Laboratory Experiment pubs.acs.org/jchemeduc
Metalloporphyrins as Oxidation Catalysts: Moving Toward “Greener” Chemistry in the Inorganic Chemistry Laboratory Rose A. Clark, Anne E. Stock, and Edward P. Zovinka* Department of Chemistry, Saint Francis University, Loretto, Pennsylvania 15940, United States S Supporting Information *
ABSTRACT: Training future chemists to be aware of the environmental impact of their work is of fundamental importance to global society. To convince chemists to embrace sustainability, the integration of green chemistry across the entire chemistry curriculum is a necessary step. This experiment expands the reach of green chemistry techniques into the inorganic laboratory curriculum. Students utilize a metalloporphyrin catalyst with hydrogen peroxide to facilitate the production of cyclooctene oxide eliminating the need for the environmentally harmful iodosylbenzene catalyst. Both chloroiron(III) tetraphenylporphyrin and chloroiron(III) tetra(pentafluorophenyl)porphyrin are incorporated into the experiment to allow comparison of catalytic activity. Product characterization includes UV−visible spectroscopy and gas chromatography−mass spectrometry.
KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Bioinorganic Chemistry, Catalysis, Gas Chromatography, Green Chemistry, Synthesis, UV−vis Spectroscopy
G
reen chemistry has become an essential component in chemical education with the driving initiative being the concept of sustainability. Sustainability involves not only creating more environmentally friendly molecules, but ensuring that their production and deterioration does not harm the environment in the process.1 In an effort to increase sustainability, chemists seek to minimize the amount of harm done to the planet by minimizing chemical byproducts and the waste generated, as well as stressing more efficient methods for synthesizing materials.1 By increasing the use of green chemistry methodologies in undergraduate education, future chemists will be better prepared to integrate sustainability into their planning processes.2 Green chemistry is widely practiced both in the classroom and in the laboratory in organic chemistry. Organic chemists have been able to incorporate green chemistry methodologies into several classic organic laboratory experiments such as the Williamson ether synthesis, Diels−Alder reaction,3 Wittig reactions,4 Knoevenagel condensations,5 synthesis of aspirin,6 and processes such as esterification, oxidation, and hydrolysis.7 Green chemistry has even been applied to the formation of biodiesel for the organic laboratory setting.8 Commercial textbooks emphasizing green chemistry are available for quick integration into the organic laboratory.9 Although there are outstanding examples of green chemistry in the organic laboratory curriculum, green chemistry needs to be expanded into the upper-division courses beyond organic chemistry. The project described here focuses on the catalytic oxidation of substrates to demonstrate the stepwise scientific process toward problem solving. During this inorganic undergraduate laboratory activity, green chemistry method© 2011 American Chemical Society and Division of Chemical Education, Inc.
ologies are used as the background setting and motivation for student learning. The green chemistry principles shown in Table 1 are highlighted throughout the experiment.10 The Table 1. Green Chemistry Principles Used in the Experiment Green Chemistry Principlea
Explanation
Prevention
5
Safer Solvents and Auxiliaries Catalysis
9 a
Heading
1
It is better to prevent waste than to treat or clean up waste after it has been created. The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
The principles are from ref 10.
experiment goal is to create an epoxide from a cyclic organic compound using a metalloporphyrin catalyst (Figure 1) and hydrogen peroxide (Scheme 1).
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DIFFICULTIES WITH APPLYING GREEN CHEMISTRY PRINCIPLES While striving to integrate green chemistry principles, synthetic chemistry often poses challenges to the ideal “green” situation. In the epoxide reaction (Scheme 1), the oxidant is switched from iodosylbenzene to hydrogen peroxide, providing an example of how green chemistry principle 1, prevention, can Published: December 7, 2011 271
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co-oxidant, an iron(IV) ferryl porphyrin π cation radical is produced from the starting metalloporphyrin without the formation of harmful materials.14 The oxidized porphyrin then oxidizes the organic compound.14 The green chemistry advantage of using H2O2 is that the desired functionalized organic compound is obtained with only water and oxygen as byproducts as shown in Scheme 1.15
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EXPERIMENTAL OVERVIEW The oxidation reaction to form the epoxide (Scheme 1) is monitored using a gas chromatography−mass spectrometry instrument (GC−MS) to detect the functionalized organic compounds. Specifically, cyclooctene is oxidized by H2O2 with the reaction catalyzed by an iron(III) porphyrin and the yield of cyclooctene oxide obtained during the reaction is determined. Literature reports have shown that the simpler and less hazardous hydrogen peroxide produces nearly equal yields to those of reactions completed using the less desirable iodosylbenzene.16 Whereas this experiment ostensibly explores the oxidation of organic compounds by hydrogen peroxide catalyzed with an iron porphyrin catalyst, the larger implications of the experiment are more important. The goal of the experiment is to examine green chemistry methods for the functionalization of an organic compound using hydrogen peroxide as the oxidizer and metalloporphyrin catalysts. The method implemented improves on previous cyclooctene oxidation methods where green chemistry principles were not a consideration.
