In-situ synthesis of zeolite X in foam geopolymer as a CO2 adsorbent

In this study, a green route that zeolite X accreted with coal fly ash (FA) based geopolymer foam materials (FGX) was firstly proposed and applied for carbon dioxide (CO2) adsorption. Zeolite X was synthesized in situ by partially transferring geopolymer gel (mainly N-A-S-H gel) in foamed geopolymer materials at hydrothermal environment. It was found that zeolite X directly crystallized on the pore walls and retained the basic pore framework of the geopolymer foam. The porous monolith-type material exhibited the bulk density as low as 400 kg/m3 and high compressive strength (3 MPa). Multigradient pores ranging from micro to macro and multi-crack distributions of FGX improved the mass transfer coefficient of the materials and allowed for packed bed CO2 adsorption. Equilibrium adsorption experiments showed that the maximum theoretical adsorption capacities of FGX reached 7.91 mmol/g for CO2 at 298.15 K. At the meantime, by investigating the CO2 dynamic adsorption processes at different adsorbent quantities, flow rates, and ratios of CO2/N2, it was concluded that higher flow rates, ratios of CO2/N2, and lower adsorbent quantities were more conducive to increasing the dynamic CO2 adsorption rate. Additionally, the composites exhibited good re-generation properties in five cyclic experiments (desorption loss <0.25%). This innovative material shows a very promising potential in CO2 adsorption.