Ind. Eng. Chem. Fundam. 1984, 23, 387-392
387
Mechanism of the Oxidation of Aqueous Phenol with Dissolved Oxygen Howard
R. Devlln’
and Iestyn J. Harris
Department of Chemical Englneerlng, University of Melbourne, Parkville, Victoria 3930, Australia
The oxidation of aqueous phenol by oxygen has been studied at elevated temperature and pressure by use of the stopped flow technique. High performance liquid chromatography has been used to identify the oxidation products and estimate their concentrations. A detailed reaction mechanism is proposed.
Introduction Chemical oxidation of phenolic waste waters represents an alternative treatment scheme when the phenol concentration is too high for direct biological systems. When the effluent is also available at elevated temperature the use of chemical oxidation may be especially favored. One such situation is the treatment of effluent produced from the thermal dewatering of Victorian brown coals (Evans and Siemon, 1970) some of which have moisture contents of 70% by weight as mined (Gloe and Higgins, 1976). If oxygen is to be used directly as a means of destroying phenol in waste waters, some detailed knowledge of the reaction intermediates and the ultimate fate of the phenol must be known so that it is clear what products may be present in the “treated” effluent and how this new effluent might itself be treated, if necessary. Knowledge of the reaction pathway also offers the possibility of the manipulation of the oxidation to allow more complete destruction of phenol or perhaps the preferential production of a particular product through appropriate variation of the process conditions. Previous investigations of the reaction between molecular oxygen and phenol in the liquid phase, for example by Sadana (1974), have concentrated on demonstrating that phenol can be destroyed by oxygen and measuring the rate of reaction between phenol and oxygen. Very little has been done to elucidate the reaction pathway by which the destruction of phenol occurs. Several workers, including Gould and Weber (1976) and Li et al. (1979), have used ozone (0,) to oxidize phenol; Eisenhauer (1968) proposed the reaction be used for the treatment of aqueous wastes. The majority of work using ozone to oxidize phenol has been carried out at ambient temperatures, since ozone is a powerful oxidant even at these temperatures. At high temperatures in aqueous solutions, the form in which oxygen participates in chemical reactions is complex. The elevated temperatures necessary can lead to the formation of oxygen radicals, o., which in turn can react with water and oxygen to form peroxide, H202,and ozone, 03, so that these four species O., 02,and 03, and H202are all capable of participating in the phenol oxidation. Phenol Oxidation Products Table I is a summary of reaction products that have been reported from the oxidation of phenol by oxygen and ozone. Some compounds have been found in both oxidation systems. The presence of muconic and fumaric acids has only been inferred and maleic acid is included because
* IC1 Australian Operations Pty Ltd, Central Research Laboratory, Newsom Street, Ascot Vale, Victoria, Australia.
maleic anhydride, which was reported in the vapor phase oxidation of phenol, would form the acid in aqueous solution. Experimental Study of Phenol Oxidation The reactor used in this work followed the approach developed by Hartridge and Roughton (1923) for kinetic studies involving a soluble gas with a nonvolatile solute. A rapid mixing stopped flow system, after that of Roughton (1934), was used, minimizing the quantities of aqueous phenol and aqueous oxygen solutions which had to be stored at the elevated temperatures and pressures required in this work. The reactor volume was approximately 0.6 cm3. An HPLC