Improving separation of CH4 and N2 by adsorption on zeolite Y Ion–Exchanged with ammonium Cations: An experimental and Grand-Canonical Monte Carlo (GCMC) simulation investigation

The separation of methane (CH ₄) from industrially–important nitrogen (N ₂)-rich vent streams such as those in liquefied natural gas processing plants is challenging. Pressure swing adsorption (PSA) is a promising, cost–effective solution in gas separation, especially for small–scale units. Further development and commercialisation of PSA–based technologies require access to low–cost, selective adsorbent materials. The current work investigates the improved separation of CH ₄ from N ₂ by TMA–Y zeolite which is obtained from the ion exchange treatment of Na–Y zeolite with the ammonium salt tetramethylammonium chloride (TMACl). TMA–Y is commercially called Ionic Liquidic Zeolite (ILZ), patented at The University of Western Australia, and has now been demonstrated on tonne–scale. In particular, we use a combination of experimental and molecular simulation techniques to provide insights into the high CH ₄–over–N ₂ selectivity of TMA–Y. For this, we obtain equilibrium isotherms of CH ₄ and N ₂ on Na–Y at different temperatures and pressures before and after ion exchange. The true selectivities of the zeolite samples are determined using the Ideal Adsorbed Solution Theory (IAST) and binary–gas grand–canonical Monte Carlo (GCMC) molecular simulations. From experiments and IAST calculations, we report an increase of >110% in the CH ₄/N ₂ selectivity of Na–Y after treatment with TMACl (from 2.2 to 4.7 at 5.0 MPa and 303.15 K for a mixture of 0.1 CH ₄+0.9 N ₂ mol.mol ⁻¹). GCMC molecular simulations provide a detailed picture of the molecular origins of this effect. Specifically, the introduction of TMA ⁺ ions into the structure of Na–Y zeolite leads to a reduction of the available adsorption volume by more than 40% (from 0.30 to 0.17 cm ³.g ⁻¹ with N ₂ as the probe molecule) for both adsorbing gas species (i.e. CH ₄ and N ₂). However, at the same time, this leads to stronger CH ₄–cation interactions due to the much higher affinity of TMA ⁺ cations (5 kJ.mol ⁻¹) than extra–framework Na ⁺ cations (1 kJ.mol ⁻¹) towards CH ₄. The overall effect of these two trends combined is the higher selectivity of the resulting TMA–Y zeolite for CH ₄. These molecular insights are useful in the systematic engineering of new materials with improved separation performance.