Nitrogen rejection in natural gas using NaZSM-25 zeolite

Abstract

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.