Hydrogen adsorption mechanism of MOF-74 metal-organic frameworks: an insight from first principles calculations

The microscopic mechanism of the H-2 adsorption of two Mg-MOF-74 isoreticular frameworks, one with a benzenedicarboxylate (BDC) linker and the other with a dihydroxyfumarate (DHF) linker, were studied on the basis of density functional theory (DFT) method. Possible adsorption sites on the internal surface of the two MOFs were detected using ab initio molecular dynamics (AIMD) annealing simulations. The simulations were able to reproduce all adsorption sites which have been experimentally observed for the BDC-based M-MOF-74 frameworks with M = Ni and Zn. In descending order of binding strengths, they are the adsorption sites primarily induced by the open metal sites P-1, the oxygen atoms of the oxido groups P-2 and the aromatic rings P-3. The H-2-framework binding strengths were properly evaluated by taking into account the vibrational zero-point energy (ZPE) contribution. An additional type of adsorption sites induced by the oxygen atoms of the carboxyl groups P-4 is predicted for the Mg-MOF-74 framework. Two types of adsorption sites primarily induced by the open metal sites P-1 and oxygen atoms of the carboxyl groups P-2 were predicted for the DHF-based Mg-MOF-74 framework. Detailed analysis of the electron density showed that the electrostatic interaction of the H-2 molecule with the charge distribution of the local framework environment within a radius of similar to 3.5 angstrom is a key factor to define adsorption positions and binding strength. The absence of the P-4 sites in the BDC-based Zn-MOF-74 framework is caused by the lower charge density at the oxygen atoms induced by less electro-positive metal. The substitution of the nonaromatic DHF linker for the aromatic BDC linker reduces the binding strength at the metal induced adsorption sites by 1.45 kJ mol(-1) due to the absence of the aromatic ring.