TY - JOUR

T1 - On the Carbonate Formation in Thermally Stressed Hydroxysodalite

T2 - Some Facts to Notice for SOD Application in Separation Processes

AU - Peschke, Irma

AU - Hildebrand, Hagen

AU - Pioch, Robert

AU - Buhl, Josef Christian

N1 - Funding Information: IP and JB gratefully acknowledge the Deutsche Forschungsgemeinschaft (DFG) for financial support of the project BU1020/22–1.

PY - 2020/10/14

Y1 - 2020/10/14

N2 - Basic sodalite (hydroxysodalite) was synthesized from two Si-Al sources: (1) kaolin to obtain |Na7.5(OH)1.5(H2O)3.5|[AlSiO4]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 |Na8(OH)2(H2O)2|[AlSiO4]6. Both SOD were exposed to thermal stress by long-time heating at 773 K under open conditions, in N2 and CO2 atmosphere and in presence of a NaOH-Na2CO3 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 CO2 atmosphere. Whereas heating in presence of the melt showed no effect, investigations in CO2 clearly indicate an intra-cage reaction between CO2 and the enclathrated [Na4OH]3+ ions as the carbonate generating mechanism instead of destruction of hydroxysodalite followed by recrystallization. A model is proposed, in which cage fillings [Na3□]3+ with a vacancy in the Na tetrahedron known from dehydrated hydrosodalites |Na6|[AlSiO4]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.

AB - Basic sodalite (hydroxysodalite) was synthesized from two Si-Al sources: (1) kaolin to obtain |Na7.5(OH)1.5(H2O)3.5|[AlSiO4]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 |Na8(OH)2(H2O)2|[AlSiO4]6. Both SOD were exposed to thermal stress by long-time heating at 773 K under open conditions, in N2 and CO2 atmosphere and in presence of a NaOH-Na2CO3 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 CO2 atmosphere. Whereas heating in presence of the melt showed no effect, investigations in CO2 clearly indicate an intra-cage reaction between CO2 and the enclathrated [Na4OH]3+ ions as the carbonate generating mechanism instead of destruction of hydroxysodalite followed by recrystallization. A model is proposed, in which cage fillings [Na3□]3+ with a vacancy in the Na tetrahedron known from dehydrated hydrosodalites |Na6|[AlSiO4]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.

KW - Aluminosilicates

KW - IR spectroscopy

KW - Microporous materials

KW - Phase transitions

KW - X-ray diffraction

UR - http://www.scopus.com/inward/record.url?scp=85087295900&partnerID=8YFLogxK

U2 - 10.1002/zaac.202000051

DO - 10.1002/zaac.202000051

M3 - Article

AN - SCOPUS:85087295900

VL - 646

SP - 1641

EP - 1649

JO - Zeitschrift fur Anorganische und Allgemeine Chemie

JF - Zeitschrift fur Anorganische und Allgemeine Chemie

SN - 0044-2313

IS - 19

ER -