Nitrogen rejection in natural gas using NaZSM-25 zeolite.

Natural gas reservoirs usually contain considerable amounts of nitrogen (N 2 ). Methane (CH 4 ) 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 N 2 rejection in natural gas. The adsorption process based on a N 2 -selective adsorbent can minimize the separation cost. However, the search for an adsorbent that can selectively reject N 2 in natural gas has lasted for decades. Here, we report a microporous zeolite called NaZSM-25 capable of adsorbing N 2 over CH 4 with an exceptional selectivity of 47 at room temperature that outperforms all previously known N 2 -selective adsorbents. At 295 K and 100 kPa, the N 2 and CH 4 uptakes on NaZSM-25 were 0.25 and 0.005 mmol g -1 , respectively. CH 4 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 CH 4 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 N 2 and CH 4 , 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 N 2 rejection units.