Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings

The crystal structure of three synthetic ferrierite samples loaded with different amounts of Co cations (2.5 wt.%, 4.3 wt.% and 13.7 wt.% respectively) were refined by means of synchrotron X-ray powder diffraction data. The unit cell content of cobalt exchanged ferrierites resulted: [Na0.18K0.03Co1.12H1.36(H2O)(17.2)][Al3.81Si32.19O72]-FER, [Na0.18K0.03Co1.89(H2O)(17.2)][Al3.81Si32.19O72]-FER and [Na0.18K0.03Co1.89(H2O)(17.2)][Al3.81Si32.19O72]-FER respectively. In the overloaded sample the excess of Co (9.4 wt.%) is located outside the zeolite structure as cobalt oxide phases. After cation exchange the space group, which was P2(1)/n in the as-synthesised form, becomes Immm. The crystal structure of the three synthetic samples is characterised by the presence of a Co(H2O) polyhedron inside the ferrierite cage. The occupancy of this polyhedron varies as a function of the Co amount in the extra-framework sites of the ferrierite structure: up to 50% for a complete ion-exchange. The residual cobalt occupies two further cation sites