Effect of aluminium source and content on the synthesis of zeolite ZSM-5 from kanemite via solid-state transformation

Abstract

The effect of the incorporation of aluminium during the synthesis of zeolite ZSM-5 from kanemite via solid-state transformation was studied using X-ray powder diffraction, 29Si and 27Al NMR spectroscopy, thermal analysis and elemental analysis. During cation exchange, which takes place in highly basic media, silicate layers of kanemite give rise to the formation of silica colloids and shorter silicate layers. For Si/Al ratios ≥19, some aluminium is incorporated into the short silicate layers, however, most of the aluminium is incorporated into the Si–O–Si network during the condensation of the silica colloids in silica polymers when the pH is lowered to 8.5, independent of the aluminium source used (NaAlO2, Al2(SO4)3 or Al(NO3)3·9H2O). Mesoporous materials are synthesized from Al-containing kanemite, that is, aluminium is added during the synthesis of kanemite. The formation of silica colloids during cation exchange is the reason why, unlike the synthesis of mesoporous materials, aluminium could be added to the kanemite-template mixture during cation exchange. The highest amount of incorporated aluminium was obtained using NaAlO2 because it is the aluminium source for which the pH is highest during cation exchange and consequently it is the aluminium source which yields the highest amount of silica colloid precipitates during the adjustment of the pH to 8.5.