Reaction of LTA and FAU zeolites with NH4H2PO4 melts yields crystalline NH4AlP2O7

Study of the reaction of zeolites A (LTA), and X and Y (FAU) with NH₄H₂PO₄ melts at 230°C for varying time periods (30 min to 36 h) found the final product formed in each case to have a powder X-ray diffraction pattern consistent with ammonium aluminum pyrophosphate, NH₄AlP₂O₇, the single crystal structure of which has not been reported. No intermediate crystalline phases were detected. ²⁹Si MAS NMR confirmed, however, that some Si was still present, and X-ray photoelectron spectroscopy revealed it to be located primarily at the crystallite surfaces. The crystallites were found to have an extremely low surface area (ca. 10m²g⁻¹), and hence not to be porous. MAS NMR studies found it to consist of six-coordinate aluminum and four-coordinate phosphorus in the aluminum pyrophosphate, and four-coordinate silicon in a highly siliceous environment. These results contradict the previous report of this reaction producing ‘phosphated aluminosilicates in which phosphorus substitutes into the zeolite framework sites thereby forming promising anion exchangers’ (A. Dyer, S.A. Malik, A. Araya, T.J. McConville, in: P.A. Williams, M.J. Hudson (Eds.), Recent Developments in Ion Exchange (Pap. Int. Conf. Ion Exch. Processes), 1987, pp. 257–263.).