A Novel Staged Form of Layered Zirconium Phosphonates with Methyl and p-Aminobenzyl Pendant Groups

Abstract

Zirconium phosphonates, Zr(O3PR)2, are layered compounds whose structure follows the basic structural motif1 of zirconium phosphate, Zr(O3POH)2. In these compounds, the zirconium atoms form stacked planes held together by the attractive interactions of the R groups that project into the interlayer region from both sides of any given zirconium plane.2,3 Mixed zirconium phosphonates, Zr(O3PR)x(O3PR′)2-x, can be formed by coprecipitation of the zirconium in the presence of two different phosphonic acids,4-7 leading to the formation of a porous material if the two organic groups on the phosphonic acids are sufficiently different in size. These mixed compounds usually contain a random distribution of the organic groups such that all layers have identical stoichiometry. In such systems, the interlayer spacing (d-space) is a function of pendant group stoichiometry, and has a generally linear dependence on component mole fraction. With this communication, we report that in the course of an extensive investigation into the physical/chemical attributes that underlie the one-dimensional interlayer spatial variation,8,9 an unusual material was found to exist at x ) 0.25 that can only be interpreted as a staged, alternating mixed/pure layered zirconium phosphonate. The systems selected for these studies incorporated p-aminobenzyl as the large group and methyl as the small group, Zr(O3PCH2C6H4NH2)x(O3PCH3)2-x. In this Zr(pab)x(me)2-x family of compounds, x was varied from 0.0 to 2.0 in regular increments. All preparations followed the general procedure9 of adding zirconyl chloride to stoichiometric amounts of the appropriate phosphonic acids as an aqueous solution, refluxing for several days, and isolating the product at room temperature. A representation of the unit cell for the compound x ) 1.0 is depicted as Figure 1. In addition, the amino moiety allows for assimilation of HCl, conveniently providing an additional system to investigate, Zr(pabHCl)x(me)2-x. The two series will also be referred to as the deprotonated and protonated systems, respectively. X-ray diffraction was employed to measure d-space values for the systems. Typical results are displayed in Figure 2, where the first-order 001 diffraction peaks occur at angles whose 2θ values are easily converted to interlayer spacing values via the Bragg equation. A plot of such values as a function of