Figure 1. Structure of the porphyrin catalysts, ClFeTPP and ClFeTPPF5.
Scheme 1. Cyclooctene Oxide Synthesis Using a Metalloporphyrin Catalyst
be used to produce less hazardous waste. Solubility issues are often a problem for synthesis but it presents the opportunity to discuss with students the choices they will face as practicing chemists. The experimental solvent system contains a mixture of 3:1 methanol to dichloromethane, which presents a conflict with green chemistry principle 5. However, other solvent systems that were examined did not produce the same yield as the methanol/dichloromethane solvent. The teachable moment at this point is whether it is better to produce the desired product using less energy with a more toxic solvent or use a much less functional solvent system, producing much less product, and therefore more energy to produce the same amount of product. Which issue is more important for the chemists? These types of roadblocks are representative of the challenges facing practicing chemists and also of integrating greener methodologies. The method published here is a step toward the total integration of green chemistry principles and can be presented as such to students as a tool to highlight the ongoing development of scientific procedures. The experiment provides a vehicle to integrate synthesis, greener methodologies, and catalytic composition into the undergraduate laboratory. By comparing two catalysts, chloroiron(III) tetraphenylporphyrin (ClFeTPP) and chloroiron(III) perfluorotetraphenylporphyrin (ClFeTPPF5), students examine green chemistry through the effect of catalyst type on the reaction products. The structure of the catalysts can be seen in Figure 1. The simple (and readily synthesized) ClFeTPP metalloporphyrin catalyst is not robust and degrades during the oxidation reactions.11 Modification of the metalloporphyrin to chloroiron(III) tetra(pentafluorophenyl)porphyrin (ClFeTPPF5) produces a higher yield of oxidized organic product.12 Comparing the yields from the two porphyrins introduces the students to a discussion on catalyst design and generational change in catalysts. Again the students are challenged to think of which would be better, a fluorinated catalyst that may not be considered green in terms of its chemistry but produced the desired product and is only used in small quantities. Traditional chemical methodology focuses solely on the desired product with little regard to the other reaction byproducts. For example, iodosylbenzene was used to functionalize organic substrates.13 However, during the redox process, iodosylbenzene breaks down into more hazardous components, such as carcinogenic benzene. By using hydrogen peroxide as a
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MATERIALS AND INSTRUMENTATION All materials used for the reactions were purchased from a commercial source and were of reagent grade. The following materials and solvents were used as obtained in the reactions: dichloromethane (ACS reagent grade, Aldrich); 30% hydrogen peroxide (Sigma-Aldrich); 95% cis-cyclooctene (Aldrich), which contained trace amounts of cyclooctene oxide; chloroiron(III) 5,10,15,20-tetraphenylporphyrin (Aldrich); chloroiron(III) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (Aldrich); methanol (Science Kit & Boreal Laboratories); and decane (Fisher Scientific). Cyclooctene oxide (Aldrich) was used as a mass spectrometry standard. A PerkinElmer Lambda 25 was used to collect UV−vis spectra of solutions. A Thermo Electron Corporation Focus GC/Trace DSQ Single Quadrupole MS system with an autosampler was used to analyze the samples. The samples were run on a 15 m × 0.25 mm TR-5 nonpolar column (Thermo Scientific).
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HAZARDS Hydrogen peroxide (30%) is a strong oxidant and should be used with caution while wearing gloves and protective goggles. Dichloromethane is a suspected carcinogen. Methanol is flammable and extremely toxic. Cyclooctene is flammable and may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Care should be taken in using methanol and dichloromethane as they are both harmful to skin and respiratory system.
