Tipo De Estudio

The mechanism by which Ti containing molecular sieves reacting with hydroperoxide promote olefin oxidations such as the epoxidation o f alkenes has been explained by the formation o f titanium peroxo complexes. These Ti-peroxo species are stable intermediates with the ability to donate an oxygen ion to the alkene (or other small molecule organic substrate) thus facilitating epoxide formation

The formation o f ‘t|^’ or side-bonded Ti-peroxo complexes in hydrogen

peroxide doped TS-1 unquestionably occurs and has been observed through UV-Vis

and ESR spectroscopy However, the exact nature, preferred coordination and

role, as oxygen-donors, are unclear. By applying gradient corrected Density

Functional Theory the formation o f a number o f structurally distinct Ti-peroxo

complexes have been examined, starting with the and r | ’ Ti-peroxo species shown

in figure 4.12; both o f which are extensively discussed in the titanosilicate literature (see section 3.9). R. H H--- Q \ O w n ...

I 9-0-R"

1

\

y | \

r

p

. -

T:?;V

Ti-T|^(peroxo) Ti-t)^ (peroxo)

Figure 4.12: Literature proposed geometries o f Ti-peroxo species in hydroperoxide doped Ti molecular sieves.

4.4.1. R groups

The reader will note the presence o f R groups in figure 4.12 (not R ’, which represent the peroxide substituent). To date, derivatised porous titanosilicates (i.e. tetrahedral Ti-OR sites) can only be directly synthesised through grafting o f Ti-OR species onto

M CM -41. In hydrothermally substituted Ti-silicas the R groups are removed during

microporous crystals, the Ti-OH or T i-Si-0 ligands will exchange for solvent species, i.e. if the solvent is methanol, the Ti-OH ligand would be replaced by Ti- OMe, forming o f course Ti-OR species. Considering the unquestionable influence that the solvent has over reaction kinetics this proposal o f direct solvent involvement with the catalytically active site is very plausible. Maschmeyer et al. showed that the nature o f the R group in Ti-silisesquioxane/H2 0 2 mixtures has a direct effect on reaction kinetics for the epoxidation o f alkenes (Ti-silsesquioxanes are molecular analogues o f Ti containing molecular sieves). Thus, examining the effects o f the R groups on titanosilicate energetics may provide valuable mechanistic insight and is therefore an integral part o f this study.

The R groups used in this work have been chosen so as to examine any electronic or steric effects o f these functional groups on the reaction pathways studied. Similar sized organic moieties, CH3, CH2F and CF3 have been selected in order to examine electronic effects, with CH3 in this case considered as neutral,

CH2F as electron withdrawing and CF3 as highly electron withdrawing (electron

withdrawing ligands are thought to retard the rates o f epoxidation reactions). Furthermore, iso-butyl and tert-butyl groups have been selected to study the effect o f sterically bulky ligands on reaction kinetics. Tert-butyl hydroperoxide (TBHP) is the most effective oxidant in Ti-mesoporous but is not reactive at all in Ti-microporous catalysts, such as TS-1. This is thought to be due to Ti active site inhibition by the presence o f bulky peroxide in the sterically confined microporous silica. Finally, SiH3^ GeH3 and SnH3 ligands have been studied to see how reaction energetics may alter on descending group 14. Doping titanosilicates with germanium has been shown to increase reactivity, but, conversely, doping with tin is known to severely retard catalytic activity. The size and electron withdrawing nature o f the R groups chosen are shown in table 4.4. The approximate size o f the R group is measured by the Connolly surface area and the strength o f the Ti-OR bond is indicated by the Ti- O bond length.

R group Ti-GR Bond Length / Â Connolly Surface Area / H 1.827 23.08 C H3 1.803 41.9 CH2F 1.83 49.71 C F3 1 . 8 8 61.56 iso-butyl 1.808 100.64 tert-butyl 1.793 99.05 SiH3 1.82 59.26 GeH3 1.802 74.27 SnH3 1.788 55.1

Table 4.4: Size and electron withdrawing/electron donating properties o f a number o f R groups.

4.4.2 Ti-peroxo complexes

Figure 4.13 shows the BP8 6/DZVP calculated reaction profile for the formation o f

the Ti-peroxo intermediate from the reaction between H2O2 and a ‘Ti-OR’

tetrahedral cluster.

4.4.2.1 The r|^ transition State

The most difficult part o f calculating any reaction pathway is the determination o f the transition state. The transition state associated with the Ti-rj^(GGH) product

(with R = H incidentally) has been proposed previously However, verification o f

whether the transition state proposed was actually that associated with the Ti- ri^(GGH) product and the suggested reactant was outside the scope o f that particular study. In this section this uncertainty is addressed directly. At the saddle point, the intrinsic reaction coordinate (IRC) is coincident with the dominant negative eigenvalue o f the Hessian matrix. Gn either side o f the saddle point is the steepest-

R _A