In this study, polyetherimide (Ultem) was chosen as a polymeric matrix, and micron‐sized Ni2+‐exchanged nano‐porous NaX zeolite was selected as an inorganic filler to fabricate novel mixed matrix membranes (MMMs) for CO2 separation. Fabricated membranes were evaluated by Differential scanning calorimeter, Fourier transform infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, and energy dispersive X‐ray. The influences of filler loading (0–5 wt.%) and feed pressure (2–10 bar) on gas separation properties were surveyed. CO2/N2 ideal selectivity of 3 wt.% NiX loaded MMM (optimum MMM) was increased to 34 (about 36%) compared to 25 for the pristine membrane. Furthermore, CO2 permeability was improved by about 45%, from 1.3 Barrer for the pure membrane to 1.88 Barrer for MMM containing 3 wt.% functionalized filler. These improvements originate from a combination of the inherent surface diffusion mechanism of zeolite X and reversible reaction between free orbitals of transition metal ions (Ni2+) and electron pairs of CO2 molecules. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.