Isolated molybdenum-based microporous POMs for selective adsorption of gases

Dodecanuclear, icosanuclear and octanuclear porous MOF-like POM materials [MoV12O₁₂(μ₂-O)₄(μ₃-O)₁₂(Htrz)₄(trz)₄]·nH₂O (n = 22, 1; n = 92, 2; Htrz = 1H-1,2,3-triazole), [MoV8O₈(μ₂-O)₁₂(Htrz)₈]_½·[MoV12O₁₂(μ₂-O)₄(μ₃-O)₁₂(Htrz)₄(trz)₄]·44H₂O (3), and [MoV8O₈(μ₂-O)₁₂(Htrz)₈]·62H₂O (4) have been obtained and well characterized. Higher Mo₁₂ agglomerates have been fabricated in acidic solution, while isolated Mo₈ species formed in weak alkaline conditions. Fortuitously, their additive 3 is precipitated in a neutral environment. Structural analyses indicate that there are diversiform pores present in 1-4, which are formed by polyoxometalates (POMs) themselves and intermolecular accumulations, or even induced by free disordered water molecules. Obvious downfield shifts have been found in the solid ¹³C NMR spectra for the coordinated 1,2,3-triazoles in 1 and 2. In particular, isolated single species of polyoxometalate-based porous framework POMOF 4 can be identified in the solution ¹³C NMR spectrum and further supported by solid and solution UV-vis spectroscopy. Moreover, gas adsorption tests indicate that 1 is the most promising CO₂ adsorbent of these four materials, compared with other common gases O₂, H₂, CH₄ and N₂ under different pressures. The peculiarities of this sort of POMOFs, as well as the relationships and interactions with gases, have been discussed.