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

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710 18 Titanium Silicalite-1

bonds, are inert. The presence of electron - withdrawing groups in 1 - chlorohexane

and methyl heptanoate strongly decreases hydroxylation while orienting the attack

on the remote methylene groups. Consistently, no dihydroxylation is observed

under the conditions of Table 18.2 , and it is negligible on C

–

n - parafﬁ ns. Both

the absence of consecutive hydroxylation and remote oxyfunctionalization reveal

the electrophilic properties of the active species.

Evidence on the latter was obtained by the use of additives, substrate probes and

labeled molecules. The addition of small amounts of protonic acids promoted

hydroxylation. Alkali metal salts and basic compounds produced the opposite

effect, with inhibition or complete deactivation of the catalyst, depending on the

amount used. The subsequent addition of hydrochloric acid restored the initial

activity, showing that inhibition and deactivation by salts and bases are completely

reversible phenomena [24, 29] . On these grounds, the active species could be

identiﬁ ed as a fairly acidic Ti hydroperoxide or an oxidant, still unknown, produced

by its further transformation [24] . However, the involvement of Ti

–

OOH as the

active species of a heterolytic mechanism is not consistent with the results of the

competitive hydroxylation of aromatic and aliphatic C

–

H bonds, pointing instead

to a homolytic pathway.

Two types of substrate probe, cis - and trans - 1,3 - dimethylcyclopentane and

ethyl - and 2 - propylcyclopropane, were used to shed light on mechanistic details of

the hydroxylation step [30] . In the use of the ﬁ rst two probes, the participation of

–

OOH species in a concerted mechanism would predict either the retention or

the inversion of conﬁ guration at the chiral center, while the stereochemistry of a

homolytic mechanism would be determined by the competition between the

epimerization of the transient tertiary carbon radical and C

–

O formation (Scheme

18.1 ). In the hydroxylation of cyclopropyl probes, the cyclopropylcarbinyl radical

clock can either rearrange to ring - opened 3 - buten - 1 - yl radical before being trapped

or rebound with the hydroxyl carrier to yield the alcohol product directly (Scheme

18.2 ). With TS - 1, nearly equal amounts of trans - and cis - 1,3 - dimethylcyclopentanol

were obtained from the ﬁ rst type of probe, while no rearranged products were

obtained with the second ones. These results suggest that the putative radical

intermediate is very short - lived, but not short enough to prevent the racemization

of the tertiary carbon radical. Incidentally, it was estimated that epimerization and

cyclopropyl rearrangement occurred with ﬁ rst order rate constants of 10

and ca

− 1

, respectively [30, 31] .

The use of radical quenchers and the competitive oxidation of cyclohexane and

cyclohexane - d

led to identiﬁ cation of the active species as a Ti - centered radical

Scheme 18.1 Hydroxylation of sterically pure cis - and trans - 1,3 - dimethylcyclopentane.

18.2 Hydroxylation of Alkanes 711

[24] . Actually, BHT (2,6 - di - tert - butyl - 4 - methylphenol), carbon tetrachloride, chloro-

form and dichloromethane neither affected the hydroxylation rate nor produced

chlorinated derivatives, thus excluding a free radical mechanism and the presence

of long - lived alkyl radicals, both in the pores of the catalyst and in the external

solution. The primary isotopic effect in methanol and t - butanol was 4.1 and 4.7,

respectively. A k

/ k

of this magnitude is not compatible with a radical chain oxi-

dation initiated by hydroxyl radicals ( k

/ k

= 1 – 2), while it is fully consistent with

substantial C

–

H bond cleavage in the transition state by a Ti - centered radical.

On the whole, it is conceivable to postulate an active species that is Ti centered,

has a radical nature and originates from a Ti

–

OOH precursor, through some sort

of reaction which the latter species may undergo. An early mechanistic proposal,

based on a diradical peroxy species, does not match all these conditions (Scheme

18.3 a). This requires that the adsorption of hydrogen peroxide on Ti leads to a

side - on bonded peroxide and, more importantly, implies the splitting of one strong

–

O bond instead of the weaker peroxidic O

–

O one. The unlikelihood of Ti

–

cleavage is conﬁ rmed by the reaction of a Rh peroxide with carbon dioxide, occur-

ring through the insertion of CO

into the O

–

O bond, to yield the corresponding

peroxycarbonate [32] .

