In Situ Synthesis and Microstructure Manipulation of SAPO-5 Films over Porous α-Al2O3 Substrates

The synthesis and microstructure manipulation of SAPO-5 films on porous alpha-Al2O3 substrates (samples F-1 through F-6) have been studied with die main focus on die effect of Si content, substrate setting, and the structure-directing function of a precoated chitosan thin layer. Without the chitosan layer, the supported SAPO-5 film synthesized under traditional heating is not continuous with either pin holes or crystal boundaries. The preferentially c-oriented SAPO-5 top-layer over a closely packed layer of SAPO-5 crystals was synthesized on the porous alpha-Al2O3 substrate using the chitosan thin layer as the structure-directing matrix. According to the characterization using scanning electron microscopy (SEM) and X-ray diffraction (XRD), the thickness of the c-oriented SAPO-5 top-layer is changed from 1.5 to 10 mu m while keeping the thickness of the packed layer of SAPO-5 crystals almost unchanged. Electron probe microanalysis (EPMA) demonstrates that P, Al, Si, and C contents within the two-layer film are uniformly distributed. The percentage of c-oriented crystal grains in the resulting SAPO-5 films evaluated by using the pole-figure analysis is over 70% (71.2% for F-4,74.2% for F-5, and 75.9% for F-6). On the basis of our experimental observations, it has been indicated that the gradual dissolution of chitosan in the acid precursor solution is accompanied with the formation of the SAPO-5 top-layer and the packed layer of SAPO-5 crystals in the hydrothermal reaction, and both the surface and the entire body of the chitosan layer serve as a three-dimensional structure-directing matrix.