Effect of AI(3+) introduction into hydrothermally prepared ZnFe2O4

The effect of aluminum introduction into the zinc ferrite ZnFe2-xAlxO4 spin el structure was studied in the concentration range of 0.0 less than or equ al to x less than or equal to 1.0. Spinel ferrites were obtained by hydroth ermal method at 140 degrees C in the whole range of Al concentration (x). T he resulting powders were calcined at 550 and 750 degrees C, and their phys icochemical and catalytic properties were compared with those prepared by c oprecipitation method [J.A. Toledo, P, Bosch, M.A. Valenzuela, A. Montoya, N. Nava, J. Mol. Catal, 125 (1997) 53]. As in coprecipitated samples, isomo rphic substitution of Fe3+ by Al3+ into the octahedral sites was observed. The aluminum introduction gives rise to a lattice distortion caused by the introduction of a metal with smaller atomic radius. This lattice distortion facilitates a charge transfer from Fe3+ to O2-, which increases the basici ty of the oxygen atoms in the Fe-O-Al bonds, increasing its proton affinity . Therefore, it favors the acid-base dissociation that takes place in the C -H bond during the abstraction of the hydrogen atom in the oxidative dehydr ogenation (OXD) process. After calcination at 550 degrees C, a maximum in t he intrinsic activity and butadiene yield was obtained for an Al concentrat ion of 0.2 less than or equal to x less than or equal to 0.5, whereas in co precipitated catalysts, a higher aluminum content was necessary (0.75 less than or equal to x less than or equal to 1.0). In the hydrothermally treate d samples calcined at 750 degrees C, the promoter effect of aluminum was no t evident. Indeed, the pure ZnFe2O4 showed the maximum activity to butadien e. However, the activity of the structural surface sites increased by a fac tor about 2 as the calcining temperature increased from 550 to 750 degrees C, (C) 2000 Elsevier Science B.V. All rights reserved.