- Email: [email protected]
Flash photolysis of lignin: II—Oxidative photodegradation of dioxane-lignin
Polymer Photochemistry 7 (1986) 461-468
Flash Photolysis of Lignin: ll--Oxidative Photodegradation of Dioxane-Lignin
M i g u e l G. N e u m a n n , R o b e r t A. M. C. D e G r o o t e and A n t o n i o E. H. M a c h a d o Instituto de Fisica e Quimica de S~o Carlos, Universidade de Sho Paulo, Caixa Postal 369, 13560 S~o Carlos, SP, Brazil (Received: 10 December, 1985)
ABSTRACT The flash photolysis of a low molecular weight dioxane-lignin in the presence of oxygen showed the same transient spectrum as de-aerated solutions. Experiments performed in the presence of DABCO (1,4-diazabicyclo(2,2,2)octane) (a singlet oxygen scavenger) and ascorbic acid (a radical scavenger) showed that ground state oxygen is the main species participating in these processes. Singlet oxygen, sensitized by species other than carbonyls, may play a role in later stages of the reaction.
INTRODUCTION We recently published a study in which the initial steps of the photodegradation of lignin in de-aerated solutions have been reinvestigated and confirmed. 1 Several authors 2-4 have studied the photodegradation of lignin in oxygen-containing solutions using continuous illumination systems and analyzing changes in the concentrations of starting material or reaction products. The experimental results had been interpreted assuming that the reaction is started by the excitation of carbonyl groups present in lignins, followed by the sensitization of triplet oxygen to its singlet state by the carbonyl 461 Polymer Photochemistry 0144-2880/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland
Miguel G. Neumann et al.
462
triplets:
CH3 OH
OH
c.3
-OH
c.3
Singlet oxygen then reacts with other portions of the lignin structure to form peroxides which eventually lead to a series of products as well as to the degradation of the original macromolecule. In this paper we present a study in which the initial steps of the photoreaction of lignin in the presence of air or oxygen have been investigated. EXPERIMENTAL Lignin was extracted from rice husks using dioxane-HCl, as described previously: The vanillin (3-methoxy-4-benzaldehyde) used as a model compound was from Aldrich and was used as delivered. The lignin solutions were prepared in 50% dioxane-water (100 mg/litre), and vanillin solutions were 10-4M, in water. Solutions were saturated with air by continuous exposure and shaking, or bubbled with oxygen when higher oxygen concentrations were required. Flash photolysis experiments were performed on an Applied Photophysics system (model KNO020) providing 30#s flashes of 10kV. Spectra and peak heights were recorded 50-500#s after irradiation. All solutions were submitted to only one flash, to avoid undesired photolysis of reaction products. Continuous illumination experiments were performed using a high-pressure mercury lamp. Infrared analyses were carried out on a Perkin-Elmer model 1320 spectrophotometer. Molecular weights were determined by GPC using a Waters high performance liquid chromatograph with a Microporasil GPC 60A column.
Flash photolysis of lignin : H
463
RESULTS AND DISCUSSION It has been found that, when submitting lignin or vanillin to flash photolysis, the initial step of the reactions involves the formation of ketyl and phenoxy radicals: 1
'l
~OCH3
)~'-OCH3 OH
CH3 OH
OH
.COH
OH
oc.;
O-
oc.
(2)
As can be seen from Fig. 1, the initial spectra of the same lignin solutions, in the presence or absence of oxygen, are practically identical. Only some decrease of the absorption maxima corresponding to the radicals can be observed. When D A B C O was added to the lignin solutions no effect on the absorption maxima corresponding to the radicals could be detected. On the other hand, the addition of ascorbic acid did practically reduce to zero the absorption at those wavelengths, as can be seen from Table 1. These results indicate that the effect of singlet oxygen on the oxidative photodegradation of lignins is not as important as was previously assumed, at least in the initial stages of the reaction. The short lifetimes of aromatic carbonyl triplets (less than a few microseconds), the efficiency of hydrogen abstraction by these triplets and the low molar absorptivity of the ketones in the visible region 6'7 contribute towards making the sensitization of singlet oxygen by triplet carbonyls much less efficient than claimed previously. 2-4'8'9 On the other hand, the reduction of the radical peaks by the addition of ascorbic acid, a good radical scavenger,
464
Miguel G. Neumann et al.
