Potential of a Water-Stable Ultramicroporous Aluminum Gallate Metal Organic Framework as Sorbent for Upgradation of Natural Gas

Abstract

Developing easily accessible metal–organic framework (MOF) sorbents with industrially relevant gas separation capabilities is desirable. This can be achieved by constructing MOFs from simple ligands and ubiquitous benign metals. Aluminum–oxygen bonds are generally stable, hence crystalline aluminum MOFs from oxygen-rich compact ligands can add new sorbents with volumetric-gravimetric advantages. Here, we present a water-stable ultramicroporous Aluminum-Gallate MOF that demonstrates good CH4 selectivity over N2 and noticeable CO2/N2 selectivity (based on IAST selectivity at 313 K, CO2/N2 = 40; heat of adsorption (HOA) for CO2 is constant over entire loading with an average value of 30 kJ/mol; CH4/N2 selectivity at 20 °C ∼6.2; HOA for CH4 = ∼23 kJ/mol). Notably, this MOF adsorbs substantially more CH4 than other transition metal gallates. At higher pressures (1–20 bar), the MOF retains this higher uptake for CH4 over N2. We have calculated the high pressure CH4/N2 selectivity values at 5 bar and 20 °C for three different compositions 85%CH4:15%N2, selectivity = 0.9; 75%CH4:25%N2, selectivity = 1.7; 65%CH4:35%N2, selectivity = 2.7. Superior adsorption of CH4 over N2 is well supported by the dynamic separation studies (dynamic breakthrough capacity for CH4 = 1.05 mmol/g). Its potential as practical natural gas purification sorbent is investigated using a 4-step PVSA process modeling. For a 0.5–5 bar pressure swing the MOF is capable of delivering 99.9% purity with greater than 80% recovery from an 85%CH4:15%N2 stream; the achieved purity of CH4 meets pipeline transportation quality. The favorable composition, structure, gas separation capacity and stability make this aluminum gallate MOF an impactful candidate for natural gas purification.