Scheme 18.3 b illustrates a recent mechanistic proposal. This bears similarities

to the oxygen - rebound mechanism, proposed for the hydroxylation of alkanes by

high - valent transition metal species in biomimetic and enzymatic systems [31, 33,

34] . The generation of the active species and other implications of the mechanism

are discussed in detail in Section 18.11.3 .

Scheme 18.2 Hydroxylation of ethylcyclopropane.

Scheme 18.3 (a) C

–

H hydroxylation by Ti( η

–

);

(b) C

–

H hydroxylation by oxygen - rebound - like mechanism.

712 18 Titanium Silicalite-1

18.2.2

Other Ti - Zeolites

Only TS - 2 and Ti,Al - β , among other Ti - zeolites, catalyze the hydroxylation of

alkanes. The close similarity of TS - 2 with TS - 1 suggests analogous catalytic proper-

ties. The somewhat poorer performances reported in the literature probably reﬂ ect

the poor quality of early catalysts, containing extra - framework impurity Ti species.

Ti,Al - β , on the other hand, is deﬁ nitely inferior to TS - 1, except in the hydroxylation

of bulky alkanes (Table 18.4 ) [35] . Interestingly enough, Ti,Al - β catalyzes the

hydroxylation of both sec - C

–

H and t - C

–

H in the same molecule, though the reac-

tivity of the latter is signiﬁ cantly higher [36] . In 1,2 - and 1,3 - dimethylcyclohexanes,

equatorial C

–

H bonds are more reactive than those at axial positions, probably

because there is less hindrance in the approach to the active species. The direct

link between the hydrophobicity of the sample used and catalytic performance is

of mechanistic interest [37] .

18.3

Hydroxylation of Aromatic Compounds

The synthesis of phenolic compounds, including the industrial production of a

major commodity such as phenol, has historically been achieved by the transfor-

mation of pre - existing aromatic functional groups. The generally poor selectivity

of direct hydroxylation routes is especially evident with molecular oxygen as the

oxidant, though sometimes relatively high yields have been claimed. The use of

hydrogen peroxide appears more promising, and is characterized by greater selec-

tivity. Actually, in the early 1970s Brichima and Rh ô ne - Poulenc commercialized

two industrial processes for the hydroxylation of phenol with H

. The develop-

ment of TS - 1 and Fe - ZSM - 5 opened new routes to selective hydroxylation pro-

cesses with hydrogen peroxide (EniChem, hydroxylation of phenol) and nitrous

oxide (Solutia, hydroxylation of benzene), respectively.

Table 18.4 Hydroxylation of alkanes on Ti,Al - β and comparison with TS - 1 [35] .

Alkane

Ti,Al - β

TS - 1

TON (mol/mol

) Selectivity (%

based on H

)

TON

(mol/mol

)

Selectivity (%

based on H

)

n - hexane 0.5 32 48.5 100

cyclohexane 2.3 51

–

methylcyclohexane 5.2 88

–

a Products below detection limits.

18.3.1

Hydroxylation of Phenol

18.3.1.1 Titanium Silicalite - 1

Normally in a review the hydroxylation of benzene should come before that of

phenol. The latter, however, has been the center of interest of most studies, par-

ticularly of early ones, whereas benzene has been considered often just for the

completeness of the range of substrates. On these grounds, an exception for

phenol is justiﬁ ed.

Hydroxylation is a consecutive reaction in which low conversion has to be main-

tained, to prevent the over - oxidation of catechol and hydroquinone to tars (Equa-

tion 18.5 ). This is normally achieved by operating with an excess of phenol over

hydrogen peroxide. The greater the H

: phenol ratio allowed in the feed, without

penalty to the selectivity, the greater the effectiveness of the catalysts (Table 18.5 )

[5, 38, 39] . A stirred reactor is normally preferred, though the use of the ﬁ xed bed

type has sometimes been reported [40 – 42] .

