Effective CH4 enrichment from N2 by SIM-1 via a strong adsorption potential SOD cage

Abstract

Methane enrichment from nitrogen is a necessary means for low-quality unconventional natural gas (coal mine methane) extraction, where adsorption-separation technology has become a research hotspot due to its low energy consumption and cost-effectiveness. In this work, three SOD-type ZIFs (ZIF-8, ZIF-90 and SIM-1, the latter also known as ZIF-94) are taken as methane sorbents for CH4/N2 separation. The experimental results show that SIM-1 (ZIF-94) has the highest CH4 adsorption capacity (1.5 mmol/g, 298 K and 1 bar) and adsorption heat (23.9 kJ/mol), although its surface area is the lowest (597 m2/g). No other known ZIF material has a higher CH4 adsorption capacity than SIM-1(ZIF-94), although some rare MOF and other porous materials can achieve such a high value. By comparing its CH4 adsorption capacity and BET surface with ZIF-93 (RHO type), which has the same units as Zn-almeIm, SIM-1(ZIF-94) has a high adsorption capacity of CH4 due to its suitable SOD cage size (0.84 nm) acting as a strong adsorption potential. Indeed, excellent enrichment–separation of the CH4/N2 mixture (30:70 and 50:50, v/v) with a high adsorption selectivity of 7.0 (IAST) rarely occurs for sorbents at room temperature and ambient pressure. Both the experimental and simulated data of the gas mixtures indicate that SIM-1 (ZIF-94) has excellent capability for enriching and removing nitrogen from methane, and it shows extraordinary industrial value.