Metakaolin-based geopolymer – Zeolite NaA composites as CO2 adsorbents

Abstract

In this work, three metakaolin-based geopolymer matrices were prepared, varying the molar ratio Si:Al (2.0 or 1.2) and the type of cation (sodium or potassium). Starting from these matrices, geopolymer-zeolite composites were synthesized and consolidated (80 °C), incorporating a commercial sodium-based zeolite, Na4A, with the aim of producing post combustion CO2 adsorbents, since the presence of Na4A crystalline phase is desirable due to its known remarkable CO2 adsorption capacity.The sodium-based geopolymer matrix with Si:Al = 1.2 allowed the in situ nucleation of the zeolite NaA, therefore this matrix was added with different amount of synthetic zeolite Na4A to verify the total conversion of the matrix into zeolite NaA, in view of an alternative low-cost synthesis method to obtain zeolite NaA as a “solid” in a complex form.The composites were deeply characterized and lastly tested for CO2 adsorption. The geopolymer matrices act as binders allowing the shaping of zeolite and producing functional composites with mutable chemical composition, microstructure and porosity according to the starting composition. The sodium-based geopolymer zeolite composite was the best performing in term of CO2 adsorption capacity being 1.0 mmol g−1 at 0.1 bar, nearly equivalent to synthetic zeolite Na4A (1.2 mmol g−1) and close to pure zeolite Na13X (1.4 mmol g−1), the current benchmark material for carbon capture application.