Postsynthesis of Hexagonally Packed Porous Zirconium Phosphate through a Novel Anion Exchange between Zirconium Oxide Mesophase and Phosphoric Acid

Abstract

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 HPO42- ions for SO42- 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.