Aymane NAJAH: Functionalization study of porous coordination polymers (MOFs) by an impulse dielectric barrier discharge (IDBD) plasma process

Abstract:
Porous coordination polymers, more commonly known as MOFs "Metal-Organic Frameworks", are an emerging class of porous materials formed by the assembly of metal ions and organic ligands. Due to their structures and properties, MOFs have attracted great interest for a wide range of applications, mainly in gas storage and separation, catalysis, optics, energy storage, etc. One of the main advantages of these materials lies in the possibility of modulating or even enhancing their properties through functionalization. The functionalization of these materials is mostly carried out by wet chemical process that often presents some constraints like the duration of the reactions, the energy cost, etc. The work presented in this thesis concerns the study of the functionalization of these materials by a dry chemical method, in this case by a plasma treatment using an impulse dielectric barrier discharge (IDBD). To the best of our knowledge, no method of functionalization of MOFs by plasma has been reported in the literature yet. The main objective of this thesis is to study the functionalization of these materials by DBD by grafting amino groups. A parametric study of the IDBD conditions made it possible to determine the most favorable discharge parameters/conditions to produce nitrogen containing radicals in the plasma. Thus, this production is favored using an NH3 plasma compared to N2/H2 or Ar/ N2/H2 plasmas. In addition, the discharge conditions optimized for carrying out DBD treatment in pulsed regime in NH3  are as follow: a voltage of 6 kVpp, a frequency of 4 kHz, an inter-electrode distance of 3 mm and a pressure of 104 Pa. A first study of pre-functionalization of terephthalic acid showed the effectiveness of the IDBD process in grafting amine groups onto the organic ligand. This work continued with the in-depth study of the DBD plasma treatment of two MOFs: zinc-based MOF-5 and aluminum-based MIL-53. These investigations have highlighted the instability of MOF-5 with respect to ammonia, thus highlighting the importance of working with trivalent or even tetravalent cations to carry out plasma treatments in NH3. The DBD treatment of MIL-53 Al has proven successful functionalization via insertion by substituting a carbon atom with a nitrogen one within the organic ligand, which opens the possibility of using this pathway to functionalize other MOFs. Finally, the analysis of the textural and hydrogen adsorption properties at room temperature and atmospheric pressure showed an improvement of the amount of hydrogen stored in the MIL-53 Al treated with DBD. Although this amount remains low, these results are encouraging to develop materials with a higher adsorption capacity at room temperature, or even at atmospheric pressure.

Keywords:
MOFs, plasma, DBD, NH3, Impulse regime, terephthalic acid, MOF-5, MIL-53 Al

Composition of the jury:
> Reporters:
- Mrs Stéphanie ROUALDES, Lecturer, IEM, CNRS Université de Montpellier
- Mrs Myrtil KAHN, Research director, LCC, CNRS, Université de Toulouse
> Examiners:
- M. Patrice RAYNAUD, Research director, Laplace, CNRS, Université de Toulouse
- M. Christophe CARDINAUD, Research director, IMN, CNRS, Université de Nantes
> Direction of thesis:
- M. Gérard HENRION, Thesis director, Research director, IJL, CNRS, Université de Lorraine
- M. Stéphane CUYNET, Thesis co-director, Chargé de recherche, IJL, CNRS, Université de Lorraine