Microcontaminants in Pentachlorophenol Synthesis. 2. Effects of

Jun 27, 2006 - Using catalyst concentrations below 0.25 wt % leads to a marked increase in the toxic equivalent (TEQ) concentration of PCDDs and PCDFs...
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Ind. Eng. Chem. Res. 2006, 45, 5205-5210

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Microcontaminants in Pentachlorophenol Synthesis. 2. Effects of Catalyst Identity, Concentration, and Addition Strategy Jianli Yu, Terry J. Nestrick,† and Phillip E. Savage* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136

Polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) are undesired byproducts that form as microcontaminants during the synthesis of pentachlorophenol. Using catalyst concentrations below 0.25 wt % leads to a marked increase in the toxic equivalent (TEQ) concentration of PCDDs and PCDFs in the pentachlorophenol product. With a catalyst concentration of 0.5 wt %, the TEQ concentration in the product was largely insensitive to the catalyst identity (AlCl3 vs FeCl3) and to the catalyst addition strategy (adding all at once or adding it incrementally). These results suggest that the microcontaminant-forming reactions are not catalyzed by the Lewis acid, which catalyzes the desired chlorination reaction. Thus, one must examine other process or reaction variables to identify the means to influence the microcontaminant level in pentachlorophenol. Introduction Pentachlorophenol is a wood preservative. It is manufactured commercially by the AlCl3-catalyzed chlorination of phenol or chlorophenol mixtures in the liquid phase.1 No solvent is used, so the reaction temperature must be maintained above the freezing point of the reactor contents, which increases from 41 °C (melting point of phenol) to about 191 °C (melting point of pentachlorophenol) as the chlorination progresses. The commercial synthesis of pentachlorophenol is accompanied by the production of small (parts per million; ppm) amounts of microcontaminants (i.e., polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) with six or more chlorine substituents).2 These compounds are thought3 to form via coupling reactions involving phenoxy radicals. The phenoxy radical adds to a polychlorophenol compound to form phenoxyphenols, which then react further to form PCDDs. The phenoxy radicals form from decomposition of polychlorocyclohexadienones produced from overchlorination of tri-, tetra-, and pentachlorophenol. Domestic pentachlorophenol manufacturers have invested considerable effort and resources into reducing the level of microcontaminants in the pentachlorophenol product, but even further reduction is desired from an environmental perspective. These efforts have been hampered, in part, by the lack of a convenient and inexpensive means of assessing the toxicity of the microcontaminants in technical-grade pentachlorophenol. High-resolution gas chromatography with mass spectrometric detection has been the standard method for determining the toxic equivalent (TEQ) concentration of PCDFs and PCDDs in a pentachlorophenol sample. These analyses reveal that a typical sample of pentachlorophenol produced commercially could contain about 103, 102, and 10 ppm, respectively, of octa-, hepta-, and hexachlorodioxin, respectively.3 Octachlorodibenzodioxin has a much lower toxic equivalency factor, however, so the latter two compounds would often be the main contributors to toxicity in commercially produced pentachlorophenol. We recently described a new method, based on a bioassay developed for dioxins, and demonstrated its viability for determining the TEQ concentration * To whom correspondence should be addressed. Tel.: (734) 7643386. Fax: (734) 763-0459. E-mail: [email protected]. † Present address: 4520 Washington St., Midland, MI 48642.

in pentachlorophenol samples.4 In the present article we use this method to determine whether the catalyst identity, concentration, and addition strategy influence the TEQ level in the pentachlorophenol product. If the formation of PCDDs and PCDFs occurs through catalyzed reactions, these variables may be important ones. This work represents the first in which a bioassay was used for assessing the effects of process variables on the TEQ concentration. Our focus in this article is on attempting to improve the commercial process with respect to microcontaminant formation. The focus is not on the underlying chemistry. This chemistry is important, of course, and as means to reduce the presence of microcontaminants are discovered, the chemistry responsible should be investigated as follow-up work. Experimental Section Previous articles have provided detailed descriptions of the experimental procedures and analytical methods.4-6 Therefore, we simply provide a brief outline of the experimental apparatus and methods in this article. Standard laboratory glassware was used to construct the semibatch reactor system. The synthesis experiments used a temperature-programmed reaction, which mimics commercial practice. Pentachlorophenol synthesis was conducted by continuously bubbling chlorine gas through a liquid phase containing, initially, about 300 g of molten 2,4,6-trichlorophenol. AlCl3 and FeCl3 served as the catalysts in the present investigations. The reactor temperature was gradually increased from 90 °C to about 190 °C during the synthesis to keep the reactor contents in the liquid phase at all times. The freezing point of the mixture increases as chlorination proceeds. Samples from the reactor were withdrawn periodically by inserting a glass rod into the reactor. The liquid on the rod quickly solidified when removed from the reactor and cooled to room temperature. At the end of the synthesis experiment, the reactor contents were poured into a disposable aluminum pan to cool. A sample of this postreaction solid product was also collected. Samples were analyzed chromatographically by capillary column gas chromatography with flame ionization detection to determine the amounts of tri-, tetra-, and pentachlorophenol. The microcontaminant level was determined by an aryl hydrocarbon receptor capture (AhRC) method that used real-time

