Effect of acidic and basic vapors on pore size distribution of alumina under hydrothermal conditions

Abstract

The development of wide pore catalysts, with surface areas and mechanical strengths for application in, for example, the hydroprocessing of petroleum residues, has received increased attention in recent years. In the present paper, we have investigated the influence of acidic and basic vapors on pore enlargement of γ-alumina supports under low-temperature hydrothermal conditions. γ-Alumina in the form of 1.5 mm extrudates was subjected to hydrothermal treatment in an autoclave in the presence of acetic acid and ammonia for various time intervals in the temperature range 150–300 °C. The treated catalyst samples were characterized for surface area, pore size distribution and crushing strength. The samples were also examined by scanning and transmission electron microscopes and X-ray diffraction to assess the extent of sintering and possible changes in the alumina phase. The results of the studies revealed that ammonia enhances, whereas, acetic acid suppresses pore enlargement of γ-alumina under hydrothermal conditions. Surface area loss for samples treated under different atmospheres were of the order: ammonia + water > water > acetic acid + water. In the presence of ammonia, selective widening of pores in the 100–250Årange with improved side crushing strength was noticed at low or moderate temperatures (e.g. 150°C). The contrasting effects of ammonia and acetic acid vapors on the sintering and pore enlargement of γ-alumina under hydrothermal conditions is discussed on the basis of the influence of these reagents on hydroxylation of Al-O-Al bonds and the subsequent formation of boehmite and its recrystallization.