Ind. Eng. Chem. Res. 1994,33, 736-739
736
Decomposition of Polyolefins and Higher Paraffins by Wet Oxidation A n n e Belinda Bjerre and Emil Sorensen* Rise National Laboratory, DK-4OOO Roskilde, Denmark
A selection of polyolefins and other paraffins has been studied with respect to the mechanism of their decomposition by wet oxidation in a batch autoclave reactor. The general conditions were 2.0 MPa of 0 2 a t 260 "C. Virtually all hydrocarbons were decomposed into water-soluble compounds and Con. An analysis of the products indicates that the decomposition proceeds mainly by fragmentation of one or two C-atoms at a time. The yield of the different products was dependent, to some extent, on the type of starting material. A reaction mechanism has been proposed to explain the intriguing occurrence of significant amounts of hydrogen in the gas phase. Introduction During our study of soil recovery by wet oxidation (Smensen et al., 1990), it has proved useful to classify pollutants according to their mode of reaction. One such class is the group of paraffins which in soil frequently arises from oil or gasoline spills. Also belonging to this group are some plastic polymers which form a significant part of household refuse. In the literature the greatest importance is attached to biological processes as regards the oxidation of petroleum hydrocarbons (Gunnison, 19911,whereas the main concern with polymers is the prevention of their degradation. The experiments cited usually have been carried out at moderate temperature on solid samples in the shape of films (Bovey et al., 1978; Decker et al., 1979; Reich and Stivala, 1983; Henry and Garton, 1990). In the present work we try to find a general scheme for the reactions of a wide range of paraffins with oxygen in water at 260 "C. Experimental Section The following materials were used: polyethylene (LDPE) (powder or granules); cross-linked polyethylene (PEX) (powderor granules);polypropylene (isotacticPP) (powder or granules); paraffin wax (Merck, mp 52-54 "C, =C24c26); n-dodecane (Koch-Light puriss.); n-pentane and 2-methylbutane (Fluka puriss.); propane (99.95%). Figure 1shows the apparatus used. It has been described previously (Sorensen et al., 19901, but the main features are repeated here. In a cylindrical vessel an impeller mounted on the lid pumps the liquid through a pipe loop. The autoclave is mounted on a rack, which makes it possible to adjust the temperature by raising or lowering it in a heating bath. After the prescribed reaction time has elapsed, the autoclave is moved to a water bath for cooling. Due to the excellent heat-transfer conditions, the heating and coolingperiods are short as shown in Figure 2. A gas valve is provided, but for liquid sampling the lid must be removed. The autoclave is made of Sandvik Sanicro 28 (27% Cr, 31% Ni, 3.5% Mo, 1%Cu). General Procedure. The reactor containing 1000 mL of deionized water was charged with the paraffin to be oxidized. NaOH may be added to regulate the pH. The lid was closed, oxygen pressure was supplied from an ordinary cylinder, and the charge was submitted to the experimental conditions listed in Table 1. The reaction times cited represent the period at which the reaction temperature was maintained, neglecting the short heating and cooling periods. This is justified by the rapid fall in
Figure 1. Autoclave with tubular loop and impeller. 300
o^ e
z
200
W
W
I 0
l
0
t
2
t
l
"
4
'
l
l
'
l
6 8 1 TIME (min)
t
l
0
1
l
2
1
4
Figure 2. Heating and cooling profile.
reaction velocity with temperature. The indication 10 + 10 + 10 min means that after 10 min a sample was taken (gas and liquid) whereupon the reactor was sealed, supplied with 0 2 , and run for another 10 min, etc. Analysis. The gas composition was determined by gas chromatography (GC) with hot-wire detection and mass spectrometry (MS) with gas inlet. The liquid was analyzed for chemical oxygen demand (COD) by the dichromate method. Low molecular weight acids were determined by ion chromatography.
