Synthesis of SAPO-34 from metakaolin: Crystallization mechanism of SAPO-34 and transformation processes of metakaolin

SAPO-34 molecular sieve was hydrothermally synthesized using metakaolin as the single Si and partial Al source. Crystallization mechanism of SAPO-34 and transformation processes of metakaolin were investigated by studying effect of metakaolin content on the products, and Si, Al and P coordination environments of the products by XRD, SEM, EDX, N2 adsorption–desorption and NMR. The results showed that SAPO-34 with high purity could be prepared within a wide range of metakaolin content. Synthesis of SAPO-34 with all Si in isolated state demonstrated that SAPO-34 crystallization proceeded by a solution–mediated transport mechanism. Increase of the Si/P ratio in the medium caused increase of Si content in the newly formed SAPO-34 lattice. Transformation processes of metakaolin were followed by (1) dissolution of the Al–O and Si–O layers, (2) appearance of the primary building units (PBUs) in a low Si content, (3) initial crystallization of SAPO-34 in isolated Si state, (4) occurrence of the PBUs in a high Si content, (5) incorporation of Si domains into SAPO-34 crystals and (6) formation of the Si-rich amorphous materials. The results drawn are useful to understand crystallization mechanisms of SAPO molecular sieves from metakaolin and from other Si and/or Al chemicals.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► SAPO-34 with all lattice Si in isolated state is prepared from metakaolin. ► SAPO-34 crystal growth proceeds via a solution-mediation mechanism. ► The Si/P ratio increase in the medium causes Si content increase in the lattice. ► Accordingly, transformation processes of metakaolin are divided in six steps.