(18.5)

In aqueous acetone and methanol, TS - 1 shows superior performance than in

t - butanol or just water. The solvent has also a major effect on the catechol : hydro-

quinone ratio (see below). This varies in the range 0.5 – 1.3, which is some way

from the value of 2 expected for a statistical attack at the ortho and para positions

(Table 18.5 , entries 1 and 2). Yet, under practical conditions, the main component

of the reaction mixture could be phenol instead of the putative solvent and, under

such conditions, the product ratio approaches unity.

The yields based on hydrogen peroxide are comparable to those obtained with

acid catalysts, but at six - fold higher conversion (Table 18.4 , entries 1 and 3). The

most important parameters for the yield are the operating temperature and the

purity, concentration and crystal size of the catalyst [5, 43] . Yields increased, at

the expense of tar production, up to a maximum of ca 83% at 100 ° C, and then

Table 18.5 Hydroxylation of phenol with hydrogen peroxide.

Catalyst Ortho/para Phenol

conversion (%)

Selectivity Ref.

(% based

on H

)

(% based

on C

OH)

1 TS - 1 0.5 – 1.3 30 82 92 [5]

2 Co

, Fe

2 – 2.3 9 66 79 [39]

3 H

1.2 – 1.5 5 85 – 90 90 [38]

18.3 Hydroxylation of Aromatic Compounds 713

714 18 Titanium Silicalite-1

decreased rapidly with further temperature rises. Extra - framework Ti phases

promote non - productive side reactions, for example decomposition of the oxidant

and radical chain oxidations. In general, this applies to any oxidation catalyzed by

TS - 1, but is crucial for aromatic hydroxylation that requires a relatively high tem-

perature. The hydroxylation of phenol, indeed, was proposed as chemical test to

assess the purity of TS - 1 [5, 43, 44] . The concentration of TS - 1 also affects the

yields, owing to greater competition of secondary reactions in the presence of

excess H

and low catalyst contents. Van der Pool and others originally showed,

and other authors conﬁ rmed, that the hydroxylation of phenol is diffusion limited

for crystal sizes larger than 0.3 µ m [45 – 47] .

Experimental data ﬁ t well with kinetic expressions that are ﬁ rst order in both

phenol and hydrogen peroxide [47] . Observed rate constants were signiﬁ cantly

larger in water than in methanol and acetone. This was ascribed to the stronger

adsorption of phenol from an aqueous solution in TS - 1 (Table 18.6 ) [47, 48] . Sur-

prisingly enough, in the presence of methanol or acetone the concentration of

phenol in the pores was about the same as that in the external solution.

An issue of debate is the relative roles of internal and external sites in the cata-

lytic process. The effects of shape selectivity, clearly present in product distribu-

tion, seem to indicate a predominance of intra - porous hydroxylation. However,

the different catechol/hydroquinone ratio in methanol (0.5) and acetone (1.3),

could indicate a signiﬁ cant contribution of sites located on the outer surface of the

crystals, particularly for crystallite sizes < 0.3 µ m. Tuel and others, studying the

time course of the reaction and the solubility of tarry deposits, went further and

concluded that catechol and hydroquinone were produced on different sites, exter-

nal and internal respectively [49] . The effect of acetone and methanol simply

reﬂ ected their ability to maintain external sites clean from tar deposits, which are

soluble in the former and insoluble in the latter. On the other hand, Wilkenh ö ner

and others concluded, with the support of kinetic constants estimated indepen-

dently for internal and external sites, that catechol was also produced in the pores

over the entire reaction proﬁ le, albeit at a lower rate [47] . The contribution of the

outer surface for crystal sizes close to 0.1 µ m ranged from 46% in methanol to

69% in acetone.

