Synthesis of zeolite Na-A from metakaolinite revisited

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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.

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