Synthesis, structure and properties of bifunctional carboxyphosphonate metal–organic frameworks

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Barker, Joseph A. ORCID: https://orcid.org/0000-0001-7742-6243 (2023). Synthesis, structure and properties of bifunctional carboxyphosphonate metal–organic frameworks. University of Birmingham. Ph.D.

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Abstract

Metal–organic frameworks (MOFs) are porous materials made of metal ions or nodes connected by organic linker molecules to form 3-dimensional frameworks with potential voids. Since the late 1990’s they have been the subject of intense scientific interest for a wide range of applications including gas storage, proton conductivity, waste remediation and catalysis to name but a few. The most common type of linker molecules are those based on carboxylate functional groups, usually containing 2, 3 or 4 of these groups per linker molecule. However, the area of carboxylate MOFs has been extremely widely researched and the variety of coordination motifs is relatively limited. There is a significant amount of research interest in phosphonate MOFs, as these have a wider variety of coordination motifs and thus should be able to form a wider variety of structures. Additionally, phosphonate groups, like carboxylates, form strong coordination bonds leading to structures with significant thermal and chemical stability. However, phosphonate linkers are not without their issues. With a tendency to form dense layered structures and undergo rapid nucleation, the synthesis of large single crystals of a phosphonate MOF is difficult. This work focused on the use of bifunctional carboxyphosphonate linker molecules in order to balance the positive and negative properties of each functional group. Starting with a study of the synthesis of the ion-exchangeable carboxyphosphonate MOF, BIRM-1 (NH\(_4\))\(_2\)[Zn\(_2\)(O\(_3\)PCH\(_2\)CH\(_2\)COO)\(_2\)·5H\(_2\)O and two of its forms with alkali metal cations in place of ammonium cations. Properties and structural behaviours of the material were also examined with brief studies on the material’s potential for applications such as proton conductivity and ammonia storage. The focus of the work then shifts towards the synthesis and characterisation of novel carboxyphosphonate materials. Firstly, using magnesium as the metal in attempts to produce a lightweight framework and then moving onto using a different linker. The linker that was chosen was 4-phosphonobenzoic acid. A total of 5 novel phases were produced in this work with one of them showing significant surface area (813 m\(^2\) g\(^{-1}\)) and meeting the definition of a MOF. This material has been denoted BIRM-10 and possesses the formula Zn\(_{1.1}\)(O\(_3\)PC\(_7\)H\(_4\)COO)(OH)\(_{0.1}\)(NH\(_4\))\(_{0.9}\)·0.33H\(_2\)O. A further 3 unsolved phases, 2 based on magnesium and 1 on cadmium, were synthesised with at least one of these also displaying significant surface area.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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