Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite

Mechanical activation of boehmite (γ-AlOOH), synthesized by thermal decomposition of gibbsite, has been carried out in a planetary mill up to 240 min. After an initial decrease in particle size up to 15 min, the particle size shows an increase with further milling; the median size (d₅₀) has increased from 1.8 to 5 μm during 15 to 240 min of milling. Quite unexpectedly, the BET specific surface area of the sample (N₂ adsorption method) decreases continuously from 264 m²/g to 67 m²/g with milling. A detailed analysis of N₂ adsorption/desorption isotherms has indicated that the decrease in surface area is associated with: (a) change in narrow slit like pores with microporosity to slit shaped pores originating from loose aggregate of platelet type particles; and (b) shift of maxima in pore size distribution plot at ~ 2 nm and ~ 4 nm to dominantly ~ 23 nm size pores. Scanning electron microscopy (SEM) studies have revealed that during milling, initial breakage is followed by agglomeration/fusion of particles with consequent loss in porosity. Amorphisation, decrease in microcrystallite dimension (MCD) and increase in microstrain (ε) are indicated from a detailed analysis of X-ray powder diffraction patterns and Fourier Transform Infrared (FTIR) spectra. Reactivity of samples, expressed in terms of increase in dissolution in alkali (in 8 M NaOH at 90 °C) and decrease in boehmite to γ-Al₂O₃ transformation temperature, increases with milling time. The nature of correlations between reactivity and physico-chemical changes during milling has been analyzed and discussed.