Postsynthesis of hexagonally packed porous zirconium phosphate through a novel anion exchange between zirconium oxide mesophase and phosphoric acid

Porous zirconium phosphates having ordered hexagonal pore system and high surface area have been prepared by post-treating surfactant-assisted zirconium oxide mesophase with phosphoric acid solution. The zirconium phosphates with P/Zr atomic ratios of 0.08-1.67 and maintaining totally the hexagonal structure were obtained by varying the phosphoric acid concentration (0.01-1 M), varying the treatment time (0.08-5 h), and repeating the treatment (1-6 times). Zirconium phosphates with P/Zr atomic ratios over 1.0 withstood the calcination up to 873 K and showed the BET specific surface area of 456-547 m² g^-1. The porous zirconium phosphates were characteristic of micropores with the pore size of 1.30-1.66 nm due to partial condensation of the mesopores during the calcination. The formation mechanism of the hexagonal zirconium phosphates was clarified to be essentially an anion exchange of HPO₄^2- ions for SO₄^2- ions which mediated the cationic zirconium species and the cationic surfactant headgroups within the as-synthesized zirconium oxide mesophase through electrostatic interactions, together with a reaction between the phosphoric acid and the Zr-OH groups. The anion exchange was confirmed to yield the phosphorus species showing two ³¹P MAS NMR bands at ca. -3.0 and -9.2 ppm, while the reaction of phosphoric acid with the Zr-OH groups was suggested to form the phosphorus species corresponding to a band at -19.4 ppm.