Unlocking the Separation Capacities of a 3D-Iron-Based Metal Organic Framework Built from Scarce Fe4O2 Core for Upgrading Natural Gas

Abstract

Methane is an important alternative fuel, and upgrading it to improve fuel efficiency is an imperative target. Solid sorbents capable of selectively removing the major impurities CO2 and N2 from the natural gas contribute immensely to this process. We report a porous 3D iron-MOF built by linking scarce Fe4O18N2 clusters through readily available terephthalate and diaminotrizaole ligands. The 1-D channels with a high density of polarizing amine groups, aromatic rings, and carboxylate oxygen adsorb CO2 and the even less polarizable CH4. The MOF uptakes 4.7 mmol/g of CO2 at 273 K, 1 bar, with an optimal heat of adsorption of ≈24.5 kJ/mol and CO2/N2 IAST selectivity of ≈26. At higher pressures, the MOF exhibits a Langmuir type isotherm for methane and nitrogen with a CH4/N2 IAST selectivity of ≈4. The MOF’s excellent cyclic stability is affirmed by the TGA- and iso-cycling. Modeling studies propound the amine’s interactions with the CO2, but more dominant is the CO2···CO2 cooperative interactions. At 20 bar, CH4 interacts with many framework sites through weak dispersive interactions. In contrast, N2 interacts specifically with the triazole moiety; thus, the MOF favors the former. The CO2, CH4, and N2 diffusion coefficients, calculated using MD simulations, are quite favorable (Dc for CO2 = 1.11 × 10–6; CH4 = 9.04 × 10–6; N2 = 1.875 × 10–5 cm2/s). The dynamic breakthrough studies confirm the potential of the Fe-MOF to separate the gas mixtures. With these advantageous sorbent characteristics of this Fe-MOF, we propose using it in a two-stage PSA for the natural gas purification process, Stage I: removal of CO2 and Stage II: removal of N2. The outcomes point to the potential of a readily accessible iron-based amine MOF as sorbent for natural gas upgrading. A process optimization using a 4-step PSA validates the ability of our MOF to yield >96% purity of CH4 as required for pipeline transportation.