A systematic study has been made of the rate of change in the specific surface area and specific pore volume of silicaalumina cracking catalysts exposed to temperatures in the range 478-950°and steam partial pressures in the range 0-7 atmospheres. The decline in specific surface area is described accurately by an empirical equation of the form -dS/dí = kSn where S is the specific surface area, t is time, and n and k are constants at any given set of conditions. These constants vary smoothly with changing temperature and partial pressure of steam. The rate of change in specific pore volume is proportional to the rate of change of specific surface area at the highest temperatures, but decreases relative to the rate of change of specific surface area as the temperature decreases; changes in the partial pressure of steam at a given temperature have a small but definite effect on this relationship. The results are discussed in terms of a model for the structure of silicaalumina catalysts and of various possible mechanisms of material transport involved in the aging process.

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The physical changes that occur to silica and alumina gel upon exposure to steam at nioderate to high temperatures were examined and the aging of these important catalyst supports was compared to that of alumina-silica cracking catalyst. The surface area and pore volume of silica gel are less stable toward prolonged steaming than those of silica-alumina cracking catalysts. This results from the greater susceptibility of silica gel to fusion of large numbers of ultimate particles. Alumina shows no evidence of "fusion," as a considerable decline of surface area was accompanied by little or no loss of pore volume.