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Reaction Pathways in Pentachlorophenol Synthesis. 2. Isothermal Reaction Jianli Yu and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
We performed isothermal pentachlorophenol synthesis experiments to obtain data useful for kinetics analysis and reaction pathway resolution. The rate of pentachlorophenol production was not a monotonic function of temperature, and no pentachlorophenol was ever observed before the tetrachlorophenol yield reached its maximum value. The results were consistent with a reaction network wherein trichlorophenol forms an intermediate product, which then forms tetrachlorophenol. Pentachlorophenol then forms from tetrachlorophenol through an autocatalytic pathway. Microcontaminants produced during pentachlorophenol synthesis included hexachlorobenzene and hexa, hepta, and octachlorodibenzodioxins. The total amount of microcontaminants at a given temperature initially increased with time, and at the higher temperatures reached a maximum and decreased. The total amount of microcontaminants was lower at a lower temperature, for a given pentachlorophenol yield. Finally, these experiments produced pentachlorophenol in roughly the same yield and with roughly the same microcontaminant level of commercially produced pentachlorophenol, which indicates a general correspondence between the laboratory experiments and the plant reactor, though the two systems operate differently. Introduction Pentachlorophenol, a wood preservative, is made commercially by the direct chlorination of phenol in a liquid-phase reaction. This synthesis reaction also leads to the formation of trace amounts of undesired microcontaminants (e.g., chlorinated dibenzodioxins and dibenzofurans). A better understanding of the chlorination reaction pathways and kinetics could lead to strategies for intercepting the formation of these microcontaminants and reducing their levels in the final product. The first paper1 in this series presented results from laboratory-scale temperature-programmed synthesis, which mimics the manner in which the plant reactor operates. This earlier study provided some insights into what variables affect mass transfer rates and microcontaminant levels, but it did not provide data useful for kinetics or reaction pathway resolution. Isothermal reaction data are convenient for performing kinetics analysis. Therefore, we initiated a set of experiments in which the reactor temperature was nominally constant during the synthesis. We recognize that results from these isothermal experiments will not be directly applicable to plant operations, but to accomplish scientific research objectives, one must often depart from the procedures and conditions used in the commercial plant. The reason for this departure is that the commercialscale reactor and the laboratory-scale reactor were designed and operated with different objectives in view. During the isothermal laboratory-scale experiments, the reaction mixture eventually solidifies when there has been sufficient chlorination to raise the melting point of the mixture above the set point temperature. This solidification marked the end of the experiment. To our knowledge, this study marks the first report of isothermal, solvent-free, pentachlorophenol synthesis reactions. * To whom correspondence should be addressed. Phone: (734) 764-3386. Fax: (734) 763-0459. E-mail: psavage@ umich.edu.
Experimental Section The reactor system, its operation, and the analytical methods were described fully in the first paper1 in this series. We give only a brief overview here. Chlorination occurred in a 500 mL round-bottom flask. The reaction was done batchwise with respect to the chlorophenols. Chlorine gas was continuously bubbled through the reactor via two spargers with 20-50 µm openings. 2,4,6Trichlorophenol served as the initial reactor charge and AlCl3 was the catalyst. Starting with trichlorophenol (rather than phenol) in these experiments saves laboratory time and reagents and is common laboratory practice in the industry. All chemicals were purchased commercially and used as received. The trichlorophenol and catalyst were brought up to the desired reaction temperature, and then the chlorine gas flow began at the desired flow rate. We explored reaction temperatures of 105, 125, 145, 165, and 188 °C. The chlorination is exothermic. We maintained nearly isothermal reaction conditions ((2 °C) by periodically flowing cool air through a Teflon tube between the heating mantle and the bottom of the reaction flask. We used a chlorine flow rate of 2.5 mol/h to run nominally isothermal reactions at different temperatures. The stirring rate was set at about 220 rpm. We conducted experiments at 188 °C with higher chlorine flow rates and found that the results obtained were insensitive to the flow rate. Thus, the stirring rate (220 rpm) and chlorine addition rate selected (2.5 mol/h) led to reactor operation in the kinetics-controlled regime. The experiment terminated as soon as the reaction mixture solidified inside the reactor. Samples were removed from the reactor periodically, and they solidified almost immediately upon withdrawal from the reactor. These samples were analyzed for chlorophenols by gas chromatography (GC) with thermal conductivity detection. We used a liquid-liquid extraction procedure to remove the microcontaminants from the chlorophenol samples. The microcontaminant-
10.1021/ie0308285 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/27/2004
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Figure 2. Temporal variation of pentachlorophenol yields at different temperatures.
