Carbon-based catalysts for toluene hydrogenation in continuous reactors

In view of the international regulations and programs promoting the reduction of greenhouse gas emissions, hydrogen is a promising energy vector. However, low volumetric energy density and safety concerns pose significant obstacles to widespread utilization. An innovative path is represented by liquid organic hydrogen carrier (LOHC) systems, in which the hydrogen is chemically bound in a liquid molecule and released directly on site, when required. Nevertheless, a lack of rigorous studies to identify the most suitable liquid components and the required catalysts for the chemical reactions can be identified in the literature. To this scope, this work firstly focused on a preliminary sustainability-based screening, showing the toluene-methylcyclohexane cycle as the most suitable in short and medium terms, and N-ethylcarbazole (NEC)/perhydro-N-ethylcarbazole (NECH) for the long-term perspective. Afterwards, a carbon-based catalyst Ru0.25%/CNF was produced and characterized to perform a comprehensive kinetic analysis on toluene hydrogenation. A batch reactor was used for a preliminary study to deepen the knowledge on phenomenological and kinetic aspects, whereas the most convenient operative conditions were identified for the case of a fixed-bed continuous reactor. Different conditions, including temperatures, pressures, and weight hourly space velocities (WHSV), were tested at this stage. Results indicate an increase in conversion with decreasing WHSV, while generally increasing trends with respect to temperature and pressure, consistent with the literature available. Therefore, the contribution of this investigation is to understand the feasibility of a transition from laboratory to industrial scale of LOHC strategies, a key point in the engineering field, by introducing an innovative and optimized process aimed at solving a global issue.