A novel pyrimidine-bridged organoalkoxysilane precursor, 4,6-bis(3-triethoxysily1-1-propoxy)-1,3-pyrimidine (BTPP), was synthesized via a platinum-catalyzed reaction and subsequently a BTPP membrane was developed via a sol-gel process. This new membrane exhibits great potential for improved CO2 separation compared with common amine-functionalized organosilica membranes. Possible CO2 transport mechanisms across amine-functionalized organosilica membranes were explored by comparing the CO2 sorption behaviors of organoalkoxysilane materials with both strong and mild affinities. The results showed that a membrane with strong affinity to CO2 does not lead to a superior CO2 separation capability. Hence, the concept of a "mild-affinity membrane" was proposed, which describes membranes with intermediate-to-low CO2 binding energy that exhibits an improved level of CO2 transport efficiency. The concept of a mild-affinity membrane offers the development of technological innovation leading to the realization of highly efficient CO2 separation membranes. (C) 2017 Elsevier B.V. All rights reserved.