Figure 1. Temporal variation of chlorophenol yields: (a) 105°C; (b) 145°C; (c) 188 °C.
containing organic phase was then analyzed by GC with flame ionization and mass spectrometric detectors. Results from these analyses provided the molar yields of the different chlorophenols and the relative amounts and identities of the microcontaminants. Results and Discussion This section provides results for the temporal variations of the yields of different products. We first discuss the chlorophenols and a reaction network for their formation and then discuss the microcontaminants. Chlorophenols. Figure 1 shows the temporal variation of molar yields of penta-, tetra-, and trichlorophenol from nominally isothermal rections at 105, 145, and 188 °C. The trichlorophenol yield decreased monotoni-
cally with time, and the pentachlorophenol yield always increased with time, except near the end of the 188 °C run. The yield of tetrachlorophenol, the intermediate product, first increased with time, reached a maximum value, and then decreased with time. Interestingly, the pentachlorophenol yield was almost always zero until the time at which the tetrachlorphenol yield reached its maximum value. At this time, pentachlorophenol first appeared and its yield then increased as the tetrachlorophenol yield decreased. The highest pentachlorophenol yield was about 84%, which is very close to the 86% pentachlorophenol content in a commercially produced sample.1 The curves in these graphs for the chlorophenols show the trends one would expect for a sequential chlorination reaction network. Figure 2 shows the temporal variation of the pentachlorophenol yields obtained at the five different isothermal temperatures examined. The highest pentachlorophenol yield was obtained at the highest temperature and the highest pentachlorophenol yield at a given temperature increased as the temperature increased. This result is reasonable, because as the temperature increased, the reaction mixture remained in a molten state longer, so higher pentachlorophenol yields could be produced. The data also clearly show that the pentachlorophenol production rate is not a monotonic function of temperature. That is, the curves for the pentachlorophenol yields do not align themselves neatly with increasing temperature. Rather, the order for the yield at a given time is 105 °C < 145 °C < 188 °C < 125 °C < 165 °C. The pentachlorophenol yield at a given time is the highest at 165 °C rather than at 188 °C. To verify that the result in Figure 2 was generally true and not attributable to some experimental artifact, we did some additional experiments. One of these experiments involved assessing the reproducibility of the results. We did pentachlorophenol synthesis runs at a nominally isothermal temperature of 188 °C on several different dates. The results, which appear later in Figure 7c, are largely consistent and lead us to conclude that the experiments can be reproduced within reasonable bounds. To investigate this issue further, we examined the influence of temperature on the yields of tri- and tetrachlorophenol from the experiments at the five different isothermal temperatures. Figures 3 and 4 provide that information. The data for the disappearance of trichlorophenol (Figure 3) show that, in general,
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Figure 5. Temporal variation of unidentified products yields at different temperatures.
Figure 3. Temporal variation of trichlorophenol yields at different temperatures.
