An overview of organic liquid phase hydrogen carriers

Abstract An overview study for assessing the feasibility of using organic compounds as hydrogen carriers is conducted. A literature survey and preliminary molecular modeling work have been carried out. Different classes of compounds have been investigated in the literature. Those are cycloalkanes, polycyclic alkanes, heteroatoms-containing hydrocarbons and even ionic liquids. In addition to experimental work where catalyst development and reactor design were sought for efficient and selective dehydrogenation of organic carriers, computational modeling work had also been conducted by different research groups, principally to search for compounds with low enthalpy of dehydrogenation. Among the different compounds considered, the ones that meet more or less the required physical (liquid in all state), toxicity/environmental, stability (to be recycled) and cost criteria for hydrogen storage and delivery purposes are within the cycloalkanes class. Methylcyclohexane only seems to be the appropriate candidate. The economic viability of using such compounds is however pending to the releasable hydrogen content as the maximum value is only about 7.2wt%. Among catalysts usually cited for the dehydrogenation of the organic compounds, noble metal supported ones and mono or bi-metallic catalysts especially are having better performances in terms of activity and selectivity. Although the combination of catalysts and organic compounds will give different reactivity and selectivity, the polycyclic alkanes and heteroatom-containing hydrocarbons will usually be more susceptible to side reactions during dehydrogenation forming coke, additional partially dehydrogenated and decomposed products as well. No data are available as to the life time of the catalyst in such reaction, but it is expected that deactivation might be a concern. Different reactor configurations were considered in pursuit of high conversions while limiting side reactions. Finally the computational modeling data obtained are in agreement with the ones found in the literature. Although heteroatom-containing polycyclic hydrocarbons are found to present lower enthalpy of dehydrogenation that would be suitable for on-board dehydrogenation, as mainly sought by the different research groups, their application as organic carrier seems quite difficult in regards of their hydrogen content, their physical properties and the cost of synthesis.