Enhancing Selective Adsorption in a Robust Pillared-Layer Metal-Organic Framework via Channel Methylation for the Recovery of C2-C3 from Natural Gas

Here, we reported a strategy for channel methylation to construct a robust ultramicroporous metal-organic framework (MOF) Ni(TMBDC)(DABCO)(0.5) through hydrothermal synthesis method and investigated its adsorption performance for recovering ethane (C-2) and propane (C-3) from natural gas. The as-synthesized Ni(TMBDC)(DABCO)(0.5) featured ultra-microporosity with a uniform pore size of 0.5 nm. The resulting sample showed a strong adsorption interaction with C3H8 and C2H6, and its C3H8 adsorption capacity at a low pressure of 1 kPa was up to 2.80 mmol/g and its C2H6 adsorption capacity at a low pressure of 10 kPa reached as high as 2.93 mmol/g, exhibiting strong binding affinity for ethane and propane. The enhanced adsorption can be attributed to the presence of the dense and accessible methyl and methylene groups in the channels of the sample. Grand Canonical Monte Carlo (GCMC) simulations also confirmed that the methylene groups from the DABCO pillar and the methyl groups from the TMBDC ligand play an important role in enhancing the adsorption of ethane and propane. Its ideal adsorbed solution theory (IAST)-predicted selectivity of C2H6/CH4 reached unprecedentedly 29, much higher than most of the reported data for MOFs. The stability test confirmed that the crystal structure of Ni(TMBDC)(DABCO)(0.5) still remained intact after it was exposed to moist air with a relative humidity of 100% for days. The breakthrough experiment demonstrated that the CH4/C2H6/C3H8 ternary mixture was completely separated using a fixed bed of Ni(TMBDC)(DABCO)(0.5) at ambient temperature, showing a great potential for recovering the low content of ethane and propane from natural gas.