(600e) Membrane Adsorbents Comprising Self-Assembled Inorganic Nanocages (SINCs) for Direct Air Capture

There is a significant interest in solid sorbents with high CO₂ sorption capacity for economically viable direct air capture (DAC) of CO₂. Although current solid sorbents such as polyethylenimine (PEI)-supported silica demonstrate high CO₂-sorption capacity, its kinetics is slow partially because CO₂ has to diffuse into the highly filled pores. In this work, we design highly porous membrane adsorbents comprising PEI and self-assembled inorganic nanocages (SINCs) supported by porous membranes such as Solupor membrane with a porosity of 86%, a high-density polyethylene porous support. PEI is a leading amine-based material for DAC with high CO₂-sorption capacity. SINCs have small sizes (< 10 nm), which obviates the need for CO₂ to diffuse throughout large crystallites (such as MOFs), reducing adsorption and desorption times. Zirconium-carboxylate cages (Zr-SINCs) were synthesized and further functionalized with amines to enhance the CO₂ sorption capacity. The membrane-sorbent demonstrates high PEI content up to 48 wt.% while still retain a high porosity of 75%, leading to high CO₂ permeance (~80,000 GPU, 1 GPU = 10^-6 cc(STP)/cm²/s/cmHg). Increasing the PEI Mw from 800 g/mol to 25,000 g/mol not only increases the PEI content from 44 wt.% to 48 wt.% but also increases the porosity from 70% to 75%, indicating that smaller PEI can fill the support pore more easily. The CO₂ gas sorption increases to 0.61 mmol CO₂/g sorbent at 1 atm for a PEI-containing polysulfone (PSf) membrane, compared to 0.23 mmol CO₂/g sorbent of pristine PSf membrane. The effect of PEI molecular weight, PEI loading, and Zr-SINCs content in the membrane-sorbents on CO₂ sorption capacity and kinetic will be discussed.