Synthesis, Characterization and Applications of Highly Stable Pyrazolate-Based Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have recently come under intense investigation for gas storage and separation applications, owing to their high internal surface area, convenient modular synthesis, and chemical tunability.1 Several efforts are devoted to engineer and prepare materials possessing elevated chemical and thermal stability as well as functional properties of technological and industrial significance. One of the current challenges in this field is the preparation of MOFs presenting structural topologies comparable to those of zeolites, which would juxtapose the thermal stability and catalytic properties typical of zeolites to the key features of MOFs- structure flexibility, modulation of pore surface, dimensions and guest affinity. Even if, in rare instances, metal-organic frameworks have displayed thermal stabilities up to 500 °C,2 none yet approach the thermal robustness of zeolites, a disadvantage further worsened by problems generally related to their low chemical stability. This is particularly true for those systems based on divalent metal cations combined with organocarboxylate bridging ligands, which can be subject to hydrolysis and thermal decomposition in the presence of moisture or moderate heat, respectively. Our strategy in this challenge has focused on the use of nitrogen-based anionic ligands, that may lead to structures with N-metal bonds, endowed with a greater stability compared to their carboxylate analogues.3 This contribution will thus survey our recent results in the field, highlighting the importance of the stability requirements of metal organic frameworks, if important industrial applications are sought. To illustrate this point, several examples of MOFs built by linear or branched pyrazolate-based ligands will be described in detail, including their extensive thermal and chemical stability characterization toward possible applications.