Adsorption of CO, CO2 and CH4 on Cu-BTC and MIL-101 metal organic frameworks: Effect of open metal sites and adsorbate polarity

A comparative adsorption study of three gases viz. CO, CO2 and CH4 on two adsorbents viz. Cu-BTC (or HKUST-1) and Cr-BDC (or MIL-101) is reported in this article. The gravimetric adsorption equilibrium measurements on the samples were performed in a Rubotherm magnetic suspension balance at three different temperatures: 295, 318 and 353 K and pressures ranging between 0 and 100 bar. Virial-Langmuir model was used to model the experimental data on Cu-BTC, whereas Dual Site Langmuir (DSL) model was used for adsorption on MIL-101. For all gases the enthalpy of adsorption at low loading was higher on MIL-101 than that on Cu-BTC, indicating the availability of open metal sites in case of MIL-101. Moreover, a sharp decrease in enthalpy of adsorption is observed in case of MIL-101, whereas only a moderate decrease is observed in case of Cu-BTC. CO has a large Henry’s constant on MIL-101, whereas at higher pressures, the solid exhibits better capacity for CO2. In case of Cu-BTC, CO2 has a higher capacity on the adsorbent as compared to the other two gases throughout the entire range of pressures studied. All the experimental data is critically analyzed by examining the role of open metal centers, adsorbate polarity and the effect of temperature on the electrostatic interactions.