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 AlPO-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 AlPO-5 crystals.