Tubular alumina substrates have been widely used as supporting membranes in gas separation. Owing to the demand for supporting membranes with a dense or ultra-micropore texture, the quality/quantity control of substrates is required to prevent the formation of defects due to rough surfaces, high curvature, and high difference in thermal expansion between the polymer precursor and the alumina substrate. This study proposes a new strategy to modify the pore texture, surface properties, and thermal expansion coefficient of the substrate by filling it with TiO2 nanoparticles and using the grinding/polishing method. The effect of CMS preparation conditions, including coating cycles and pyrolysis temperature on the microstructure of the carbon matrix is also discussed. A tubular CMS membrane with excellent permselectivity toward H-2/CO, H-2/N-2, and O-2/N-2 (77.52, 162.94, and 13.87, respectively), and permeabilities of 55.9 barrer and 6.49 barrer are obtained for H-2 and O-2, respectively. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.