Al and Ti-containing mesoporous molecular sieves:: Synthesis, characterization and redox activity in the anthracene oxidation

Mesoporous materials containing aluminum and titanium were obtained by direct synthesis at room temperature, using a sol-gel method with two reaction steps (acid and alkaline hydrolysis). After solvent extraction and calcination (823 K), three solids, H-MCM-41, Al-MCM-41(10) and Ti-MCM-41(30) were obtained, where the numbers in parenthesis denote the Si/metal molar ratio. A hexagonal mesoporous silica containing titanium, Ti-HMS(30), was also prepared, using n-dodecylamine as a template at pH 10. Characterization of samples was made by ICP-OES, ATR-FTIR. XRD, N-2 adsorption at 77 K, XPS and ammonia TPD. The results indicate that the samples have low crystallinity, surface areas of 500-750 m(2)/g high degree of isomorphic substitution and, in certain cases, higher acidity (Bronsted and Lewis) as compared to a commercial Y zeolite (Si/Al = 1.5). Catalytic studies of anthracene oxidation in liquid phase at 348 K, using tert-butylhydroperoxide as oxidant agent showed that the Ti-HMS(30) sample is much more prone to produce 9,10-anthraquinone (yield >90%) than the other mesoporous samples. This performance is directly related to parameters such as surface area, nature of the heteroatom introduced, Si/Ti molar ratio and strength and concentration of acid sites. The reaction kinetic experiments show a great dependency of specific reaction rates with temperature (in the range 320-348 K) and an activation energy value of 15.2 kcal/mol. A set of experiments performed under factorial design indicated that an optimal oxidant/aromatic ratio is 5.5 and the optimal amount of catalyst is about 44.3 mg. In general, our results confirm that the synthesized materials are promising catalysts (redox and acid) for petrochemical applications. (C) 2007 Elsevier B.V. All rights reserved.