X
i i
I 45O
5OO
WAVELENSl"H(nm)
Transient spectra of lignin in deaerated ( solutions, 50/~s after flash.
) and aerated ( . . . .
)
points to the importance of free radicals in the early stages of the reaction. Thus, it may be assumed that oxygen will react thermally with both radicals formed in the initial hydrogen abstraction reaction. The decrease in the molecular weight of photolyzed lignin, as shown in Fig. 2, may be explained by the cleavage of the radicals formed by TABLE 1 Effect of Ascorbic Acid on the Peak Heights of the Transients of Flash Photolyzed Lignin
Absorbance Wavelength Without ascorbic With10-3M (rim) acid ascorbic acid 415 435 460
0.0049 0-0038 0.0033
0.0015 0.0005 0.0000
465
Flash photolysis of lignin : H 8OO
Mw
2OO
i I
i 2
l 3
"rIME ( h}
Fig. 2. Molecular weightof dioxane-ligninafter irradiationin aerated solutions.
the addition of triplet oxygen to phenoxy radicals:
O--O. CH3 O.
CH3
CH3
O
O
OCH3 O
(3)
In that case the ketyl radical will also react with ground-state oxygen to re-form the original carbonyl group:
•COH
CO + 302
' ~
+ HO2" OCH3
OCH3 OH
(4)
OH
Later stages of the reaction may include the sensitization of singlet oxygen by some reaction product, such as a quinone, that has a lifetime well over the microsecond range and absorbs much more strongly in the visible region. Steady-state irradiation of lignin in the presence of oxygen increased the amount of carbonyl groups present
466
Miguel G. Neumann I
et al. SOLVENT: THF V : 1641cnt"1
1.0
| 0.8
TIME (h)
Fig. 3. Carbonylgroup concentrationin photolyzedaerated ligninsolutions. in the solution, as can be seen from Fig. 3. This effect is due to the reaction of excited double bonds with triplet oxygen, which later will form tr- or fl-phenones and aliphatic ketones: J
X-CJ R
~OCH3 OH
hY ),
)~OCH3 OH
\C-o
02 OH + RCO
(5) ~OCH3 OH As a result of the experiments presented in this investigation it must be assumed that the sensitization of single oxygen by triplet carbonyls does not play an important role in the oxidative photo-
Flash photolysis of lignin : H
467
degradation of lignin during the early stages of the reaction. A tentative reaction scheme to explain the effects of singlet oxygen quenchers observed by us and by other authors follows. Initial step (determined from flash photolysis experiments)
[Ligninl
"-COH ( "O h~ . - ~ .
+
~ O
CH3
'OH
O-
(6)
CH3
Dark reaction
~
OCH3
O.
302'+OCH3+~O O
(7)
O
Continuous irradiation (oxygen sensitization)
hv ~i
0
302
(8) 0
+ 102 ~
Degradation products
(9)
Studies to confirm the importance of the sensitization of oxygen by compounds other than carbonyls are in progress. ACKNOWLEDGEMENTS Financial support by CNPq (Conselho Nacional de Desenvolvimento Cientlfico e Tecnol6gico, Brasil) is gratefully acknowledged.
468
Miguel G. Neumann et al. REFERENCES
1. Neumann, M. G., De Groote, R. A. M. C. and Machado, E. A. H., Part I of this series, Polym. Photochem., 7 (1986) 409. 2. Gellerstedt, G. and Petterson, E.-L., Svensk Papperstidn., 80 (1977) 15. 3. Forsskahl, I., J. Photochem., 25 (1984) 197. 4. Brunow, G. and Sivonen, M., Papperi Ja Puu, 57 (1975) 215. 5. Pepper, J. M. and Siddiqueullah, M., Canad. J. Chem., 39 (1964) 1454. 6. Das, P. K., Encinas, M. V. and Scaiano, J. C., J. Amer. Chem. Soc., 103 (1981) 4154. 7. Turro, N. J., Modern molecular photochemistry, Benjamin, New York, 1978, pp. 296-361. 8. Nimz, H. H. and Turznik, G., Cellulose Chem. Technol., 14 (1980) 727. 9. Brunow, G., in Singlet oxygen reactions with organic compounds and polymers, Ranby, B. and Rabek, J. F. (eds), Wiley, London, 1978, pp. 311-15.
