Hydrogen storage for stationary and mobile applications is an expanding research topic. One of the more promising hydrogen storage techniques relies on the reversibility and high selectivity of liquid organic hydrides, in particular, methylcyclohexane (MCH). The use of liquid organic hydrides in hydrogen storage also provides high gravimetric and volumetric hydrogen density, low potential risk, and low capital investment because it is largely compatible with the current transport infrastructure. Despite its technical, economical, and environmental advantages, the concept of hydrogen storage in liquid organic carriers has not been commercially established because of technical limitations related to the amount of energy required to extract the hydrogen from liquid organic hydride and the insufficient stability of the dehydrogenation catalyst. This paper provides a review for the effort that has been directed toward the development of this concept over the past few decades and mainly focuses on the catalytic production of hydrogen from MCH. The topics that have been covered are the kinetics of MCH dehydrogenation over Pt/Al2O3 and Pt–Re/Al2O3 catalysts, the kinetics of catalyst deactivation, the thermodynamic equilibrium in MCH dehydrogenation, and the sulfur impact on the MCH dehydrogenation reaction.

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Three vacuum residual oils (VR) derived from Ratawi Burgan (RB), Lower Fars (LF), and Eocene (EOC) crude oils were subjected to thermal cracking in a pilot plant, which simulates the Eureka process, to produce cracked distillate petroleum products and residual pitch. The cracking reaction was performed at 430 °C for 50 min. The chemical composition of the produced cracked petroleum products and byproduct pitch was studied to determine its relationship to the variations in the properties of the feedstock. Saturates, aromatics, resins and asphaltenes (SARA) analysis of the vacuum residues, cracked oils, and pitch show that the residues and pitch consist mainly of aromatic hydrocarbons (VR: 94 wt %; pitch: 99 wt %), while the oils themselves contain about 42 wt % saturated hydrocarbons (oil RB : 46 wt %; oil EOC : 44 wt %; oil LF : 36 wt %). 1 H and 13 C NMR revealed that the VRs consist predominantly of alkyl aromatics with di-, tri-(aromatics, resins), and polyaromatic rings (asphaltenes) that thermally decompose splitting the molecules into saturated lower molecular weight hydrocarbons and aromatics having lower aliphatic carbon attached to it. Regardless of the feed, all oils contain more aliphatic (∼62 wt %) than aromatic carbon (∼21 wt %). The cata-condensed aromatic moiety in the oil is triaromatic. The effect of feedstock on the chemical composition of the oil and pitch is most prominent for the aromatic and asphaltenic fractions.

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Various kinetic models were developed for methylcyclohexane (MCH) dehydrogenation over supported Pt catalysts. The best fitting mechanistic model is of the non-Langmuirian/noncompetitive Horiuti-Polanyi type. In this model, the Horiuti-Polanyi aromatic hydrogenation mechanism, which assumes an atomic hydrogen addition to aromatics on the catalyst surface, is applied in reverse to MCH dehydrogenation. The model also assumes that hydrogen and MCH molecules adsorb noncompetitively on two different types of sites to accommodate the observed near zero-order dependence of reaction rate on MCH and the negative order dependence upon hydrogen. To account for the increase in the hydrogen inhibition effect with pressure, a non-Langmuirian adsorption isotherm is adopted, which assumes a nonlinear dependency between the adsorption equilibrium constant for hydrogen and the system pressure. The reversible and irreversible deactivation kinetics are satisfactorily included in the kinetic model.