Enhancing Water Stability of Metal-Organic Frameworks via Phosphonate Monoester Linkers

A new porous metal organic framework (MOF), barium tetraethyl-1,3,6,8-pyrenetetraphosphonate (CALF-25), which contains a new phosphonate monoester ligand, was synthesized through a hydrothermal method. The MOF is a three-dimensional structure containing 4.6 angstrom x 3.9 angstrom rectangular one-dimensional pores lined with the ethyl ester groups from the ligand. The presence of the ethyl ester groups makes the pores hydrophobic in nature, as determined by the low heats of adsorption of CH4, CO2, and H2O (14.5, 23.9, and 45 kJ mol(-1), respectively) despite the polar and acidic barium phosphonate ester backbone. The ethyl ester groups within the pores also protect CALF-25 from decomposition by water vapor, with crystallinity and porosity being retained after exposure to harsh humid conditions (90% relative humidity at 353 K). The use of phosphonate esters as linkers for the construction of MOFs provides a method to protect hydrolytically susceptible coordination backbones through kinetic blocking.