0888-5885/9412633-0736$04.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 737 Table 3
Table 1 expt 1 2 3
4 5 6 7 8 9 10 11 12
charge 10 g of LDPE 10 g of LDPE 10 g of LDPE + 26.7 g of NaOH 10 g of PEX 10 g of PEX 10 g of PP 10 g of paraffin wax 10 g of n-dodecane 5 g of n-dodecane 10 g of n-pentane 10 g of 2-methylbutane 3.9 g of propane
pop, MPa
reaction time, min
temp, OC
30 10 + 10 + 10 30
260 260 260
2 2 2
30
30
260 260 260 260
2 2 2 2
40 30 30 30
240-260 240-260 240-260 240-260
1 2 2 1.8
+
10 + 10 10 10 + 10 + 10
Table 2 time, min 10 20 30 time, min 10 20 30
H2 1.3 1.5 0.4
NP 7.0 3.3 2.6
gas composition, % CO 02 9.2 59 6.0 71 0 90
liquid composition (acids), ppm formic acetic glycolic 1050 2275 933 588 3500 1250 100 3525 825
AI
coz
0.08 0.05 0.03
24 18.5 7.3
pH 2.64 2.74 3.03
COD, mgof Oz/L
time, min 10 20 30 time, min 10 20 30
Hz 2.5 1.8 0.65
Nz 6.2 4.3 3.7
gas composition, % CO 0 2 Ar 12.5 50.4 0.84 5.1 71.9 0.5 0.5 89.4 0.43
liquid composition (acids), ppm formic acetic glycolic 1190 868 166
1430 2530 2580
484 lo00 713
COz 28.2 16.9 5.7
pH
COD, mgof Oz/L
2.48 2.56 2.94
3800
Table 4 time, min 10 20 30 time, min 10 20 30
gas composition, % H2 1.1 1.0 0.8
Nz
CO
02
Ar
COz
8.5 3.5 3.4
6.8 2.0 0.4
54.7 80.3 90.3
0.1 0.05 0.05
28.9 13.2 4.7
liquid composition (acids),ppm formic acetic nlvcolic 963 2607 34 654 4308 104 283 4740 153
DH
COD, mn of OdL
2.60 2.69 2.84
6600
Table 5 5100
Results Experiment 1. The resulting liquid was smoke-colored and slightly turbid. After filtration on a Millipore filter ( 2.
-
-CH2CHO
+
4
/
-CH2CHCH2-
I
I
J
recombine
-CH$CH2-
‘CH2-
+ OH’
+ H20
6 ketone -CH2COOH acid
Th Dreferred site of attack are tertiarv hvdroeen and secondary positions next to the end of ;chain. ?he very reactive alkoxy radicals can undergo a @-scissionto generate a carbonyl compound and an alkyl radical. The bulkier alkyl group tends to be the departing radical. If the alkoxy radical is primary, formaldehyde is generated. If it is secondary, next to the end of a chain, the carbonyl product is acetaldehyde. RCHR’ I 0‘
-
RCHO
+ R”
These aldehydes will subsequently oxidize to the corresponding acids. Liquid-phase oxidation (not wet oxidation) of light hydrocarbons has been used technically for the manufacture of acetic acid (Saunby and Kiff, 1976). The formation of glycolicacidis more difficult to explain, but it appears regularly as a not negligible product. Other hydroxy acids have not been detected. They probably decompose under the reaction conditions. Pyruvic and oxalic acids are known to decarboxylate. Referring to Table 9, it is remarkable how the product distribution keeps within narrow limits for the higher paraffins. Still, cross-linking in PE gives rise to a markedly higher proportion of CI in the oxidation products and a low percentage of compounds larger than Cp. It seems unlikely that this should be due to the change in chemical structure, affecting perhaps 1out of 70 C-atoms. More conspicuous is the shift in physical properties which means that the polymer remains solid under reaction conditions where the other materials are melted and undergo vigorous dispersion. For polypropylene the main feature is the formation of more acetic acid at the expense of glycolic acid. The side chains do not accelerate the reaction. On the contrary, more material with large molecules is left, as seen from
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 739 the measured COD and the darker appearence of the solution. The same is the case in experiment 3 under alkaline conditions, many carboxyl compounds being stabilized at the elevated pH (Bjerre and Sorensen, 1990). In experiment 7 the high residual COD after 30 min is due to the fact that the 2 MPA of 0 2 has been introduced only once rather than three times as in other cases (at 10,20, and 30 rnin). It must be noted here that formic acid is an important intermediate. In an acid environment it quickly passes on to C02 (Bjerre and Sorensen, 1992). With shorter chains as in the dodecane case, the CZyield is relatively high and C3 is also seen for the first time in significant amounts, namely as propionic acid. This is understandable on statistical grounds since the stub is a significant part of a short chain and the survival of propionic acid for 30 min is fairly probable, whereas butyric acid is not detected. The rate constants are for acetic acid, 0.0010; propionic acid, 0.0022; and for butyric acid, 0.0046 (Bjerre and Smensen, 1990). The unidentified products are considered to be mediumsized oxygen-containing species which must have a considerable resistance toward further oxidation. The occasional detection of benzoic acid shows that cyclization may also occur. The presence of H2 and CO in the strongly oxidizing environment is somewhat intriguing. It must be assumed that H' is generated from a member of the reaction chain, e.g., H
I
H
I
-CH&--CCHz-
I
H
I
I
I
II
H
-
H
-CHzCHO
+ 6H
4
O
-CH&Hz
0'
-CH&O
0-
I I
RCHO or
R k O
+ OH-
-
0.-
I
RCOH
which prevents the abstraction of CO. The runaway reaction occurring with dodecane at 260 "C is similar to the autoignition in gasoline engines, but it is rarely seen to start as low as 260 "C (Miller and Fisk, 1987). Apart from this the oxidation passes off much more quickly in the aqueous phase although both fuel and 02 are more abundant in the gas phase. The effect of the aqueous medium is supposed to be promoting the hydroperoxide decomposition to radical products (Henry and Garton, 1990). It appears that with shorter chains the stability of the paraffins increases, so that a faint reaction is seen with Cg but not with C3.
