The Effects of Ligand Substitution on MOF-808 Thermal Cycling Stability and Negative Thermal Expansion

The highly porous and crystalline nature of MOFs has garnered interest for a wide array of applications in the energy industry from large volume applications such as gas separation/storage and heterogeneous catalysis to specialty materials applications like thermoelectrics and thermal expansion composites. The modularity of their structures allows for the modification and sometimes direct tunability of both molecular and microstructural properties. The utilization of MOF materials in industrial applications requires the materials to exhibit suitable operational thermal stability. This is important for materials to perform efficiently in temperature cycling processes commonly found in industrial applications. Herein, we present a thermal stability analysis of various ligand-substituted MOF-808 derivatives along with an evaluation method to determine how these and other structural changes can directly affect thermal cycling stability. Through this analysis, high magnitude (−119 ppm/K) and cyclable negative thermal expansion (NTE) is identified in MOF-808-SO₄. These results suggest that application-targeted ligand modifications are capable of drastically changing the thermal stability and thermal expansion of MOFs.