Little is known of the active species and the hydroxylation mechanism. It is even

unclear whether the mechanism is homolytic or heterolytic. Accordingly, mecha-

nisms of both types have been proposed (Schemes 18.4 and 18.5 ).

Table 18.6 Henry constants ( K

) for the adsorption of phenol in TS - 1.

Solvent K

Water 84.4

Methanol 0.7

Acetone 0.6

In the mechanism of Scheme 18.4 , hydroxylation is carried out homolytically by

the same species proposed for alkane hydroxylation [25] . The oxidation of the

cyclohexadienyl intermediate by hydrogen peroxide produces the diphenol and

regenerates the active species, closing the catalytic cycle. Scheme 18.5 illustrates

the heterolytic routes proposed by Wilkenh ö ner and others for hydroquinone and

catechol production, based on cationic peroxy intermediates [47] . Both types of

mechanism, however, are little more than working hypotheses, needing validation

by experimental evidence.

18.3.1.2 Other Ti - Zeolites

TS - 2 was shown to be almost indistinguishable from TS - 1, as predicted by similar-

ity of structures and active sites [46] . Ti - Beta zeolites, with and without Al in the

structure, were less effective than TS - 1. The yields based on hydrogen peroxide,

just above 60%, were typical of rather modest catalysts. Apparently, product selec-

tivity was inﬂ uenced by the Al content. The relatively hydrophilic Ti,Al - β produced

catechol and hydroquinone in nearly equimolar amounts [50] . The Al - free Ti - β

showed a higher catechol selectivity, with an ortho / para ratio of 2 [47] . In both

cases, the greater spaciousness of pores favoured ortho hydroxylation. For a useful

comparison, the ortho / para ratio on medium - pore TS - 1 was 0.77 under analogous

conditions.

Scheme 18.4 Homolytic hydroxylation of phenol by Ti

–

•

species.

Scheme 18.5 Heterolytic hydroxylation of phenol by Ti

–

OOH

permission of Elsevier).

18.3 Hydroxylation of Aromatic Compounds 715

716 18 Titanium Silicalite-1

18.3.2

Hydroxylation of Benzene

The hydroxylation of benzene on TS - 1 produces phenol as the primary product

and, by consecutive oxidation, hydroquinone, catechol, benzoquinone and tarry

products (Equation 18.6 ) [5, 30, 51] . The selectivity to phenol generally falls below

50% even at only 3 – 5% benzene conversion. Hydroxylation with a mixture of

hydrogen and oxygen on Pd/TS - 1 proved to be even less effective [52] .

(18.6)

Recently, the use of sulfolane solvent allowed better kinetic control of the oxida-

tion chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene

conversion. The by - products were catechol (7%), hydroquinone (4%), 1,4 - benzo-

quinone (1%) and tar (5%) [53, 54] . According to these authors, a rather stable

complex, formed by hydrogen bonding with sulfolane, promoted desorption and

hindered the re - adsorption of phenol, protecting it from consecutive oxidation

(Equation 18.7 ). Actually, the rate of oxidation of phenol in the presence of sulfo-

lane was only 1.6 times that of benzene, while it was 10 times higher in the pres-

ence of acetone.

(18.7)

The pretreatment of TS - 1 with a solution of ammonium ﬂ uoride and hydrogen

peroxide further increased the conversion (ca 9%) and selectivity. The recovery of

catechol and hydroquinone, by their hydrogenation back to phenol, was also con-

sidered [55] . It is worth noting that, at a threshold yield value of ca 9%, the hydrox-

ylation of benzene could become competitive with the cumene process, considering

that the overall per pass yield in the latter does not exceed 8 – 9%.

Far greater yields were reported for the hydroxylation of benzene under the so -

called triphasic conditions, that is with the solid catalyst, aqueous hydrogen per-

oxide and an immiscible aromatic phase [56] . Others, however, could not reproduce

these results [54] .

