Jackson S.D., Hargreaves J.S.J. Metal Oxide Catalysis

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740 18 Titanium Silicalite-1

from the other, no catalyst of direct synthesis is known that is sufﬁ ciently active

and selective in a basic medium. Process integration, however, is conceivable and

is present in the patent literature. In an early patent, a mixture of cyclohexanone

and hydrogen peroxide was produced by the autoxidation of cyclohexanol and fed

directly into the ammoximation reactor [164] . Hydrogen peroxide from the autoxi-

dation of anthrahydroquinones was extracted with aqueous methanol or t - butanol

and used as obtained [165, 166] .

18.11

Adsorption, Active Species and Oxidation Mechanisms

In previous sections, mechanistic proposals have been illustrated with a some-

times limited discussion of active species and of the relations existing between

them. A deeper insight is also needed into adsorption phenomena and their role

in the network of reactions catalyzed by TS - 1. In this section, these issues will be

examined more closely, with the aim of identifying the chemical and physical

factors determining the direction of the oxidation pathway from the perspective

of a general mechanistic scheme.

18.11.1

Adsorption and Catalytic Performances

It is clear from the ﬁ rst part of this chapter that the external medium has a major

effect on the catalytic properties of TS - 1 and other Ti - zeolites. Early rationalization

suggested that the adsorption of a protic molecule, ROH, had a stabilizing effect

for Ti

–

OOH species. In this view, water and alcohol solvents inﬂ uenced the cata-

lytic performance through the electronic and steric effects exerted in the oxidation

step by the ROH molecule present in the active species (Section 18.4.1.1 ) [62] .

Subsequently, Langhendries and others shed light on the key role the exter -

nal medium has in the adsorption of oleﬁ ns, alkanes and alcohols [167] . They

showed that partition coefﬁ cients, deﬁ ned as the ratio of intra - porous to extra -

porous concentrations, decreased in the order of decreasing solvent polarity,

OH > C

OH > 1 - C

OH, CH

CN > CH

COCH

, and for a given solvent

with decreasing hydrophobicity of the molecular sieve, TS - 1 > Ti,Al - β >

Ti - MCM - 41. Partition coefﬁ cients increased also with the chain length of the

adsorbate. The reactivity orders in the epoxidation of a series of oleﬁ ns were

fully consistent with the trends. On these grounds, the authors concluded that

the adsorption phenomena were sufﬁ cient to explain the effect of the solvent

on kinetics, without the need to invoke other roles.

Actually, the study provides an experimental demonstration of the primary role

the surface has in the catalytic performance of Ti - zeolites. TS - 1 is superior to

Ti,Al - β because its surface is more hydrophobic and, therefore, the intra - porous

concentration of the oleﬁ ns and other apolar reagents is greater, rather than

because the activity of Ti - sites is substantially different. In this regard, the micro-

porous nature of the catalyst has a major role in that it enhances both attractive

and repulsive interactions between surface and adsorbate and, through them, the

selective adsorption of some compounds over others. It is useful, in this regard,

to recall the concept of zeolites as solid solvents [168] . Nonetheless, the partition

coefﬁ cients alone are unable to account for the catalytic behavior of Ti - zeolites in

all circumstances. So, the role of a protic adsorbate on Ti

–

OOH species is again

of assistance [25] . Three pieces of evidence in the literature are interesting in this

regard:

• If a correlation was apparent between adsorption and epoxidation rates within a

homogeneous series of solvents, this no longer held when the solvents were

considered all together and no distinction was made between protic and aprotic

ones [25, 167] .

• In a study by Corma and coworkers, the rate of epoxidation of 1 - hexene on

Ti,Al - β matched, for a homogeneous series of solvents, the trend of adsorption.

However, it was twice as fast in acetonitrile than in methanol, in contrast to

partition coefﬁ cients which are ordered in the reverse direction [77, 167] . The

relationship for cyclohexanol oxidation was more complex, the rate increasing

with the polarity of aprotic solvents and decreasing with polarity increase in

protic ones [77] .

• The hydroxylation of n - hexane on TS - 1, in contrast to the epoxidation of propene,

reached its maximum rate in the least polar solvent, t - butanol (Table 18.13 ).

