A Novel Eutectic Mixture of Biphenyl and Diphenylmethane as a Potential Liquid Organic Hydrogen Carrier: Catalytic Hydrogenation

Abstract

A novel, liquid‐phase hydrogen storage material was produced by mixing biphenyl (C12H10, solid) and diphenylmethane (C13H12, liquid) with a mass ratio (wt %) of 35:65. The eutectic mixture was proved to be liquid at ambient temperature with a viscosity of 3.0 cP at 25 °C and a vapor pressure of 1.4 Pa at 25 °C, which potentially meets the requirements for an efficient hydrogen storage system. A theoretical study using COSMO‐RS method further confirms that an optimum composition of biphenyl and diphenylmethane for the formation of eutectic mixture is 37:63 by mass, which is very close to the composition found experimentally. The eutectic mixture exhibited a superior activity for hydrogenation reactions conducted at 120 °C under 50 bar of H2 pressure in the presence of a Ru (5 wt %)/Al2O3 catalyst. Hydrogen uptake of the eutectic mixture tested was close to full conversion, which corresponds to 6.9 wt % and 60.1 g L−1 of gravimetric and volumetric hydrogen storage capacities, respectively.

A study on hydrogen uptake and release of a eutectic mixture of biphenyl and diphenyl ether

“…However, oxygen-containing LO-HCs received less attention due to their limited aromatic structures and chemically labile nature toward the hydrogenolysis (i.e., C-O bond cleavage) [17] compared to N-heterocycles, despite the apparent thermodynamic benefit of heteroatom effect. Given these previous studies in conjunction with our recent results of biphenyl and diphenylmethane [10] , use of an oxygen-containing compound with biphenyl could provide improved dehydrogenation thermodynamics compared to the mixture of biphenyl and diphenylmethane if the hydrogenolysis issue is solved. We report here on different hydrogen storage properties of a novel liquid carrier, the mixture of biphenyl and diphenyl ether ( Scheme 1 ).…”

“…Full thermochemical properties (i.e., reaction enthalpy and Gibbs free energy) of stepwise dehydrogenation of H 2 -rich form ( 6, 10 ) are also presented in Tables S3-S6. The obtained thermodynamic data suggest that a biphenyl and diphenyl ether mixture can also be a plausible LOHC candidate as the case of the biphenyl and diphenylmethane mixture [10] . We calculate the solid-liquid equilibria of the binary mixture of biphenyl and diphenyl ether to estimate a eutectic point.…”

“…Full product distributions were analyzed by GC-MS and are presented in Table S8. Our previous hydrogenation activities of the H 2 -lean form 1 showed that the fastest reaction kinetics can be reached with Ru/Al 2 O 3 and the slowest kinetics are achieved with Pd-based catalysts [10] . Fig.…”

“…Therefore, mixing biphenyl and diphenylmethane to a eutectic composition allows the material to be liquid at ambient temperatures and still provides a high hydrogen storage capacity (6.9 wt%, 60 g-H 2 L −1 ). The hydrogenation reaction of 1 to a hydrogen-rich mixture 2 and the dehydrogenation of 2 to 1 were demonstrated successfully with a high conversion over 94% with the aid of proper catalysts [10,11] . DFT calculations indicated that dicyclohexylmethane requires a higher temperature (603 K) than bicyclohexyl (568 K) for complete dehydrogenation [11] .…”

“…Although biphenyl contains a high gravimetric Scheme 1. (Top) A eutectic mixture of biphenyl and diphenylmethane employed in the previous study [10,11] . (Bottom) A eutectic mixture of biphenyl and diphenyl ether employed in this study.…”