Kinetics of sintering of polyethilene glycol and lanthanum doped aluminum oxide obtained by the sol-gel method

Abstract

Sintering and crystallization of low-density polyethylene glycol (PEG) and lanthanum, La(III)-doped Al2O3 aerogels prepared from aluminum isopropoxide were investigated. The sintering behavior of non-doped and doped aerogels was examined by following the change of specific surface area with isothermal heat-treatment. The specific surface area and crystalline phases of non-doped and PEG+ +La(III)-doped aerogels were determined, and the effects of dopants on the sintering and crystallization of Al2O3 aerogels are discussed. Isothermal sintering experiments showed that the sintering mechanism of non-doped and PEG+La(III)-doped Al2O3 aerogels is surface diffusion. The specific surface areas of alumina samples decrease rapidly during the initial period of sintering, and more slowly with prolonged sintering time. The change of the porous structure is correlated with the phase transformation of γ-Al2O3 during calcinations of Al2O3 aerogels. The surface area of non-doped Al2O3 aerogels came to about 20 m2 g-1 with heat-treatment at 1100°C because of crystallization of α-Al2O3 after densification. In the case of heat-treatment at 1200°C, the largest surface area was observed for PEG+La(III) doped Al2O3 aerogels and the XRD pattern showed only low ordered θ-Al2O3. These indicate that the addition of PEG+La(III) to boehmite sol prevents Al2O3 aerogels from sintering and crystallizing to the α-Al2O3 phase. Even after 20 h at 1000°C, PEG+La (III)-doped alumina samples maintain a rather good specific surface area (108 m2 g-1) in comparison to the non-doped, containing mainly θ-Al2O3 and minor amounts of δ-Al2O3. Aluminum-oxides with these structural and textural properties are widely used as coatings and catalyst supports in various fields of catalysis.

U ovom radu je ispitivan uticaj dodataka polietilen-glikola i lantana na mehanizam sinterovanja i termostabilnost aktivnog poroznog aluminijum-oksida dobijenog sol-gel metodom. Rezultati su pokazali da se proces izotermskog sinterovanja odvija mehanizmom površinske difuzije materijala i da sa dodatkom polietilen-glikola i lantana ne dolazi do promene mehanizma procesa sinterovanja. Zapaženo je da specifična površina aluminijum-oksida sa dodatkom i bez dodatka polietilen-glikola i lantana na početku procesa sinterovanja brzo opada, dok je tokom vremena taj pad sve sporiji. Promene parametara porozne strukture su korelisane sa faznim transformacijama γ-Al2O3 nastalim u toku procesa sinterovanja. Rendgeno-strukturna analiza je pokazala da uzorak bez dodataka sinterovan na 1000°C sadrži uglavnom θ-Al2O3 fazu, dok se na 1100°C potpuno transformiše u α-Al2O3 fazu. Međutim, kod uzorka aluminijum-oksida sa dodatkom polietilen-glikola i lantana nakon 20 h žarenja na 1000°C primećen je tek začetak θ-Al2O3 faze.