Effect of lithium doping into MIL-53(Al) through thermal decomposition of anion species on hydrogen adsorption

Lithium-doped MIL-53(Al) (Li-MIL-53(Al)) is prepared by impregnating MIL-53(Al) with an ethanol solution of LiNO ₃, followed by heat treatment in vacuum. The nitrate anion is thermally decomposed and removed in the form of NO and N ₂O at 573 K. This temperature is much lower than the decomposition temperature of bulk LiNO ₃, which can be attributed to the smaller size of LiNO ₃ in the pores as well as to the high charge density of aluminum in the MIL-53(Al) skeleton. The doped amount can be varied by changing the concentration of the LiNO ₃ solution. The lithium doping enhances the hydrogen uptake from 1.66 to 1.84 wt % at 77 K and 1 atm when the doped amount is Li/Al = 0.036. This enhancement suggests that one lithium cation can adsorb two hydrogen molecules. However, the isosteric heat of hydrogen adsorption is not enhanced, possibly due to the interaction of the doped lithium cations with carboxyl groups, as suggested by ¹³C CP/MAS NMR. Electron-withdrawing oxygen atoms of the carboxyl group should weaken the affinity of the doped lithium cation to hydrogen molecules. Thus, the lithium cations only act as the additional adsorption sites with an affinity to hydrogen molecules similar to that of the internal surface of MIL-53(Al). Similarly, other alkaline/alkaline earth metal cations, such as Na ⁺, Mg ²⁺, and Ca ²⁺, can also be doped into MIL-53(Al), resulting in the increase in the hydrogen uptakes to 1.76, 1.76, and 1.69 wt % for Na ⁺, Mg ²⁺, and Ca ²⁺, respectively.