Materiais com estrutura mesoporosa ordenada contendo titânio: Preparação, Caracterização, Estudos de Adsorção e Propriedades Catalíticas

Abstract

his PhD thesis presents studies on the preparation, characterisation, thermal stability, stability during long term exposure to water vapour at 298K and evaluation of the catalytic properties of Ti-MCM-41 materials. Studies related to the synthesis and structural characterisation of Si-MCM-41 materials, prepared at room temperature, from structure directing agents with different chain lengths and by direct addition of the silica source or by its addition in alcoholic solution, allowed selection of tetra-, hexa- and octadecyltrimethylammonium bromides as the most suitable structure directing agents for the preparation of M41S type materials of high quality and exclusion of BuOH as the alcohol in the process of synthesis of the these materials. After developing and implementing a new method for the synthesis of Ti-MCM-41 materials, carried out at room temperature and based on the procedure for preparing Si-MCM-41 solids, a systematic studied of the effect of the principal synthesis parameters on the structural and chemical properties of the materials, namely, chain length of the structure directing agent, order of addition of the silicon and titanium precursors, direct or alcoholic solution addition of the titanium alcoxide, nature of the metal alcoxide used as titanium source and silicon/titanium molar synthesis ratio, was carried out. The results of structural characterisation, obtained by nitrogen adsorption at 77K and X-ray diffraction, show that it is possible to prepare Ti-MCM-41 materials with molar synthesis ratio Si/Ti ≥ 30 of high structural quality, and that the incorporation of titanium does not lead to a significant loss of quality of the substituted materials in comparison to the corresponding samples composed only of silica. Higher titanium contents imply a progressive decrease in the principal structural characterisation parameters without verifying the complete disappearance of the characteristic structure of MCM-41 materials. It was also shown that for low Si/Ti molar synthesis ratios the use of Ti(OEt)4 as titanium source leads to Ti-MCM-41 materials of slightly higher structural quality than the analogous samples synthesised from Ti(OPri)4. In the present work, the post-synthesis deposition of titanium in Si-MCM-41 precursor samples, in accordance with a nominal Si/Ti ratio of 10, did not prove to be an efficient synthesis method for obtaining substituted materials with the ideal structural characteristics of MCM-41. The results of chemical composition determination confirmed that the preparation of titanosilicates of the Ti-MCM-41 type by this new process leads to samples with a final Si/Ti molar ratio in agreement with the synthesis molar ratio, that is, it is possible to define, a priori, the final Si/Ti ratio intended for Ti-MCM-41 materials. The comparative analysis of the structural properties of the Ti-MCM-41 materials prepared in this work and those obtained by hydrothermal routes by different authors, allow us to conclude that in general the former present higher structural quality. In addition, the synthesis method presented allows Ti-MCM-41 materials to be obtained in a considerably short time, 60 to 90 minutes, without the need to apply external heat or pressure, which makes it very attractive for large scale production revealing itself to be an extremely convincing procedure for the preparation of Ti-MCM-41 materials. The results of the thermal stability studies show that the samples substituted with titanium have higher thermal stability than Si-MCM-41 samples, the collapse of the mesoporous structure being observed at 1373 and 1273K, respectively. Adsorption isotherms of nitrogen at 77K and n-pentane at 298K and X-ray diffraction patterns were determined on selected samples of Ti-MCM-41 before and after prolonged exposure to water vapour at room temperature (by carrying out two consecutive water vapour adsorption-desorption isotherms at 298K by the gravimetric method) and the results obtained also show that isomorphic substitution of silicon by titanium leads to a significant increase in stability. They also provided unequivocal confirmation that Ti-MCM-41 samples with a Si/Ti synthesis molar ratio ≤ 50 retain the initial structural uniformity associated to high pore volumes. It was also possible to show that the structural alterations occur principally during the determination of the first water vapour adsorption-desorption isotherm at 298K and that these alterations can be considered barely significant according to the criteria adopted by various authors. Studies of the evaluation of the oxidative catalytic properties of selected Ti-MCM-41 samples, using as model reaction system the epoxidation of cyclo-hexene, CH, with tert-butyl-hydroperoxide, TBHP, as oxidising agent, indicate that the presence of isolated and tetracoordinated titanium centres in the catalyst matrix is not a necessary or sufficient condition for obtaining high cyclohexene conversions coupled with selectivity in relation to cyclohexene oxide of 100%. The results show that the Ti-MCM-41 materials prepared with a Si/Ti synthesis molar ratio ≥ 10 exhibit good catalytic performance for the reaction tested, translated by high conversions and selectivities of 100%, coupled with practically residual acid catalytic reactivity and high structural stability in relation to the effect of leaching. We can therefore conclude that the tested materials possess structural properties, namely, high surface area and pore sizes in the meso range, suitable for an efficient dispersion of the active redox sites and for the diffusion of the substrate molecules, CH, oxidising agent, TBHP, and the principal reaction product, cyclohexene oxide, E.