MIL-68(Al) and MIL-53(Al) are carboxylate-based metal-organic frameworks (MOFs) with the same chemical composition but different structures (polymorphs). In this study, MIL-53(Al) nanosheets of ca. 150 nm in size with an average thickness of 3.5 ± 0.9 nm were obtained after immersion of a sample composed of MIL-68(Al) and MIL-53(Al) in water under different conditions (ultrasound, stirring, reflux, 60 °C and room temperature). The disaggregated MIL-53(Al) nanosheets produced under more severe conditions were suspended in a PDMS solution and then deposited on asymmetric polyimide P84® supports under vacuum filtration to form supported mixed matrix membranes (MMMs). When applied to the separation of CO2/CH4 and CO2/N2 mixtures, the MMM with MIL-53(Al) nanosheets improved the CO2/CH4 (28.4-28.7 vs. 22.4) and CO2/N2 (19.9-23.2 vs. 17.5) selectivities of the conventional MIL-53(Al) MMM with higher CO2 permeances (20.8-29.6 GPU vs. 9.5 GPU for CO2/CH4 and 17.7-26.8 GPU vs. 11.2 GPU for CO2/N2).