Synthesis and chemistry of chromium in CrAPO-5 molecular sieves

CrAPO-5 molecular sieves were synthesized hydrothermally starting with different Cr precursors and Cr and template contents. The behavior of Cr was investigated spectroscopically by diffuse reflectance spectroscopy (d.r.s.) and electron spin resonance (e.s.r.). In the gels, Cr³⁺ and Cr⁶⁺ are present as, respectively, octahedral Cr(H₂O)₆³⁺ and mainly dichromate. Both ions are in the liquid phase of the gels. In all as-synthesized CrAPO-5′s, Cr³⁺ is octahedrally coordinated to six oxygen ligands. The corresponding e.s.r. spectra show a broad isotropic signal centered around 1.97, following the Curie-Weiss law. This behavior is comparable with Cr³⁺ impregnated on AIPO-5 and assigned to hexa-aquo Cr³⁺ ions. Upon calcination, this signal decreases in intensity, a new sharp signal of Cr⁵⁺ is formed, and, in d.r.s., Cr⁶⁺ is formed at the expense of Cr³⁺. However, the broad e.s.r. signal of Cr³⁺ does not follow the Curie-Weiss law and is due to Cr³⁺ clusters. Cr⁶⁺ and Cr⁵⁺ are easily reduced with CO to octahedral Cr³⁺ and octahedral and tetrahedral Cr²⁺, whereas recalcination reestablishes the original Cr⁶⁺ and Cr⁵⁺ species. All these results support the idea that Cr³⁺ is not incorporated into the framework, but is present as octahedral ions at the surface of the AIPO-5 crystals.