In this study, three types of modified silica microspheres were functionalized with carboxyl, sulfonic acid group and pyridine group via a facile distillation-precipitation polymerization method, respectively. Each type of functionalized silica microspheres was incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate mixed matrix membranes (MMMs) for gas separation. Scanning electron microscopy (SEM) characterization indicated that all the three types of functionalized silica microspheres could disperse homogenously within the SPEEK matrix via tuning the polymer-particle interaction. The incorporation of silica microspheres and sulfonic acid functionalized silica microspheres resulted in increased free volume cavity (r(3)) in MMMs, whereas the pyridine functionalized silica microspheres led to decreased r(3). The relation between r(3) and gas diffusion coefficient was revealed: the higher r(3) displayed higher gas diffusion coefficient. The functionalized silica microspheres could construct CO2 transport pathways due to the increased CO2 adsorption, imparting MMMs with remarkably enhanced CO2 separation performance. In particular, the pyridine functionalized silica microspheres loaded MMMs showed the optimum gas separation performance with CO2/CH4(N-2) selectivity of 64.5 (68.3) and CO2 permeability of 2043 Barrer at loading of 20 wt% in humidified state, surpassing the Robeson upper bound reported in 2008, while the pristine SPEEK membrane exhibited CO2/CH4(N-2) selectivity of 26.7 (35.1) and CO2 permeability of 525 Barrer. (C) 2016 Elsevier B.V. All rights reserved.