Hybrid membranes are promising materials for the purification of natural gas from carbon dioxide. The present paper investigates the effect of the incorporation of primary and secondary amine functional groups on the performance of an organic inorganic hybrid silica membrane for CO2/CH4 separation. Hybrid membranes were synthesized by chemical vapor deposition using 3-aminopropyltrimethoxysilane and (3-methylaminopropyl)trimethoxysilane as primary and secondary alkylamine-silica precursors, respectively. The amino functionalized membranes were compared to an amine-free membrane prepared using propyltrimethoxysilane as precusor. The amine-free membrane had a pore size of 0.37 nrn, and at 393 K a CO2 permeance of 2.1 x 10(-8) mol m(-2) s(-1) Pa-1 and a CO2/CH4 selectivity of 4. The primary amine membrane had a pore size of 0.36 nm, and at 393 K displayed a CO2 permeance of 2.1 x 10(-8) mol m(-2) s(-1) Pa-1 and a CO2/CH4 selectivity of 70. The secondary amine hmembrane had a pore size of 0.43 nm and achieved a CO2 permeance of 1.3 x 10(-7) mol m(-2) s(-1) Pa-1 and a CO2/CH4 selectivity of 140. The pore sizes were estimated by Tsuru`s method. The transport mechanism of CO2 throughout the amino silica hybrid membranes was surface diffusion. The secondary amino silica hybrid membrane was stable for 60 h under a relative humidity of 20%. (C) 2014 Elsevier B.V. All rights reserved.