Metal Alkoxide Functionalization in Metal−Organic Frameworks for Enhanced Ambient-Temperature Hydrogen Storage

Abstract

Metal−organic frameworks (MOFs) are permanently porous solids, which are promising hydrogen storage materials. However, the maximum H₂ adsorption energies in MOFs are only around 10 kJ·mol⁻¹, leading to small adsorption capacities at ambient temperature. In this work we use ab initio calculations and grand canonical Monte Carlo (GCMC) simulations to explore metal alkoxide functionalization for improving H₂ storage in IRMOF-1, IRMOF-10, IRMOF-16, UiO-68, and UMCM-150. We examine functionalization with lithium, magnesium, manganese, nickel, and copper alkoxides. We show that lithium and magnesium alkoxides physically bind H₂ and manganese, nickel, and copper alkoxides chemically bind H₂. H₂ binding energies calculated with quantum mechanics are −10, −22, −20, −78, and −84 kJ·mol⁻¹, respectively, for the first hydrogen molecule. Of these, lithium and manganese alkoxides bind H₂ too weakly to enhance adsorption at ambient temperature, even at 100 bar. Owing to the strong binding energies, Ni and Cu exhibit high uptake at low pressure, but metal alkoxide sites saturate at pressures as low as 1 bar. They thus exhibit poor deliverable capacities [wt % (100 bar) − wt % (2 bar)]. Magnesium alkoxide exhibits low uptake at low pressure and high uptake at high pressure and is a promising functional group for enhanced ambient-temperature hydrogen storage in all MOFs studied.