Incorporación de Ti(III) en aluminofosfatos cristalinos y su comportamiento catalítico en reacciones de oxidación selectiva

Abstract

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química Inorgánica. Fecha de lectura: 20-11-2015This work focuses on the development of a synthesis procedure for the preparation of Ti-doped microporous aluminophosphate (TAPO) materials, by means of the isomorphic substitution of Al(III) atoms in tetrahedral sites by Ti(III) atoms. The main aim consists on modifying the chemical environment of the resultant titanium centers with respect to those in the conventional TAPO materials, prepared with Ti(IV), in an attempt to enhance their catalytic behavior and reduce its difference with the one shown by Ti-zeolites in liquid phase oxidation reactions. The materials have been synthesized using two Ti(III) sources, different titanium contents and different structure directing agents, to generate the so-called Ti(III)APO materials. They have been prepared with several structures, mainly AlPO- 5 (AFI) and to a lesser extent with AlPO-11 (AEL) and AlPO-41 (AFO) structures. The characterization of the materials obtained by hydrothermal treatment of the synthesis gels prepared under nitrogen atmosphere, in order to minimize Ti(III) oxidation before its incorporation in the structure, confirms the presence of those ions. This is also visible at first sight by the purple color of the samples. On the other hand, the characterization shows the changes in the TAPO materials framework caused by the incorporation of Ti(III), as the different crystal morphology, the modification of the phosphorous environment or the high hydration degree of the materials. The analysis of the results allows proposing the incorporation mechanisms of this heteroatom, being favored the formation of environments Ti(OP)4, by substitution of isolated Al(III), which lead to Ti–OH y P–OH species, by oxidation of these Ti(III) atoms when the sample is exposed to air. However, Ti–O–Ti species are also likely if a partial oxidation of Ti(III) atoms to Ti(IV) takes place during the synthesis process. The proportion of both mechanisms is, in some way, regulated by the titanium content. The catalytic behavior of the materials has been studied in the oxidation reaction of cyclohexene with H2O2 as oxidant and acetonitrile as solvent. It has been observed that the presence of water in the reaction media reduces considerably the catalytic activity of the Ti(III)APO materials. This effect has led to the design of a procedure for the removal of the water that accompanies the commercial oxidant H2O2 30 wt. %, by a Soxhlet system and molecular sieve 3A as selective adsorbent. This procedure allows removing around 90 % of the water present. Furthermore, a reproducible quantification method of water and H2O2 has been developed by means of FTIR spectroscopy. The catalytic activity studies show that titanium centers in Ti(III)APO-5 materials are more active than those in the conventional Ti(IV)APO-5, and the first ones can reach similar conversion rates to those of the Ti-beta zeolite under anhydrous reaction conditions. The Ti(III)APO-5 materials are highly selective to the allylic oxidation products, and 2-cyclohexenyl hydroperoxide is the main one. In addition, synthesis strategies to enhance the stability of Ti(III) and favor its incorporation in the AlPO4 framework in that oxidation state have been designed. The synthesis were carried out, on the one hand, with urea as complexing agent of Ti(III) and, on the other hand, with addition of HCl to the initial gel. The former have given rise to the crystallization of AlPO-15 structure instead of AlPO-5. The latter have not had a significant effect on Ti(III) incorporation, but on its textural properties leading to higher conversion of cyclohexene over these materials. On the other hand, an additional modification of the titanium coordination environment, as well as of the hydrophilic degree of the materials, was caused by the addition of a silicon source together with the Ti(III) one. The synthesis gels were prepared in aqueous and in biphasic media, making use of synthesis strategies of SAPO materials. The presence of silicon induces the Ti(III) incorporation in isolated Al(III) sites and, furthermore, some Ti–O–Si are formed, while there is no evidence of the existence of Ti(OSi)4 environments. This synthesis strategy has led to titanium centers with higher intrinsic activity than those in the Si-free Ti(III)APO-5 materials. The crystallization of the AlPO-11 or the AlPO-41 structures seems to be directed by the Ti(III) content in the synthesis gel, being favored the latter at higher content. The intrinsic activity of the titanium centers in the Ti(III)APO-11 and Ti(III)APO-41 materials, especially in the first ones, is higher than that in the Ti(III)APO-5 in the oxidation reaction of cyclohexene. Nevertheless, the chemical nature of the active centers is similar in the three materials, since all of them selectively lead to the allylic oxidation productsConcesión del CSIC de una beca JAE-Predoc para la realización de esta tesis y la financiación de las estancias en el extranjero y de los Ministerios de Ciencia e Innovación y de Economía y Competitividad de los proyectos MAT2009-13569 y MAT2012-31127 que han permitido la financiación de este trabaj