Nitrogen rejection in natural gas using NaZSM-25 zeolite

Natural gas reservoirs usually contain considerable amounts of nitrogen (N₂). Methane (CH₄) as the main component in natural gas must be purified before transferring to the pipeline or storing as liquified natural gas (LNG). Currently, energy-intensive cryogenic distillation is the only industrial approach for N₂ rejection in natural gas. The adsorption process based on a N₂-selective adsorbent can minimize the separation cost. However, the search for an adsorbent that can selectively reject N₂ in natural gas has lasted for decades. Here, we report a microporous zeolite called NaZSM-25 capable of adsorbing N₂ over CH₄ with an exceptional selectivity of 47 at room temperature that outperforms all previously known N₂-selective adsorbents. At 295 K and 100 kPa, the N₂ and CH₄ uptakes on NaZSM-25 were 0.25 and 0.005 mmol g^-1, respectively. CH₄ showed negligible external surface adsorption in the whole temperature range of 273-323 K. Theoretical studies through replica exchanged Monte Carlo, molecular dynamics, and ab initio density functional theory (DFT) proved the diffusion limitation of CH₄ as a result of 8-membered ring (8MR) pore opening deformation by Na⁺ cation. The DFT results showed the diffusion energy barriers of 63 and 96 kJ mol^-1 for N₂ and CH₄, respectively, when passing an 8MR occupied with a Na⁺. NaZSM-25 is a promising adsorbent to be utilized in a pressure swing adsorption process at room temperature to minimize the energy consumption in N₂ rejection units.