Thermally Induced Conversion of Sr-Exchanged LTA- and FAU-Framework Zeolites. Syntheses, Characterization, and Polymorphism of Ordered and Disordered Sr1-xAl2-2xSi2+2xO8 (x = 0; 0.15), Diphyllosilicate, and Feldspar Phases

The thermal behavior of fully Sr-exchanged LTA- and FAU-framework zeolites was followed in the temperature range from ambient to 1500 °C. Both zeolite frameworks collapsed into amorphous intermediate substances after heating between 780 and 950 °C. Prolonged annealing of these products above 950 °C induces their recrystallization to ordered hexagonal SrAl 2 Si 2 O 8, [ a 0 = 5.1970(2) Å, c 0 = 15.200(1) Å] and disordered hexagonal Sr 0.85 □ 0.15 Al 1.71 Si 2.29 O 8 [ a 0 = 5.2039(2) Å, c 0 = 15.158(1) Å], diphyllosilicate phases, respectively. These phases are unstable under prolonged heating, and near 1100 °C, they can be polymorphously transformed into corresponding Sr-feldspar LTA [ a 0 = 8.358(2) Å, b 0 = 12.941(3) Å, c 0 = 7.114(1) Å, β = 115.26(2)°] and Sr-feldspar FAU [ a 0 = 8.354(2) Å, b 0 = 12.957(3) Å, c 0 = 7.128(2) Å, β = 115.27(2)°] phases, maintaining framework cation-ordering characteristics. The phase conversions in the temperature range investigated were followed by thermal (DTA, TGA, and DSC), XRPD, and SEM analyses. The structural mechanism for M 1- x Al 2-2 x Si 2+2 x O 8 (M = Ca, Sr, Ba, Pb) polymorphous transformation from diphyllosilicate to feldspar topology is discussed.