Enhanced Gas Adsorption on Cu3(BTC)2 Metal-Organic Framework by Post-Synthetic Cation Exchange and Computational Analysis

Increased gas adsorption in a series of post-syntheticallymodifiedmetal-organic frameworks (MOFs) of the type HKUST-1 was achievedby the partial cation exchange process. Manipulation of post-syntheticconditions demonstrates high tunability in the site substitution andgas adsorption properties during the dynamic equilibrium process.In this work, post-synthetic modification of Cu-3(BTC)(2) is carried on by exposure to TM2+ solutions (TM= Mn, Fe, Co, Ni) at different time intervals. The crystal structure,composition, and morphology were studied by powder X-ray diffraction,Fourier-transform infrared spectroscopy, inductively coupled plasmaoptical emission spectroscopy, and scanning electron microscopy. Structuralanalysis supports the retention of the crystal structure and partialsubstitution of the Cu metal nodes within the framework. A linearincrease in the transmetalation process is observed with Fe and Cowith a maximum percentage of 39 and 18%, respectively. Conversely,relatively low cation exchange is observed with Mn having a maximumpercentage of 2.40% and Ni with only 2.02%. Gas adsorption measurementsand surface area analysis were determined for each species. Interestingly,(Cu/Mn)(3)(BTC)(2) revealed the highest CO2 adsorption capacity of 5.47 mmol/g, compared to 3.08 mmol/g forCu(3)(BTC)(2). The overall increased gas adsorptioncan be attributed to the formation of defects in the crystal structureduring the cation exchange process. These results demonstrate theoutstanding potential of post-synthetic ion exchange as a generalapproach to fine-tuning the physical properties of existing MOF architectures.