Incorporation of Ti(III) into the AlPO4-5 framework by direct synthesis

Abstract

This work focuses on the synthesis and characterization of a conceptually innovative series of TAPO materials, particularly centered in the AFI-structured TAPO-5. Unlike the conventional TAPOs, the so-called Ti(III)APO-5 are prepared by using Ti(III) sources. This strategy aims to avoid the presence of both Ti–O–Al and Ti–O–Ti bonds in TAPOs, undesired for catalytic purposes. The thermodynamic stability of this oxidation state of Ti in water and in the presence of air is low, but the preparation of the gel and the filling of the autoclaves under N2 atmosphere allowed Ti to maintain its 3+ oxidation state in a considerable proportion in the hydrothermally-crystallized solids. The materials were characterized by powder XRD, ICP chemical analysis, SEM, DR-UV–visible spectroscopy, TGA and 31P and 27Al MAS NMR. DR-UV–visible spectroscopy certified the presence of Ti(III) in both tetrahedral and octahedral environments in the as-made samples before being exposed to air, whereas 31P MAS NMR indicated the vicinity of Ti in the second coordination sphere of P in the already oxidized sample. Although once dried and calcined in air Ti(III) is entirely oxidized to Ti(IV), Ti(III)APO-5 materials present key singularities against conventional TAPO-5. Thus, apart from the presence of Ti(OP)4 environments, the charge transfer transition detected by DR-UV–vis appears at lower wavelength, whereas both TGA and 27Al NMR indicates a peculiar hydration of the as-prepared samples, which is unprecedented in conventional TAPO-5. Based on these characterization features, the possible nature of the Ti centers generated by this synthesis approach is discussed.