Flash Photolysis of Lignin: ll--Oxidative Photodegradation of Dioxane-Lignin
M i g u e l G. N e u m a n n , R o b e r t A. M. C. D e G r o o t e and A n t o n i o E. H. M a c h a d o Instituto de Fisica e Quimica de S~o Carlos, Universidade de Sho Paulo, Caixa Postal 369, 13560 S~o Carlos, SP, Brazil (Received: 10 December, 1985)
ABSTRACT The flash photolysis of a low molecular weight dioxane-lignin in the presence of oxygen showed the same transient spectrum as de-aerated solutions. Experiments performed in the presence of DABCO (1,4-diazabicyclo(2,2,2)octane) (a singlet oxygen scavenger) and ascorbic acid (a radical scavenger) showed that ground state oxygen is the main species participating in these processes. Singlet oxygen, sensitized by species other than carbonyls, may play a role in later stages of the reaction.
INTRODUCTION We recently published a study in which the initial steps of the photodegradation of lignin in de-aerated solutions have been reinvestigated and confirmed. 1 Several authors 2-4 have studied the photodegradation of lignin in oxygen-containing solutions using continuous illumination systems and analyzing changes in the concentrations of starting material or reaction products. The experimental results had been interpreted assuming that the reaction is started by the excitation of carbonyl groups present in lignins, followed by the sensitization of triplet oxygen to its singlet state by the carbonyl 461 Polymer Photochemistry 0144-2880/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Northern Ireland
Miguel G. Neumann et al.
462
triplets:
CH3 OH
OH
c.3
-OH
c.3
Singlet oxygen then reacts with other portions of the lignin structure to form peroxides which eventually lead to a series of products as well as to the degradation of the original macromolecule. In this paper we present a study in which the initial steps of the photoreaction of lignin in the presence of air or oxygen have been investigated. EXPERIMENTAL Lignin was extracted from rice husks using dioxane-HCl, as described previously: The vanillin (3-methoxy-4-benzaldehyde) used as a model compound was from Aldrich and was used as delivered. The lignin solutions were prepared in 50% dioxane-water (100 mg/litre), and vanillin solutions were 10-4M, in water. Solutions were saturated with air by continuous exposure and shaking, or bubbled with oxygen when higher oxygen concentrations were required. Flash photolysis experiments were performed on an Applied Photophysics system (model KNO020) providing 30#s flashes of 10kV. Spectra and peak heights were recorded 50-500#s after irradiation. All solutions were submitted to only one flash, to avoid undesired photolysis of reaction products. Continuous illumination experiments were performed using a high-pressure mercury lamp. Infrared analyses were carried out on a Perkin-Elmer model 1320 spectrophotometer. Molecular weights were determined by GPC using a Waters high performance liquid chromatograph with a Microporasil GPC 60A column.
Flash photolysis of lignin : H
463
RESULTS AND DISCUSSION It has been found that, when submitting lignin or vanillin to flash photolysis, the initial step of the reactions involves the formation of ketyl and phenoxy radicals: 1
'l
~OCH3
)~'-OCH3 OH
CH3 OH
OH
.COH
OH
oc.;
O-
oc.
(2)
As can be seen from Fig. 1, the initial spectra of the same lignin solutions, in the presence or absence of oxygen, are practically identical. Only some decrease of the absorption maxima corresponding to the radicals can be observed. When D A B C O was added to the lignin solutions no effect on the absorption maxima corresponding to the radicals could be detected. On the other hand, the addition of ascorbic acid did practically reduce to zero the absorption at those wavelengths, as can be seen from Table 1. These results indicate that the effect of singlet oxygen on the oxidative photodegradation of lignins is not as important as was previously assumed, at least in the initial stages of the reaction. The short lifetimes of aromatic carbonyl triplets (less than a few microseconds), the efficiency of hydrogen abstraction by these triplets and the low molar absorptivity of the ketones in the visible region 6'7 contribute towards making the sensitization of singlet oxygen by triplet carbonyls much less efficient than claimed previously. 2-4'8'9 On the other hand, the reduction of the radical peaks by the addition of ascorbic acid, a good radical scavenger,
464
Miguel G. Neumann et al.
