The dependence of Co2+-exchange into zeolite FAU on its Si/Al ratio

Three single crystals of fully dehydrated, largely Co²⁺-exchanged zeolites X and Y were prepared by the exchange of Na₈₀-X (|Na₈₀|Si₁₁₂Al₈₀O₃₈₄]-FAU, Si/Al = 1.40), Na₇₅-Y (|Na₇₅|Si₁₁₇Al₇₅O₃₈₄]-FAU, Si/Al = 1.56), and Na₇₁-Y (|Na₇₁|Si₁₂₁Al₇₁O₃₈₄]-FAU, Si/Al = 1.70) with aqueous streams 0.05 M in Co(NO₃)₂, pH = 5.1, at 294 K for 3 days. This was followed by vacuum dehydration at 673 K. Their crystal structures were determined by synchrotron X-ray diffraction techniques in the cubic space group Fd \(\overline{3}\) m at 100(1) K. In all three crystals Co²⁺ ions occupy the 6-ring sites I, I’, and II; Na⁺ ions occupy sites II’ and II. The number of Co²⁺ ions exchanging into the zeolite was about 30 per unit cell for all three crystals. The number of residual Na⁺ ions, however, decreased sharply as Si/Al ratio increased, and the number of H⁺ ions co-exchanging into the zeolite decreased nearly to zero. Some dealumination of the zeolite framework was seen in the first crystal (initial Si/Al = 1.40).