Microspectroscopic studies of template interactions in AlPO4-5 and SAPO-5 crystals

Abstract

Individual crystals of the aluminophosphate AlPO₄-5 and the silicon-substituted aluminophosphate SAPO-5 synthesized with a triethylamine template have been studied by FTIR and Raman microspectroscopy. Both materials as synthesized contain the template in the protonated form. In AlPO₄-5, the protonated template is charge balanced by hydroxide ions. Dehydration causes elimination of water leaving the template in the free amine form, but this process is reversible. Measurements with polarized infrared radiation reveal that both the protonated and free template molecules are oriented with the C₃ axes perpendicular to the channel direction. In SAPO-5, deprotonation of the template also occurs on dehydration but to a lesser extent, and protonated template is still present after heating in acuo to 500 K. The balance between protonated and free template depends on the extent of hydration of the sample and the presence or absence of negative charge on the framework. After thermal calcination to remove the template completely, the single crystals of AlPO₄-5 are devoid of hydroxy groups, whereas SAPO-5 shows two types of hydroxy groups oriented perpendicular to the channel axes.