Metal−Organic Frameworks Based on Double-Bond-Coupled Di-Isophthalate Linkers with High Hydrogen and Methane Uptakes

Solvothermal reactions of Cu(NO₃)₂ with azoxybenzene-3,3′,5,5′-tetracarboxylic acid (H₄aobtc) or trans-stilbene-3,3′,5,5′-tetracarboxylic acid (H₄sbtc) give rise to two isostructural microporous metal−organic frameworks, Cu₂(abtc)(H₂O)₂·3DMA (PCN-10, abtc = azobenzene-3,3′,5,5′-tetracarboxylate) and Cu₂(sbtc)(H₂O)₂·3DMA (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 m²/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.