Temperature dependent photoacoustic measurements of the structural transformation of VPI-5 to AlPO4-8

Abstract

The structural transformation of VPI-5 into AIPO$_4$-8 has been investigated by temperature dependent photoacoustic (PA) measurements. The photoacoustic amplitude in general decreases with increasing temperature. This fact is superimposed on the specific effects of dehydration and phase transition. The effect of dehydration and phase transition(endothermal process) on the PA signal intensity has been studied using powdered sulfur as an example of having only phase transition and ZSM-5, AIPO$_4$-11 and alumina samples with chemisorbed water molecules but no structural transformations. On the other hand, VPI-5 sample has a water molecules chemisorbed within tye framework structure and also undergo the structural transformation. The observed enhancements of PA signal intensity for the VPI-5 samples evacuated below 200$^\circ\!$C are considered to be due to the periodic evaporation of adsorbed water molecules from the sample surfaces by absorbing the chopped light. These evaporated water molecules creat periodic pressure fluctuations in the boundary layer between sample surface and gas layer. Positive enhancement of the PA signal intensity was observed even though the dehydration is endothermal process. The sharp decreases in PA signal around 165$^\circ\!$C and 190$^\circ\!$C when the temperature was raised from room temperature are attributed to the Al-O-P bond breakage. These decreases can be considered as the measure of structural transformation of VPI-5 into AIPO$_4$-8. The dehydration induce the enhancement of PA signal intensity while the structural transformation in VPI-5 caused the large decrease in PA signal intensity offsetting the enhancement of PA signal intensity due to dehydration. The decrease of PA signal intensity due to Al-O-P bond breakage was observed more distinctly by the enhancement effect of PA signal when the water molecules were desorbed during the structural transformation process. Two endothermal processes, namely dehydration and bond br...