RESUMO
Alkane metathesis transforms small alkanes into their higher and lower homologues. The reaction is catalyzed by either supported d0 metal hydrides (M = Ta, W) or d0 alkyl alkylidene complexes (M = Ta, Mo, W, Re). For the silica-supported tantalum hydrides, several reaction mechanisms have been proposed. We performed DFT-D3 calculations to analyze the viability of the proposed pathways and compare them with alkane hydrogenolysis, which is a competitive process observed at the early stages of the reaction. The results show that the reaction mechanisms for alkane metathesis and for alkane hydrogenolysis present similar energetics, and this is consistent with the fact that the process taking place depends on the concentrations of the initial reactants. Overall, a modified version of the so-called one-site mechanism that involves alkyl alkylidene intermediates appears to be more likely and consistent with experiments. According to this proposal, tantalum hydrides are precursors of the alkyl alkylidene active species. During precursor activation, H2 is released and this allows alkane hydrogenolysis to occur. In contrast, the catalytic cycle implies only the reaction with alkane molecules in excess and does not form H2. Thus, the activity for alkane hydrogenolysis decreases. The catalytic cycle proposed here implies three stages: (i) ß-H elimination from the alkyl ligand, liberating ethene, (ii) alkene cross-metathesis, allowing olefin substituent exchange, and (iii) formation of the final products and alkyl alkylidene regeneration by olefin insertion and three successive 1,2-CH insertions to the alkylidene followed by α abstraction. These results relate the reactivity of silica-supported hydrides with that of the alkyl alkylidene complexes, the other common catalyst for alkane metathesis.
RESUMO
O(2) adsorption in proton, sodium and copper exchanged chabazite has been studied using periodic and cluster approaches by means of density functional theory. The Grimme's correction has been used to include the dispersion contribution to B3LYP. Two cation locations have been considered: one with the cation at the six-membered ring (MCHA(I)) and the other with the cation at the 8-membered ring (MCHA(IV)). The O(2)-HCHA and O(2)-NaCHA adsorption complexes present a eta(1)-O(2) bent coordination. The adsorption energies, which are due to dispersion, are between -15 and -19 kJ mol(-1), in agreement with the experimental values. On the other hand, the O(2) coordination to CuCHA is through a eta(2)-side-on mode with a square planar coordination around the metal center. This structure favors the Cu d -->pi* O(2) charge transfer which becomes the predominant stabilizing factor. The adsorption of singlet states of O(2) in HCHA and NaCHA, modeled with an ONIOM M12T:48T, is of the same nature as that of the ground state, and only the highest in energy (1)Sigma is significantly more stabilized in MCHA than the triplet state by 14 to 24 kJ mol(-1). The adsorption of singlet O(2) in Cu exchanged zeolites presents a higher electron transfer from Cu(+) to O(2) than that calculated for the triplet species and thus both singlet states are stabilized with respect to the ground state O(2). Generally, singlet oxygen appears more attractive to active zeolite models than those calculated with triplet oxygen, indicating a source of oxidative efficiency for designed structures.