Efficient N2/CH4 separation in a stable metal–organic framework with high density of open Cr sites

Natural gas purification by economic and low-energy adsorptive separation processes is important. However, as a difficult global problem, N2 removal from N2/CH4 mixtures is challenging because of their similar physical properties. In this study, the separation of N2/CH4 by Cr-metal–organic frameworks (MOFs) (MIL-100Cr, MIL-101Cr and TYUT-96Cr) with various open Cr site densities was studied. Gas sorption isotherms showed that TYUT-96Cr exhibited the highest N2/CH4 uptake ratio (13.67 at 0.01 bar and 298 K) of the selected Cr-MOFs, which was consistent with the highest density of Cr (III) sites. This material provided the highest CH4 productivity (0.716 mmol/cm3) per cycle for N2 removal from a 5:95 N2/CH4 mixture to produce 99.99% pure CH4, as shown by the breakthrough experiments. Pressure-swing adsorption simulations indicated that, by a two-bed six-step method, pristine 95% methane was enriched to 98.9% by TYUT-96Cr. The largest CH4 productivity, high breakthrough selectivity and thermal stability of TYUT-96Cr among the Cr-MOFs, made it a promising adsorbent in trace N2 removal for practical industrial application.