Abstract
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•DFT calculations show that the first CH dissociation from an alkyl substituent of an aromatic ring on Pt(1 0 0) and Ni(1 0 0) flat surfaces is an exothermic process.•This is unique relative to dissociation of an aromatic or an alkane CH bond, both processes usually endothermic.•This conclusion holds for large volume aromatics, such as toluene and xylenes, for mesitylene, and for ethylbenzene.•Our calculations provide a molecular understanding of the essential role of cofed (or recycled) hydrogen to produce alkyl aromatics in practical applications.
This study reports the relevance of alkyl substituents on the chemisorption of alkylaromatics on metal surfaces, namely Pt(1 0 0) and Ni(1 0 0). Density functional theory (DFT) based calculations are used to calculate the chemisorption energies of benzene, methylbenzene (toluene), 1,2-, 1,3-, and 1,4-dimethylbenzenes (xylenes), and 1,3,5-trimethylbenzene (mesitylene), and ethylbenzene on Pt and Ni. Removal of hydrogens from the adsorbed benzene substantially reduces the stability of the adsorbed species on both Pt(1 0 0) and Ni(1 0 0) surfaces. Methyl substituents on the benzene ring, i.e., toluene, xylenes and mesitylene, result in exothermic dissociation of a CH bond from each methyl substituent on both Pt and Ni surfaces. This leads to formation of CH2 groups connected to the aromatic ring, resulting in a π-conjugated system stacked over the metal surface, and expanding over the entire molecule. The energy gain associated with dehydrogenation of each hydrogen from the methyl groups cumulates, so that complete π-conjugated configuration by dehydrogenation of the methyl groups of xylenes and mesitylene is expected. Further removal of hydrogen from aromatic rings of the π-conjugated system is always endothermic on both surfaces. The study is extended to ethylbenzene on the same surface, and results suggest that dehydrogenation of methylene group followed by dehydrogenation of the methyl group, leading to formation of adsorbed styrene is favored. These results may justify more explicitly the essential role of cofed (or recycled) hydrogen to produce alkylaromatics in practical applications, enabling to recover sp3 alkyl substituents to desorb the products. Although the actual catalysis involves more complexities on other types of metal sites, such as edges and kinks, the current study already demonstrates one significant aspect of the hydrogenation-dehydrogenation catalysis on metal surfaces.