High surface area nanosized α-alumina has been obtained by thermally treating a sol−gel-derived mesophase at 1200 °C; the mesophase was synthesized by a sol−gel route involving evaporation induced self-assembly (EISA) of a hydrolyzed gel from Al-tri-sec-butoxide in s-BuOH in the presence of a nonionic surfactant (EO 20 PO 70 EO 20 ), HCl as catalyst, and water (H 2 O/Al = 6). The activated material renders moderate surface areas of about 8.4−10 m 2 g −1 , associated with significant crystallite coarsening. The key aspect to produce smaller crystallites is making the mesophase more resistant to coarsening. This was achieved by enhancing the condensation step by treating the hydrolyzed gel with tetrabutyl ammonium hydroxide (TBAOH) before evaporation. The characteristics of the mesophase indicate condensation of the primary particles with less AlO 5 unsaturated sites, at the expense of a lower solid yield due to small crystallites dissolution. The activated TBAOH condensed EISA material is composed of α-alumina aggregated crystallites of about 60−100 nm, and the material possesses surface areas ranging from 16 to 24 m 2 g −1 due to the improved resistance to coarsening. At least two aspects are suggested to play a role in this. The worm-hole morphology of the mesophase aggregates yields high particle coordination, which favors densification rather than coarsening. Furthermore, the decrease of the AlO 5 defect sites by the TBAOH condensation makes the mesophase less reactive and consequently more resistant to coarsening.
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Commercially available g-Al 2 O 3 was calcined at temperatures between 500 and 1200 1C and tested for its performance in the oxidative ethylbenzene dehydrogenation (ODH) over a wide range of industrially-relevant conditions. The original g-Al 2 O 3 , as well as Zand a-Al 2 O 3 , were tested. A calcination temperature around 1000/1050 1C turned out to be optimal for the ODH performance.Upon calcination the number of acid sites (from 637 to 436 mmol g À1 ) and surface area (from 272 to 119 m 2 g À1 ) decrease, whereas the acid site density increases (from 1.4 to 2.4 sites per nm 2 ). Less coke, being the active catalyst, is formed during ODH on the Al-1000 sample compared to g-Al 2 O 3 (30.8 wt% vs. 21.6 wt%), but the coke surface coverage increases. Compared with g-Al 2 O 3 , the EB conversion increased from 36% to 42% and the ST selectivity increased from 83% to 87%. For an optimal ST selectivity the catalyst should contain enough coke to attain full conversion of the limiting reactant oxygen. The reactivity of the coke is changed due to the higher density and strength of the Lewis acid sites that are formed by the high temperature calcination. The Al-1000 sample and all other investigated catalysts lost ODH activity with time on stream. The loss of selectivity towards more CO X formation is directly correlated with the amount of coke.
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