The thermal stability of metal-organic frameworks

In order to systematically engineer the properties of crystalline and amorphous metal-organic frameworks (MOFs) towards practical application, a thorough understanding of their high-temperature behaviour is required. However, key properties such as the thermal decomposition temperature of a given MOF are often overlooked in the literature, leading to a huge gap in our understanding of the thermal stability of this vast class of materials. Herein, we undertake a critical review of thermogravimetric analysis (TGA) data from the MOF literature, and question the consistency, accuracy and meaning of the data provided. We use existing data to collate thermal decomposition temperature (Td) values for a series of archetypal coordination materials and their analogues. Several factors were identified which influence the thermal stability of MOFs. In particular: (i) the nature and position of functional groups, (ii) metal hardness, and, (iii) the presence of coordinated solvent molecules were all found to have a significant impact on decomposition temperature. Isoreticular expansion and interpenetration on the other hand were found to have a relatively modest impact. Moreover, we propose that decomposition mechanisms in MOFs may be broadly separated into two categories, depending on whether the decomposition is ligand-centred, or inorganic node-centred. Whilst ligand stability appears to be the dominant factor in determining overall thermal decomposition temperature, the stability of the inorganic node is key in realising solid-liquid transitions and high-temperature recrystallisation within the family. Thus, both ligand and node stability must be considered when attempting to engineer the high-temperature properties of MOFs.