The transformation of kaolin to low-silica X zeolite

Abstract

The thermal transformation of kaolin extrudates to metakaolin and the subsequent crystallization of low-silica X extrudates has been investigated by X-ray diffraction, scanning electron microscopy, FTIR spectroscopy, N₂-adsorption, ²⁷Al and ²⁹Si MAS NMR, and surface analysis. Kaolin transforms to metakaolin at 973 K. The macroporous metakaolin extrudates are converted to zeolite X plus a small amount of zeolite A in alkaline solution at 324 K. The ²⁷Al NMR signals of octahedral and pentacoordinated aluminium in metakaolin are removed after 30 min at 324 K, whereas the first XRD features of zeolites X and A are not detected until 48 h. Likewise, ²⁹Si NMR and FTIR spectra show that zeolite is first present at 48 h synthesis time. The amount of zeolite A decreases with increasing synthesis time, but there is an overall increase in product crystallinity and surface area up to 240 h. SEM analyses show that zeolite crystals first form in the interior of extrudates at the edges of macropores; with time the metakaolin matrix is consumed to produce well-crystalline zeolite extrudates consisting of ca. 95% low-silica X and 5% zeolite A.