Acetonitrile behaved quite similarly to methanol and water [24, 25, 169] . On the

assumption that t - butanol was comparable to i - propanol for the effects on

adsorption, a clear relationship between rates and partition coefﬁ cients was

lacking. Considering that hydroxylation and epoxidation involve different active

species and mechanisms, a diverse role of the solvent in the two active species

could contribute to the differences, whereas the partition coefﬁ cients alone

could not.

Table 18.13 Solvent effects in the oxidation of n - hexane on TS - 1.

Solvent K

Vms

HO0

⋅⋅

()

−−

water – 1.1 (two liquid phases)

methanol 16.5 1.2

acetonitrile 2.5 0.9

i - propanol 0.8 –

t - butanol/H

O (27%) – 2.4

t - butanol – 4.3

Thomas Vieth Publisher.

b Partition coefﬁ cients of n - hexane.

c Rates at 15 – 30% conversion.

18.11 Adsorption, Active Species and Oxidation Mechanisms 741

742 18 Titanium Silicalite-1

In principle, the solvent can affect catalysis in a multiplicity of modes, for

example through the adsorption of the reagents, the stabilization of Ti

–

OOH

species or interaction with oxidation intermediates [25] . It is worth noting that the

ﬁ rst two phenomena produce effects in the same direction, and their individual

contributions are not easily distinguished. For instance, the rate of epoxidation in

t - butanol may be slower than in methanol owing to smaller partition coefﬁ cients

of the oleﬁ n, to more sterically hindered active species and to less electrophilic

peroxidic oxygen.

18.11.2

The Structure of Ti

–

OOH Species

The structural and chemical properties of active species in TS - 1 are strictly related

to the tetrahedral conﬁ guration that Ti is forced to assume in the silicalite frame-

work, to its preference for the octahedral one, which can be achieved by the adsorp-

tion of polar molecules, and to the reversible cleavage of one Ti

–

OSi bond by a

protic molecule [8, 170, 171] . On this basis, Scheme 18.17 illustrates the formation

of Ti

–

OH and Ti

–

OOH species, by the chemisorption of water and hydrogen

peroxide, respectively. The use of an alcoholic solution leads to analogous

results. Other Ti - species generally predicted by theoretical studies, for example,

bearing an undissociated H

molecule weakly adsorbed on Ti or consisting of

–

Si peroxides, are not very compatible with the chemistry of TS - 1 and are

not considered further in this context.

Scheme 18.17 Adsorption and reactions of Ti - sites.

The structures proposed for Ti

–

OOH have been illustrated in Section 18.4.1.1

and show η

(monodentate) or η

(bidentate) coordination (Figure 18.1 a – c). Such

a dualism in Ti

–

OOH binding parallels that in the role of a protic adsorbate. A

spectator function for the latter and even its absence in the active species have

generally been envisaged by theoretical calculations (Figure 18.1 b) [20] . The oppo-

site view, usually visualized as structure a in Figure 18.1 , was originally based on

the interpretation of early experimental results [24, 61, 62, 171] . Currently, the

likelihood of structures a and c in Figure 18.1 , entailing an active role for the protic

adsorbate, can rely upon additional evidence in the literature.

• Freely diffusing oleﬁ ns reveal large steric constraints in the approach to peroxidic

oxygen, in the epoxidation on TS - 1 and Ti - Beta zeolites (Tables 18.8 and 18.9 )

[62, 81, 83] . For the active species b in Figure 18.1 , the repulsive effect exerted

by the surface on the alkyl substituents is the only possible explanation. For

species a and c , the R group would produce additional, and probably greater,

steric constraints. Whether the surface alone is sufﬁ cient to account for the

magnitude of the effect is a matter of debate. However, the epoxidation of

2 - butene compared to that of terminal oleﬁ ns shows that the steric effects are

large even for cis - isomers, in which both methyl groups can point away from

the surface. Clearly, steric effects should be larger for trans - 2 - butene, as

experimentally found, for the opposite reason.

