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

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720 18 Titanium Silicalite-1

the rate of epoxidation of cyclohexene, found to be slower than that of 1 - hexene

by almost two orders of magnitude at near room temperature [62] . The second is

apparent in the epoxidation of freely diffusing oleﬁ ns, for example of the methyl -

substituted butenes mentioned above, in which the reactivity order is the expected

one, but the relative rates are greatly different from those of a purely electrophilic

oxidant (Table 18.8 ) [65] . Most striking is 2 - methyl - 2 - butene compared to

1 - pentene, with a rate somewhat slower on TS - 1/H

and 240 times faster on

peracetic acid. Transition - state shape selectivity in these and other epoxidations is

likely to result from steric repulsions exerted by the surface and by the species

chemisorbed on Ti on alkyl substituents of the double bond.

The evidence provided so far, that is the quantitative yields (including glycols),

the absence of allylic oxidation, the substituent effects and the role of shape selec-

tivity, are consistent with a heterolytic mechanism, involving an electrophilic

oxidant species subject to major steric constraints. The retention of conﬁ guration

in the epoxidation of sterically pure cis - and trans - butenes is further and convincing

support for this proposition. By analogy with many known transition metal per-

oxides, the active species may be identiﬁ ed among Ti( η

–

OOH), Ti( η

–

OOH) and

Ti( η

–

) peroxy species (Figure 18.1 a – d). The detection of Ti superoxide radicals

(Figure 18.1 e), on addition of hydrogen peroxide to TS - 1, and the homolytic nature

of C

–

H abstraction do not weaken the idea of a heterolytic mechanism. Based on

the author ’ s and others ’ experience [66] (and in contrast to some other reports [9] ),

the superoxide species involve only a negligible fraction of Ti - sites.

The competition of one - electron pathways is sometimes detectable in the epoxi-

dations catalyzed by transition metal catalysts [67] . However, in the epoxidation of

unhindered oleﬁ ns on TS - 1, the typical radical products are below the detection

limits. Their presence could no longer be neglected when the rate of epoxidation

is so slow as to become comparable to that of homolytic side reactions, for example

with bulky oleﬁ ns (see also Section 18.11 ). It is possible that, within these limits

only, the epoxide is produced in part through the addition of a radical peroxy

intermediate to the double bond [68, 69] . Even so, a homolytic pathway has again

been proposed as a generally valid epoxidation mechanism [7] .

In early proposals based upon the mechanisms of Mo and V peroxides,

Ti( η

–

) was thought to be the active species [70, 71] . The epoxidation of the

oleﬁ n occurred either by a coordinative mechanism, with the oleﬁ n adsorbing on

Ti prior to the insertion into one Ti - peroxide bond, or by the attack of one peroxy

oxygen directly on the double bond. However, the chemical inertness of known

Ti( η

–

) peroxides, the unlikelihood of oleﬁ n adsorption on oxophilic Ti in com-

petition with the protic medium and other evidence led subsequently to the loss

Figure 18.1 Active species proposed for epoxidations on TS - 1.

of credibility for this proposal. Currently, a general consensus involving a central

role of a Ti

–

OOH species seems to exist. There is less agreement on the details

of its structure, particularly on the presence and the function of a co - adsorbed

protic molecule (Figure 18.1 a and c) and on the mono - (Figure 18.1 a) or di - hapto

(Figure 18.1 b and c) coordination of the hydroperoxide group. In an early proposal,

the latter was monodentate and was stabilized in a ﬁ ve - membered ring by a hydro-

gen - bonded protic molecule, typically methanol or water (Figure 18.1 a) [24, 62] .

This proposal was based on adsorption properties of Ti sites, on the reactivity

behaviour of organic and inorganic peroxides and on the smooth formation of

cyclic structures in hydroperoxides and other compounds. In the alternative

species, based on quantum chemical studies, the hydroperoxide group was η

coordinated and did not require stabilization (Figure 18.1 b) [72] . In another pro-

posal, also based on DFT calculations, η

coordination and hydrogen bonding with

a protic molecule coexist in the same structure (Figure 18.1 c) [73, 74] .

