On the Carbonate Formation in Thermally Stressed Hydroxysodalite – Some Facts to Notice for SOD Application in Separation Processes

Basic sodalite (hydroxysodalite) was synthesized from two Si‐Al sources: (1) kaolin to obtain |Na 7.5 (OH) 1.5 (H 2 O) 3.5 |[AlSiO 4 ] 6 sodalites (SOD) with small crystals (< 0.5 μm) and (2) a mixture of cristobalite and corundum (CC) to obtain larger microcrystals (1–5 μm) with ideal composition |Na 8 (OH) 2 (H 2 O) 2 |[AlSiO 4 ] 6 . Both SOD were exposed to thermal stress by long‐time heating at 773 K under open conditions, in N 2 and CO 2 atmosphere and in presence of a NaOH‐Na 2 CO 3 melt. The crystals obtained from kaolin were dehydrated and developed remarkable degrees of carbonate cage fillings already under open conditions. The large microcrystals obtained from CC exhibit this effect only at very low scale even after long‐time heating in CO 2 atmosphere. Whereas heating in presence of the melt showed no effect, investigations in CO 2 clearly indicate an intra‐cage reaction between CO 2 and the enclathrated [Na 4 OH] 3+ ions as the carbonate generating mechanism instead of destruction of hydroxysodalite followed by recrystallization. A model is proposed, in which cage fillings [Na 3 □] 3+ with a vacancy in the Na tetrahedron known from dehydrated hydrosodalites |Na 6 |[AlSiO 4 ] 6 are required to induce the intra‐cage reaction of hydroxysodalite. As those fillings only occur in a noticeable number in dehydrated hydroxysodalite obtained from kaolin, the large extent of the carbonate formation inside this sample becomes obvious. The results are significant for future improvement of hydroxysodalite membranes.