10.1021/ie0605783 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/27/2006

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polymerase chain reaction (PCR) for quantification.4 This AhRC PCR immunoassay provided the TEQ concentration of PCDDs and PCDFs in the sample. Results reported herein are from a single analysis of each sample, which is expected to be accurate to within (40% of the reported value. Results and Discussion We explored three main variables associated with the catalyst used in the chlorination reactions. These were the catalyst identity (i.e., AlCl3 vs FeCl3), catalyst concentration (0.125, 0.25, and 0.50 wt %), and catalyst addition strategy (i.e., add all catalyst at once prior to starting the chlorine flow or add it incrementally throughout the course of the temperatureprogrammed reaction). We begin by discussing the first two variables and then conclude with results from experiments using different catalyst addition strategies. Effect of Catalyst Concentration: AlCl3. We conducted a set of temperature-programmed synthesis experiments to determine the effect that varying the concentration of the AlCl3 catalyst has on the reaction rate and TEQ concentration in the product. These experiments used catalyst concentrations of 0.125, 0.25, and 0.50 wt %. The temporal variation of the molar yields of tri-, tetra-, and pentachlorophenol appear in Figure 1. Regardless of the catalyst concentration used, the same general trends appear. The yield of tetrachlorophenol increases with time to a maximum and then decreases with time to zero. The yield of pentachlorophenol increases with time to a maximum value and then it too decreases at longer times. The maximum pentachlorophenol yields were achieved at 140, 204, and 265 min, respectively, for runs using 0.5, 0.25, and 0.125 wt % AlCl3 catalyst. These results show that using more catalyst produces more pentachlorophenol in a shorter amount of time. Also included in Figure 1 are the TEQ concentrations, as determined by the AhRC PCR method,3 for selected samples. The experiment with the highest catalyst concentration produced pentachlorophenol with the lowest TEQ concentration at the maximum pentachlorophenol yield. The run with the lowest catalyst concentration produced pentachlorophenol with the highest TEQ concentration. Figure 2 shows more explicitly how the concentration of AlCl3 catalyst added affected the TEQ concentration in the samples that contained the maximum tetra- and pentachlorophenol yields. Increasing the AlCl3 concentration from 0.125 to 0.5 wt % reduces the TEQ concentration at the maximum pentachlorophenol yield from 2.0 to 0.8 ppm. Likewise, the TEQ concentration at the maximum tetrachlorophenol yield is reduced from 1.2 to 0.6 ppm. Our typical experimental procedure uses 0.25 wt % AlCl3 catalyst. Reducing the catalyst concentration below this level seems to have an adverse impact on microcontaminant content. Thus, there seems to be little good accompanying a decrease in the amount of catalyst. The reaction is slower and the TEQ content is higher. Increasing beyond our typical concentration of 0.25 wt % may reduce the TEQ concentration, but it appears from Figure 2 that a catalyst concentration around 0.25 wt % may already be near a point of diminishing returns. Effect of Catalyst Concentration: FeCl3. Experiments similar to those discussed in the previous section were conducted with FeCl3 as the catalyst. In these experiments, however, we reduced the chlorine flow rate during the run from 2.5 to 2.1 and then to 1.7 mol/h. In the previous experiments the chlorine flow rate was constant at 2.5 mol/h. Reducing the chlorine flow allowed us to better control the reactor temperature during this

Figure 1. Temporal variation of chlorophenol molar yields with different catalyst concentrations: (a) 0.125 wt % AlCl3; (b) 0.25 wt % AlCl3; (c) 0.50 wt % AlCl3.

exothermic chlorination reaction. Hence, we used this approach in all subsequent experiments. After making this finding, we conducted a separate experiment with AlCl3, which confirmed that this reduction in the chlorine flow rate did not affect the TEQ concentration in the product. Figure 3 provides the temporal variations of the yields of tri-, tetra-, and pentachlorophenol for these experiments along with TEQ values for selected samples. Inspecting Figure 3a, one notes that, during synthesis with 0.125 wt % FeCl3, the yield of tetrachlorophenol increases with time to a maximum of 69% at 65 min, and then decreases with

