Nitrogen rejection in natural gas using NaZSM-25 zeolite

Natural gas reservoirs usually contain considerable amounts of nitrogen (N-2). 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 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 CH4 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 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 N-2 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 N-2 rejection units.

Automated summary

Natural gas reservoirs often contain a significant amount of nitrogen (N2), which needs to be removed before transferring or storing natural gas. Currently, cryogenic distillation is the main industrial method for nitrogen rejection, but it consumes a lot of energy. This study introduces NaZSM-25, a zeolite that can selectively adsorb N2 over CH4 at room temperature, with exceptional selectivity and performance compared to previous adsorbents.