Role of Exposed Metal Sites in Hydrogen Storage in MOFs

Abstract

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.

Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives

“…The accessibility of these sites has been proven both experimentally [30] and based on simulations [31][32][33]. Consequently, Cu-BTC has been used in some catalytic applications up to now.…”

Hetero-bimetallic metal–organic polyhedra

“…27 These bimetallic MOPs show significantly stronger affinity for hydrogen than the Cu 2 (L) structure which has an enthalpy of approximately -8 kJ/mol over the hydrogen loading range. In contrast to the other bimetallic MOPs the adsorption enthalpy of CuPd(L) shows a gradual decline from -10 kJ/mol to a value of -8.60 kJ/mol.…”

Microwave Synthesis of a Porous Metal-Organic Framework, Nickel(II) Dihydroxyterephthalate and its Catalytic Properties in Oxidation of Cyclohexene

“…Among them, nickel dihydroxyterephthalate [Ni2(dhtp)(H2O)2]· 8H 2 O (where, dhtp = 2,5-dihydroxyterephthalate, hereafter denoted by Ni-DHTP which was named by Ni-CPO-27 is one of the frequently studied metal-organic framework (MOF) material because of a large surface area, a relatively high thermal stability and a large number of open metal sites after removal of solvent or water molecules from the framework pores. [11][12][13] These properties enable the above material to be a potential candidate for gas sorption and delivery. [14][15][16] Since the discovery of Ni-DHTP which has equivalent structure of MOF-74 reported by Rosi et al,17 this has been synthesized by conventional hydrothermal or solvothermal method (CH) for improving the purity of crystal.…”