A new methodology has been developed consisting of the use of the same solvent (tetrahydrofuran, THF) for both nanosized MOF MIL-68(Al) crystallization and polysulfone (PSF) mixed matrix membrane (MMM) casting, using this MOF as filler. The usual solvent for MIL-68 is dimethylformamide. However, the synthesis of this MOF has been studied here in THF (a solvent almost never used for MOFs) which makes possible the one-pot processability of both MOF and PSF, without the stages of crystal separation and drying from the mother liquor. The 8 wt% MIL-68(Al)@PSF MMMs were superior to the pure PSF membrane in the separation of both H2/CH4 and CO2/CH4 mixtures.