Glycine-Modified HKUST‑1 with Simultaneously Enhanced Moisture Stability and Improved Adsorption for Light Hydrocarbons Separation

In this work, a novel Gly@HKUST-1 with simultaneously enhanced moisture stability and improved adsorption capacity was developed by grafting glycine (Gly) onto the open metal sites of HKUST-1, in order to recover light hydrocarbons from natural gas efficiently. The resultant Gly@HKUST-1 had higher C2H6 and C3H8 adsorption capacities compared to HKUST-1 and some other metal–organic frameworks, which reached as high as 6.47 and 7.80 mmol/g at ambient conditions. Its C2H6/CH4 and C3H8/CH4 adsorption selectivities were up to 12.6 and 173.5, respectively. Fixed-bed experiments showed that 0.3Gly@HKUST-1 could separate the CH4/C2H6/C3H8 mixtures completely at ambient conditions, showing excellent separation property toward the light hydrocarbons. More importantly, stability experiments confirmed that after the two samples were exposed to moist air (RH of 55%) for 20 days, 0.3Gly@HKUST-1 still remained its crystal structure, while parent HKUST-1 lost most of its crystallinity. Computational simulations showed that Gly grafting onto unsaturated Cu site played a critical role in enhancing stability against water vapor and improving capacity due to its shielding of unsaturated Cu site as well as introduction of −COOH or −NH2 as the H-binding sites. Gly@HKUST-1 is promising for practical application of efficiently recovering ethane and propane from natural gas by adsorption.