Kinetic Compensation Effect In The Epoxidation Reaction Over Titanosilicate/H₂O₂ System

Titanosilicates are a kind of important heterogeneous catalyst.In the oxidation of olefins,titanosilicates exhibit high activity and selectivity,and the reaction conditions are mild.Therefore,the structural characteristics and catalytic properties of titanosilicates have become the focus of attention.However,it is found that the solvent also plays an important role in the epoxidation of titanosilicalite/H₂O₂ system.In general,three hypotheses containing the influence to the adsorption behavior,intraporous diffusivities and intrinsic reaction kinetics at the active site of various catalysts,have been carried out to describe the role of solvents in various liquid-phase oxidation reactions.However,up to now,the nature and laws of solvent effect remains unclear.This dissertation is aimed to unveil the nature of the solvent effect in titanosilicalite/H₂O₂ system by the research on the thermodynamic properties and reaction mechanism of the reaction.The solvent effects was investigated via the typical probe reaction in various solvent（methanol,acetonitrile,acetone and tert.-butyl alcohol）and various titanosilicates（TS-1、TS-1+、TS-1s、Ti-MWW、F-Ti-MWW）.The results of this paper show that,the solvent effect of the system obeys the compensation effect of the kinetics,and the apparent activation energy and the reaction rate constant show a certain trend（lnA=0.3647Ea-2.4284）,and there exists the same kinetic temperature Tiso（56.7℃）.The faster activation of H₂O₂ has higher activation energy with the actual temperature above the isokinetic temperature,while the faster activation of H₂O₂ has the lower activation energy with the actual temperature below the isokinetic temperature.In a series of reactions,there are the same reactive intermediate,and solvent also involved in the formation of the intermediate.This study demonstrates that the KCE in titanosilicate/H₂O₂ system provides a possible method to select solvent for achieving high catalytic efficiency under different tempertures.This conclusion makes us have a deeper understanding of the solvent effect of the titanosilicalite/H₂O₂ system.