Origin of the Temperature-Dependent Rate Decline in Toluene Hydrogenation

Abstract

Efficient hydrogenation of toluene (TOL), a representative liquid organic hydrogen carrier (LOHC), is important for establishing a carbon-neutral H2 supply chain. Although the temperature-dependent decline in aromatic hydrogenation rates has often been attributed to site blocking by strongly adsorbed species, direct experimental evidence for the formation and evolution of these species has remained limited. Here, TOL hydrogenation over Ni/Al2O3 and Pt/Al2O3 was investigated using kinetic measurements, temperature-programmed surface reaction (TPSR) with D2, temperature-programmed reduction coupled with mass spectrometry (TPR-MS), H2 regeneration tests, gas-switching experiments, and Raman spectroscopy. Over Ni/Al2O3, TOL was selectively hydrogenated to methylcyclohexane at 120 °C, whereas the TOL consumption rate decreased markedly at 180 °C under conditions far from thermodynamic equilibrium. A similarly suppressed hydrogenation regime was observed over Pt/Al2O3 at approximately 250–350 °C, demonstrating a strong dependence on the catalyst metal. Over Ni/Al2O3 at 180 °C, the apparent reaction order with respect to H2 exceeded 2, and the initial activity was fully restored by treatment under H2 at 600 °C, indicating that H2 participated in both TOL hydrogenation and the removal of persistent site-blocking species. During TPR-MS, TOL and benzene evolved from approximately 200 °C, supporting preservation of the six-membered carbon-ring framework in retained adsorbates. CH4 evolution began above approximately 250 °C and persisted beyond 500 °C, consistent with methyl-derived fragments remaining after benzene release and undergoing subsequent hydrogenative removal. At 300 °C over Ni/Al2O3, hydrodemethylation became the dominant reaction pathway. Gas-switching experiments from TOL/H2 to H2 produced a transient CH4 peak, indicating the accumulation of hydrogenatable carbonaceous species, and Raman spectroscopy confirmed the presence of disordered graphite-like carbon. These findings provide experimental support for kinetic inhibition by persistent ring-containing adsorbates at intermediate temperatures and associate higher-temperature hydrodemethylation with carbon accumulation and graphite-like carbon formation.