Scheme 18.7 illustrates a proposed early mechanism based on the intermediate

of Figure 18.1 a [62, 75] . The ﬁ rst steps in the catalytic cycle are reversible and

envisage the adsorption of one molecule of H

, the formation of a Ti

–

OOH

species and its hydrogen bonding with one alcohol or water molecule. In the irre-

versible epoxidation step, the attack on the double bond by the peroxy oxygen

vicinal to Ti, leads to the formation of the epoxide, a Ti - alkoxide and a water mol-

ecule. Finally, the desorption of the epoxide and the reaction of Ti - alkoxide with

water, to form again the initial Ti - site, complete the catalytic cycle. As an alterna-

tive to it, H

could chemisorb directly on Ti - alkoxide to regenerate the Ti

–

OOH

species. The desorption of the product is an important step for the activity, since

the epoxide behaves as an inhibitor.

Scheme 18.8 illustrates the oxygen - transfer steps of species b and c in Figure

18.1 [72 – 74] . Both mechanisms are based on DFT studies and are similar to the

epoxidations with organic hydroperoxides on Group IV – VI metal catalysts [1] . The

Scheme 18.7 Epoxidation mechanism of species ( a ) from Figure 18.1 .

18.4 Oxidation of Oleﬁ nic Compounds 721

722 18 Titanium Silicalite-1

coordination of distal peroxy oxygen on Ti facilitates its removal as a titanol. A

weakness of Scheme 18.8 a is the absence of adsorbates other than hydrogen per-

oxide on Ti. The hypothesis could be correct for epoxidations with organic hydro-

peroxides carried out under anhydrous conditions, but not for the use of aqueous

hydrogen peroxide. It is fully consistent, however, with epoxidations conducted

under reduced pressure, in the conditions studied by Lin and Frei [76] . The same

arguments for Scheme 18.7 could apply also to Scheme 18.8 b.

18.4.1.2 Other Ti - Zeolites

The number of metal zeolites and their application to the epoxidation of oleﬁ ns

rose in parallel from the late 1980s. TS - 2, Ti,Al - β , Ti - β , Ti - MWW and, rarely,

Ti - MOR are catalysts that have been studied in some detail [7 – 9, 35, 77 – 84] . TS - 2

behaves, according to the few studies published, similarly to TS - 1. The greater

spaciousness of pores in Ti - Beta zeolites and of external cups in Ti - MWW allows

the epoxidation, under mild conditions, of oleﬁ ns unable to diffuse in TS - 1 and

TS - 2, such as methylcyclohexenes, cyclododecene, norbornene, camphene and

methyl oleate [80 – 83] . Steric constraints still prevail over electronic factors,

however, as in medium pore Ti - zeolites, even in the epoxidation of linear oleﬁ ns

(Table 18.9 ). It is generally believed that active sites and epoxidation mechanisms

are not signiﬁ cantly different from those of TS - 1.

There are major differences that distinguish Ti,Al - β , Ti - β and Ti - MWW. The

ﬁ rst concerns the activity and selectivity, which, under optimum conditions for

each catalyst, are considerably lower than for TS - 1. The activity in methanol

decreases in the order, TS - 1 > Ti - β > Ti,Al - β ≥ Ti - MWW, in a parallel trend with

the decrease of the hydrophobicity. Actually, the density of surface Si

–

OH species

increases in the reverse order: TS - 1 < Ti - β < Ti,Al - β ≤ Ti - MWW. The selectivity, in

turn, drops owing to a greater incidence of solvolysis and hydrogen peroxide

decomposition. Even in the absence of framework Al sites, as in Ti - β , the solvolysis

can signiﬁ cantly reduce epoxide yields. Ion exchange with basic compounds or

just their addition in the reaction medium is an effective tool to limit the losses

of product [84] .

A second difference compared to TS - 1 concerns the solvent and its effects on

kinetics and selectivity. The choice is again restricted to alcohols, ketones and

acetonitrile, but the latter is now preferable to methanol for higher rates and lower

solvolysis [77, 78] . As a general rule, methanol is the best solvent for oxidations

catalyzed by TS - 1, whereas acetonitrile is preferable for Ti - β , Ti,Al - β and Ti - MWW

Scheme 18.8 (a) Epoxidation mechanism of species ( b ) from

Figure 18.1 ; (b) Epoxidation mechanism of species ( c ) from

Figure 18.1 .