Figure 4. Temporal variation of tetrachlorophenol yields at different temperatures.
the rate of trichlorophenol consumption was slower at the lower temperatures and faster at the higher temperatures. This effect of temperature on trichlorophenol disappearance was small, however. The data for tetrachlorophenol (Figure 4) show that the maximum yield attained at a given temperature is a decreasing function of temperature. That is, the maximum in the tetrachlorophenol yield curve is highest for the run at 105 °C and it is lowest for the run at 188 °C. Also, the tetrachlorophenol yields at short times are lowest for the run at 188 °C, even though the trichlorophenol disappearance was fastest at this temperature. This behavior suggests that there exists a product from trichlorophenol consumption other than tetrachlorophenol. This product might be an intermediate that subsequently reacts to form tetrachlorophenol. Modest support for this hypothesis comes from an inspection of the initial slopes of the tetrachlorophenol yield vs time curves. One expects a primary product (one formed directly from the reactant) to display a yield vs time curve with a derivative (i.e., formation rate) that decreases as reaction time increases. The curves in Figure 4, however, show a nearly constant slope for the first 60 min of reaction, suggesting that tetrachlorophenol might not be a primary product. Additional support for the hypothesis comes from the data in Figure 5. Here we plot the molar yield of
unidentified products at each temperature as a function of time. This yield was calculated as 100% minus the sum of the yields of the three chlorophenols we quantified. It is a measure of the initial material that is not present in the three chlorophenols we quantified at any instant in time. These yields exceed 50% at 188 °C, so they can represent a significant fraction of the total material. It is known2 that products in addition to the three chlorophenols (e.g., chlorinated phenoxyphenols) are produced during the chlorination reaction. Products of this type could account for a portion (about 10% yield) of these unidentified products. Additionally, the experimental uncertainty in the yields of the chlorophenols is about 5-10%. Given this uncertainty, yields of “unidentified” products up to about 15% might be attributable to experimental error. The yields shown in Figure 5 are clearly higher than this level in most instances. Therefore, we believe that these data provide an estimate of the amount of product(s) formed that we were unable to identify and quantify directly. The initial portions of the curves in Figure 5 have derivatives that decrease with increasing time, indicating that the products represented here are primary products. That is, they arise directly from trichlorophenol. The curves also exhibit maxima, which indicates that they are reactive and form some other product(s). The maximum in the yield of unidentified product at a given temperature occurs earlier than does the maximum in the tetrachlorophenol yield at that temperature. This behavior is qualitatively consistent with the reaction mathematics for a reaction network in which trichlorophenol forms unidentified products, which subsequently form tetrachlorophenol. Having been led by the data to hypothesize the existence of an intermediate product between tri- and tetrachlorophenol, we turned to the archival chemical literature for information about its possible identity. Agnini and Casale3,4 report the existence of a pentachlorophenol isomer, 1,2,4,6,6-pentachlorocyclohexa-1,4dien-3-one, that they produced in high yield by the room-temperature chlorination of phenol in carbon tetrachloride. de la Mare5 later showed that cyclohexadienone intermediates are the initial products from the addition of a chlorine atom to an aromatic ring. The chlorinated cyclohexadienones can then isomerize to form the corresponding chlorophenols. On the basis of this literature evidence, we find support for the existence of a tetrachlorocyclohexadienone intermediate between tri- and tetrachlorophenol. By analogy, then, one would expect a chlorinated cyclohexadienone inter-
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Figure 6. Autocatalytic reaction network for pentachlorophenol synthesis.
