Porous materials for catalytic applications: from coordination polymers to metal organic frameworks

Abstract

Porous solids are materials characterized by the presence of structural voids (channels or confined cavities). These materials have always attracted a considerable interest in chemistry. Nowadays many porous materials can be made in the laboratory and they can even be produced on a large industrial scale and used as catalyst, adsorbent, and for small molecule separation. As far as catalysis is concerned, one of the main goals of the present day research is the design of hetereogeneous systems that could work in mild conditions, and which could be synthetized in a simple and economical way. This research work had the aim to investigate porous materials for electrocatalytic applications. The study initially focused on the investigation of bis(3,5-dimethylpyrazolyl)methane silver complexes to generate molecular architectures based on weak interactions as halogen bond. The coordination architecture synthetized was potentially porous with large channels of 30 Å of diameter filled with THF, but the structure was not stable after the removal of the solvent and gave rise to new phases. Due to their metastable nature, we moved our interest towards more robust materials, which could sustain harsher conditions usually found in catalytic processes. For this reason, we focused on a class of amorphous porous materials namely, porous organic polymers (POPs) containing phosphine oxide groups, which was able to bind transition metals centres such as cobalt, iron, nichel or molybdenum. The composition, the thermal and adsorption properties of these materials were determined. POPs functionalized with different metal ions were studied (a proof-of-concept) for the electrocatalytic activity towards the hydrogen evolution reaction (HER). The major drawbacks of these amorphous POPs were related to their amorphous nature and to the inherent inability to control their pore size. To circumvent this issue, we moved our attention towards crystalline materials such as metal organic frameworks (MOFs), which are characterized by a well-defined framework thus opening the possibility to tune their properties and to adapt the material for a specific application. For this purpose, we designed several phosphorous and sulphur-based linkers, which could act as anchor sites for soft transition metal in order to incorporate metal ions within the pores of the framework. We prepared a novel crystalline and porous network belonging to UiO-68 family of MOFs. The compound showed a surface area of 1440 m2/g and, it could be easily functionalized with various metals such as palladium, platinum, cobalt and nickel to generate a potentially heterogeneous electrocalyst for electrochemical reactions.