Ionothermal synthesis of FeAPO-5 in the presence of phosphorous acid.

The ionothermal crystallization of FeAPO-5 molecular sieves in the presence of phosphorous acid (H3PO3) has been investigated. The use of H3PO3 enabled the formation of a metastable intermediate phase (FeNKX-2) that transforms into a more open-framework crystalline phase (FeAPO-5). The initial raw materials dissolved rapidly in the presence of the [bdmim]Cl polar ionic liquid, and the addition of the Fe3+ salt resulted in the crystallization of the FeNKX-2 intermediate. At this stage, the [bdmim]+ cation did not play the role of a pore filler for the FeNKX-2 crystalline structure. Consecutive phase transformation from the FeNKX-2 to the FeAPO-5 phase occurred under prolonged ionothermal treatment, and during this stage, the tetrahedral Fe3+ species was found to not only participate in the construction of the FeAPO-5 framework but also act as an intermediary electron-transfer medium. The fast crystallization of FeAPO-5 was explained by the presence of Fe3+ as an intermediate electron-transfer medium promoting the fast release of phosphorus nutrients (P5+) from the phosphite (P3+) reservoir that were further required for the crystallization of the FeAPO-5 molecular sieves. The use of ionic liquids as dual solvents and templates in combination with H3PO3 as an alternative phosphorus source thus opens the possibility to synthesize other microporous materials via a phase transformation approach. [ABSTRACT FROM AUTHOR]