Adsorption properties of acetylene, ethylene and ethane in UiO-66 with linker defects and NO2 functionalization

Abstract

Separation of small hydrocarbon gases is crucial for many industrial processes, but the high energy demand of current separation techniques encourages exploration of alternative sorption-based approaches. Here, we investigate the metal organic framework UiO-66(Zr) as possible solid adsorbent with a focus on the fundamental sorption mechanisms of C₂ hydrocarbons in the presence of linker defects and NO₂ functionalization. To this end, we directly measure heats of adsorption with calorimetry and correlate our results with ab initio calculations. In particular, we study adsorption of C₂H₂, C₂H₄ and C₂H₆ at 293 K and 195 K. At 293 K, the measured low-coverage enthalpy is found to be an average enthalpy of multiple adsorption sites. However, at 195 K, the low-coverage enthalpy corresponded to the strongest binding site, i.e. the linker defect site. We find that C₂H₂ exhibits the highest adsorption enthalpy with a significant gap of ∼6 kJ mol⁻¹ to C₂H₄ and ∼9 kJ mol⁻¹ to C₂H₆. Our ab initio calculations show excellent agreement with our experiments and are crucial to uncover the mechanisms that lead to the observed differences in binding affinity as a function of coverage. We also find that binding enthalpies increase for all three gases by adding –NO₂ functional groups, which even further increase the gap to ∼9 kJ mol⁻¹ and ∼10 kJ mol⁻¹ for C₂H₄ and C₂H₆, respectively. Our findings provide mechanistic insight that suggests that linker defects in UiO-66(Zr) have promise to improve its effectiveness as a solid adsorbent in C₂H₂, C₂H₄ and C₂H₆ applications and that functionalization may be used to increase specificity.