PEG-templated synthesis of ultramicroporous n-ZIF-67 nanoparticles with high selectivity for the adsorption and uptake of CO2 over CH4 and N2

Challenges remain concerning the development of rapid and low-cost adsorbents that combine good separation performance with high adsorption capacity, especially for CO2. Herein, we report for the first time a new polyethylene glycol templated synthesis method to obtain a nanocrystalline zeolitic imidazolate framework (n-ZIF-67) at room temperature in 5 min. The n-ZIF-67 nanoparticles were characterized using X-ray powder diffraction with Rietveld refinement (space group = I-43 m, a = 17.0545(4) Å), Fourier-transform infrared, transmission electron microscopy, thermogravimetric analysis, and N2 adsorption (Brunauer-Emmett-Teller) measurements and exhibited excellent properties, including a total pore volume of 0.86 cm3/g, a high surface area equal to 1891 m2/g, and 0.64 nm size ultramicropores.). The highly porous ZIF-67 nanoparticles were explored for the adsorption and desorption of CH4, CO2, and N2 gases at pressures up to 40 bar and isotherm temperatures of 273, 298, 323, and 353 K. The adsorption isotherms revealed a high capacity for CO2 of 681 mg/g at 298 K and an adsorption enthalpy of 29.19 to 34.44 KJ/mol, in part linked to the ultramicroporous structure. The n-ZIF-67 particles exhibited gas uptake values for CH4 and N2 of 241 mg/g and 219 mg/g, respectively. As far as the authors are concerned, these are the highest capacities ever reported for zeolitic framework metal organic frameworks such as ZIF-67 and ZIF-8. The Langmuir adsorption isotherm was employed to obtain the maximum adsorption capacity, qm, and adsorption equilibrium constant, KL. The isosteric heat of adsorption data sheds light on a CO2 physisorption process. The n-ZIF-67 also exhibited high CO2/N2 and CO2/CH4 mixed gases selectivity, with the preferential adsorption of CO2 over N2 or CH4 confirmed by breakthrough experiments. The n-ZIF-67 with ultramicropores is therefore an effective new adsorbent for greenhouse gas capture with high CO2 gas selectivity over competing gases. Zeolitic framework MOFs merit further development as low-cost and easy-to-synthesize adsorbents to help address the pressing need to mitigate CO2 emissions.