• The addition of modest quantities of water to acetonitrile solvent increases the

rate of oxidation of 1 - hexene but decreases that of cyclohexanol [77] . The

contrasting effects were attributed to the adsorption of water on Ti

–

OOH to

form the cyclic structure of a (Figure 18.1 ). In the epoxidation mechanism

H), this was reputed to be the active species and, therefore, the addition of

water promoted the rate through its formation. In the oxidation of the alcohol, a

species with the same structure (R

cyclohexyl) was the oxidation intermediate

and water, in competition with cyclohexanol for adsorption on Ti (R

H), could

only decrease the rate. It should be added that the study was performed with

Ti,Al - β as the catalyst, but, as mentioned in Section 18.4.1.2 , the same active

species is believed to apply generally [77 – 79] .

• The diffusivity and adsorption capacity of aromatic molecules, in TS - 1 and in

Ti - MOR, were both reduced by the simultaneous presence of H

and H

O, in

a linear relationship with the Ti content [172] . No such effects were produced by

the use of H

O alone under analogous conditions or by that of H

mixtures with Ti - free zeolites. The underlying rationale, in the authors ’ view, is

the formation of bulky peroxy species, with the structure a of Figure 18.1 ,

protruding into the pores. Their presence hinders the movement of adsorbing

reagents and reduces the free volume available for them in the pores. No such

bulky species can be formed when water alone adsorbs on Ti - sites.

The evidence above and a number of other studies, of various strength, based

upon IR and UV - Vis spectroscopy, together with XANES and EXAFS studies and

the kinetic behavior of the oxidation of alcohols and allyl chloride (Equations 18.8

and 18.11 ), demonstrate the co - adsorption of H

and, at least one further mole-

cule of water or alcohol on Ti. In some cases, they also conﬁ rm the formation of

cyclic structures like a and c in Figure 18.1 , meaning that structure b is only pos-

sible under vacuum conditions. Their formation under the reaction conditions

also favor, albeit not proving, their participation in the oxygen - transfer step to the

double bond. Actually, the steric constraints in the epoxidation of linear oleﬁ ns

and the promoter or inhibitor effect of water, according to the type of substrate,

point in this direction. The opposite view, based on theoretical models, is weak-

ened by the variety of signiﬁ cantly different active species they lead to.

18.11.3

Reactive Intermediates and Oxidation Mechanisms

The heterolytic mechanism and the Ti

–

OOH nature of the active species in the

epoxidation of oleﬁ ns have the support of several chemical, spectroscopic and

18.11 Adsorption, Active Species and Oxidation Mechanisms 743

744 18 Titanium Silicalite-1

theoretical studies. For the hydroxylation of alkanes, some indirect evidence favors

homolytic H - abstraction by a Ti - centered radical, the nature of which, however,

remains unknown. Scarce information exists on the oxidation mechanisms of

arenes and alcohols, for which mechanistic proposals have been advanced by

analogy with other oxidations. Even less is available for N - and S - compounds and

for the decomposition of hydrogen peroxide. On the whole, the understanding of

TS - 1 catalysis is still limited, with epoxidation the only exception. Nonetheless,

two deductions are possible from available information. Ti

–

OOH can directly

perform the oxidation of certain substrates, for example, oleﬁ ns and, maybe, O - ,

S - and N - compounds, or undergo a redox process, converting into a radical oxidant

species. As a result, competitive heterolytic and homolytic pathways can co - exist

in TS - 1. Precedents in this regard can be found in the literature [67] .

The epoxidation species received adequate attention in previous sections, with

Figure 18.1 and Schemes 18.7 , 18.8 and 18.17 illustrating the mode of generation,

the possible structures and the mechanisms of oxygen transfer proposed in the lit-

erature. Conversely, the species responsible for the cleavage of saturated C

–

bonds necessitates at this stage a deeper insight, particularly in the light of Scheme

