Functionalized mixed matrix membranes (MMMs) have the potential to upgrade the separation performance of conventional polymeric membranes while as yet keeping their processing benefits and lower costs. This study investigated the highly stable zirconium based metal-organic framework UiO-66 functionalized with sulfonic group, which was grafted by silane coupling agent mercaptopropyl trimethoxysilane. Sulfonic group imparted high separation properties to UiO-66. Polysulfone (PSf) was used as a polymer matrix and MMMs were synthesized for non-functionalized and functionalized UiO-66; and their results were compared. The characterization and permeation results ascribed that functionalized UiO-66 adhered well with the PSf matrix. The results demonstrated that increasing sulfonic group concentration resulted in higher selectivities owing to their high affinity towards CO2 gas molecules. The MMMs exhibited high CO2 permeabilities and selectivities under pure and mixed gas conditions; and proved this material promising for CO2 separation. In order to evaluate the practical commercial viability of these MMMs, investigation was conducted under different temperatures and compared with existing literature.