Thermal stabilization of alumina modified by lanthanum

Transition alumina, with different La loadings, were synthesized from boehmite (AlOOH) hydrosols containing a PEO/PPO/PEO triblock copolymer (Pluronics (R) P123) and lanthanum nitrate. After calcination at 500 degrees C, the xerogels prepared from these sols have large specific surface area (approximate to 400 m(2)/g) and very large porous volumes, increasing with the amount of La to reach 2.5 cm(3)/g for La/(La + Al) = 0.036. This material still kept a surface area close to 180 m(2)/g and a pore volume of 2.3 cm(3)/g when it was calcined at 1000 degrees C. However, after calcination at 1200 degrees C, the best textural properties (70 m(2)/g and 0.6 cm(3)/g) were observed for La/(La + Al) = 0.015. Larger La loadings led to the formation of LaAl11O18 and LaAlO3 and the detection of these mixed oxides was associated with a decrease of surface area and pore volume. The improvement of the thermal stability of these materials can be explained by the synergy of two effects: (i) the adsorption of copolymer onto fiber-like boehmite nanoparticles, preventing their compact rearrangement during the drying and maintaining a large porosity after calcination; (ii) the inhibition of the sintering process through the formation of thermally stable species between reactive surface sites (strong Lewis acid sites) and lanthanum atoms. (C) 2014 Elsevier Inc. All rights reserved.