Pore control of Al-based MIL-53 isomorphs for the preferential capture of ethane in an ethane/ethylene mixture

Abstract

Preferential ethane adsorption from an ethane (C₂H₆)/ethylene (C₂H₄) mixture has emerged as a promising alternative to energy-intensive cryogenic distillation for obtaining high-purity ethylene. In this study, MIL-53(Al) isomorphs were prepared using various organic linkers such as 1,4-benezenedicarboxylic acid (MIL-53-BDC), 2,6-naphthalenedicarboxylic acid (MIL-53-NDCA), and 4,4′-biphenyldicarboxylic acid (MIL-53-BPDC) to investigate their C₂H₆/C₂H₄ separation properties. C₂H₆/C₂H₄ separation performances were rigorously investigated by combining single-component gas adsorption, ideal adsorbed solution theory calculations, breakthrough experiments, and molecular simulations (Monte Carlo simulation and density functional theory calculations). All MIL-53 isomorphs exhibited preferential uptake of C₂H₆ in the C₂H₆/C₂H₄ mixture, among which MIL-53-NDCA exhibited a reasonably good C₂H₆/C₂H₄ selectivity (1.53) and the largest C₂H₆ uptake (4.24 mmol g⁻¹) at 298 K and 1 bar. Molecular simulations revealed that the good C₂H₆/C₂H₄ separation performance can be attributed to efficient C–H⋯π interactions between C₂H₆ and the NDCA linker (Site I) and suppressed olefinic π⋯OH interactions between C₂H₄ and μ₂-OH groups (Site II). In the breakthrough experiment using MIL-53-NDCA, a binary mixture of C₂H₆/C₂H₄ (50/50 v/v) was separated into high-purity ethylene (>99.95%) with ethylene productivity of 11.2 L_STP kg⁻¹ at 298 K.