Functionalized UiO-66 by single and binary (OH)₂ and NO₂ groups for uptake of CO₂ and CH₄

Abstract

The metal-organic frameworks UiO-66 with single or binary −(OH)₂ and −NO₂ functional groups were synthesized, and their structures and properties were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and nitrogen adsorption isotherms. Their performances in CO₂ and CH₄ adsorption at high pressures and different temperatures (273 and 298 K) were also determined. Singly functionalized UiO-66-(OH)₂ had a lower thermal stability than UiO-66-NO₂, but the thermal stability of binary UiO-66-(OH)₂-NO₂ significantly improved to 650 K. The binary-functionalized samples UiO-66-(OH)₂-NO₂ and UiO-66-NO₂-(OH)₂ displayed higher CH₄ and CO₂ adsorption than UiO-66-(OH)₂ and UiO-66-NO₂. The CO₂ adsorption capacities of UiO-66-(OH)₂-NO₂ and UiO-66-NO₂-(OH)₂ were 4.65 and 7.35 mmol/g, respectively, at 273 K and 900 kPa. UiO-66-(OH)₂ had a higher separation factor for selective separation of CO₂ from CH₄, especially at low pressures up to 100 kPa; the separation factor ranged from 4.9 at about 100 kPa to 5.8 at 60 kPa.