Autoxidation

The term “autoxidation” comprises the oxidation with dioxygen [137] and a multitude

of free radical reactions catalyzed by transients in the system such as hydroxyl

radicals (autocatalysis) and superoxide anion radicals [1,7,138]. This chain process

with various phases of autoxidation starting with the initiation of the reaction

of dioxygen [131], being the least reactive, with the activated substrate (particularly

a phenolate) requires an elevated temperature [137] and/or the presence of heavy

metals [132], acting as redox catalysts [3], forming a superoxide anion radical and

a substrate radical [Eq. (9)]. As noted, oxygen bleaching must be conducted in an

alkaline environment (pH > 10) and a temperature of about 80–100 °C and beyond

to ensure reasonable rates. At higher pH (alkali charge) and temperature (about

120 °C), hydroperoxides decay homolytically to hydroxyl radicals. Whilst activation

644 7Pulp Bleaching

of the substrate and elevated temperatures are needed to initiate the reaction [Eq.

(9)], dioxygen, on the other hand, reacts very rapidly with any substrate radical to

the corresponding peroxyl radical [Eq. (10), propagation]. The recombination and

termination respectively is accomplished by coupling of two radicals and does not

require activation by ionization, as is needed for electron transfer.

Eq. (9): Initiation [4]

R__O2→R_ _ _O_2

RH _ HO_→R_ _ H2O

RH _ _O_2 →R_ _ HOO_

RH _ O2→R_ _ HOO_

Eq. (10): Propagation [4]

R_ _ O2→ROO_

ROO_ _ RÇ→ROOH _ R_

ROO_→R_ _ _O_2

R_ _ RÇ→RH _ R_

Eq. (11): Recombination – Termination

R_ _ HO_→ROH

R_ _ _O_2 →ROO_

R_ _ R_→R__R

ROO_ _ ROO_→ROOR _ O2

ROO_ _ R_→ROOR

The initiation step above occurs mostly at C atoms which can produce the most

stable free radicals (allylic, benzylic position, and 3 > 2 >> 1c arbons).Hence, unsaturated

fatty acids are extra-reactive at themethyleneCthat separates the double bonds.

7.3.2.3.5 Singlet O2 – Excited State

Singlet dioxygen, with a lifetime of about 0.06 s, can be generated from triplet

dioxygen by photoexcitation [127,139]. Alternatively, it can be made from triplet

oxygen through collision with an excited molecule (photosensitizer), which relaxes

to the ground state after a radiationless transfer of energy to triplet oxygen to form

reactive singlet oxygen [Eq. (12)] [125,140,141]. Furthermore, singlet oxygen is

generated by the reaction between a hydroxyl radical and a hydroperyl radical [Eq.

(13)], and between HOCl and peroxide [Eq. (14)] [142], the oxidation of the superoxide

anion radicals with heavy metals [Eq. (15)] or ozone [Eq. (16)], and in consequence

of the decay of polyoxides.

7.3 Oxygen Delignification 645

O2

h m

Photosensitizer _____

O2

_1D g _ _12_

HO_ _ HOO_→O2

1D g _ __H2O _13_

HOOH _ OCl_→Cl__H2O _ O2

1D g _ _ _14_

_O_2 _Fe3_→Fe2_ _O2

1D g _ _ _15_

O3 _ _O_2 →O_3 _O2

_1D g _ _16_

Alkenes (double-bonds) react with oxygen to form hydroperoxides, potentially

through an epoxide intermediate, and dienes reacts with oxygen in a Diels–Alderlike

reaction to form endoperoxides.

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