was used to identify and quantify the reaction products. A detailed description of the analytical equipment and its calibration is given by Devlin (1982). Reaction Conditions The complete study examined three separate conditions of phenol to oxygen ratio, these being summarized in Table 11. A series of reaction temperatures between 150 and 225 OC was used to determine the effect of temperature on the reaction pathway. The initial oxygen concentrations were the equilibrium values calculated from the data of Pray et al. (1952) at the appropriate temperature and the total system pressure of 20690 kPa less the contribution due to the partial pressure of water. Reaction Products Table I11 lists all of the reaction products identified in this work. Three of these, malonic, succinic, and acrylic acids, had not previously been reported in phenol oxidation. Carbon dioxide formation was confirmed by gas chromatography, but the small reactor volume made quantification difficult so its production was calculated by difference from a carbon balance. Reaction Pathway To determine the reaction pathway for the reaction between phenol and oxygen, a detailed analysis of the concentration history of reaction products and intermediate has been undertaken. Development of the mechanisms for such a complex reaction system requires some knowledge of the short-lived intermediates as well as the final reaction products. Typical concentration histories are shown in Figures 1 and 2. These clearly distinguish between intermediate products such as maleic and acrylic acids and the eventual end products such as formic and acetic acids. Excess phenol conditions (Figure 2) also demonstrate the intermediate nature of the dihydric phenols. At the elevated temperatures of oxygen oxidation of phenol, although the oxidation of phenol and its reaction products is likely to be a significant process, there is scope 0 1984 American Chemical Society
388
Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
Table I. Summary of Reported Products from Oxidation of Phenol with Molecular Oxygen or Ozone
Structure
Comoound phenol
Ozone
Oxvcren
Ozone
Structure
CornpoLind
Oxygen
maleic acid
OH
as above
fi,
'1
fumaric acid catechol
Gould and Weber Jnt
&OH
119761 Li e t a l ( 19791 GIL LIL
1
hydroquinone
Sadana (19741 Sadana and Katzer ( 1 9741 G'L
Gould dnd Weber Jnr. 11976) Eisenhauer
p- ben zoqu i none
Gould and Weber Jnr 11976)
0
Li e t ai ( 1 9791 L IL
H
HOOC
cat
Sadana i 1974) Sadana arid Katzer (19741 G;L
H
H
0
oxalic acid
Sadana (19741 Sadana and Katzer ( 1 9741 Walsh and Katzer ( 19731 Ahmad e t ai ( 19731 cat, uncat GIL
Gould and Weber Jnr. Li e t al (1979) GIL LIL
OH
-
Eisenhauer (1968) GIL
glyoxylic acid
";c-c+" HO
\H
glyoxal
Gould and Weber Jnr. (1976) GIL as above G/L
-
o-benzoquinone
as above cat. uncat
a c a c acid
Schmidt (1970) GIL uncat
H
I
40
1
\OH
H-C-C
H formic acid c.c muconic acid
Eisenhauer
gp8)
H Y c o F g O H
Li e t al ( 19791 L 'L
H-C carbon dioxide
as above GIL uncat
go \OH
COZ
Bauch e t a l (1970)
GIL H Ahmad e t a l (1970)
0
maleic anhydride
I/
V.c a t
HC,C\
carbon monoxide
Sadana (1974) Sadana and Katzer (1974) Schmidt (1970) Walsh and Katzer (1973) Ahmad e t (19701 GIL cat, uncat Ahmad e t ai (1970) GIL uncat
co
11
0
G/L Gashiquid system L/L Liquid/liquid system cat Catalysed uncat Uncatalysed
Table 11. Summary of Reaction Conditions Used temp, "C
phenol
150
100
170 200 225 150 200 200
100 100 100 840 840 1680
concn
O,(actual)/ O,(for concn complete oxidn) 0 2
2.4 2.7 3.1 3.8 2.4 3.11 3.11
10.0
11.3 13.0 15.4 1.2 1.5 0.6
for other types of reactions to occur, such as electrophilic addition and decarboxylation. H H H\
H/c=c,
/H
H+ H 2 0 +
R-COOH
R represents a carbon-hydrogen chain which may contain both saturated and unsaturated carbon-carbon bonds.
