Positron annihilation spectroscopic investigation of Al-pillared montmorillonites

We report, for the first time, our result on characterisation of Al-pillared clay using positron annihilation spectroscopy (PAS) along with XRD and BET techniques. Positron life-time spectra for Al-pillared montmorillonite, as received and calcined at different temperature (100-500 degrees C), along with the parent natural clay are measured. The annihilation rate of ortho-positronium (o-Ps) increases monotonically with calcination temperature up to 300 degrees C and falls subsequently. Similar behaviour is seen for the fraction of o-Ps formed. We interpret the initial rise in annihilation rate of o-Ps to be due to the increase in Bronsted acidity with dehydroxylation of pillars. The reduction in annihilation rate on calcination at higher temperatures could be due to loss of acidity and/or due to mesopore formation following delamination of clay structure.