Influence of aluminium source on the crystal structure and framework coordination of Al and Si in fly ash-based zeolite NaA

Abstract

In this study zeolite NaA with different crystal sizes and % crystallinity was prepared from a clear solution extract of fused fly ash. Sodium aluminate or aluminium hydroxide was used to adjust the aluminium content in the fused fly ash extract and hence adjust the Si/Al ratio of the various synthesis molar regimes for the synthesis of zeolite NaA. Each molar regime was hydrothermally treated for 3 h at 100 °C. Chemical, mineralogical, morphological and structural composition of the synthesized products was determined using XRF, laser ablation ICP-MS, XRD, SEM and 29Si and 27Al NMR. This study showed that the variation of Si/Al ratio of the synthesis mixture (0.27 to 0.80) allowed control of zeolite NaA crystal/sizes that ranged from 0.67 to 3.18 µm. The decrease in Si/Al ratio of the synthesis mixture was proportional to the crystal size (µm) and the 29Si NMR peak width and area of the synthesized zeolite NaA. The increase in crystallinity and decreases in crystal size caused the 29Si NMR peak width to be narrower as the peak area reduces. Interestingly, the Si/Al ratio of the synthesis mixture was inversely proportional to the crystallinity of zeolite NaA when Na-aluminate was used as Al source. The effect of Al source was also noticed by the appearance of a small additional distorted tetrahedral Al peak between 15 and 17 ppm (27Al NMR) when Al-hydroxide was added to fused fly ash extract.