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Multiple Functional Groups of Varying Ratios in Metal-Organic Frameworks

Science  12 Feb 2010:
Vol. 327, Issue 5967, pp. 846-850
DOI: 10.1126/science.1181761

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Many Mixed Linkers in MOFs

Crystallization can separate different molecules because different molecules cannot generally be accommodated equally well in the same crystal lattice. However, in metal-organic framework (MOF) compounds, the organic linkers do not pack closely to other parts of the lattice, so it may be possible to mix several linkers that are derivatives of a parent compound with the same end groups. Deng et al. (p. 846) show that zinc-based MOFs can be made that mix 1,4-benzenedicarboxylate and up to eight of its derivatives in a random fashion. The effects of such mixing on porosity and absorption characteristics is nonlinear; in one case, a mixed-linker compound was four times better for selecting CO2 versus CO compared with the best MOF bearing only one of the component linkers.

Abstract

We show that metal-organic frameworks (MOFs) can incorporate a large number of different functionalities on linking groups in a way that mixes the linker, rather than forming separate domains. We made complex MOFs from 1,4-benzenedicarboxylate (denoted by “A” in this work) and its derivatives -NH2, -Br, -(Cl)2, -NO2, -(CH3)2, -C4H4, -(OC3H5)2, and -(OC7H7)2 (denoted by “B” to “I,” respectively) to synthesize 18 multivariate (MTV) MOF-5 type structures that contain up to eight distinct functionalities in one phase. The backbone (zinc oxide and phenylene units) of these structures is ordered, but the distribution of functional groups is disordered. The complex arrangements of several functional groups within the pores can lead to properties that are not simply linear sums of those of the pure components. For example, a member of this series, MTV-MOF-5-EHI, exhibits up to 400% better selectivity for carbon dioxide over carbon monoxide compared with its best same-link counterparts.

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