Related surface and heat of immersion characterization of some alumina samples II: Water-soaked activated alumina

Abstract

Soaked alumina was prepared by leaving Rhône Poulenc activated alumina in doubly distilled water for 24 h. The activated alumina was then air dried. Both processes were carried out at room temperature. The soaked alumina was thermally dehydrated in vacuo at various temperatures in the range 20–500 °C. Water losses (in grams of water per 100 g of the sample) for all the heat-treated samples were determined; it was shown that the soaked alumina samples exhibit higher water losses than those determined for the parent activated alumina samples. This was considered to be ample evidence for chemisorption and/or strong uptake of water molecules at the surface and in the pore system of the bulk alumina. Nitrogen adsorption measurements were carried out at liquid nitrogen temperature for all the samples. All the isotherms were of type II according to the classification of Emmett and Brunauer and were characterized by the occurrence of hysteresis loops. The areas of the hysteresis loops initially increased up to 200 °C; decreases in the areas of the hysteresis loops were then noted up to 400 °C. The increase in area of the hysteresis loops below 200 °C is attributed to the increased loss of physisorbed water. The decreased rate of water loss (water loss from the surface and/or structural water loss) corresponds to the observed decrease in area of the hysteresis loops up to 400 °C. Analysis suggested that the surface areas are mostly located in wide pores. The presence of some narrow pores (or micropores) was detected mainly at temperatures above 300 °C. Heat of immersion measurements were carried out using water as the immersion liquid for all the samples investigated. Despite the observed increase in water loss and nitrogen surface area for the soaked samples, the parent activated and soaked alumina samples exhibit almost the same integral heats of immersion up to 300 °C. At higher temperatures, the soaked alumina exhibited higher heats of immersion in water, which is directly related to the high extent of dehydration (or to the dehydroxylation of the surface hydroxyl groups) and the exposure of both aluminium ions (Al 3+) and oxygen anions (O 2-) to the surface, which enhance the formation of excess hydrous alumina and/or aluminium hydroxide. On calculation of the heats of immersion in water per unit area, it is found that activated alumina has higher heats of immersion up to 300 °C, whereas the soaked alumina preheated above 300 °C has higher heats of immersion because of the higher concentration of surface hydroxyl groups as well as the increased Al 3+ and O 2- ion content of these soaked samples.