[81, 85, 86] . The enhanced activity is likely the result of the effects of different

factors, while the greater selectivity has to be related to the reduction of acidity by

the mildly basic medium. Interestingly enough, in various instances Ti - β , Ti,Al - β

and Ti - MWW turn out to be more active catalysts than TS - 1, when acetonitrile is

a common solvent. In one study, however, triﬂ uorethanol emerged as the best

solvent for Ti,Al - β [84] .

A third difference concerns Ti - MWW only. The siting of Ti in different porous

environments, that is in external pockets, in internal supercages and in sinusoidal

10 - MR channels, leads to active species associated with different diffusional and

steric constraints [79] . Thus, the epoxidation of bulky oleﬁ ns can occur exclusively

in external pockets, whereas the linear ones are not subject to site limitations.

Ti - MWW is also an unusual catalyst in the epoxidation of stereoisomers. At odds

with TS - 1 and Ti - Beta zeolites, trans - oleﬁ ns are epoxidized faster than their cis

analogues [85] . Though the mechanism is still unclear, a better ﬁ tting of the trans

conﬁ guration to the tortuous nature of 10 - MR channels could be an explanation.

Ti - Beta zeolites and, even more, mesoporous Ti - silicates can be somewhat

unstable to aqueous hydrogen peroxide and to strongly chelating agents. A partial

collapse of the lattice and the release of Ti, in the form of TiO

particles or soluble

Ti peroxides, was sometimes observed under these conditions (see also Section

18.4.2 ). The structural instability grows in parallel with the hydrophilicity of the

surface and the defectiveness of the silica matrix: Ti - β < Ti,Al - β << Ti - MCM - 41

[87 – 89] . For the same reason, the stability of the catalyst is indirectly related to the

method of synthesis, as far as this is able to produce materials with a different

content of connectivity defects.

Table 18.9 Epoxidation of oleﬁ ns over Ti - β , Ti,Al - β , TS - 1 and CH

Relative rates.

Oleﬁ n

Ti,Al - β

Ti - β

TS - 1

C H

1.0 1.0 1.0 1.0

0.70 0.77 0.63 ca 1

– 1.55 – 20 – 24

1.1 – 2.2 20 – 24

0.83 – – 20 – 24

1.6 1.2 – 27

1.6 – 0.63 ca 240

1.2 0.72 0

( ≥ 240)

a Data from Ref. [83] .

b Data from Ref. [81] .

c Data from Ref. [65] .

18.4 Oxidation of Oleﬁ nic Compounds 723

724 18 Titanium Silicalite-1

Since water is detrimental to catalytic performance, and its presence is unavoid-

able with hydrogen peroxide as the oxidant, an alternative consists of the use of

t - butyl hydroperoxide ( TBHP ), compatible with the pores of Ti - Beta zeolites and

with external pockets of Ti - MWW. In the case of the former catalysts, rates are

lower than with hydrogen peroxide while with Ti - MWW they are comparable [79,

83] . It should be considered, however, that in this use both large - pore Ti - zeolites

and mesoporous Ti - silicates are in competition with the cheaper and easier to

prepare Ti/SiO

In summary, the poorer activity, the stronger acidity and sometimes the struc-

tural instability are major drawbacks that minimize the chances of application of

large - pore Ti - zeolites. The reduced adsorption of alkanes, alkenes and other apolar

reagents in favor of that of water, and the defectiveness of the surface, are the

principal reasons for this (for a further treatment, the reader is referred to Section

18.11 ). In relation to these issues, synthetic methods for the production of materi-

als with fewer defective sites and better connectivity, have been developed for both

Ti - Beta zeolites and mesoporous Ti - silicates [7 – 9] .