Benzene hydroxylation on Ti,Al - MOR was studied by three different groups

[57 – 59] . Despite the spaciousness of its pores, the activity of this catalyst was lower

than that of TS - 1, as expected for a more hydrophilic catalyst. Accordingly, increase

in the Al content caused a decrease in the conversion, probably because of reduced

adsorption of benzene.

18.3.3

Oxidation of Substituted Benzenes

Electron - withdrawing substituents decrease the rate of oxidation, revealing the

electrophilic nature of the oxidant species. Accordingly, negligible or no yields

were reported for the hydroxylation of chlorobenzene, nitrobenzene, benzonitrile,

benzaldehyde and benzoic acid on TS - 1 [5] . Alkyl groups produced contrasting

effects: a rate enhancement by electron donation and a rate decrease by steric and

transport restrictions. In this regard, the size of the methyl group in toluene virtu-

ally compensated for the increase of electron density [5] . For other alkylbenzenes,

the nuclear reactivity trend was in the order: toluene > p - xylene ≥ ethylbenzene >

p - methylethylbenzene, showing again the predominance of steric hindrance [5, 30,

59] . Bulkier substituents suppressed hydroxylation, for example in 2 - propylben-

zene [24] .

The oxidation of alkylbenzenes also produced sec - alcohols and ketones, with no

reaction at tertiary and primary carbons. At variance with the rule that double

bonds are preferentially oxidized over other functionalities, side - chain epoxidation

had to compete with aromatic hydroxylation in the oxidation of α - methylstyrene,

1 - phenyl - 2 - butene and 4 - phenyl - 1 - butene [60] .

Ti - MOR promoted the ring hydroxylation of toluene, ethylbenzene and xylenes

with negligible oxidation of the ethyl side chain [59] . In the same study, however,

and in contrast to earlier ones, a similar result was also reported for TS - 1. No oxi-

dation of benzylic methyls was observed. Cumene yielded mainly the decomposi-

tion products of cumyl hydroperoxide. The oxidation of t - butylbenzene was

negligibly low. The reactivity order, toluene > benzene > ethylbenzene > cumene,

reﬂ ects the reduced steric constraints in the large pores of mordenite. Accordingly,

the rate of hydroxylation of xylene isomers increased in the order para < ortho < meta ,

in contrast to the sterically controlled one, ortho < meta << para , shown on TS - 1. It

is worth mentioning that the least hindered p - xylene exhibited the same reactivity

on either catalyst.

18.4

Oxidation of Oleﬁ nic Compounds

18.4.1

Epoxidation of Simple Oleﬁ ns

The oxidation of oleﬁ ns with aqueous hydrogen peroxide in methanol can produce

several products, by different reaction paths: double bond epoxidation, allylic H -

abstraction, epoxide solvolysis, alcohol and glycol oxidation (Scheme 18.6 ). Nor-

mally, oxide catalysts of Group IV – VI metals are poorly selective, because of their

acidic properties, the inhibition they are subject to in aqueous media and homo-

lytic side reactions with hydrogen peroxide. The only exception concerns the

epoxidation of α , β - unsaturated alcohols and acids, which are able to bind on the

18.4 Oxidation of Oleﬁ nic Compounds 717

718 18 Titanium Silicalite-1

catalyst through the oxygenated functionality. However, TS - 1, a mixed oxide of

silica and titania similar in composition and Ti dispersion, but not structure,

to the Ti/SiO

Shell catalyst, unexpectedly showed high activity and selectivity.

Actually, a most remarkable feature of TS - 1 is the smooth and preferential epoxida-

tion of the double bond, over any other functionality that may be present in the

molecule.