18.3 b, based on the atypical Ti

–

•

radical. Actually, no precedent exists in the

chemistry of Ti for such a species. Nonetheless, other conceivable mechanisms

based on the participation of the species a – d of Figure 18.1 , of Ti

–

•

superoxide

and of a Ti

III

–

•

diradical are even less credible. Actually, it is hard to reconcile

–

H hydroxylation with Ti( η

–

) and Ti

–

OOH species, considering that the reac-

tion is homolytic whereas the oxidant is not. The implausibility of diradical

III

–

•

has been discussed (Section 18.2.1 ). The fact that no major role is

assigned to Ti

–

•

, is not only because this represents a negligible fraction of

overall Ti content [66] . Actually, the reactivity of metal superoxides with alkanes was

shown to be generally low or zero [1] . The generation of Ti

–

•

requires the inter-

vention of an OH

•

radical, whose origin remains unknown. No evidence of its reac-

tivity was detected in the oxidation of oleﬁ ns, in which TiO

•

is expected to add

across the double bond (e.g. the stereo scrambling of cis and trans oleﬁ ns by

addition – elimination).

It is useful to note, at this stage, that TS - 1 shows similarities in C

–

H hydroxyl-

ation with biomimetic catalysts and simple Fe

ions dissolved in acetonitrile, for

example the homolytic nature of the reaction, the electrophilic character of the

active species, a signiﬁ cant primary H/D isotope effect, the competition between

hydroxylation and H

decomposition and, often, the stereoscrambling ability.

Similarities exist also with enzymatic systems, notably with cytochrome P - 450 and

the methane monooxygenase ( MMO ) type of enzymes [31, 173] . On these grounds,

it is tempting to extend further the analogy, proposing Ti

–

•

as the active species

related to the Fe

–

•

of biomimetic and enzymatic systems and Fe

III

–

•

of Fe

/CH

CN (Fe

–

•

and Fe

III

–

•

are radical notations of Fe

–

O and Fe

–

respectively). Indirect support for Ti

–

•

comes from a study by Bonchio

and others on the hydroxylation of benzene by the peroxide VO(O

)(H

(L = picolinic acid), in which evidence exists for the participation of a V

–

•

species, instead of the diradical V

–

•

previously suggested by others [174] .

However, while the generation of high - valent iron oxo species from d

III

–

OOH

and d

–

OOH hydroperoxides is straightforward, an external reducing agent

would be required for that of Ti

–

•

from d

–

OOH (Scheme 18.18 ). A redox

process had been proposed also for V

–

•

formation from precursor

VO(O

)(H

The regeneration of the active species from Ti

III

represents a second problem to

solve. Actually, the one - electron oxidation of low - valent metal ions with H

nor-

mally produces free hydroxyl radicals, which, however, are not detected with TS - 1.

A possible solution is suggested by the redox behavior of Fe

perchlorate in ace-

tonitrile solvent [33, 34] . At odds with aqueous media promoting Fenton - like reac-

tions, acetonitrile favored the formation of an iron - bound oxidant, able to

homolytically hydroxylate parafﬁ nic C

–

H bonds. A parallel is apparent between

the aprotic medium of Fe

and the intra - porous volume of TS - 1, in which surface

–

OH and protic adsorbates are negligible under the hydroxylation conditions.

Thus, hydrogen peroxide could regenerate Ti

–

•

from Ti

III

, as it does with Fe

from Fe

, without being decomposed into free OH

•

radicals (Scheme 18.19 ).

Whether this process passes through Ti

III

–

OOH, is of little interest at this stage.

Scheme 18.19 only means that H

regenerates the active species, by the oxida-

tion of transient Ti

III

intermediate, without decomposition into free hydroxyl radi-

cals. The organophilic nature of the intra - porous volume of TS - 1 promotes the

process. It is remarkable, in this regard, that biomimetic catalysts are generally

used in aprotic solvents and that the active species of MMO is thought to be located

in a hydrophobic pocket of the enzyme.

Scheme 18.18 Generation of Ti

–

•

active species and C

–

H hydroxylation.

Scheme 18.19 Regeneration of Ti

–

•

species from Ti

III

intermediate by hydrogen peroxide.

18.11 Adsorption, Active Species and Oxidation Mechanisms 745

–

•

is also an excellent candidate as an initiator of H

decomposition

(Scheme 18.20 ). The subsequent decomposition of HO

•

species into molecular

oxygen could follow, at least in part, known pathways [1] . The details of oxygen

evolution, however, is an issue that goes beyond the scope of this chapter.

