Hydrogenation and dehydrogenation of methylcyclohexane/toluene as a liquid organic hydrogen carrier

Liquid organic hydrogen carriers (LOHCs) present a promising technology for long-distance hydrogen transport. The methylcyclohexane/toluene (MCH/TOL) system, known for its high hydrogen storage capacity and low toxicity, is a leading LOHC candidate. However, its commercial feasibility depends heavily on efficient catalysis for both the dehydrogenation of MCH and the hydrogenation of TOL, typically using Pt-based catalysts. MCH dehydrogenation is particularly challenging, as it is highly endothermic and requires high temperatures that often lead to rapid catalyst deactivation. Therefore, developing highly active, stable catalysts with low Pt loading is essential for this step. Conversely, TOL hydrogenation is thermodynamically favorable and proceeds under milder conditions. Yet, developing non-noble catalysts with activity comparable to standard Pd/Pt systems, despite their cost advantage, is challenging. In the MCH dehydrogenation reaction, Pt catalysts often suffer from performance limitations in terms of poor stability at elevated temperatures, due to sintering and/or poisoning by side products and eventually coke. Developing bimetallic Pt alloys by tuning the electronic and structural properties of Pt presents a promising strategy to overcome these issues. Although extensive research has been conducted on the development of bimetallic or multi-metallic Pt catalysts, the influence of the dispersion state of the promoter metal has seldom been investigated, although it is critical to catalyst design. In the first part of this thesis work, it was demonstrated that the initial dispersion of Fe on SiO2, as isolated atoms or aggregated nanoparticles, profoundly influences the final structure and activity of Pt–Fe catalysts. Atomically dispersed Fe atoms, exhibiting strong interactions with the SiO2 support, facilitate the formation of smaller Pt–Fe ensembles after loading Pt. Conversely, when Fe exists as larger clusters or nanoparticles, its weak support interaction leads to negligible alloy formation and Pt particles resembling monometallic catalysts in both size and performance. The catalyst incorporating atomically dispersed Fe, Pt–Fe0.2/SiO2-SEA (0.9 wt% Pt and 0.2 wt% Fe) shows higher stability, with conversion decreasing only from 99.2% to 97.7% after 26 h, whereas Pt-Fe0.2/SiO2-Nit (with aggregated Fe species) drops from 99.8% to 92.1%. Under a pure MCH feed at a weight hourly space velocity (WHSV) of 15.5 h-1, Pt–Fe0.2/SiO2-SEA maintains 99.7% conversion over 240 hours with >99.9% selectivity. Furthermore, at a WHSV of 70.6 h⁻¹, a remarkable hydrogen evolution rate of 3967 mmol·gₚₜ⁻¹·min⁻¹ can be obtained. These results highlight the importance of precise promoter dispersion in the rational design of high-performance bimetallic catalysts, offering a clear route to optimize dehydrogenation through structural control. While Pt-Fe bimetallic alloy catalyst has demonstrated promising performance compared to bare Pt, further improving the stability and reducing the Pt loading is required for practical applications. In the 2nd work, Pt-Beta zeolite catalyst was developed with a lower loading of Pt and exceptional stability. With as low as 0.3 wt% Pt nanoparticles confined within dealuminated Beta zeolite (DAl-Beta), the catalyst exhibits exceptional stability. Unlike conventional approaches, where zeolites primarily confine the metal catalysts, it is demonstrated that the zeolite framework strongly interacts with MCH, TOL, and key reaction intermediates via dispersive van der Waals forces. This interaction mechanism is supported by both experimental and theoretical studies. MCH is strongly adsorbed and polarized by the zeolite framework, facilitating subsequent adsorption onto adjacent confined Pt sites. Furthermore, due to its strong interaction with the zeolite framework, TOL readily desorbs from the Pt surface once formed. Consequently, this catalyst with only 0.3 wt% Pt displays exceptional stability. Under a pure MCH feed at a WHSV of 15.9 h⁻¹, 99.8% of conversion can be maintained over 1000 hours with >99.8% of selectivity. Furthermore, at a WHSV of 277 h⁻¹, a remarkable hydrogen evolution rate of 39052 mmol·gₚₜ-1·min-1 can be obtained. To complete the TOL/MCH LOHC cycle, TOL hydrogenation needs to be effectively performed alongside MCH dehydrogenation. While TOL hydrogenation has been extensively studied using noble metals like Pt and Ru, developing non-noble metal-based catalysts is essential to reduce the overall cost. In this context, among various silica-based materials, including SiO2, Al2O3, and dealuminated Beta, 5 wt% Ni catalyst supported on dealuminted Beta has demonstrated high activity. It achieves a turnover frequency (TOF) of 312 mmol·gₐₜ-1·h-1 at 160 °C and atmospheric pressure. Furthermore, under a WHSV of 14 h-1, it maintains >85% conversion with ~99.9% selectivity for 160 hours. This performance demonstrates the viability of replacing noble metals and enabling continuous operation under milder reaction conditions.