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Figure 2. Effect of AlCl3 catalyst concentration on TEQ concentration at maximum tetra- and pentachlorophenol yields.

time to zero at 130 min. The pentachlorophenol yield increases with time to a maximum of 87% at 140 min. Figures 3b and 3c show very similar results for both the maximum yields and the times at which they occur. This similarity indicates that using different amounts of FeCl3 catalyst does not have much effect on the reaction rate. This result is in contrast to the behavior observed with AlCl3, wherein the rate increased with increased catalyst loading. Figure 3 also provides results for the TEQ concentration of different samples. In all cases, the TEQ concentration increased during the course of the reaction, and there were large increases (a factor of 3-5) during the final 10-20 min of the experiment. When 0.125 wt % FeCl3 was used, the TEQ concentration was 0.13 ppm at the maximum tetrachlorophenol yield (65 min), 0.93 ppm at 127 min, and 4.7 ppm at 140 min. When 0.25 wt % FeCl3 was used, the TEQ concentration was 0.1 ppm at the maximum tetrachlorophenol yield (65 min), 1.1 ppm at 121 min, 2.6 ppm at 126 min, and 3.2 ppm at 138 min. Similarly, when 0.50 wt % FeCl3 was used, the TEQ concentration was 0.44 ppm at the maximum tetrachlorophenol yield (60 min), 0.32 ppm at 130 min, 0.77 ppm at 140 min, and 1.68 ppm at 150 min. Figure 4 shows more explicitly how the concentration of FeCl3 catalyst affected the TEQ concentration at the maximum pentachlorophenol yields. Increasing the FeCl3 catalyst concentration from 0.125 to 0.50 wt % leads to the TEQ concentration at the maximum pentachlorophenol yield being reduced from 4.7 to 0.8 ppm. The TEQ concentration at the maximum tetrachlorophenol yield, however, did not show this trend. Rather, the highest catalyst concentration led to the highest TEQ concentration at this point in the reaction. The results in this section show that AlCl3 is a more desirable catalyst for pentachlorophenol synthesis than is FeCl3 because it provides a product with a lower TEQ concentration. Effect of Mixed Catalysts. Two experiments were conducted to investigate the effect of using two different catalysts together (AlCl3 and FeCl3) on pentachlorophenol synthesis. In one experiment, both catalysts (0.25 wt % each) were added to the reactor before chlorine flow began, which is the normal procedure. In another experiment, 0.25 wt % FeCl3 was added before chlorine flow began, and 0.25 wt % AlCl3 was added later (at t ) 12 min where T ) 120 °C), after chlorine flow had begun. The yields of tri-, tetra-, and pentachlorophenol as a function of reaction time appear in Figure 5. The temporal variation of the yields in parts a and b of Figure 5 are very similar, indicating that delaying cocatalyst addition had little

Figure 3. Temporal variation of chlorophenol molar yields with different catalyst concentrations: (a) 0.125 wt % FeCl3; (b) 0.25 wt % FeCl3; (c) 0.50 wt % g FeCl3.

effect on the progress of the synthesis reaction. Figure 5 also shows that delaying cocatalyst addition had little effect on the TEQ concentration in the sample at a given reaction time. In both experiments, the TEQ concentration was about 0.2 ppm at the maximum tertrachlorophenol yield and about 0.7 ppm at the maximum pentachlorophenol yield. These results, along with those in Figures 1c and 3c, all of which are from experiments with 0.50 wt % catalyst, show that the use of two catalysts has no measurable effect on TEQ near the maximum pentachlorophenol yield. The TEQ concentration at the maximum pentachlorophenol yield was between 0.69 and 0.77 ppm for all of the experiments that used 0.5 wt % catalyst, regardless of the catalyst identity. To summarize this first section on the identity and concentration of the catalyst, we find that using higher concentrations of

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Figure 4. Effect of FeCl3 catalyst concentration on TEQ concentration at maximum tetra- and pentachlorophenol yields.