746 18 Titanium Silicalite-1

The acidic character of Ti

–

OOH species is the ﬁ nal property to be addressed.

An acid – base equilibrium is established under reaction conditions, with mildly

basic compounds, including water and alcohols, adsorbed in the pores (Scheme

18.21 ) [24, 171, 175] . The addition of basic compounds in small amounts improves

the selectivity of epoxidation through its effect on the equilibrium. On increasing

the local basicity, this shifts completely to the right, with the formation of anionic

- peroxides [176] . Since Ti( η

–

)

−

species are characterized by low electrophilic-

ity, their formation has a profound effect on the reactivity of TS - 1, with inhibition

and eventually deactivation in most reactions (ammoximation is one exception).

Protonic acids regenerate initial Ti

–

OOH species. Owing to the equilibrium

nature of the reaction, any attempt to prepare anionic TS - 1 peroxides in a pure

state failed. Hydrogen peroxide and the base gradually leached into water wash-

ings, to be eventually completely removed with them [176] .

Scheme 18.20 Decomposition of hydrogen peroxide initiated by Ti

–

•

species.

Scheme 18.21 Acid – base properties of Ti

–

OOH hydroperoxide.

18.11.4

Proposal for a General Mechanistic Scheme

In conclusion, different active species and different mechanisms are operative in

TS - 1, in a network of competing reactions. Scheme 18.22 is a tentative rationaliza-

tion, in a simpliﬁ ed form, of information presented in previous sections on most

studied reactions. Different factors determine the direction of the oxidation

pathway, for example the temperature, the solvent, the nature of the surface and

the substrate itself [169] .

Scheme 18.22 pivots upon species a of Figure 18.1 (although the participation

of c is equally conceivable). Each oxidation path is in competition with the others,

becoming predominant under speciﬁ c conditions. For instance, unhindered

oleﬁ ns in a methanol solution of hydrogen peroxide undergo fast epoxidation even

at below room temperature (mechanism A ). The oxidation of allylic C

–

H (mecha-

nism B ), the oxidation of CH

OH (mechanism C ) and the decomposition of H

(reactions D ), although equally possible, are too slow to be detectable to a signiﬁ -

cant extent. At the same time, the acid – base equilibrium can be controlled by the

addition of parts per million quantities of basic compounds, to minimize the sol-

volysis of the product. On raising the temperature and contact time to assist the

epoxidation of sterically hindered oleﬁ ns, the reaction of Ti

–

OOH with H

, and

therefore the homolytic pathways B and D , may compete signiﬁ cantly, to the point

that with the least reactive oleﬁ ns these can prevail over epoxidation A. The litera-

ture offers several examples in which epoxidation, allylic oxidation and H

decomposition are all present, in different proportions, even in the oxidations with

TS - 1 and particularly in those with Ti - Beta catalysts.

The hydroxylation of alkanes is considerably slower than epoxidation and neces-

sitates signiﬁ cantly higher temperatures to occur at a comparable rate. As a matter

of fact, H

decomposition D is never negligible in the hydroxylation of alkanes,

its competition being signiﬁ cant even in the fastest one observed on TS - 1, the

hydroxylation of n - hexane (Table 18.2 ). It is also a side reaction in the generation

of Ti

–

•

species, but for the most part it should originate from the direct reaction

of H

(ﬁ rst reaction of Schemes 18.18 and 18.20 ). With the least activated hydro-

carbons, for example with bulky alkanes, the decomposition of H

becomes the

main reaction. Analogous considerations hold also for the competition of the oxi-

dation of methanol solvent, a side reaction that occurs to a signiﬁ cant extent with

the least reactive substrates, together with decomposition of the oxidant. Table

18.11 shows, in this regard, why other alcohols are inappropriate for use as

solvents.

The relative weight of different pathways in Scheme 18.22 is determined also

by the combination of surface/solvent, through the control of the substrate - to -

oxidant ratio in the pores. At a ﬁ xed composition of external medium, the H

to - hydrocarbon and the H

O - to - hydrocarbon ratios increase in the order

TS - 1 < Ti - β < Ti,Al - β << Ti - MCM - 41, owing to increasingly hydrophilic pores.

