The hydrogen storage capacity of mono-substituted MOF-5 derivatives: An experimental and computational approach

Herein we report the synthesis and hydrogen storage capability of four simple MOF-5 modifications, i.e. CH3-MOF-5, OCH3-MOF-5, Br-MOF-5 and Cl-MOF-5. The mono-substituted MOF-5s, with the exception of OCH3-MOF-5, exhibit the same topology as that of unsubstituted MOF-5. The BET surface areas are in the range of 680-2750 m(2) g(-1) for this series. Introducing these functional groups appears to have a significant effect on the thermal stability of the resulting frameworks. The thermal stability of CH3-MOF-5 is comparable to that of MOF-5 after heat-treatment, i.e. vacuum dried at 200 degrees C for 40 h, whereas the structure of Cl-MOF-5 collapses under the same conditions. Activated MOF-5, CH3-MOF-5, Br-MOF-5 and Cl-MOF-5 show 1.44 wt.%, 1.47 wt.%, 1.08 wt.% and 0.99 wt.% of hydrogen uptake capacities at 77 K and 1 bar, respectively. Experimental and computational results reveal that the introduction of - CH3, -Br and -Cl has only a minor effect on the isosteric heat of hydrogen adsorption for MOF-5 and the experimental values are in the range of 2.8 -3.0 kJ mol(-1) for all derivatives. This explains the similar hydrogen adsorption capacities of MOF-5 and CH3-MOF-5. The poor porous structures of Br-MOF-5 and Cl-MOF-5, however, result in lower hydrogen adsorption capacities compared to MOF-5 despite their similar isosteric heat of hydrogen adsorption. At high pressures (>20 bar), the excess hydrogen capacity appears to be a strong function of the specific surface area. (C) 2013 Elsevier Inc. All rights reserved.