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

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 Al4B6O15 lattice with Co2+ leads to the formation of unsaturated Co2+ (4-fold coordination) along with oxygen vacancies (O-v). The former one behaves as a medium-strength Lewis acid site, and can adsorb and activate molecules with a nitro group (e. g., beta-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 O-v, 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.

Automated summary

The dual-site synergistic effect in heterogeneous catalysis is intriguing but complex. It involves two spatially separated and functionally independent active sites for the adsorption and activation of at least two substrate molecules. This complexity becomes more challenging when the substrate molecules are large. In this study, the replacement of Al3+ with Co2+ in Al4B6O15 lattice creates unsaturated Co2+ (4-fold coordination) and oxygen vacancies (O-v). These dual sites, acting as Lewis acid and oxygen adsorption sites, synergistically catalyze the Friedel-Crafts alkylation reactions of nitro-substituted molecules and indole derivatives. The catalyst exhibits high durability and the addition of hydrogen further enhances its catalytic activity, providing a novel perspective on the development of dual-site catalysts.