18.4.2

Epoxidation of Unsaturated Alcohols

In principle, the oxidation of allyl alcohols may concern the double bond or the

alcohol group. In the absence of steric constraints, TS - 1, TS - 2, Ti,Al - β , Ti - β and

Ti - MWW catalyze the epoxidation with generally high chemoselectivity [81, 88,

90 – 97] . Carbonyl compounds may form by competitive oxidation, to an extent that

depends on steric constraints (Table 18.10 ). Normally, TS - 1 is the most effective

Table 18.10 Oxidation of unsaturated alcohols over TS - 1 and Ti - β .

Oleﬁ n TS - 1

Ti - β

Epoxide (%) Aldehyde (%) Epoxide (%) Aldehyde (%)

100 –

> 90

–

100 –

> 90

–

90 10 85 11

– – 8 9 1 1

7 6 2 4 – –

82 18 96 –

6 4 3 6 – –

a Data from Ref. [90] .

b Data from Ref. [81] .

catalyst for unhindered substrates. However, Ti - MWW showed the best activity

and selectivity for the synthesis of glycidol in acetonitrile and water: Ti - MWW >

TS - 1 > Ti - β [96] . Selective poisoning tests showed that the epoxidation occurred in

the external pockets of crystallites.

Stereo - , diastereo - and regioselectivity can be very high (Scheme 18.9 ). This

implies OH - assistance in the oxygen - transfer step, through the hydrogen bonding

of the alcohol group with the oxidant species, as for vanadium catalysts and per-

acids. Thus, with TS - 1, 3 - cyclohexenol and 3 - cyclopentenol yielded the cis epoxides

with 90% selectivity [91] . Chiral allyl alcohols produced, with TS - 1 and Ti,Al - β , a

mixture of threo and erythro epoxy alcohols, in a ratio that was apparently indepen-

dent of constraints produced by the pores [97] . The epoxidation of geraniol and

nerol on TS - 1 occurred on the vicinal double bond, while the remote and more

nucleophilic one was reported to be inert [91] . However, Schoﬁ eld and others

observed epoxidation at either double bond, on TS - 1 and Ti,Al - β , in the ratio of ca

2 : 1 in favor of the distant one [93] . Interestingly enough, the use of methanol

completely inhibited epoxidation at the vicinal position, favoring the electron - rich

distant one. This suggests that the competition of methanol for adsorption on Ti

sites suppresses the coordinative interaction of the allyl alcohol with the active

species.

The use of aqueous methanol, through the opening of oxirane rings, can have

a detrimental effect on the stability of the catalyst. By - product triols coordinate

strongly on Ti, facilitating its removal from the silica matrix, to the point that after

an induction period the epoxidation of allyl alcohols on Ti,Al - β and Ti - β became

an essentially homogeneous process [88] . In the epoxidation of crotyl alcohol, the

trend of Ti leaching was: Ti,Al - β > Ti - β >> TS - 1, the process being negligible on

the latter. The use of acetonitrile and the adoption of anhydrous conditions pro-

tected the catalyst from structural degradation, by the reduction of solvolysis.

Speciﬁ c tests revealed that the simultaneous presence of triol and hydrogen per-

oxide was necessary for leaching to occur, while taken alone they did not produce

any signiﬁ cant structural damage.

Scheme 18.9 Oxidation of allyl alcohols.

18.4 Oxidation of Oleﬁ nic Compounds 725

726 18 Titanium Silicalite-1

It is likely that the commercial value of glycidol is the unmentioned background

of most studies. However, process parameters were speciﬁ cally studied in only

one case, in which TS - 2 was the catalyst. The conversion of the oleﬁ n, under

optimum conditions, was ca 88% with 100% selectivity [94] . Methallyl alcohol was

the subject of a similar study [95] .

18.4.3

Epoxidation of Allyl Chloride and other Substituted Oleﬁ ns

Several patents and two papers deal with the epoxidation of allyl chloride [98, 99] .

Actually, a process based on TS - 1 would represent an environmentally cleaner

alternative to current production of epichlorohydrin. In this regard, one study has

addressed the cost of commercial hydrogen peroxide with the in situ production

of the oxidant, by the use of molecular oxygen and anthrahydroquinone com-

pounds [99] . In a mechanistic study, the kinetic data were interpreted on the basis

of Eley – Rideal isotherms, with the rate of reaction being ﬁ rst order on TS - 1 and

between 0 and 1 on H

and C

Cl (Equation 18.8 ) [98] .

