Isolated molybdenum-based microporous POMs for selective adsorption of gases

Abstract

Dodecanuclear, icosanuclear and octanuclear porous MOF-like POM materials [Mo^V₁₂O₁₂(μ₂-O)₄(μ₃-O)₁₂(Htrz)₄(trz)₄]·nH₂O (n = 22, 1; n = 92, 2; Htrz = 1H-1,2,3-triazole), [Mo^V₈O₈(μ₂-O)₁₂(Htrz)₈]_½·[Mo^V₁₂O₁₂(μ₂-O)₄(μ₃-O)₁₂(Htrz)₄(trz)₄]·44H₂O (3), and [Mo^V₈O₈(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.