Co2+ to Al3+ Substitution Induced Dual-Site Synergistic Catalysis to Friedel-Crafts Alkylation Reactions.

ABSTRACT

The dual-site synergistic effect in heterogeneous catalysis is quite interesting, and also complex because at least two substrate molecules are adsorbed or activated on the catalyst surface, which apparently needs two spatially separated and functionally independent active sites. It would become more difficult when the substrate molecules are large ones. The replacement of Al3+ in Al4 B6 O15 lattice with Co2+ leads to the formation of unsaturated Co2+ (4-fold coordination) along with oxygen vacancies (Ov ). The former one behaves as a medium-strength Lewis acid site, and can adsorb and activate molecules with a nitro group (e. g., ß-nitrostyrene). The latter one can adsorb and activate oxygen species, which further activates the indole derivatives. Next, the spatially separated dual sites on the catalyst surface can synergistically and efficiently catalyze their Friedel-Crafts alkylation reactions under mild conditions. The high durability can be proved by the as-maintained high yields, that is, 98, 93, 96, 92 and 90 % for 5 runs, respectively. The reaction kinetics obey the second-order characteristic. Annealing under hydrogen condition can further generate more surficial Ov , leading to an improvement to the catalytic activity. A simple and probably routine aliovalent doping endows such a complex synergistic catalysis involving two large substrate molecules, providing an inspired perspective of developing dual-site catalysts.