18.4.1.1 Titanium Silicalite - 1

Epoxidation occurs in dilute solutions of aqueous hydrogen peroxide and at near

room temperature [61, 62] . The rate is rapid even at H

concentrations of 1%

or lower and at temperatures as low as − 5 ° C. Electron - deﬁ cient oleﬁ ns can also

be oxidized at near room temperature (Table 18.7 ). Preferred solvents are alcohols

and, in general, protic or polar media. The rate of epoxidation decreases in the

solvent order: methanol > ethanol > i - propanol > acetone > acetonitrile > t - butanol,

with a difference greater than one order of magnitude between the two extremes

of the series [62, 63] . An exception could be the oxidation of cyclopentene, for

which acetone and acetonitrile were reported to be less effective than t - butanol

[64] . In the epoxidation of propene, the rates in water were possibly faster than in

methanol, though the rapid decay of the catalyst did not allow unequivocal conclu-

sions. As a matter of fact, water contents up to 50 wt% in methanol produced only

a moderate reduction of the rate, probably because of the lower solubility of

propene in aqueous media (TOF 1 – 2 s

− 1

, at 40 ° C).

Scheme 18.6 Reaction pathways in the oxidation of propene with hydrogen peroxide.

Table 18.7 Epoxidation of linear oleﬁ ns.

Oleﬁ n T ( ° C) t (min) t

1/2

(min)

Conversion (%) Selectivity (%

based on H

)

1 - butene

− 5

60 – 96 96

1 - pentene 25 60 5 94 91

1 - hexene 25 70 8 88 90

cyclohexene 25 90 – 9 –

1 - octene 45 45 5 81 91

allyl chloride 45 30 7 98 92

allyl alcohol 45

35 16 81 72

Solvent, methanol; oleﬁ n, 0.90 mol l

− 1

; TS - 1, 6.2 g l

− 1

b t

1/2

is the time necessary to 50% H

conversion.

c TS - 1 4.0 g/l

The epoxidation of unhindered oleﬁ ns is nearly quantitative and the incidence

of side reactions is negligible [62] . The epoxide selectivity can be higher than 90%.

The by - products are merely those produced by the acid solvolysis of the oxirane

ring, namely the corresponding glycol and methyl ethers. Basic compounds able

to diffuse inside the pores of TS - 1, such as sodium acetate, sodium hydroxide and

ammonia, decrease the intra - porous acidity generated by Ti

–

OOH species and,

sometimes, by lattice Al impurities. Their addition in parts per million quantities

minimizes the cleavage of the epoxide, thus enhancing the selectivity up to 97 –

98%. Spontaneous hydrolysis, fairly slow but always present in protic media, pre-

vents the achievement of quantitative yields [61] . Further increasing the

concentration of the base gradually decreases the catalytic activity up to complete

inhibition. Bulky tetrapropylammonium hydroxide, unable to diffuse into the

pores of TS - 1, does not affect the epoxidation process at any concentration.

The epoxidation rate is related to the electron density of the double bond, increas-

ing with it. Thus, the epoxidation of propene is much faster than that of ethene.

The formal substitution of one methyl, chloro or hydroxyl group at the allylic posi-

tion of propene results into the reactivity order: 1 - butene > allyl chloride > allyl

alcohol. Methyl substitution on butenes also produces the expected ordering:

2 - methyl - 2 - butene > 2 - methyl - 1 - butene > 3 - methyl - 1 - butene (Table 18.8 ).

However, since epoxidation occurs within pores of cross - section comparable to

that of the oleﬁ n, steric restrictions generally prevail over inductive effects, leading

to anomalous reactivity orders. They result from restrictions to diffusion in the

pores ( reactant shape selectivity ) and to the approach of the double bond to the

active species ( transition state shape selectivity ). The ﬁ rst is sufﬁ cient to explain

18.4 Oxidation of Oleﬁ nic Compounds 719

Table 18.8 Relative rates in the epoxidation of C

and C

oleﬁ ns.

TS - 1 TS - 1 CH

/ r

Oleﬁ n r

/ r

Oleﬁ n r

/ r

1.0

1.0 1.0

0.59

0.13 ca 1.0

0.71

0.87

20 – 24

0.17

0.19 20 – 24

2.7

– – 2 0 – 2 4

– –

0.79 240

a Data taken from Refs [62, 65] .

b Averaged values from competition kinetics.

c Epoxidation occurred with complete retention of conﬁ guration.