High-performance SPEEK/amino acid salt membranes for CO2 separation

Abstract

Three types of amino acid salts (sodium lysine, sodium arginine, sodium histidine) were introduced into SPEEK polymer matrix to fabricate a series of SPEEK/amino acid salt membranes for CO₂ separation. The membranes could efficiently separate CO₂ from a CO₂/CH₄ mixture due to the simultaneous improvement of CO₂ permeability and separation performance. Firstly, the amino acid salt contained a larger number of amino groups (–NH₂), which could reversibly react with CO₂, improving the CO₂ molecules transfer performance. Secondly, the ether oxygen structure (–O–) in the polymer chain and the carboxyl groups (–COO⁻) in the amino acid salt had an excellent affinity for CO₂, enhancing the solubility selectivity. Thirdly, the introduction of amino acid salt increased the water uptake, and water was favored over CO₂ dissolution; this was also good for the increase of solubility selectivity. Among the three types of amino acid salts, the membrane doped with 20 wt% sodium lysine (SPEEK/LLS-20) showed the highest CO₂ permeability of 295.6 barrer and CO₂/CH₄ selectivity of 71.8 at 2 bar and room temperature, surpassing the 2008 Robeson upper bound line.