Nanoscale composition inhomogeneity in silica-aluminas prepared by variousmethods

A strong heterogeneity of composition was found in all the silica-aluminas prepared by sol-gel methods that we have examined. The Si/Al ratio at 50 to 75 points in each sample was determined by EDS X-ray spectroscopy using an analytical electron microscope. The area measured at each point was smalle r than 10(3) nm(2). Relative fluctuations of the Si/Al ratio ranged from 10 % to 20% for the most homogeneous samples, up to a factor of 100 in other c ases. Ln some samples particles were found which contained solely silica. We have examined on the one hand a series of laboratory samples prepared by the co-hydrolysis of alkoxides over a large range of composition and, on t he other hand, industrial catalysts of very similar compositions. The latte r ones were prepared by various sol-gel methods probably based on using pre cursors other than alkoxides. impurities were also analysed. On the whole, the industrial samples are often more heterogeneous than the laboratory one s, although one of them was almost as homogeneous as the laboratory samples . Most of the industrial catalysts contained less impurities than those pre pared in the laboratory. (C) 1999 Elsevier Science B.V. All rights reserved .