Synthesis of zeolite Na-A from metakaolinite revisited

Abstract

²⁹ Si and ²⁷Al NMR, XRD, IR, SEM and EDAX have been used to examine the synthesis of zeolite Na-A from metakaolinite. When metakaolinite is treated with 5 mol dm^–3 NaOH, five- and six-coordinated Al in the aluminous matrix is rapidly converted into four-coordination. The siliceous matrix of kaolinite is partially preserved in metakaolinite and is responsible for the residual long-range order in the ab plane. The siliceous matrix is much more stable to the base than the aluminous matrix. ²⁷Al NMR spectra recorded with fast MAS (> 12 kHz) allow quantification of Al and show that metakaolinite is most reactive after treatment at ca. 700 °C, when the populations of four- and five-coordinated Al are at a maximum and the population of six-coordinated Al at a minimum. Upon treatment with alkali, metakaolinite slowly dissolves. SiO⁴⁺₄ and Al(OH)4^–₄ ions in solution condense to an amorphous phase with Si/Al = 1. This direct precursor transforms into zeolite A by a solid-state rearrangement.