mediate to appear between tetra- and pentachlorophenol. Interestingly, chlorinated cyclohexadienones can catalyze chlorophenol coupling reactions to form chlorinated dioxins.6,7 The interpretation of the experimental data for the chlorophenols and unidentified products, along with the accompanying discussion of the relevant chemical literature, suggests the presence of several specific features in the chlorination reaction network. Trichlorophenol forms some product(s) other than or in addition to tetrachlorophenol in a primary reaction path. These unidentified products react further. Tetrachlorophenol is not a primary product, but rather forms, at least in part, via a secondary reaction path. Tetrachlorophenol likely reacts to form an intermediate product, which subsequently converts to pentachlorophenol. Pentachlorophenol is not unreactive. Once formed, this desired product can react away as evidenced by the maxima in the pentachlorophenol yields that appeared at 188 °C. Finally, each of the chlorinated phenols is capable of participating in coupling reactions, which can be catalyzed by the dienone intermediates.6,7 In the next section, we present a reaction network that possesses these features and assess its ability to describe quantitatively the experimental data. Chlorination Network. To verify that simple, irreversible, first-order, series-type reactions, where the starting material, trichlorophenol, converts exclusively to tetrachlorophenol, which then forms pentachlorophenol, were inadequate for describing the experimental data, we used a series reaction model to fit the experimental concentration profiles. The results indicated that such a model consistently overestimated the yields of tetra- and pentachlorophenol at short reaction times, failed to capture the rapid decrease of tetrachlorophenol and corresponding rapid increase in pentachlorophenol at longer times, and failed to capture the pentachlorophenol yield remaining at essentially zero until the tetrachlorophenol yield reaches its maximum value. Instead, the series reaction model displayed a slow increase in the yield and continuous production of pentachlorophenol. A different type of reaction network is required to capture the observed behavior. We offer the reaction pathways in Figure 6 as one possible reaction network. This scheme postulates the existence of products X1 and X2, which are intermediates between tri- and tetrachlorophenol and between tetraand pentachlorophenol, respectively. We did not identify these products but infer their existence from the experimental data and the chemical literature. The network also postulates the existence of an autocatalytic pathway for production of pentachlorophenol from tet-
rachlorophenol. We use Y to designate the autocatalyst. We found that the inclusion of autocatalysis was essential to fit the data for tetrachlorophenol disappearance and pentachlorophenol appearance. These curves are too steep to be described by a first-order series reaction. Additionally, we found that including pathways from the different chlorophenols to coupling products did not improve the agreement between the data and the model. Therefore, these paths, though certainly occurring, are not treated explicitly in the model. According to this network and assuming that reaction orders match stoichiometries and that the chlorine and catalyst concentrations remain constant throughout the experiment, the differential equations describing the temporal variations of the concentrations of the different species are
d[Tri] ) -k1[Tri] dt d[X1] ) k1[Tri] - k2[X1] dt d[Tetra] ) k2[X1] - k3[Tetra] - k6[Tetra][Y] dt d[X2] ) k3[Tetra] - k4[X2] dt d[Y] ) k4[X2] dt d[Penta] ) k4[X2] + k6[Tetra][Y] - k5[Penta] dt We solved this set of equations numerically and simultaneously performed parameter estimation to find values of the six rate constants that minimized the sum of the squared differences between calculated and experimental molar yields of tri-, tetra-, and pentachlorophenol. We employed Euler’s method to solve the differential equations and performed the parameter estimation using the Solver routine in Microsoft Excel. Table 1 displays the numerical values of the rate constants obtained from data at different temperatures. The parameter k1 shows little variation with temperature, whereas k2 decreases with temperature in this range. Neither of these trends is consistent with the Arrhenius equation, but this relationship does not always apply when complex, multistep reactions are modeled as single global steps. Moreover, the solubility of chlorine in the molten solution is likely temperature dependent, and the concentration of this species is embedded within the global rate coefficients. The parameters k3 and k6[Tri]0 were very strongly correlated and exhibited a compensation effect. That is, a large change in k3 in one direction could compensate for a large change in the value of k6[Tri]0 in the opposite direction. The minimum in the objective function appears to be quite shallow with respect to even order-ofmagnitude variations in these two parameters. In summary, the parameters in Table 1 are simply those that provide the best fit of the experimental data at the different temperatures. They should not be viewed as possessing any chemical significance.
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Table 1. Optimized Values of Kinetic Parameters in the Autocatalytic Reaction Network param (h-1)
105 °C
125 °C
145 °C
165 °C
188 °C
k1 k2 k3 k4 k5 k6[Tri]0
1.6 2.6 3.0 × 10-3 5.2 0 2.2 × 102
2.3 3.6 2.0 × 10-4 1.2 0 2.4 × 104
1.6 1.8 1.6 × 10-4 1.7 0.029 1.6 × 104
2.4 2.2 1.7 × 10-4 0.086 0.086 6.2 × 105
2.3 1.0 7.2 × 10-5 7.2 × 10-5 0.026 2.1 × 109
Figure 8. Temporal variation of the total amount of microcontaminants at different temperatures.