Acknowledgment The authors thank H. Egsgaard for the execution of the comprehensive analysis work and for valuable comments on the interpretation of the results. Literature Cited
Bjerre, A. B.; Ssrensen, E.Thermal Decompositionof Dilute Aqueous Formic Acid Solutions. Ind. Eng. Chem. Res. 1992, 31, 1574. Bovey, F. A,; Schilling, F. C.; Cheng, H. N. Stabilisation and Degradation of Polymers. Adu. Chem. Ser. 1978, 169, 133. Decker, C.; Mayo, F. R.; Richardson, H. Aging and Degradation of Polyolefins. 111. Polyethylene and Ethylene-Propylene Copolymers. J. Polym. Sci.; Polym. Chem. Ed. 1973, 11, 2879. Gunnison, D. "Evaluation of the potential use of microorganisms in the cleanup of petroleum hydrocarbon spills in soil"; WES/TR/
EL-91-13;1991. Henry, J. L.; Garton, A. Kinetics of PolyolefinsOxidation in Aqueous Media. Polym. Mater. Sci. Eng. 1990, 63, 277. Miller, J. A.; Fisk, G. A. Combustion Chemistry, Special Report. Chem. Eng. New 1987, Aug, 22-46. Reich, L.; Stivala, S. S. Elements of Polymer Degradation; McGraw Hill Inc.: New York, 1983. Saunby, J. B.; Kiff, B. W. Liquid-phase Oxidation. .Hydrocarbons to Petrochemical. Hydrocarbon Process. 1976, Nov, 247-252. Ssrensen, E.;Bjerre, A. B.; Rasmussen, E. Soil Recovery by Wet Oxidation. Enuiron. Technol. Lett. 1990, 1 1 , 429.
.
Receiued for review June 15, 1993 Revised manuscript received October 5 , 1993 Accepted December 8,1993O
+ HCCYII
0
+ HzO;
-
H
H
I 1 -CHzC-CCHzI 1
+ OH-
R-577;1990.
+
--c-CHzC--CCY-
and that H' subsequently abstracts another H to form H2. By comparing experiments 1and 2 or 4 and 5 it is seen that treating for 30 min produces more hydrogen than for 3 X 10 min, where the same material is supplied each time with a fresh charge of 0 2 . This indicates that H' has less chance to survive the higher the oxygen pressure. With decreasing chain length the hydrogen formation also decreases, which is attributed to the fewer degrees of freedom in the smaller molecules. It seems natural that CO might be produced by way of formic acid, but HCOOH is known to oxidize rapidly to CO2 under the conditions prevailing in these experiments without releasing CO (Bjerre and Smensen, 1992) so another route must be imagined: H
RCHO
Bjerre, A. B.; Ssrensen, E."Thermal and oxidative decomposition of lower fatty acids with special attention to formic acid"; Riss-
H
0'
The distinctive suppression of CO formation at high [OH-] in the aqueous phase which appears in experiment 3 indicates that the configuration of the active species is changed by a reaction like
-CHz60
-
-6Hz
+CO
Abstract published in Aduance ACS Abstracts, February
15, 1994.
