Impact of NH4OH treatment on the ion exchange and pore characteristics of a metakaolin-based geopolymer

Abstract

We investigated the viability and influence of NH₄OH post-synthetic treatment on the pore characteristics of geopolymers. Geopolymers are a class of materials with amorphous aluminosilicate three-dimensional frameworks, regarded as amorphous analogues of zeolites. Similar to zeolites, when geopolymers are used in catalysis or adsorption applications, post-synthetic treatments such as ion exchange with NH₄⁺ salts (e.g., NH₄Cl and NH₄NO₃) and desilication (using strong bases such as NaOH) are necessary to introduce active sites and modify their pore structure, respectively. Recently, it has been shown that treatment with NH₄OH combines these two steps, in which acidic sites are introduced and the pore structures of zeolites are modified simultaneously. Considering the increasing interest in geopolymers in catalysis and adsorption applications, understanding the impact of such treatment on the structure of geopolymers is needed. Our diffuse reflectance infrared Fourier-transform spectra show that NH₄⁺ exchanges Na⁺ in the geopolymer, and laser diffraction with scanning electron microscopy images show that the particle size of the powdered geopolymer decreases after NH₄OH treatment. N₂ sorption isotherms and ¹²⁹Xe and ¹H NMR measurements revealed information about the changes in pore structures: micropores were larger than mesopores and inborn mesopores increased in diameter, thereby reducing the surface area to volume ratio. However, pore accessibility and pore connectivity were not altered by NH₄OH treatment. Since solid-state NMR and X-ray fluorescence revealed desilication, these changes in particle size and pore characteristics are considered to be due to desilication caused by NH₄OH treatment.