BACKGROUND Copper-benzene-1, 3, 5-tricarboxylic acid (CuBTC) is a pre-eminent member of the metal-organic framework material family with excellent CO2 adsorption capacity and size-sieving characteristics for CO2/methane (CH4) separation. Although a few reports have addressed the use of micro-sized CuBTC particles in mixed matrix membranes (MMMs), the application of nano-sized particles has not yet been explored. RESULTS Micro-sized and nano-sized CuBTC particles were synthesized by Cu(NO3)(2)center dot 3H(2)O and Cu(OH)(2) precursors, respectively. Mixed matrix membranes composed of 5-35 wt% CuBTC in poly (amide-6-b-ethylene oxide) (PEBA) were fabricated to investigate the CO2/CH4 separation performance of the samples. Synthesized particles and membranes were characterized by X-ray diffraction, Fourier transform infrared and field emission scanning electron microscopy techniques. Membrane separation performance was determined at different pressures of pure CO2, CH4 and N-2. At 35 wt% loading of nano-sized CuBTC, CO2 permeability and CO2/CH4 selectivity were enhanced to 178.9 Barrer and 34.6; 80.3% and 13.8% higher than those of pristine polymer, respectively. The corresponding results for micro-sized CuBTC were 138.9 Barrer and 33.9. A mixed gas permeation test carried out on a mixture of CO2-CH4 (10:90 mol%) at 12 bar showed the best separation performance at 25 wt% loading of nano-sized CuBTC with CO2 permeability of 92.6 Barrer and CO2/CH4 selectivity of 23.1. CONCLUSION Filler size had a great impact on the MMM separation performance. As a result of the nano-sizing of CuBTC particles, higher CO2 permeability and CO2/CH4 selectivity could be obtained because of the uniform dispersion of the filler in the matrix and higher interface area/volume. (c) 2020 Society of Chemical Industry