A combination of gas adsorption and gas breakthrough measurements show that the metal-organic framework, Al(HCOO) ₃ (ALF), which can be made inexpensively from commodity chemicals, exhibits excellent CO ₂ adsorption capacities and outstanding CO ₂ /N ₂ selectivity that enable it to remove CO ₂ from dried CO ₂ -containing gas streams at elevated temperatures (323 kelvin). Notably, ALF is scalable, readily pelletized, stable to SO ₂ and NO, and simple to regenerate. Density functional theory calculations and in situ neutron diffraction studies reveal that the preferential adsorption of CO ₂ is a size-selective separation that depends on the subtle difference between the kinetic diameters of CO ₂ and N ₂ . The findings are supported by additional measurements, including Fourier transform infrared spectroscopy, thermogravimetric analysis, and variable temperature powder and single-crystal x-ray diffraction.