Methylcyclohexane-toluene (MCH-TOL) as liquid energy carriers provide means to store and release H₂, which can be catalyzed by Pd catalyst. MCH dehydrogenation to TOL was investigated on Pd nanoparticles dispersed on SiO₂, Al₂O₃, CeO₂ and MgO, which underwent six sequential H abstraction steps to form TOL. Partial dehydrogenation of MCH gave methylcyclohexenes as detectable intermediates that were not equilibrated with MCH and H₂ from 453 K to 553 K, irrespective of the identity of the support, suggesting the kinetically relevant H-abstraction step preceded methylcyclohexene formation (i.e., the first two H abstraction steps). TOL formation turnover rate showed Langmuir-type dependence on MCH pressures and negative one-half order with respect to H₂ pressures from 453 K to 503 K regardless of the coverage of surface species on Pd particles, suggesting the second H abstraction step limited the overall TOL rate. From 513 K to 553 K on Pd/Al₂O₃, TOL rate was independent of H₂ pressures regardless of the coverage of surface species on Pd, indicative of the first H abstraction step as the kinetically relevant step. Increasing temperature rendered endothermic H abstraction steps irreversible since the reverse steps (H addition) were exothermic and not favored at higher temperatures. TOL formation turnover rates were not affected by diameters of Pd nanoparticles (2.1 to 11 nm) on SiO₂ or Al₂O₃ supports, but physical mixtures of Pd/SiO₂ and Al₂O₃ increased TOL formation turnover rate by a factor of ~ 1.5, suggesting such a rate enhancement did not require atomic contact between Pd and Al₂O₃. Instead, the less reactive species on Pd particles can desorb and be scavenged on Lewis acid-base sites on Al₂O₃ to release more Pd sites for TOL formation, which was also corroborated for TOL hydrogenation on Pt catalysts supported by oxides with Lewis acid-base sites.

Zhongyao Zhang

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