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

Abstract

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.