Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolite frameworks: Rietveld refinement of the hexagonal CaAl<inf>2</inf>Si<inf>2</inf>O<inf>8</inf> diphyllosilicate structure

Abstract

Thermally induced phase transformations of Ca-exchanged LTA and FAU zeolites are followed in the range from room temperature to 1500°C. Both frameworks collapse into amorphous intermediate products after heating between 800 and 900°C. Prolonged heating of the intermediate product obtained from the Ca-LTA zeolite over 900°C induces recrystallization of CaAl2Si2O8 phases (hexagonal CaAl2Si2O8 phase and anorthite) into a mixture. The crystal structure of the hexagonal CaAl2Si2O8 phase [a0 = 5.1154 (Å), c0= 14.767(9) Å] is refined by the Rietveld method from X-ray powder diffraction data to final agreement factors RP = 8.99%, RWP = 11.87%, and RB = 4.31%. The refined hexagonal Ca-diphyllosilicate phase is unstable in prolonged calcination, and at 1050°C it can be transformed polymorphously into an highly ordered anorthiteLTA. Prolonged heating of the amorphous intermediate product obtained from Ca-FAU zeolite over 900°C results directly in formation of a less ordered anorthiteFAU Ca0.88D□0.12Al1.77Si2.23O 8.00 framework with vacant Ca2+ sites. © Elsevier Science Inc. 1996.