Impact of metal precursor and molar ratios on adsorption and separation of CO2 and CH4 by SOD-ZIF-67 prepared using green solvent-free synthesis

Abstract

The subclass of metal-organic frameworks (MOFs) known as SOD-ZIF-67 (Sodalite Co (mIm)2, with mIm = 2-methylimidazole) has demonstrated exceptional performance in carbon dioxide (CO2) adsorption and separation. Traditional synthesis methods for SOD-ZIF-67 are often time-consuming and require significant quantities of organic solvents. This study introduces an enhanced, eco-friendly, solvent-free method for synthesizing SOD-ZIF-67. In this approach, SOD-ZIF-67 is produced within 1 h by mechanically grinding a cobalt (Co) metal precursor with an organic linker (mIm), eliminating the need for solvents. The properties of the synthesized SOD-ZIF-67 samples were confirmed through various characterization techniques. Thermal stability tests indicated that the synthesized SOD-ZIF-67 could endure temperatures up to 673 K. Among the samples prepared with different molar ratios, the SOD-ZIF-67 sample with a Co to HmIm molar ratio of 2:1 (denoted as Z-2) demonstrated the highest BET surface area and pore volume, measuring 1923 m2/g and 0.72 cm3/g, respectively. Z-2 exhibited uniform and narrow ultramicropores measuring 0.54 nm, and demonstrated a CO2 adsorption capacity of 91.52 mg (CO2)/g (2.08 mmol/g) at 298 K and 100 kPa. The material showed high selectivity for CO2 over N2 and CH4, with selectivities of 20.44 for CO2/N2 (0.15:0.85) and 4.28 for CO2/CH4 (0.5:0.5). Additionally, breakthrough experiments with CO2/N2 and CO2/CH4 mixtures validated its dynamic separation efficiency, underscoring Z-2′s suitability for selective CO2 capture from flue gases and natural gas purification.