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.