Thermal collapse of SAPO-34 molecular sieve towards a perfect glass

Controlled collapse of a silicoaluminophosphate molecular sieve CHA-type SAPO-34 by thermal aging was studied in a multi-method approach. Structural changes and the mechanism of order–disorder–order transitions during collapse were followed for varying annealing conditions between 1000 and 1100 °C. Collapse of SAPO-34 occurs via the formation of an intermediate amorphous state by the loss of long-range order but preservation to a high degree of short-range order and macroscopic morphology. This intermediate amorphous state with low entropy may be classified as a “perfect glass”. Prolonged aging in the collapse regime results in the precipitation of crystalline AlPO4 while the SiO2 species initially remain in an amorphous phase. Further increasing the aging temperature leads to virtually complete recrystallization and, ultimately, melting.