Enhancing light hydrocarbon storage and separation through introducing Lewis basic nitrogen sites within a carboxylate-decorated copper–organic framework

Abstract

A novel nanoporous Cu metal–organic framework (NEM-4) with open Cu^II sites, Lewis basic nitrogen sites, and uncoordinated –COO⁻ groups exhibits both outstanding uptake capacities (in cm³ (STP) g⁻¹) for C₂H₂ (204), C₂H₄ (164.1), C₂H₆ (172.2), C₃H₆ (197.4), and C₃H₈ (196.1) and high selectivities for C₂H₂/CH₄ (63.2), C₃H₆/CH₄ (174.8), and C₃H₈/CH₄ (168.3) under ambient conditions. After eight cycles of adsorption–desorption tests, only 8.2% and 10.3% decrease in the acetylene and propene storage capacities was observed, indicating an excellent repeatability. Compared with 1 (carboxylate decorated NOTT-101), when nitrogen sites are inserted, the C₂–C₃ hydrocarbon uptakes of NEM-4 can be significantly enhanced. Grand Canonical Monte Carlo and first-principles calculations reveal that not only the open Cu^II sites but also the uncoordinated –COO⁻ groups and the nitrogen sites play significant roles in its high C₂–C₃ hydrocarbon uptakes. Moreover, the adsorption and separation of cationic dyes in NEM-4 are highly size and charge state dependent, and the adsorbed methylene blue (MB⁺) in NEM-4 can be efficiently released in an NaCl-containing CH₃OH solution. This study reveals that the combination of open metal sites, carboxylate groups, Lewis basic pyridyl sites, and appropriate pore geometry is responsible for the high adsorption/separation of light hydrocarbons in NEM-4.