Metal−Organic Frameworks Based on Double-Bond-Coupled Di-Isophthalate Linkers with High Hydrogen and Methane Uptakes
2008-05-13T00:00:00Z (GMT) by
Solvothermal reactions of Cu(NO<sub>3</sub>)<sub>2</sub> with azoxybenzene-3,3′,5,5′-tetracarboxylic acid (H<sub>4</sub>aobtc) or <i>trans</i>-stilbene-3,3′,5,5′-tetracarboxylic acid (H<sub>4</sub>sbtc) give rise to two isostructural microporous metal−organic frameworks, Cu<sub>2</sub>(abtc)(H<sub>2</sub>O)<sub>2</sub>·3DMA (PCN-10, abtc = azobenzene-3,3′,5,5′-tetracarboxylate) and Cu<sub>2</sub>(sbtc)(H<sub>2</sub>O)<sub>2</sub>·3DMA (PCN-11, sbtc = <i>trans</i>-stilbene-3,3′,5,5′-tetracarboxylate), respectively. Both PCN-10 and PCN-11 possess significant enduring porosity with Langmuir surface areas of 1779 and 2442 m<sup>2</sup>/g (corresponding to BET surface areas of 1407 or 1931 m<sup>2</sup>/g, respectively) and contain nanoscopic cages and coordinatively unsaturated metal centers. At 77 K, 760 Torr, the excess gravimetric (volumetric) hydrogen uptake of PCN-10 is 2.34 wt % (18.0 g/L) and that of PCN-11 can reach 2.55 wt % (19.1 g/L). Gas-adsorption studies also suggest that MOFs containing CC double bonds are more favorable than those with NN double bond in retaining enduring porosity after thermal activation, although NN has slightly higher H<sub>2</sub> affinity. The excess gravimetric (volumetric) adsorption at 77 K saturates around 20 atm and reaches values of 4.33% (33.2 g/L) and 5.05% (37.8 g/L) for PCN-10 and PCN-11, respectively. In addition to its appreciable hydrogen uptake, PCN-11 has an excess methane uptake of 171 cm<sup>3</sup>(STP)/cm<sup>3</sup> at 298 K and 35 bar, approaching the DOE target of 180 v(STP)/v for methane storage at ambient temperature. Thus, PCN-11 represents one of the few materials that is applicable to both hydrogen and methane storage applications.
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