X
i i
I 45O
5OO
WAVELENSl"H(nm)
Transient spectra of lignin in deaerated ( solutions, 50/~s after flash.
) and aerated ( . . . .
)
points to the importance of free radicals in the early stages of the reaction. Thus, it may be assumed that oxygen will react thermally with both radicals formed in the initial hydrogen abstraction reaction. The decrease in the molecular weight of photolyzed lignin, as shown in Fig. 2, may be explained by the cleavage of the radicals formed by TABLE 1 Effect of Ascorbic Acid on the Peak Heights of the Transients of Flash Photolyzed Lignin
Absorbance Wavelength Without ascorbic With10-3M (rim) acid ascorbic acid 415 435 460
0.0049 0-0038 0.0033
0.0015 0.0005 0.0000
465
Flash photolysis of lignin : H 8OO
Mw
2OO
i I
i 2
l 3
"rIME ( h}
Fig. 2. Molecular weightof dioxane-ligninafter irradiationin aerated solutions.
the addition of triplet oxygen to phenoxy radicals:
O--O. CH3 O.
CH3
CH3
O
O
OCH3 O
(3)
In that case the ketyl radical will also react with ground-state oxygen to re-form the original carbonyl group:
•COH
CO + 302
' ~
+ HO2" OCH3
OCH3 OH
(4)
OH
Later stages of the reaction may include the sensitization of singlet oxygen by some reaction product, such as a quinone, that has a lifetime well over the microsecond range and absorbs much more strongly in the visible region. Steady-state irradiation of lignin in the presence of oxygen increased the amount of carbonyl groups present
466
Miguel G. Neumann I
et al. SOLVENT: THF V : 1641cnt"1
1.0
| 0.8
TIME (h)
Fig. 3. Carbonylgroup concentrationin photolyzedaerated ligninsolutions. in the solution, as can be seen from Fig. 3. This effect is due to the reaction of excited double bonds with triplet oxygen, which later will form tr- or fl-phenones and aliphatic ketones: J
X-CJ R
~OCH3 OH
hY ),
)~OCH3 OH
\C-o
02 OH + RCO
(5) ~OCH3 OH As a result of the experiments presented in this investigation it must be assumed that the sensitization of single oxygen by triplet carbonyls does not play an important role in the oxidative photo-
Flash photolysis of lignin : H
467
degradation of lignin during the early stages of the reaction. A tentative reaction scheme to explain the effects of singlet oxygen quenchers observed by us and by other authors follows. Initial step (determined from flash photolysis experiments)
[Ligninl
"-COH ( "O h~ . - ~ .
+
~ O
CH3
'OH
O-
(6)
CH3
Dark reaction
~
OCH3
O.
302'+OCH3+~O O
(7)
O
Continuous irradiation (oxygen sensitization)
hv ~i
0
302
(8) 0
+ 102 ~
Degradation products
(9)
Studies to confirm the importance of the sensitization of oxygen by compounds other than carbonyls are in progress. ACKNOWLEDGEMENTS Financial support by CNPq (Conselho Nacional de Desenvolvimento Cientlfico e Tecnol6gico, Brasil) is gratefully acknowledged.
468
Miguel G. Neumann et al. REFERENCES
1. Neumann, M. G., De Groote, R. A. M. C. and Machado, E. A. H., Part I of this series, Polym. Photochem., 7 (1986) 409. 2. Gellerstedt, G. and Petterson, E.-L., Svensk Papperstidn., 80 (1977) 15. 3. Forsskahl, I., J. Photochem., 25 (1984) 197. 4. Brunow, G. and Sivonen, M., Papperi Ja Puu, 57 (1975) 215. 5. Pepper, J. M. and Siddiqueullah, M., Canad. J. Chem., 39 (1964) 1454. 6. Das, P. K., Encinas, M. V. and Scaiano, J. C., J. Amer. Chem. Soc., 103 (1981) 4154. 7. Turro, N. J., Modern molecular photochemistry, Benjamin, New York, 1978, pp. 296-361. 8. Nimz, H. H. and Turznik, G., Cellulose Chem. Technol., 14 (1980) 727. 9. Brunow, G., in Singlet oxygen reactions with organic compounds and polymers, Ranby, B. and Rabek, J. F. (eds), Wiley, London, 1978, pp. 311-15.