One-step fabrication of hollow aluminosilicate microspheres with a composite zeolite/glass crystalline shell

One-step procedure to fabricate hollow aluminosilicate microspheres with a composite NaA(NaX)/glass/mullite composite shell was demonstrated by the direct conversion of low-silica cenospheres (Si/Al = 1.8) separated from coal fly ash. The reported method does not involve seeding and structure-directing reagents. Composition of cenospheres and the stirring of reactants at the optimal synthesis temperature (T = 80 °C) and alkaline molarity (2.5 M NaOH) are parameters affected the formation of a monozeolitic phase (NaA or NaX). The influence of the silica–alumina composition of cenospheres on the type of zeolite phase formation is only revealed under stirring conditions. In the case of the low-silica cenospheres, NaA (LTA) zeolite crystallizes as a dominant phase (43 wt.%) (Si/Al = 1.8) under stirring of reactants, while the static conditions favor formation of the NaX (FAU) zeolite phase (45 wt.%).

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► The synthetic route to hollow zeolite microspheres without seeding was realized.
► Microspheres with a NaA(NaX)/glass/mullite shell were obtained from cenospheres.
► We determined the quantitative phase composition of zeolitized cenospheres.
► Parameters for prevailing formation of NaA or NaX zeolite were determined.