Nickel catalysts in the hydrogenation of benzyltoluene using impure hydrogen streams

Abstract

Biomass-derived hydrogen is frequently contaminated with CO and CO2, which limits its direct utilization in downstream applications. Cyclic hydrogenation/dehydrogenation of liquid organic hydrogen carriers (LOHCs) such as benzyltoluene (BT) provides a pathway to simultaneously store hydrogen and purify such "low-grade" hydrogen feeds. In this context, Ni-based catalysts are particularly attractive because they are cheap and can hydrogenate aromatic systems under CO-containing atmosphere. Here, we systematically investigate BT hydrogenation over a commercial Ni/Al2O3/SiO2 catalyst under processrelevant conditions (T = 170-230 °C, p = 10-50 bar) in the presence of CO (0-6 %) and demonstrate that BT conversion remains feasible even at high CO levels, while the hydrogenation rate decreases due to competitive CO adsorption. Timeresolved analysis of both gas-and liquid-phase compositions further reveals CO methanation during BT hydrogenation, indicating that both reactions proceed in parallel while competing for active sites. By varying temperature and total pressure, the balance between BT hydrogenation and methanation can be steered, with 230 °C and 50 bar identified as the most favorable conditions for efficient BT hydrogenation under CO presence. In addition, introducing CO2 at biomass-typical concentrations (30 vol%) shows that it likewise adsorbs competitively on Ni and is also converted to methane. Conclusively, Ni-based catalysts are effective for mixed-gas hydrogenation and LOHC-based purification and storage of biogenic hydrogen.However, co-methanation consumes part of the hydrogen as unrecoverable CH₄, which may serve as an energy-dense coproduct.