CAPITULO V: FACULTAD DE INGENIERIA INDUSTRIAL UNIVERSIDAD UTP
5.1 Sus comienzos
5.1.3 Estructura de la Facultad
5.1.3.2 Estructura Organizacional
The high selectivities observed for a wide range o f olefins and the stereospecifity
o f the reaction are consistent only with a heterolytic mechanism. In early studies,
two types o f mechanism have been considered
1. Formation o f Ti-peroxides.
\
^ + ROOM - f \ . 0 / " O - O R\l
"
/ T i + R O H > = <Figure 3.14 Suggested epoxidation mechanism via a Ti-peroxide species 75
2. Formation o f Ti-hydroperoxides. Ill IV
#(
0R
I:
'O R + R OHThe formation o f a peroxo complex between H2O2 and the titanium atoms in
titanium molecular sieves is certain. UV-Vis studies o f Titanosilicate/H202
systems confirm the presence o f side-bonded (species II, figure 3.12) otherwise
known as peroxo species, with an absorption band at 26,000cm'' attributed to
ligand to metal charge transfer o f 02^'^Ti"'. This assignment is based on a band
at the same frequency for [TiF5(02)]’^ complexes. However, UV-vis cannot
determine the exact structure o f Ti-peroxo complexes. The strong repulsion
between formally unshared electrons in planar H2O2 can be reduced by transition-
metal ions such as titanium, as they accept electron density from the filled anti
bonding orbitals o f H2O2 interacting with the empty metal d orbitals o f
appropriate symmetry. It is for this reason that even hydroperoxo (-0 0 H )
complexes may prefer the side-on configuration that can provide added stability. Thus, three types o f T i-r|2(00) complex have been suggested as the oxygen-donating species in titanium molecular sieves. 1) Formation o f Ti-r|^(02)
complexes that are readily formed in aqueous TiR4/peroxide mixtures
(species (a), figure 3.16). 2) Binding o f hydroperoxo or organoperoxo ligands (with H2O2 or ROOH respectively) in an fashion to the titanium centre (species (b) figure 3.16) and 3) the anionic analogue o f the aforementioned complex (species (c), figure 3.16)
V
°\r
\ f
(a) (b) (c)
Figure 3.16 Suggested structures for the Ti-peroxo species in titanium molecular sieves with hydroperoxide.
Since discrete Ti-q^(02) complexes are known to readily form and be very stable in the liquid phase a number o f authors have assumed species (a) to be the oxygen-donating group in titanium molecular sieve catalysts. However, density functional theory cluster calculations indicate that in alkene epoxidation reactions.
Ti-T|^(0 2) complexes will indeed form, but only in the absence o f the alkene and
therefore cannot be the catalytic mediating species Furthermore, Ti-r|^(-02)
complexes do not explain the observed influence o f varied peroxide substituents
on the reaction kinetics o f porous titanosilicate catalysts In addition, the
acidic properties o f TS-I/H2O2 mixtures are thought to be due to the presence o f Ti-peroxo complexes and thus, Ti-T|^(-02) type complexes cannot directly account for this phenomenon.
A number o f quantum chemical studies have suggested that Ti-rj^(OOR)
species are stable and energetically accessible and that they provide an
explanation for the aforementioned peroxide and acid-base effects observed
for titanosilicate/peroxide mixtures. Density functional theory calculations also suggest that Ti-r|^(OOH) are the key catalytic species, donating an oxygen to a
weakly bound alkene molecule An activation barrier o f < 60 kJmof* has been
quoted for the formation o f Ti-r|^(OOH) species.
-27 w + 56
X'
'H H O -o'" -V 7 -H.0\^j
o _ '[ -53 Ti. transition stateFigure 3.17 Suggested mechanism for the formation o f Ti-ri^(OOH) complexes
and their role as oxygen donors to alkenes Energies are in kJmol'X
Transfer o f electron density from the partially filled 7t* 0 - 0 anti-bonding orbitals o f the peroxidic oxygen(s) to the Ti^^ centre is thought to stabilise r|^-OOH species.
Hartree Fock cluster calculations on a number o f Ti-peroxo models found the Ti-T|^(02") ion pair model to be the most stable The authors characterised the complex by a calculated 0 - 0 vibration which was in good agreement with an IR and Raman analysis o f a hydrogen peroxide doped TS-l catalyst.
However, all o f the Ti-T)^(peroxo) species have one major failing; they do not explain the well-documented dependence o f solvents on the catalytic
reactivity of titanium molecular sieve systems with peroxide 14,47,48,52,53,59,65,77 Thus, many workers champion a Ti-r|'(OOR) complex or a 5-membered ring intermediate 26,42,44,51,59,92-95 solvent or adsorbate is an integral part of the compound, offering stabilisation of the peroxide moiety through hydrogen bonding (species (a) and (b), figure 3.18). The five-membered ring complex was introduced in section 3.8.1. H - - 0 H - - 0
/
\
/
\
H — 0 O _H+ H — O O \ / ■ - \ / o V +H+ o V/TA-1<
1<
(a) (b)Figure 3.18 Suggested structures of the Ti-peroxo complexes in peroxide doped titanium molecular sieves.
Species (a), in the figure above is also consistent with the known tendency of titanium to expand its coordination shell above 4 and has several organic analogues However, quantum mechanical simulations of small model Ti(0H)4
clusters found the Ti-rj’(OOH) complex to be 33 kJm of' less stable than the Ti- Tj^(OOH) complex
Alkyl hydroperoxides (ROOH) are known to form both rj^(OOR) and ri'(OOR) complexes with transition metals depending on the binding strength of other ligands. The observation that different peroxides lead to variations on the regioselectivity of epoxidations has also been considered to indicate that the whole -OOR group is involved in the active oxygen donating species Furthermore, Sheldon et al. have demonstrated that the structure of the ROOH hydroperoxide can play an important role in determining the selectivities in Ti-Oi- Si02 mixed oxide catalysed epoxidations another indication that the peroxide R substituents must be part of the Ti-peroxo catalytic species. If this is the case, electron withdrawing substituents in the hydroperoxide are likely to increase the
rate o f epoxidation by enhancing the electrophilic nature o f the Ti-peroxo complex.
3.10 Summary
UV-vis and ESR experimental techniques have shown that Ti-peroxo complexes definitely do form but give very limited information on the structure o f such species. Although a large number o f theoretical studies have been undertaken, the nature o f the oxygen-donating species is still unclear. No one study has presented a thorough, systematic and quantitative interrogation o f the formation, structure and coordination of all Ti-peroxo complexes suggested in the literature, until now. This thesis reports the energetics and structure o f all o f the literature postulated Ti-peroxo complexes described in the preceding section in a detailed and systematic manner. The effect o f hydration and proton transfer mechanisms is also explored which lead to a new Ti-peroxo species, not previously reported in the literature. This work also shows for the first time, evidence that particular Ti- peroxo complexes, predicted though quantum mechanical calculations, do exist in titanium molecular sieves through excellent agreement with EXAFS spectroscopy. Identifying the nature o f Ti-peroxo complexes in titanium molecular sieves is crucial in elucidating the mechanism o f epoxidations.