Influence of Si Incorporation into the Novel Ti(III)APO-5 Catalysts on the Oxidation of Cyclohexene in Liquid Phase

Abstract

This work focuses on the influence of Si incorporation into the Ti(III)APO-5 materials on their catalytic performance in the oxidation of cyclohexene with hydrogen peroxide under anhydrous reaction conditions. A series of Ti(III)APSO-5 materials were synthesized from gels containing a fixed amount of powder TiCl3 and varied amounts of TEOS as Ti(III) and Si sources, respectively under N2 atmosphere. The as-prepared and calcined materials were characterized by powder XRD, ICP chemical analysis, SEM, DRUV-visible spectroscopy, TGA, multinuclear MAS NMR and XPS. They confirm the presence of Si within the AlPO4 framework, although the existence of Ti(OSi)4 environments could be rejected, according to the XPS spectra. However, the presence of some Si atoms in the first tetrahedral coordination sphere of the Ti centers, i.e., as Ti–O–Si bonds, could be proposed from the DRUV and MAS NMR spectra. Furthermore, high Si content can provide certain hydrophobicity to the material, what influence the competitive adsorption of the polar hydrogen peroxide and the organic substrate. A Si-rich Ti(III)APSO-5 and a Si-free Ti(III)APO-5 samples were catalytically tested in the oxidation of cyclohexene. The incorporation of Si in Ti(III)APO-5 materials enhanced their catalytic activity but scarcely modified the selectivity, which was dominated by the products formed through the allylic oxidation pathway, in agreement with the absence of Ti-zeolitic environments Ti(OSi)4.