Thus, the competition of H

with the hydrocarbon for reaction with Ti

–

OOH

increases in the same order and with it the generation of Ti

–

•

radicals in the

pores. Allylic oxidation (path B ) and H

decomposition (path D ) increase accord-

ingly, at the expense of epoxidation (path A ). As a matter of fact, the epoxidation

of oleﬁ ns on large - pore catalysts is generally slower and less selective than on TS - 1,

becoming negligible on mesoporous Ti - silicates, owing to a lower adsorption of

the substrate and a greater incidence of such side reactions.

Scheme 18.22 Reactive intermediates and oxidation pathways in TS - 1/H

18.11 Adsorption, Active Species and Oxidation Mechanisms 747

748 18 Titanium Silicalite-1

Analogous considerations on the effect of the surface/solvent couple also apply

to the oxidation of alkanes and aromatics. The lower reactivity of C

–

H bonds

combines with decreasing alkane/H

ratios in the order TS - 1 > Ti - β > Ti,Al -

β >> Ti - MCM - 41, favoring the competition of hydrogen peroxide for Ti

–

•

and,

consequently, its decomposition over alkane hydroxylation in a similar trend. Actu-

ally, only a few alkanes are oxidized with relatively good yields on TS - 1, while their

hydroxylation on large - pore zeolites is normally an exception. With hydrophilic

Ti - MCM - 41, alcohols are likely produced by a free radical chain oxidation of the

alkane occurring in the external medium. The hydroxylation of aromatics com-

petes in a similar way with solvent oxidation and hydrogen peroxide decomposi-

tion. It is interesting to notice, in this regard, that oxygen evolution is signiﬁ cantly

smaller in the hydroxylation of phenol than in that of alkanes, a possible indication

of the quenching effect of phenolic species on the generation and the reactions of

•

radicals.

18.12

Conclusions

Progress in the area of Ti - zeolites, since early papers published at the end of the

1980s, has been impressive. Synthesis, catalysis, characterization and modeling

have been the subject of numerous studies. Most synthetic efforts concentrate on

methods to obtain TS - 1 by the use of cheaper template and silica sources. The

lower grade TS - 1 thus obtained is generally intended for the epoxidation of

propene, in which the catalyst inventory is relatively high and a lower cost for

it could compensate for moderate losses of selectivity. The search for large - pore

Ti - zeolites is also the subject of many studies. Though their number in the patent

literature is now of the order of several tens, only a few have been studied effec-

tively and, moreover, these show little applicative potential. Currently, no new

Ti - zeolite, sufﬁ ciently effective to at least supplement TS - 1 in the oxidation of

bulky substrates, is on the horizon. Combining the spaciousness of pores with a

highly hydrophobic surface appears a task of maximum difﬁ culty.

Characterization has conﬁ rmed, with plenty of data, the isomorphous substitu-

tion and tetrahedral structure of Ti, the reversible splitting of one Ti

–

OSi bond by

water and methanol and the formation of Ti

–

OOH active species. Mechanistic

proposals have been advanced on these and on chemical grounds, albeit limited

for the most part to the epoxidation reaction. Modeling has in several cases pro-

vided valuable support in this regard. For further mechanistic insights, in situ

studies might be helped by the recent availability of gaseous H

in a pure state

[175] .

Catalytic studies have extended the applications of TS - 1 from early aromatic

hydroxylation to the oxidation of oleﬁ ns, alkanes, alcohols, amines and thioethers,

and to other minor reactions. A combination of three factors is the basis of its

effectiveness: the high activity of Ti peroxy species, the organophilic properties of

the surface and the size of the pores in the range of molecular dimensions. For

References 749

the role of propene oxide in the chemical industry, most of the research interest

has focused on epoxidation, leading eventually to the development of a new

process. On the whole, three oxidation processes are now commercial – HPPO,

ammoximation and phenol hydroxylation – with prospects for other applications,

particularly in the event that cheaper hydrogen peroxide becomes available in the

future.

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