(18.8)

Diallyl ether and diallyl carbonate produced monoglycidyl and diglycidyl ethers

in a ratio that depended on the conversion. Allyl methacrylate yielded the corre-

sponding glycidyl methacrylate [5] . Ti - MWW, in acetonitrile and acetone solvents,

was again reported to be more active and selective than TS - 1 [100] .

The epoxidation of α , β - unsaturated ketones produced corresponding epoxides

and minor amounts of glycols [101] . Methyl substitution on the double bond

reduced, or even suppressed, the oxidation. α , β - Unsaturated aldehydes yielded the

corresponding carboxylic acids and minor amounts of epoxides. α , β - Unsaturated

acids were inert under analogous reaction conditions. In this regard, Ti - zeolites

behave differently from homogeneous tungsten catalysts, in which epoxidation

does occur through a coordinative mechanism [102] . Consistently, acrylic acid

esters were inert to both kinds of catalyst.

18.4.4

Epoxidation with Solvolysis/Rearrangement of Intermediate Epoxide

The presence of acidity leads to a variety of end - products, depending on the cata-

lyst, the oleﬁ nic substrate and the reaction conditions (e.g. the sort of solvent, the

temperature and the use of acidity moderators). The acid properties of Ti,Al - β in

this regard, and the use of basic additives to reduce their effects, have been illus-

trated [61, 84] . Ti - MOR is also acidic owing to residual Al in the framework [59] .

TS - 1, with Al, Ga, B or Fe inserted in the framework, behaves similarly [103] . Al -

free Ti - β and Ti - MWW are less acidic catalysts than the former ones, but signiﬁ -

cantly more acidic than TS - 1. Most likely, the acid sites in the latter are the

–

OOH species a of Figure 18.1 (see also Section 18.11.3 ).

In some instances, combined redox/acid catalysis might be desirable, as in the

one - pot syntheses of β - phenylacetaldehyde by the oxidation of styrene. The primary

product is generally believed to be 1,2 - styrene oxide, which, in a second stage,

undergoes acid - catalyzed rearrangement to the aldehyde. Acidic zeolites are known

to behave as good catalysts in the latter reaction [104] . According to one study,

however, 1,2 - styrene oxide and β - phenyl acetaldehyde could be formed indepen-

dently by two competing mechanisms [105] .

Derouane and coworkers exploited the dual catalytic function and the spacious-

ness of pores of Ti,Al - β to prepare trans - 2 - alkoxycycloalkanols, of interest as phar-

maceutical intermediates, by the oxidation of cyclohexene and cyclopentene [106] .

The one - pot epoxidation/cyclization of unsaturated terpene alcohols, for example

linalool and α - terpineol, to mono - and bicyclic derivatives also required the cataly-

sis of large - pore Ti,Al - β (Scheme 18.10 ) [93, 107] .

Scheme 18.10 Oxidation/cyclization of unsaturated alcohols on Ti,Al - β .

18.5 Oxidation of Alcohol and Other Oxygenated Compounds 727

On TS - 1, Ti - β , Ti,Al - β and Ti - MWW in water and, to a lesser extent, in acetoni-

trile, tetralkyl - substituted oleﬁ ns yielded corresponding pinacols (tetralkyl - substi-

tuted glycols) [108] . In the oxidation of 3,4 - and 4,5 - unsaturated alcohols on TS - 1,

the intramolecular cyclization, by internal attack of the hydroxy group on the

oxirane ring, competed successfully with hydrolysis by external attack of water

(Scheme 18.11 ) [107] . Cyclization occurred regioselectively on the less remote

carbon, with yields often higher than 85%. Oxidation of 5 - hexen - 1 - ol, in which the

ﬁ ve - membered ring cannot form, produced the tetrahydropyran derivative. Triol

by - products increased with temperature and water content. Conversely, nearly

anhydrous conditions and the control of acidity with basic additives turned the

oxidation into epoxy alcohol production.

