Nitrogen Rejection from Methane via a "Trapdoor" K-ZSM-25 Zeolite.

Nitrogen (N) rejection from methane (CH) 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 N had 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 N in favor of CH with 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. N and CH molecules were differentiated by different admission-trigger temperatures. A mild working temperature range of 240-300 K was determined wherein N gas molecules were able to access the internal pores of K-ZSM-25 while CH was rejected. As confirmed by experimental, molecular dynamic, and density functional theory studies, the outstanding N/CH selectivity is achieved within a specific temperature range where the thermal oscillation of door-blocking K provides enough space only for the relatively smaller molecule (N) 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 N from CH in the gas industry without deploying energy-intensive cryogenic distillation around 100 K.