Figure 5. Temporal variation of chlorophenol molar yields and TEQ concentrations for synthesis with equal amounts of FeCl3 and AlCl3 (0.5 wt % total). (a) Both catalysts added before starting chlorine flow. (b) FeCl3 added before starting chlorine flow, AlCl3 added later.

either AlCl3 and FeCl3 leads to lower TEQ concentrations in the final pentachlorophenol product, but this benefit diminishes as the catalyst concentration increases. Additionally, using higher concentrations of AlCl3 accelerated the reaction rate, whereas this effect was not observed with FeCl3. There appear to be no synergetic effects that arise from using mixed catalysts, and the TEQ concentration at the maximum pentachlorophenol yield was about the same regardless of the identity of the catalyst, when 0.5 wt % was used. Effect of AlCl3 Addition Strategy. We performed four synthesis experiments to investigate the effect of the AlCl3 addition time on pentachlorophenol synthesis. The chlorine flow rate was 1.5 mol/h initially, then reduced to 1.1 mol/h after about 2.5 h, and then reduced further to 0.9 mol/h after 1/2 h of the temperature ramp. The chlorine flow was stopped at the end of

the temperature ramp. We were interested primarily in the TEQ concentrations near the maximum tetra- and pentachlorophenol yields. Thus, the sample with the highest tetrachlorophenol yield, the last three samples from the reactor, and one final postreaction sample (from the disposable aluminum pan into which the reactor contents are dumped at the end of the run) were selected for AhRC PCR analysis to determine their TEQ concentrations. The chlorophenol yields and TEQ results appear in Figure 6. The trends in the temporal variations of the chlorophenol yields were similar to those in previous experiments. That is, the trichlorophenol yield decreased with time to zero. The tetrachlorophenol yield increased to a maximum, and then decreased to zero. Pentachlorophenol first appears after a certain amount of tetrachlorophenol forms, and then its yield increases with time. Figure 6a shows results obtained from using 0.25 wt % AlCl3, where half the catalyst was added before starting the chlorine flow, and the other half added 202 min after chlorine flow began (T ) 130 °C). Figure 6b gives the results obtained by using 0.5 wt % AlCl3 with half added initially and half at 163 min (T ) 130 °C). The maximum pentachlorophenol yields for these two experiments were both 94%, but they formed at 306 min (Figure 6a) and at 264 min (Figure 6b), respectively. As noted earlier in this article, using more AlCl3 accelerates the rate of pentachlorophenol production. The pentachlorophenol yields in the postreaction samples were 91% and 94%, respectively. The pentachlorophenol yield in a control sample of a commercial flaked pentachlorophenol product was determined to be 91%. This reference sample was determined by the manufacturer to contain 86% pentachlorophenol, which indicates that the yields reported for the samples we synthesized may be high by a few percent. As can be seen in Figure 6a, the TEQ concentrations increased from 1.0 ppm before the maximum pentachlorophenol yield to 1.3 ppm at the maximum pentachlorophenol yield, and then to 2.3 ppm just 5 min later. The TEQ concentration was 3.9 ppm for the postreaction pentachlorophenol sample, which is 3 times the value at the maximum pentachlorophenol yield. When using twice as much AlCl3 (Figure 6b), the TEQ values increased from 0.27 ppm before the maximum pentachlorophenol yield to 0.29 ppm at the maximum yield, and then to 1.2 ppm after an additional 8 min of reaction. The TEQ concentration was 1.6 ppm for the final postreaction pentachlorophenol sample, which is almost 5 times the value at the maximum pentachlorophenol yield. These results are consistent with previous ones in that the larger amount of catalyst led to lower TEQ concentrations. Figure 6c shows the experimental results obtained by using 0.25 wt % AlCl3, where one-third was added initially, one-third was added at t ) 131 min, when the reactor temperature was 126 °C, and one-third was added at 274 min (T ) 160 °C). Figure 6d shows the results from an experiment with the same addition strategy (add catalyst at 126 and 160 °C) but with a 0.5 wt % AlCl3 loading. TEQ concentrations for the postreaction samples were 1.4 and 2.7 ppm when 0.25 and 0.5 wt % AlCl3, respectively, were used. These TEQ values in the final product were higher than those in the last three samples from the reactor. The pentachlorophenol yields in theses samples were 97 and 99%, but these yields are likely high because the yield of the flaked reference sample was determined to be 94% (rather than its actual value of 86%). Comparing the results in Figures 1b, 6a, and 6c and those in Figures 1c, 6b, and 6d provides insight into the effect of the catalyst addition strategy on the TEQ concentration. The first set of figures contains results from syntheses with 0.25 wt %

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Figure 6. Temporal variation of chlorophenol molar yields and TEQ concentrations for synthesis with AlCl3 catalyst added incrementally: (a) 0.25 wt % total, half added initially, half added at T ) 130 °C; (b) 0.50 wt % total, half added initially, half added at T ) 130 °C; (c) 0.25 wt % total, one-third added initially, one-third added at 126 °C, one-third added at 160 °C; (d) 0.50 wt % total, one-third added initially, one-third added at 126 °C, one-third added at 160 °C.

