Study of the influence of the nature of metals, anions, and linker functionalities on the stability of Metal-Organic Frameworks (MOFs) with the MTN topology.

Details

Supervisors

Filinchuk, Yaroslav

Faculty

Faculté des sciences

Degree label

Master [120] en sciences chimiques, à finalité approfondie

Abstract

MIL-100(M) and MIL-101(M) (MIL = Matériaux de l’Institut Lavoisier) are two types of mesoporous metal-organic frameworks with respectively trimesate and terephthalate as linker. They are known to possess high surface areas, reaching a few thousands m2 g-1. These materials find applications in catalysis, gas sorption and separation, and in even as electrode materials in batteries. The stability of MOFs, either thermal or with respect to solvents and solutions, is important but somewhat neglected, so we worked on this, focusing on two specific frameworks: MIL 100 and MIL 101. We synthesized a range of MIL 100 and MIL-101 MOFs based on different metals (Fe, Al, V, Mn, Sc, Cr, In and Ga), and some MIL 100 and MIL 101 derivatives with an amine function on the linker. We succeeded in synthesizing MIL 100(Ga) which has not been reported in the literature yet. However, like other MOFs (MIL 100(In), MIL-101(Sc)), it was not obtained as a diffraction-pure sample. A new MOF was obtained with manganese with an overall oxidation state of +III and called MIL 53(Mn) OMe as there are bridging methoxy moieties between metal centres. The as-synthesized structure maintains closed pores, therefore we tried to open them by different methods. Promising results were obtained as a change of structure was obtained when soaking the MOF in some solvents (water, methanol) however, we could not identify the new structures. Thermodiffraction reveals decomposition temperatures of the framework between 120°C for MIL 101(V) NH2 to 440°C for MIL 100(Al) while DTA reveals decomposition temperatures for the linker between 256°C for MIL 100(Fe) NH2 and 550°C for MIL 101(Al).The stability of the synthesized MOFs towards solutions (HCl, NaOH, KF, Na2HPO4, …) and solvents (methanol, acetonitrile, dichloromethane, …) was also analyzed. Vanadium MOFs were found to be unstable in almost every tested solvent and solution while, at the opposite, chromium MOFs were stable in almost all of them. The ability of our MOFs to store reversibly carbon dioxide was investigated using a thermogravimetric setup. Weight gains from 4% for MIL 100(Cr) to 12% for MIL 101(Al)-NH2 were observed with a general tendency of MIL-101 MOFs to adsorb more carbon dioxide than the MIL-100 ones because of their bigger pores. Our MOFs were also tested as catalyst of both the hydrogenation of nitrobenzene into aniline and the Knoevenagel condensation between n-heptanal and malononitrile. All the MIL 100 MOFs tested for the first reaction only allowed the conversion of about 15% or less nitrobenzene in 4 hours while full completion is obtained with a commonly used hydrogenation catalyst Pd/C in only 30 minutes. The conversions and selectivities concerning the Knoevenagel condensation brought us better results as most of our MOFs lead to full conversion with good selectivities (>90%). All three vanadium MIL 100 and MIL 101 lead to low conversions of about 10% or less while MIL 101 MOFs lead to lower selectivities than MIL 100 MOFs.