A range of porous solid adsorbents were synthesised and their ability to separate and capture carbon dioxide (CO₂) from gas mixtures was examined. CO₂ separation from flue gas – a type of exhaust gas from fossil fuel combustion that consists of CO₂ mixed with mainly nitrogen and biogas (consists of CO₂ mixed with mainly methane) were explicitly considered. The selected adsorbents were chosen partly due to their narrow pore sizes. Narrow pores can differentiate gas molecules of different sizes via a kinetic separation mechanism: a large gas molecule should find it more difficult to enter a narrow pore. CO₂ has the smallest kinetic diameter in zeolites when compared with the other two gases in this study. Narrow pore adsorbents can therefore, show enhanced kinetic selectivity to adsorb CO₂ from a gas mixture.

The adsorbents tested in this study included mixed cation zeolite A, zeolite ZK-4, a range of aluminophosphates and silicoaluminophosphates, as well as two types of titanium silicates (ETS-4, CTS-1). These adsorbents were compared with one another from different aspects such as CO₂ capacity, CO₂ selectivity, cyclic performance, working capacity, cost of synthesis, etc. Each of the tested adsorbents has its advantages and disadvantages. Serval phosphates were identified as potentially good CO₂ adsorbents, but the high cost of their synthesis must be addressed in order to develop these adsorbents for applications.