Related surface and heat of immersion characterization of some alumina samples I: Rhône poulenc activated alumina

Abstract

Activated alumina was thermally dehydrated in vacuo at different temperatures in the range 20 – 500 °C. Nitrogen adsorption measurements, heats of immersion in water, methanol and carbon tetrachloride, and water losses (in grams of water per 100 g of the sample) of the different heat-treated samples were estimated and correlated. Nitrogen adsorption measurements show the occurrence of type II adsorption isotherms which are characterized by hysteresis phenomena. The area of the hydteresis loop increases up to 200 °C; thereafter, it decreases with further increase temperature until 500 °C is reached. An initial increase in the area of the hysteresis loop (below 200 °C) is interpreted on the basis of the increased physisorbed water loss up to 200 °C. The decreased area of the hysteresis loop at higher temperatures (above 200 °C) is attributed to the decreased rate of water loss at these high temperatures. Water losses were found to increase abruptly between 200 and 300 °C and to increase gradually in the lower and higher temperatures range until 500 °C is reached. These changes in water loss are consistent with the changes in the heats of immersion in water, to a lesser extent with the changes in the heats of immersion in methanol and to a limited extent with the changes in the heats of immersion in carbon tetrachloride. Generally, the heats of immersion follow the order water > methanol > carbon tetrachloride; this has been related to changes in the chemistry of the alumina surface and is also discussed in relation to the porosity characteristics. From pore structure studies it was found that these samples contain both mesopores (or wide pores) and narrow pores (or micropores). After an analysis for the surface areas located in wide pores, the heats of immersion in water were found to follow the fraction of the area located in wide pores (or mesopores); this has been interpreted on the basis of a reduction in the mutual repulsions encountered between water molecules inside these mesopores or wide pores.