Metal-organic frameworks as basic catalysts: a combined synthetic-spectroscopic-catalytic study.

Metal-organic frameworks (MOFs) are a class of hybrid, often crystalline, solids that have received a lot of research attention over the past two decades. In their structure, inorganic nodes are connected with each other through coordinatively bonded organic linkers, forming a three-dimensional, porous network. Most MOFs combine a high porosity with a diverse spectrum of potential chemical functionalities, making them excellent candidates for a wide range of applications. Among the large variety of MOF structures, those constructed from Group IV metal cations (Ti4+, Zr4+, Hf4+) are particulary interesting because of their high thermal, chemical and mechanical stability. In this PhD dissertation, new synthetic strategies for the formation and manipulation of Group IV MOFs are presented, which open new possibilities for their use in catalysis or adsorption.

The synthesis of Ti-MOFs is challenging due to the high reactivity of the commonly employed Ti-sources, like alkoxides, as precursors. Therefore, in the first part of this thesis, a hydrolytically more stable Ti-source, titanocene dichloride, is investigated as an alternative for the synthesis of Ti-MOFs. Combining titanocene dichloride with trans-1,4-cyclohexanedicarboxylate (cdc) leads to the formation of COK-69. This framework features photoactive Ti3O-clusters, and shows a reversible change in structure upon ad- or desorption of guests from its pores. This so-called breathing behavior is due to the conformationally flexible nature of the cdc linker.

In the second part of this work, breathing behavior is introduced in the well-known rigid Zr-terephthalate UiO-66 by replacing its terephthalate (bdc) linkers with cdc. Because of the high symmetry structure, this framework loses long-range order in the transition from its cubic, open-pore, adsorbate-filled form to a tetragonal, closed-pore, evacuated phase. By selectively adsorbing hydrogen-bond donating molecules, the crystalline open-pore phase can be recovered. To fine-tune this breathing behavior, mixed-linker bdc/cdc materials are synthesized. While a preference for bdc incorporation is observed, solid solutions are formed over a wide range of compositions. The mechanical properties of these materials are found to be intermediate to those of the pure phases: cdc incorporation diminishes the mechanical stability, while sufficiently high bdc contents prevent the cdc linkers from changing their conformation upon guest removal, thus inhibiting breathing.

In the third part, defects are introduced into the structure of Zr-MOFs in two ways, to fine-tune their porosity. First, the mixed-linker bdc/cdc materials are subjected to a heat treatment to selectively decompose the cdc linkers from their structure. The resulting materials clearly contain (additional) missing linker defects, however the expected increase in internal surface area could not be confirmed, possibly due to the partial collapse of the pores upon cdc decomposition. Secondly, the concept of missing linker defects is exploited to turn a non-porous Zr-MOF into a porous one. ZrSQU, with the same basic structure as UiO-66 but containing the much smaller squarate as linker, can only be formed in the presence of monocarboxylic acids in its synthesis mixture. These monocarboxylates are incorporated into the structure by substituting some squarates, creating missing-linker defects. In this way, porosity is enabled in this theoretically non-porous solid, and depending on the employed acid (formic or acetic acid), different gases (H2, N2) can be selectively adsorbed.

In the final part of this dissertation, the preparation of transparent, monolithic and hierarchically porous Zr-MOF structures is presented. By using sufficiently concentrated synthesis solutions, gels consisting of nanoparticulate MOF crystals can be formed. Solvent can be removed from these gels by drying, resulting in monolithic aggregates in which the nanocrystallites show a random packing. This leads to highly porous materials due to the additional interparticle mesopores. Being able to prepare these Zr-MOFs as a gel opens up possibilities to directly prepare them as shaped objects. As a proof-of-principle, binder-free, mesoporous beads of UiO-66 were synthesized using a drop-casting setup.