Thermally induced conversion of Sr-exchanged LTA- and FAU-framework zeolites. Syntheses, characterization, and polymorphism of ordered and disordered Sr<inf>1-x</inf>Al<inf>2-2x</inf>Si<inf>2+2x</inf>O<inf>8</inf> (x = 0; 0.15), diphyllosilicate, and feldspar phases

Abstract

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 SrAl2Si2O8, [a0 = 5.1970(2) Å, c0 = 15.200(1) Å] and disordered hexagonal Sr0.85□0.15Al1.71Si2.29O 8 [a0 = 5.2039(2) Å, c0 = 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-feldsparLTA [a0 = 8.358(2) Å, b0 = 12.941(3) Å, c0 = 7.114(1) Å, β = 115.26(2)°] and Sr-feldsparFAU [a0 = 8.354(2) Å, b0 = 12.957(3) Å, c0 = 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 M1-xAl2-2xSi2+2xO8 (M = Ca, Sr, Ba, Pb) polymorphous transformation from diphyllosilicate to feldspar topology is discussed.