Control of interpenetration and gas-sorption properties of metal-organic frameworks by a simple change in ligand design

Thazhe Kootteri Prasad, Myunghyun Paik Suh
Chemistry: a European Journal 2012 July 9, 18 (28): 8673-80
In metal-organic framework (MOF) chemistry, interpenetration greatly affects the gas-sorption properties. However, there is a lack of a systematic study on how to control the interpenetration and whether the interpenetration enhances gas uptake capacities or not. Herein, we report an example of interpenetration that is simply controlled by the presence of a carbon-carbon double or single bond in identical organic building blocks, and provide a comparison of gas-sorption properties for these similar frameworks, which differ only in their degree of interpenetration. Noninterpenetrated (SNU-70) and doubly interpenetrated (SNU-71) cubic nets were prepared by a solvothermal reaction of [Zn(NO(3))(2)]⋅6 H(2)O in N,N-diethylformamide (DEF) with 4-(2-carboxyvinyl)benzoic acid and 4-(2-carboxyethyl)benzoic acid, respectively. They have almost-identical structures, but the noninterpenetrated framework has a much bigger pore size (ca. 9.0×9.0 Å) than the interpenetrated framework (ca. 2.5×2.5 Å). Activation of the MOFs by using supercritical CO(2) gave SNU-70' and SNU-71'. The simulation of the PXRD pattern of SNU-71' indicates the rearrangement of the interpenetrated networks on guest removal, which increases pore size. SNU-70' has a Brunauer-Emmett-Teller (BET) surface area of 5290 m(2) g(-1), which is the highest value reported to date for a MOF with a cubic-net structure, whereas SNU-71' has a BET surface area of 1770 m(2) g(-1). In general, noninterpenetrated SNU-70' exhibits much higher gas-adsorption capacities than interpenetrated SNU-71' at high pressures, regardless of the temperature. However, at P<1 atm, the gas-adsorption capacities for N(2) at 77 K and CO(2) at 195 K are higher for noninterpenetrated SNU-70' than for interpenetrated SNU-71', but the capacities for H(2) and CH(4) are the opposite; SNU-71' has higher uptake capacities than SNU-70' due to the higher isosteric heat of gas adsorption that results from the smaller pores. In particular, SNU-70' has exceptionally high H(2) and CO(2) uptake capacities. By using a post-synthetic method, the CC double bond in SNU-70 was quantitatively brominated at room temperature, and the MOF still showed very high porosity (BET surface area of 2285 m(2) g(-1)).

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