DFT Study of Solvent Coordination Effects on Titanium-Based Epoxidation Catalysts. Part One:  Formation of the Titanium Hydroperoxo Intermediate

Abstract

Density functional theory has been used to study the effects of solvent coordination on the structure and formation of the titanium hydroperoxo species believed to be the active intermediate in oxidation reactions with Ti(IV)−H2O2 catalytic systems. Titanium hydroperoxo intermediates possessing a variety of coordination environments have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. Titanium hydroperoxo intermediates bearing one or two solvent ligands may assume several degenerate structures with the hydroperoxo moiety coordinated to titanium in a monodentate or bidentate manner. Hydrogen bonding of the hydroperoxo moiety with a protic solvent ligand via a monodentate five-membered ring structure does not confer any significant additional stabilization to the titanium hydroperoxo intermediate. The presence of a single solvent ligand on the titanium center lowers the Gibbs free energy of activation for the formation of titanium hydroperoxo intermediates by 4−8.5 kcal/mol. Hydrogen-bonded protic molecules can facilitate proton transfer from a hydrogen peroxide ligand on titanium and thereby lower the Gibbs activation barrier for intermediate formation by 5−6 kcal/mol. NBO analysis provides an enriched understanding of how solvent ligands alter structural and electronic properties and reduce activation barriers. Consideration of thermodynamic and kinetic results indicates that the most abundant titanium hydroperoxo intermediates formed under liquid-phase reaction conditions will most likely possess a single solvent ligand on titanium and possibly a hydrogen-bonded protic solvent molecule.

Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

“…All attempts to obtain a TiOO-H side-on coordination structure were, however, unsuccessful because the optimization always evolved towards geometries with a g 1 -coordination. This result contrasts with the recent B3LYP study of Sever and Root on the Ti(OH) 3 OOH model clusters who found structures with a g 2 -coordination for the OOH group [92]. This dissimilarity may be due to the different coordination number of titanium in the Ti-POM and Ti(OH) 3 OOH as well as due to the different basicity of Ti-O-W and Ti-O-H bonds.…”

“…For successful designing new catalysts and processes, we needed a basic knowledge about the mechanism of the formation of active titanium species and the mechanisms of their interaction with organic substrates. Although considerable efforts, both theoretical and experimental, have been directed to understanding activity observed in TS-1 and related materials [9,[66][67][68][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94], much remained to be realised regarding the nature of the active oxidising species and chemical interactions at the molecular level. This stimulated our interest to model mechanistic studies on the titanium-single-site-catalysed oxidations.…”

Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid over a Recyclable Fe3O4–CoOx Magnetite Nanocatalyst

“…40 Other oxidation products including DFF and HMFCA were also detected, and believed to be the intermediates. 40 Other oxidation products including DFF and HMFCA were also detected, and believed to be the intermediates.…”

Selective aerobic oxidation of the biomass-derived precursor 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid under mild conditions over a magnetic palladium nanocatalyst

“…36 The aerobic oxidation of HMF over γ-Fe 2 O 3 @-HAP-Pd(0) catalyst was therefore initially carried out in a range of solvents in order to assess the effect of the solvent. Table 1 shows the results of aerobic oxidation of HMF in each solvent, the dielectric constant of the solvent being indicated.…”