EFFECT OF AGING ON NONHYDROLYTIC ALUMINA XEROGELS

Nonhydrolytic sol-gel route is a relatively recent process which enabl es production of complex, multicomponent oxide materials. This process has some advantages over the conventional hydrolytic sol-gel route du e to the ability to produce low-shrinkage, homogeneous, multicomponent gels. The objective of this work was to determine the effects of agin g of nonhydrolytic gels on the composition, yield, phase transformatio ns and morphology. Xerogels were prepared from aluminum chloride and i sopropyl ether. Properties were studied using AgNO3 titrations, TGA/DT A, XRD, and BET analysis. We have found that the gels contain signific ant amount of chlorine where the Cl/Al atomic ratio ranges from 1.1-0. 6 depending on the aging time. The crystallization temperature and ent halpy of crystallization decreased with aging time. The decrease of th e surface area near the crystallization temperature correlates well wi th the decrease of the enthalpy of crystallization as a function of ag ing time. A closed pore phenomenon has been observed in the nonhydroly tic alumina system. Finally, analysis of the condensation degree (CD) yielding Al-O-Al bonds suggests that the rate determining step before the gel point is the alkoxy groups formation. However, during aging of the gels, the CD remains constant since the condensation of chloride with isopropoxy groups is stericly inhibited. Surface areas in the 300 -650 m(2)/g range were obtained depending on the aging time.