The purpose of this study is the surface modification of a micro-sized nanoporous zeolite through a silylation reaction and the incorporation of the silylated particles into a homogeneous cellulose acetate (CA) membrane to achieve better polymer-zeolite adhesion in the corresponding mixed matrix membranes (MMMs). In the current study, 3-aminopropyl(diethoxy)methylsilane (APDEMS) was used as the silane coupling agent and micro-sized nanoporous sodium zeolite-Y (NaY) as the precursor zeolite. The unmodified pure form of the zeolite (NaY), the silane-modified zeolite (NaY-sm) and the corresponding MMMs were characterized using DLS, BET, FTIR-ATR, XRD, SEM, and TG/DTA analyses. Moreover, the CO₂/N₂ separation performances of the prepared membranes were evaluated through the gas permeation measurements. The results demonstrated that the modification results in an increase in average particle diameter, external surface area, and overall volume (or size) as well as a decrease in the micropore surface areas, volumes, and the crystallinities of the resultant modified zeolite particles. In addition, the modification led to an improvement in the uniformity of the particle distributions all over the MMM structure and a considerable reduction in the numbers/sizes of the undesirable cracks and agglomerates. The CO₂ permeability of CA increased about 77.19% at 4 bar for the CA/NaY-sm 20 wt% membrane. It could be almost concluded that not only did the CO₂ permeability of CA/NaY-sm membranes not decrease when compared to CA/NaY membranes, but the permeability also actually showed an average 4.67% increase. Moreover, an average CO₂/N₂ selectivity of 6.34% was obtained for NaY-sm filled MMMs compared to those filled with the pure NaY (≥4 bar). This can be a significant contribution to developing new materials in the field.