Effect of Synthesis Temperature Formation of the Structure and Properties of Silicoaluminophosphate with the AEI Structure

Abstract

Changes in the phase, morphological, and textural properties of silicoaluminophosphates with the AEI structure during crystallization at 150, 170, and 190°С are studied. Using a combination of physicochemical methods, evolution of the solid phase extracted at different steps of synthesis is shown. It is established that at 150°С the microporous phase containing silicon in tetrahedral positions of the framework appears already at the initial stages of crystallization and further nanostructuring of the solid phase is associated with the incorporation of silicon via the SM3 mechanism. At 170°С, crystallization proceeds through formation of the aluminophosphate and aluminosilicate mixed mesoporous composite. The transformation of mixed composite into the microporous crystalline material is accompanied by the incorporation of [SiO4] isolated tetrahedra into the forming crystalline framework according to the SM2 mechanism; this provides a strong acidity of the material. The intermediate mesoporous semiproduct extracted at the initial stages of synthesis at 190°С is a mixture of disordered aluminophosphate and coarse mesoporous agglomerates; their transformation into silicoaluminophosphate with the AEI structure occurs only upon silica transfer to the liquid phase, and the incorporation of silicon into the AEI crystalline framework proceeds via the SM3 mechanism with the subsequent formation of silicon islands.