Figure 7. Comparison of experimental results from the autocatalytic reaction network: (a) 105 °C; (b) 145 °C; (c) 188 °C.
Figure 7 compares model and experimental results for synthesis runs at 105, 145, and 188 °C. The data at 188 °C are experimental results from four independent runs. These replicated experiments show that the results are largely consistent from run to run and that the results are reproducible. The figures show that the model does a good job of capturing the trends in the data. These trends could not be captured by a series reaction model without intermediate products (Xi) and without autocatalysis. Therefore, we take the model
presented herein to be consistent with Occam’s razor. That is, it is the least complex model we considered that can describe the experimental data. The experimental data and this kinetics analysis suggest that phenol chlorination might be autocatalytic. We are not aware of any previous work intimating autocatalysis for this reaction, but the present results show that this feature, or perhaps some other competing network features that could describe the apparent induction time and subsequent rapid increase in pentachlorophenol yield, are worthy of additional investigation. The chemical literature8,9 indicates that the chlorination of aromatics is an electrophilic substitution reaction. The active electrophile is a complex between AlCl3 and Cl2. Electronic-structure calculations have shown that this complex remains intact throughout the reaction. The reaction coordinate for chlorination of benzene has been determined, and it includes two main transition states, two π complexes, and one σ complex.8 The rate-determining step is conversion of the first π complex into the first σ complex. We are aware of no similar work having been published for the chlorination of phenol or chlorophenols. Thus, the literature offers insufficient guidance on mechanistic details to speculate as to the identity of autocatalyst Y in the reaction network. Nevertheless, postulating its existence provides a means to explain the isothermal reaction data. Microcontaminants. All of the data presented thus far have been for the chlorophenols. We now present information on the microcontaminant levels from different experiments. We present these results in terms of the total chromatogram peak area for all extracted products in each run. We used the extraction and analysis procedures reported previously.1 Figure 8 shows the total peak area of all extractable compounds (viewed collectively as an indicator of the
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Figure 9. Variation of the total amount of microcontaminants with pentachlorophenol yield at different temperatures. Figure 11. Temporal variation of heptachlorodibenzodioxin amount at different temperatures.
Figure 10. Temporal variation of hexachlorobenzene amount at different temperatures (peak areas at 105 and 125 °C were too small to quantify).
microcontaminant level) as a function of the reaction time for experiments at different temperatures. We also show, for comparison, the total peak area for microcontaminants in a representative sample of commercially produced (Vulcan Chemical) flaked pentachlorophenol. At the lower temperatures (105 and 125 °C), the total peak area slowly increased with increasing reaction time. At the higher temperatures (145, 165, and 188 °C), the total peak area first increased and then decreased with time. This behavior suggests that some of the peaks corresponded to intermediates that were formed early in the reaction and then consumed at longer times. At 188 °C, the total peak area displays the same trend as it does at the other temperatures, but it increases sharply at the end of reaction. The reason for this increase at the end of the reaction (at 188 °C) is possibly over chlorination, which results in more byproducts being formed. Figure 9 shows the same data plotted as a function of the pentachlorophenol yield. From this plot we learn that for a given pentachlorophenol yield, micrcontaminant levels are generally lower at the lower temperatures. Additionally, these experiments produced pentachlorophenol with a total microcontaminant level comparable to that present in the commercial product. Figures 10-12 show peak areas as a function of the reaction time for three specific individual microcontaminants. Each of these individual peaks (microcontami-
Figure 12. Temporal variation of octachlorodibenzodioxin amount at different temperatures.
