Effects of Open Metal Site Availability on Adsorption Capacity and Olefin/Paraffin Selectivity in the Metal–Organic Framework Cu3(BTC)2

We report a detailed investigation of the factors influencing olefin/paraffin separation in the metal–organic framework (MOF) adsorbent Cu3(BTC)2 (BTC, benzene-1,3,5-tricarboxylate). The effects of synthesis and activation parameters on the availability of open Cu metal sites in the MOF were studied via measurements of crystallinity, textural properties, structural defects, and water desorption, as well as binary 1-hexene/hexane vapor breakthrough chracteristics. A reduced crystallization temperature and improved solvent exchange with methanol were found to lead to CuBTC adsorbents with higher olefin selectivity and capacity. In situ Fourier transform infrared and mass spectrometry measurements were used to quantify the structural defects and availability of open metal sites in the MOF as a function of the activation conditions. Based upon these measurements, we identify the relationships between the defects, activation conditions, open metal site availability, and consequent separation performance. Recommendations are made for appropriate activation and desorption conditions that improve olefin selectivity while maintaining total capacity. This investigation led to the identification of a functional CuBTC adsorbent with a selectivity of ∼5 for a 10%/90% 1-hexene/hexane liquid feed mixture at 50 °C, and also suggests a systematic methodology to determine operating conditions for MOF adsorbents with open metal sites.