18.5

Oxidation of Alcohol and Other Oxygenated Compounds

The oxidation of primary and secondary alcohols using TS - 1 and Ti,Al - β produces

corresponding aldehydes and ketones [5, 77, 109 – 112] . While the latter are sufﬁ -

ciently stable under the reaction conditions, the former can undergo further oxida-

tion to carboxylic acids. Acetals and esters are also produced when operating in

728 18 Titanium Silicalite-1

neat alcohol or in methanol solvent (Equations 18.9 and 18.10 ). Tertiary alcohols

yield corresponding hydroperoxides.

(18.9)

(18.10)

Primary alcohols react at a much slower rate than secondary ones, suggesting

an electrophilic oxidation mechanism (Table 18.11 ). The relative inertness of

methanol justiﬁ es its use as an oxidation solvent for TS - 1. Within a homologous

series, the rate increases with the chain length of the alcohol up to a maximum

for octanol, then decreasing again [109] . It is likely that this trend involves the

combined effects of electronic factors, deactivation of the catalyst and adsorption

phenomena. A true kinetic ordering, only obtainable from initial rates, is substan-

tially lacking for this as for other oxidations. Shape selectivity is apparent in the

decrease of the rate of alcohols with the hydroxy group at more internal positions

or with vicinal methyl groups, the decrease being larger with the increase of the

number of methyl substituents [110, 111] .

Two papers deal with the kinetics of the oxidation on TS - 1 and Ti,Al - β [110, 111] .

Equation 18.11 illustrates the general rate law valid for both catalysts, according

to a Langmuir – Hinshelwood model. It is consistent with the competition of

alcohol/solvent for Ti sites and the adsorption of the substrate and the oxidant on

the same site. The inhibitory effect of water, implicit in the competitive adsorption

of the solvent, was proved by oxidation tests performed in acetonitrile containing

variable quantities of water [77, 111] .

Scheme 18.11 Oxidation of 3,4 - , 4,5 - and 5,6 - unsaturated alcohols on acidic Ti - zeolites.

(18.11)

The oxidation mechanism, based on an early one proposed for a tungsten cata-

lyst, entails a concerted step in which a C

–

H bond undergoes electrophilic attack

by the distal peroxy oxygen of Ti

–

OOH (Scheme 18.12 ) [110, 111, 113] .

The one - step synthesis of isoamyl butyrate from isoamyl alcohol and n - butyral-

dehyde, possibly through the formation and subsequent oxidation of an acetal

intermediate, could represent a special case in TS - 1 catalysis, since the oxidant

was molecular oxygen [114] . The authors did not advance any mechanistic hypo-

thesis. n - Butyl hydroperoxide, however, produced in situ by the autoxidation of

n - butyraldehyde, could have been the true oxidant, by virtue of its dimensional

compatibility with the narrow pores of TS - 1.

The oxidation of glycols occurred preferentially at the secondary alcohol group.

Thus, propene glycol and 2 - phenyl - 1,2 - ethanediol afforded hydroxyacetone and

β - hydroxyacetophenone, respectively, as the major product [115] . Ethene glycol

Table 18.11 Oxidation of alcohols catalyzed by TS - 1.

Alcohol t

1/2

(h)

decomposition (%)

methanol 8.5 13

ethanol 0.7 2.5

1 - propanol 1.0 5

1 - butanol 1.3 6

1 - octanol 3.0 –

2 - methyl - 1 - propanol 3.4

–

2 - propanol 0.01

< 0.5

2 - butanol 0.05

< 0.5

2 - pentanol 0.06

< 0.5

3 - pentanol 0.9 5

cyclohexanol 35 50

Solvent, pure alcohol; T , 45 ° C; TS - 1, 5 wt%; H

, 0.5 mol l

− 1

b Time required for 50% conversion.

c Fraction of H

decomposed at total conversion of alcohol.

d The product is t - butyl hydroperoxide.

Scheme 18.12 Oxidation of alcohols on TS - 1 and Ti,Al - β .

18.5 Oxidation of Alcohol and Other Oxygenated Compounds 729