Analysis of H2 Release from Organic Polycyclics over Pd Catalysts Using DFT

Density functional theory (DFT) was used to study H-2 release from dodecahydro-N-ethylcarbazole over Pd. Dehydrogenation of the molecule starts from the five-membered ring by weakening the axial C-H bonds adjacent to the N atom. In the adsorption mode identified for dodecahydro-N-ethylcarbazole, the five-membered ring was located on a hollow site and the axial H atoms adjacent to N were positioned atop Pd atoms with an adsorption energy of -95 kJ/mol. The two axial C-H bonds of the five-membered ring showed a large increase in bond distance upon adsorption, from 1.10 to 1.16 angstrom, that was responsible for the weakening of these bonds. The adsorption mode for dodecahydro-N-ethylcarbazole explained the product distribution obtained from the dehydrogenation reaction with octahydro-N-ethylcarbazole and tetrahydro-N-ethylcarbazole identified as primary and secondary products, respectively. DFT was also used to investigate the structure sensitivity of dodecahydro-N-ethylcarbazole dehydrogenation observed on Pd catalysts. The effect of the ethyl group and N atom on the rate of dodecahydro-N-ethylcarbazole dehydrogenation was also investigated by comparing the adsorption energies of dodecahydro-N-ethylcarbazole with two other H-2 storage candidates, dodecahydrocarbazole and dodecahydrofluorene. The latter compounds had much slower H-2 recovery rates, a consequence of the strong adsorption of these compounds relative to dodecahydro-N-ethylcarbazole.