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 −NO 2 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 CO 2 and CH 4 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-NO 2, but the thermal stability of binary UiO-66-(OH) 2 -NO 2 significantly improved to 650 K. The binary-functionalized samples UiO-66-(OH) 2 -NO 2 and UiO-66-NO 2 -(OH) 2 displayed higher CH 4 and CO 2 adsorption than UiO-66-(OH) 2 and UiO-66-NO 2 . The CO 2 adsorption capacities of UiO-66-(OH) 2 -NO 2 and UiO-66-NO 2 -(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 CO 2 from CH 4, 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.