I
1
I
t
H-C-C-R-COOH HO H
R-C
' 4 \OH
Heat
R-H
+ CO,
A t elevated temperatures and in the presence of water, oxygen is capable of three different oxidation reactions. It can substitute an oxygen atom into an aromatic ring to form a dihydric phenol or quinone (Scheme I). Oxygen is also capable of attacking carbon to carbon double bonds to form carbonyl compounds, and in oxidizing alcohols and carbonyl groups to form carboxylic acids. Intermediate Ring Compounds Intermediate ring compounds, dihydric phenols, and quinones were not observed under conditions of excess oxygen. They were produced under conditions near the
Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
389
Table 111. Reaction Products Identified in This Work compd
structure
phenol catechol hydroquinone p-benzoquinone maleic acid fumaric acid succinic acid propionic acid malonic acid acrylic acid
excess 0,
near stoichiometry
excess phenol
X
X
X X X
C6H, OH C6H4(OH1, C6H4(0H)2 6'
H4
(
X
A
12
HOOCCHCHCOOH
X X X X X X X l
HOOCCH,CH,COOH CH,CH,COOH HOOCCH,COOH CH,CHCOOH HOOCCOOH HOOCCHO OHCCHO
oxalic acid glyoxylic acid glyoxal acetic acid formic acid carbon dioxide
comments
low concentration trace amounts trace amounts trace amounts low concentration low concentration
"1
not separated by HPLC
X
CH,COOH HCOOH
X X X
co,
3.0 0
Maleic acid 0
2.01
Acrylic acid
P-benzoquinone
2.0
10
'
20
, 30
40
50
60 O
10
20
sb
40
30
C
+s
8
B-.
~
$0
0
o Phenol
o Phenol
o Acetic acid
Acetic acid
A Oxalic* acid Formic acid
A Oxalic' acid
A
Hydroquinone Formic acid Catechol
/
n
-
1
I
I
4b
20 30 40 50 60 Reaction time (min) Figure 1. Concentration histories of some reaction products at 200 "C using excess oxygen.
20 30 i0 I Reaction time (min) Figure 2. Concentration histories of some reaction products at 200 "C using excess phenol.
stoichiometric ratio of phenol and oxygen and under conditions of excess phenol, appearing in greater proportions (percent of total carbon) as the ratio of phenol/oxygen in the initial feed increased. Measurable quantities of the ring compounds do not appear until after 15 min. Since the reactions which produce these compounds are known to occur even at low temperature, their nonappearance under conditions of excess oxygen indicates that their breakdown is rapid. Under conditions of excess phenol and near stoichiometric proportions, the oxygen level drops rapidly as the reaction proceeds and apparently so does the rate of the ring degradation reactions. The clear dependence on oxygen level indicates that oxidation is the major mechanism by which these intermediate ring compounds are degraded, thermal degradation being less significant.
These unsaturated acids seem to be intermediates in the main reaction path rather than products formed by a minor side reaction. The acids, formic, acetic and oxalic* also formed under the three phenol/oxygen ratios used. All were produced in reasonable quantities (Figures 1 and 2) and were more stable than acrylic and maleic acids. The three compounds included as oxalic* acid are related by the oxidation series shown
10
The Formation of Carboxylic Acids For the three phenol to oxygen ratios used, the unsaturated acids, maleic and acrylic, appear in all situations.
10
--
glyoxal
-
glyoxylic acid
-
oxalic acid
The behavior of the s u m of these products for the present work, under the three conditions of phenol to oxygen ratios, can be seen in Figure 3. For excess oxygen conditions there appears to be an almost instantaneous production of one or more of these compounds, followed by a reasonably rapid decay. The total amount of these compounds then passes through a minimum and then again begins to rise. This general
390
Ind. Eng. Chem. Fundam., Vol. 23, No. 4, 1984
0
Scheme I OH
OH
0
/
II
P--benzoquinone
H,o,"+
H/c, C+o I
HO
1
HO
0 , attack on
,C+
0
formic acid
H \ C$0
-
I
,C*
I
oxidation
H-C-H 1 HO
acrylic acid
0
OH hydroquinone
co,
H
I HO-C-H
H-C-ti I
0
H
decarb-
-
okvfation
I H-C-H I
HOHC*O
HOyC*O
3-hydroxy propanoic acid
malonic acid
acetic acid
HO/'*O
HO/'*O
glyoxylic acid
oxalic acid
Figure 4. Reaction pathway for the production of acetic acid.
