Diphenyl Ether Derivatives as Potential Liquid Organic Hydrogen Carriers: Thermochemical and Computational Study

The liquid organic hydrogen carriers (LOHCs) are promising materials for hydrogen storage. The standard molar enthalpies of formation, Delta H-f(m)o (liq/cr) of diphenyl ether substituted with methyl, methoxy, and hydroxyl substituents were measured by the high-precision combustion calorimetry. Enthalpies of vaporization/sublimation Delta(g)(l,cr) H-o(m) (298.15 K) of these compounds were obtained by the transpiration method. Enthalpies of fusion of 2- and 4-hydroxy-diphenyl ether were measured by differential scanning calorimeter. Gas-phase enthalpies of formation Delta H-f(o)m (g, 298.15 K) of substituted diphenyl ethers calculated by the G4 method were in an excellent agreement with the experimental results. Enthalpies of the hydrogenation/dehydrogenation reactions of diphenyl ether derivatives were estimated and compared with the data available for other potential LOHC. A set of thermodynamic properties of diphenyl ether derivatives of benchmark quality was recommended for thermochemical calculations. Equilibrium temperatures of hydrogenation/dehydrogenation processes were calculated and compared with those of benzene derivatives. Because of their high hydrogen storage capacity, as well as very low volatility, these compounds could be considered as seminal candidates for LOHC.