Zeolite Synthesis: General Aspects

Peroxide-Promoted Disassembly Reassembly of Zr-Polyoxocations

Zr/Hf aqueous-acid clusters are relevant to inorganic nanolithography, metal-organic frameworks (MOFs), catalysis, and nuclear fuel reprocessing, but only two topologies have been identified. The (Zr₄) polyoxocation is the ubiquitous square aqueous Zr/Hf-oxysalt of all halides (except fluoride), and prior-debated for perchlorate. Simply adding peroxide to a Zr oxyperchlorate solution leads to a striking modification of Zr₄, yielding two structures identified by single-crystal X-ray diffraction. Zr₂₅, isolated from a reaction solution of 1:1 peroxide/Zr, is fully formulated [Zr₂₅O₁₀(OH)₅₀(O₂)₅(H₂O)₄₀](ClO₄)₁₀·xH₂O. Zr₂₅ is a pentagonal assembly of 25 Zr-oxy/peroxo/hydroxyl polyhedra and is the largest Zr/Hf cluster topology identified to date. Yet it is completely soluble in common organic solvents. ZrT_d, an oxo-centered tetrahedron fully formulated [Zr₄(OH)₄(μ-O₂)₂(μ₄-O)(H₂O)₁₂](ClO₄)₆·xH₂O, is isolated from a 10:1 peroxide/Zr reaction solution. The formation pathways of ZrT_d and Zr₂₅ in water were described by small-angle X-ray scattering (SAXS), pair distribution function (PDF), and electrospray ionization mass spectrometry (ESI-MS). Zr₄ undergoes disassembly by 1 equiv of peroxide (per Zr) to yield small oligomers of Zr₂₅ that assemble predominantly in the solid state, an unusual crystal growth mechanism. The self-buffering acidity of the Zr-center prevents Zr₂₅ from remaining intact in water. Identical species distribution and cluster fragments are observed in the assembly of Zr₂₅ and upon redissolution of Zr₂₅. On the other hand, the 10:1 peroxide/Zr ratio of the ZrT_d reaction solution yields larger prenucleation clusters before undergoing peroxide-promote disassembly into smaller fragments. Neither these larger cluster intermediates of ZrT_d nor the smaller intermediates of Zr₂₅ have yet been isolated and structurally characterized, and they represent an opportunity to expand this new class of group IV polycations, obtained by peroxide reactivity and ligation.