phenol
I uu
Es catechol
o Excess Oxvgen Near Stoichiometry
801
A Excess Phenol
o-benzoquinone 60
25
aJ .-U
o Excess Oxygen
X
.-0 n
Near Stoichiometry A Excess Phenol
5m
151
0
1
40
20
I
L
--
Ib
-
3b
io
210 io $0 Reaction time (min) Figure 5. Carbon dioxide production history at 200 "C for three different reactant ratios. O
G +-*
4b
2b 3b 5b 60 React ion ti me ( rni n) Figure 3. Concentration histories of oxalic acid* at 200 OC for three different reactant ratios.
pattern is repeated for conditions of excess phenol, but is also much less pronounced than for excess oxygen. Since the response of the HPLC detedor is assumed the same for each compound (glyoxal, glyoxylic acid, and oxalic acid) the concentration profiles of Figure 3 cannot be explained by interconversion of these three components. Thus one is forced to provide for a pathway similar to that proposed by Gould et al. (1976) for the degradation of these products; that is, glyoxylic acid can be degraded to carbon dioxide without passing through oxalic acid. The formation and possible degradation pathways for these compounds is shown as part of Figure 6. Malonic, propionic, and succinic acid were identified only under conditions of limited oxygen (Table 111). Malonic acid appeared in amounts sufficient to quantify, but succinic and propionic acids were in trace amounts only. The presence of these acids in the reaction products was unexpected because they are not conjugated. Acetic acid also fits into this unconjugated group but as can be seen from Figures 1 and 2 is present as a major stable product under all conditions used in this work. Malonic acid, like other @-ketoacids undergoes decarboxylation to produce acetic acid and carbon dioxide. A possible reaction pathway for the production of acetic acid is shown in Figure 4. To produce succinic acid requires the hydrogenation of the carbon to carbon double bonds in maleic acid, which would not be expected to occur to a significant degree under the oxidizing conditions prevailing for the majority
lb
of the present work. The formation of these two acids was considered as side reactions. Carbon Dioxide Formation The rate of formation of carbon dioxide was calculated by difference from material balances, assuming that all carbon not accounted for was carbon dioxide. Carbon dioxide production for the three reactant ratios is shown in Figure 5. The rate of formation of carbon dioxide is dependent on the phenol to oxygen ratio but in each case it was produced from the start of the reaction, which indicates that some mechanism for carbon dioxide formation must occur early in the reaction pathway. This agrees with the observations of Sadana et al. (1974) and Ahmad et al. (1970). The proposed mechanism based on the previous discussion is shown in Figure 6. The six-carbon products formed from the opening of the benzene ring are shown as the dicarboxylic acids-muconic acid and 2,5-dioxo-3hexenedioic acid. These two compounds could have been shown in dicarbonyl and carbonyl-carboxylic acid form. No evidence exists from this or other work on the exact nature of the six carbon products; hence the choice was arbitrary. It is believed the oxidation pathway of any of the alternatives would be materially the same. Fumaric acid has not been included in the pathway since it will be in equilibrium with its isomer, maleic acid. Fumaric acid was found in only trace amounts in this work. Maleic acid, the cis isomer is expected from benzene ring opening. Other compounds which have been included in the pathway but were not isolated in the reaction products are shown in Figure 7. Verification of the Reaction Pathway A single experiment was conducted at 200 O C using a 100 mg/L acrylic acid solution and an oxygen concentra-
Ind. E .
.)em. Fundam., Vol. 23, No. 4, 1984
391
OH
phenol OH
OH I
catechol OH hydroquinone
1
H H
HO, I I 4C-C-C-H 0 I 1
+ C02
H H
succinic acid
3-hydroxy-propanoic acid
malonic acid
H,O
C0-,
H-C
c-L-c
