H2O2-based selective epoxidations: Nb-silicates versus Ti-silicates

Abstract

This paper aims at a comparison of the stability and catalytic performance of niobium- and titanium-containing mesoporous silicates prepared by evaporation-induced self-assembly (EISA) in selective oxidations with hydrogen peroxide. The catalysts comprised evenly dispersed dimeric and/or small oligomeric active sites (MNb(V) or Ti(IV)), and the Nb sites revealed better hydrolytic stability than the Ti ones, as demonstrated by DRS UV–vis. An attempt has been made to rationalize the differences observed in chemo- and regioselectivity of alkene epoxidation using kinetic modeling tools. The rate law established for cyclooctene epoxidation over the Nb- and Ti-catalysts is consistent with an Eley–Rideal mechanism that involves adsorption of H2O2 on M sites, interaction between M and H2O2 to afford a hydroperoxo species ‘MOOH’ and water, followed by oxygen transfer from ‘MOOH’ to CC bond, producing epoxide and regenerating M. The higher epoxidation (heterolytic pathway) selectivity of the Nb catalysts is, at least partially, related to their lower activity in H2O2 unproductive decomposition. The apparent activation energy of H2O2 degradation is higher for Nb-silicate (20.5 vs 14.8 kcal/mol for Ti-silicate), while the activation energies of epoxidation follow an opposite trend (11.9 vs 14.2 kcal/mol). The heterolytic pathway selectivity of Nb catalysts can be greatly improved by decreasing the reaction temperature. The difference in regioselectivity of limonene epoxidation over Nb- and Ti-silicates observed in MeCN, where the nature of the catalysis is truly heterogeneous, indicates different structures of the active ‘NbOOH’ and ‘TiOOH’ species and/or different mechanisms of the oxygen transfer step. The inversion of regioselectivity observed for Nb catalyst in MeOH may be caused by Nb leaching in this solvent and/or changing the structure of ‘NbOOH’.