Nitrogen rejection from methane via a “trapdoor” K-ZSM-25 zeolite

Nitrogen (N₂) rejection from methane (CH₄) is the most challenging step in natural gas processing because of the close similarity of their physical-chemical properties. For decades, efforts to find a functioning material that can selectively discriminate N₂had little outcome. Here, we report a molecular trapdoor zeolite K-ZSM-25 that has the largest unit cell among all zeolites, with the ability to capture N₂in favor of CH₄with a selectivity as high as 34. This zeolite was found to show a temperature-regulated gas adsorption wherein gas molecules’ accessibility to the internal pores of the crystal is determined by the effect of the gas-cation interaction on the thermal oscillation of the “door-keeping” cation. N₂and CH₄molecules were differentiated by different admission-trigger temperatures. A mild working temperature range of 240-300 K was determined wherein N₂gas molecules were able to access the internal pores of K-ZSM-25 while CH₄was rejected. As confirmed by experimental, molecular dynamic, andab initiodensity functional theory studies, the outstanding N₂/CH₄selectivity is achieved within a specific temperature range where the thermal oscillation of door-blocking K⁺provides enough space only for the relatively smaller molecule (N₂) to diffuse into and through the zeolite supercages. Such temperature-regulated adsorption of the K-ZSM-25 trapdoor zeolite opens up a new approach for rejecting N₂from CH₄in the gas industry without deploying energy-intensive cryogenic distillation around 100 K.