Metal-organic frameworks based on double-bond-coupled di-isophthalate linkers with high hydrogen and methane uptakes
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abstract
Solvothermal reactions of Cu(NO3)2 with azoxybenzene-3,3,5,5-tetracarboxylic acid (ILaobtc) or transstilbene-3,3,5,5-tetracarboxylic acid (H4sbtc) give rise to two isostructural microporous metal-organic frameworks, Cu 2(abtc)(H2O)23DMA (PCN-10, abtc = azobenzene-3,3,5,5-tetracarboxylate) and Cu2(sbtc)- (H2O)23DMA (PCN-11, sbtc = trans-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 m2/g (corresponding to BET surface areas of 1407 or 1931 m 2/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 C=C double bonds are more favorable than those with N=N double bond in retaining enduring porosity after thermal activation, although N=N has slightly higher H2 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 cm3(STP)/cm3 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. 2008 American Chemical Society.
