1,4-Butanediol as a Hydrogen Carrier: Liquid- versus Gas-Phase Dehydrogenation

Hydrogen storage is a key technology for enabling the decarbonization of future energy systems, but is subject to low volumetric density in the case of compressed hydrogen. Chemical conversion to a liquid hydrogen carrier can overcome this challenging issue. For example, the reversible reaction of 1,4-butanediol (BDO) to γ-butyrolactone releasing two moles of hydrogen could be an attractive option for storing and transporting hydrogen. Herein, the thermodynamics of both, the reaction and the process, are evaluated. An important question that should be answered is whether there is a difference in energy efficiency between this reaction being performed in the liquid and in the gas phase. The thermodynamic analysis showed that heat demand for evaporation from the liquid to the gas phase is a major drawback of gas-phase reactions even though the enthalpy of reaction is somewhat smaller in the gas than in the liquid phase. However, due to the vapor pressure of BDO, there is some evaporation occurring in the case of liquid-phase reactions at reaction temperatures required to release hydrogen. This evaporation reduces the energy benefits of the liquid-phase reaction. As a consequence, the round-trip efficiencies of both options (liquid- and gas-phase reaction) are similar (varying by <5%). This is an advantageous finding as it allows selection of reaction conditions (liquid- or gas-phase reaction) solely based on catalytic considerations, without major restrictions due to concerns from process engineering.