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

Abstract 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 AlPO 4 while the SiO 2 species initially remain in an amorphous phase. Further increasing the aging temperature leads to virtually complete recrystallization and, ultimately, melting.