nants) was also present in the Vulcan sample we used for comparison. The dashed line in each figure provides the corresponding peak area found in the extract from the flaked pentachlorophenol sample from Vulcan. No hexachlorobenzene (Figure 10) was detected in the samples at 105 and 125 °C. At the other temperatures, the amount of this microcontaminant increased with time. The hexachlorobenzene level at a given time was an increasing function of temperature. The peak area for one of the heptachlorodibenzodioxins (Figure 11) increased with time, reached a maximum, and then decreased to very low levels. This microcontaminant appears to react away as the main chlorination reaction proceeds. The amount of octachlorodibenzodioxin (Figure 12) was also a function of temperature, with the highest levels occurring at the highest reaction temperature. We note that this peak could also include octachlorodibenzofuran, if present. Conclusions The reaction network for pentachlorophenol synthesis from 2,4,6-trichlorophenol includes a pathway from trichlorophenol to a product other than tetrachlorophenol and perhaps autocatalytic formation of pentachlo-
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rophenol. Quantitative comparison of the experimental data with the reaction mathematics accompanying this reaction network showed reasonably good correspondence. Reaction temperature has a positive correlation with the total amount of microcontaminants and with the amount of specific microcontaminants present in pentachlorophenol. Developing practical means to make pentachlorophenol in high yields but at temperatures lower than its melting point (about 190 °C) could lead to a product with a lower level of microcontaminants. To this end, it is significant that pentachlorophenol can be synthesized in almost 40% yield by direct chlorination at 105 °C, before the reaction mixture solidifies. Developing a practical means of keeping the chlorophenols in a liquid phase at lower temperatures may be a path to an improved pentachlorophenol synthesis process. Acknowledgment We thank Buffy Branam and Terry Nestrick for experimental and analytical assistance. David Hildebrand provided valuable guidance regarding the identities of the chlorinated intermediates. We also gratefully acknowledge financial support from the Pentachlorophenol Task Force, an industry group that sponsors research related to pentachlorophenol.
Literature Cited (1) Yu, J.; Savage, P. E. Reaction Pathways in Chlorophenol Synthesis. 1. Temperature-Programmed Reaction. Ind. Eng. Chem. Res. 2004, 43, 5021-5026. (2) Hildebrand, D. (Vulcan Chemicals). Personal communication, 2003. (3) Agnini, G.; Casale, L. Isomer of Pentachlorophenol. Patent Specification 721,929, London Patent Office, 1955. (4) Agnini, G.; Casale, L. Note sulla formazione di un isomero del pentaclorofenolo e sulla specifica differenza di azione dell’ AlCl3 dal FeCl3 nella clorurazione del fenolo. Chim. Ind. (Milan) 1951, 33, 490-491. (5) de la Mare, P. B. D. Pathways in Electrophilic Aromatic Substitutions. Cyclohexadienes and Related Compounds as Intermediates in Halogenation. Acc. Chem. Res. 1974, 7, 361-368. (6) Kulka, M. Octahalogenodibenzo-p-dioxins. Can. J. Chem. 1961, 39, 1973-1976. (7) Kulka, M. Preparation of Polyhalodibenzo-p-dioxins. U.S. Patent 3,251,859, 1966. (8) Osamura, Y.; Terada, K.; Kobayashi, Y.; Okazaki, R.; Ishiyama, Y. A Molecular Orbital Study of the Mechanism of Chlorination Reaction of Benzene Catalyzed by Lewis Acid. THEOCHEM 1999, 461-462, 399-416. (9) Olah, G. A.; Renner, R.; Schilling, P.; Mo, Y. K. Electrophilic Reactions at Single Bonds. XVII. SbF5, AlCl3 and AgSbF6 Catalyzed Chlorination and Chlorolysis of Alkanes and Cycloalkanes. J. Am. Chem. Soc. 1973, 95, 7686-7692.
Received for review November 11, 2003 Revised manuscript received June 14, 2004 Accepted June 28, 2004 IE0308285
