnection of the ligand molecules and the 1D IBU chains consisting of alternating edge sharing CeO 7 polyhedra [11,12] . To the best of our knowledge, this is the first IBU that is not based on the hexanuclear Ce−O cluster in Ce IV ‐MOFs.…”

“…[7,8] Very recently, several Ce IV -MOFs with MIL-140 topology have been reported, in these frameworks the three-dimensional (3D) network are constructed through the interconnection of the ligand molecules and the 1D IBU chains consisting of alternating edge sharing CeO 7 polyhedra. [11,12] To the best of our knowledge, this is the first IBU that is not based on the hexanuclear CeÀ O cluster in Ce IV -MOFs. Therefore, new synthetic routes to Ce IV -MOFs, particularly these with novel IBUs, are highly desirable for the development and the multifunctional applications of Ce IV -MOFs.…”

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues

“…20,22−24 We have recently reported on the water-based synthesis of two perfluorinated Ce IV analogues of the wellknown UiO-66 and MIL-140A framework types. 21,25 The ultramicroporous F4-MIL-140A(Ce) displays an S-shaped isotherm and outstanding CO 2 /N 2 selectivity, and it was also found to display inverse CO 2 /C 2 H 2 selectivity. 26 As a result of our continued effort in developing highly stable (per)fluorinated MOFs based on tetravalent metals for CCS applications, 27 we report herein on the preparation and solid-state characterization of a perfluorinated analogue of the well-known MOF-801, namely, Zr IV tetrafluorosuccinate (ZrTFS), and of the two mixed-linker perfluorinated MOFs (PF-MOFs) PF-MOF1 and PF-MOF2 through postsynthetic tetrafluorosuccinate/fumarate (Scheme 1) linker exchange starting from pure zirconium fumarate MOF-801.…”

“…Typical CO 2 adsorption sites in MOFs include coordinatively unsaturated metal ions and functional groups with a basic character on the organic linker. ,, A promising approach that is gaining momentum is to introduce fluorine atoms in the backbone of ultramicroporous MOFs to prepare new materials labeled as perfluorinated MOFs (PF-MOFs). , Besides increasing the thermodynamic affinity for CO 2 (as evidenced by a high isosteric heat of adsorption, Q st ), the presence of (per)­fluorinated groups can also render the framework hydrophobic, thus preserving the CO 2 capture performance in humid conditions, which is crucial for real-life applications. The family of hybrid ultramicroporous materials containing fluorinated anions such as SiF 6 2– , NbOF 5 2– , and TiF 6 2– represents an excellent example of how the presence of fluorine atoms exposed in narrow pores provides superior affinity for CO 2 while preserving a physisorption-based mechanism and minimizing competitive water adsorption. ,− We have recently reported on the water-based synthesis of two perfluorinated Ce IV analogues of the well-known UiO-66 and MIL-140A framework types. , The ultramicroporous F4-MIL-140A­(Ce) displays an S-shaped isotherm and outstanding CO 2 /N 2 selectivity, and it was also found to display inverse CO 2 /C 2 H 2 selectivity…”

Exploring the Role of Cluster Formation in UiO Family Hf Metal–Organic Frameworks with in Situ X-ray Pair Distribution Function Analysis

“…Our previous work showed that, due to the similarities between the powder X-ray diffraction (PXRD) patterns of the fcu and hcp MOFs, , it is not always immediately obvious that a MOF structure with a different cluster has formed. Despite advances in understanding, − including studies on the effects of different variables (including acid modulator, water, and organic linker) on the crystallization of UiO-66 (zirconium , and cerium analogues), the focus of much current research is on the crystallization process rather than the metal cluster formation, which directs the crystallization. A full picture of the formation routes of MOFs is therefore still far off.…”