Enhancing the natural gas upgrading and acetylene extraction performance of stable zirconium-organic frameworks PCN-605 by ligand functionalization

Abstract

Exploring stable adsorbents for efficient separation of C2H2 and CO2 from CH4 is of great significance to meet the requirements of natural gas upgrading and high-purity C2H2 production. Despite excellent structure stability and large surface area/pore volume, low adsorbate affinity and co-adsorption of mixtures in zirconium-organic frameworks (Zr-MOFs) seriously limit their applications in gas separation. To overcome these limitations, we designed a series of stable adsorbents based on Zr-based PCN-605 by replacing two H atoms from two central ligand-aromatic rings with different functional groups (–F, –OH, –NH2, –SO3H, and –SO3Ag), which are expected to gain steric hindrance effect and charge-tuning effect in varying degrees. The adsorption and separation performances of C2H2/CH4 and CO2/CH4 in PCN-605 materials were theoretically investigated by combining grand canonical Monte Carlo and density functional theory. Thereinto, PCN-605-SO3Ag shows the largest selectivity of C2H2/CH4 (1031.5) and CO2/CH4 (271.1) under molar ratio 1:99 at 298 K, or increased by 43.3 and 11.9 times, as against the parent PCN-605. This improvement is attributed to a good synergism of steric hindrance effect with charge-tuning effect exerted by –SO3Ag, which modifies not only the pore surface structure but also the charge distribution, favoring special three-way adsorption to recognize C2H2, strengthening CO2 affinity with –SO3Ag, but suppressing CH4 uptake, etc. This work highlights the effect of precise surface-structure and charge-distribution control by ligand functionalization on C2H2/CH4 and CO2/CH4 separation in PCN-605 materials, and provides a feasible strategy to design function-oriented Zr-MOF adsorbents for applications of natural gas upgrading and C2H2 extraction.