AlCl3 but different addition strategies. The TEQ concentrations at the maximum tetra- and pentachlorophenol yields were about 0.7 ( 0.2 ppm and 1.15 ( 0.2 ppm, respectively, regardless of how the catalyst was added to the reactor. Likewise, the second set of figures shows that the TEQ concentrations at the maximum tetra- and pentachlorophenol yields were about 0.57 and 0.75 ppm, respectively, regardless of the catalyst addition strategy when 0.5 wt % AlCl3 was used. In both cases, the catalyst addition strategy has no apparent effect on the TEQ concentration. In Figure 6, the TEQ concentration in the postreaction sample taken from the aluminum pan was about 3-5 times higher than the TEQ concentration in the sample at the maximum tetrachlorophenol yield. This result indicates that most of the TEQ concentration in the final postreaction solidified product forms after the tetrachlorophenol yield reaches its maximum value. Therefore, future studies that focus on the influence of reaction variables near the end of the run may lead to strategies that can reduce the microcontaminant level. Summary and Conclusions 1. There exists a narrow window in time near the maximum pentachlorophenol yield wherein the TEQ concentration increases very quickly (by about 1 ppm in 10 min). Also, the

TEQ concentration in the postreaction sample was always higher (by about 50-200%) than the TEQ concentration in the last sample withdrawn from the reactor. These observations suggest that PCDDs and PCDFs form primarily near the end of the reaction and that they continue to be formed even as the molten pentachlorophenol cools and solidifies after the synthesis reaction. Strategies to stop these reactions from occurring could lead to a reduction in the TEQ concentration in the final product. Additionally, this rapid increase in TEQ concentration near the end of the run demonstrates that a method of TEQ analysis that is fast, cheap, and reliable would be invaluable for process monitoring. 2. The highest catalyst concentration examined (0.5 wt %) produced pentachlorophenol with the lowest TEQ concentrations. The TEQ concentration at the maximum pentachlorophenol yield with 0.5 wt % catalyst was always about 0.8 ppm regardless of the catalyst identity or addition strategy used. These results lead us to conclude that the PCDD- and PCDF-forming reactions are not catalyzed by FeCl3 or AlCl3. 3. In general, pentachlorophenol with the highest TEQ concentrations formed in experiments with low concentrations of FeCl3 as the catalyst. Pentachlorophenol from AlCl3-catalyzed synthesis at lower catalyst concentrations generally had lower TEQ concentrations.

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4. For synthesis with AlCl3, the TEQ concentration present in the sample with the highest tetrachlorophenol yield is about half the concentration that will exist in the sample with the highest pentachlorophenol yield. This observation indicates that attention should be directed to the earlier stages of the reaction to reduce the TEQ concentration at the maximum tetrachlorophenol yield. Acknowledgment We thank one of the manuscript reviewers for helpful suggestions and insightful comments. We also acknowledge the financial support of the Microcontaminant Reduction Venture, an industry-sponsored consortium committed to reducing microcontaminant levels in technical-grade pentachlorophenol. Literature Cited (1) Muller, F.; Caillard, L. Chlorophenols. In Ullmann’s Encyclopedia of Industrial Chemistry, on-line edition; posted June 15, 2000, DOI 10.1002/ 14356007/a07_001.

(2) Buser, H. R.; Bosshardt, H. P. Determination of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Commercial Pentachlorophenols by Combined Gas Chromatography-Mass Spectrometry. J.sAssoc. Off. Anal. Chem. 1976, 59 (3), 562-569. (3) Report of the Ad Hoc Study Group on Pentachlorophenol Contaminants; EPA-SAB-EHC-78-003; Environmental Health Advisory Committee, Science Advisory Board; US EPA, Dec 29, 1978. (4) Yu, J.; Nestrick, T. J.; Allen, R.; Savage, P. E. Microcontaminants in Pentachlorophenol Synthesis. 1. New Bioassay for Microcontaminant Quantification. Ind. Eng. Chem. Res. 2006, 45, http://dx.doi.org/10.1021/ ie0602106. (5) Yu, J.; Savage, P. E. Reaction Pathways in Pentachlorophenol Synthesis. 1. Temperature Programmed Reaction. Ind. Eng. Chem. Res. 2004, 43, 5021-5026. (6) Yu, J.; Savage, P. E. Reaction Pathways in Pentachlorophenol Synthesis. 2. Isothermal Reaction. Ind. Eng. Chem. Res. 2004, 43, 62926298.

ReceiVed for reView May 9, 2006 Accepted May 13, 2006 IE0605783