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LABORATORY PROCEDURE The experiment was conducted in a third-year undergraduate laboratory where students had some previous knowledge of the necessary instrumentation and lab techniques. Previous 272
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laboratory experiments in general chemistry and organic chemistry had introduced green chemistry methods to the students, including atom economy and microwave synthesis techniques. The students, working in pairs, performed the experiment within a 4 h period. Each pair of students examined the control and one of the porphyrin catalyst reactions described below. The class data was pooled at the end of the lab period and shared electronically. The reaction solutions were prepared using 3:1 methanol/ dichloromethane with 5.9 × 10−2 M decane added an as internal GC standard. Into each of two quartz cuvettes, 3 mL of the solvent system and 195 μL cyclooctene were added. Into one of the cuvettes, 80 μL of metalloporphyrin (1 × 10−3 M) was added to catalyze the reaction. No catalyst was added to the second cuvette and it remained as the control. A 1 μL aliquot of each solution before reaction was injected into the GC−MS and run using the following conditions: initial temperature, 50 °C; hold time, 1.5 min; ramp, 15 °C/min to 310 °C; hold time, 1 min. The oxidation reaction is initiated with the addition of 30% hydrogen peroxide in ∼15 μL increments at 8 min intervals until the total volume of peroxide is ∼90 μL. The cuvettes were kept in an ice bath during the addition of H2O2. Upon the addition of the last increment of H2O2, an aliquot of each solution was removed from the reaction mixture and injected into the GC−MS. The conditions for each run were exactly the same as the first run containing only cyclooctene, the solvent, and the porphyrin. The GC−MS analysis of the samples was conducted before the reaction and after the six additions of H2O2 when the catalyst was exhausted.17 Some factors that must be stressed so that students obtain quality results include maintaining ice bath conditions during the reaction, reducing the amount of light exposure, and maintaining the slow addition of hydrogen peroxide to the reaction mixture. A calibration curve for cyclooctene oxide was prepared to determine the concentration of cyclooctene oxide created in the substrate oxidation reactions. Five cyclooctene oxide standards, 8 × 10−3, 4 × 10−3, 6 × 10−4, 2 × 10−4, and 1 × 10−5 M were prepared and run on the GC−MS using the same conditions as the reaction. The cyclooctene oxide standards were diluted to volume using a stock solution (100 mL of a 3:1 mixture of methanol/dichloromethane as the solvent with 2 mL of 5.9 × 10−2 M decane solution added as an internal standard was prepared for the whole class). A ratio of peak area (or peak height) for the cyclooctene oxide/decane peaks was plotted against the concentration of cycylooctene oxide/decane. The decane internal standard helps alleviate errors in concentration due to small changes in injection volume. UV−vis scans were collected at 1, 3, 5, 10, 15 min after the final addition of the hydrogen peroxide.
Figure 2. GC−MS response for cyclooctene prior to the oxidation. The decane peak is not visible compared to the relative abundance of cyclooctene. The inset on the right shows the magnified peak for the decane internal standard at 3.33 min.
with the cyclooctene peak having a rentention time of 2.3 min. After oxidation in the presence of the metalloporphyrin catalyst activated by H2O2, the reaction solutions were analyzed through GC−MS again to identify the cyclooctene oxide product. The chromatogram after oxidation with the fluorocatalyst is shown in Figure 3. The cyclooctene oxide peak with a retention time of 4.6 min is well resolved from the cyclooctene starting material peak. The decane internal standard has a retention time of 3.33 min in all runs; however, the decane is not visible at the abundance observed for the analytes. An inset is added in Figures 2 and 3 to show the decane response. All calibration plots and analyses use decane as the internal standard to enhance reproducibility of the concentration determined. Two different catalyst systems were compared and the reaction catalyzed by ClFeTPPF5 yielded much better results relative to the ClFeTPP-catalyzed reaction (Table 1 in the Supporting Information). The cyclooctene obtained from Aldrich contained trace amounts of cyclooctene oxide that was quantified in the control experiment. After addition of ClFeTPPF5, the concentration of cyclooctene oxide increased by a factor of 50, from 1.8 to 100 mM. In comparison, the ClFeTPP-catalyzed reaction showed no increase in cyclooctene oxide concentration (staying at ∼2 mM, Table 1 in the Supporting Information). The degradation of both catalysts over the course of the reaction is shown through the disappearance of the 403 and 401 nm λmax bands in the visible spectra (Figures 4 and 5). These spectra are collected by the students to give a visual representation of the catalyst degradation and provide an explanation for the lack of desired product by the reaction catalyzed by ClFeTPP. Although kinetics experiments could be performed using this data, they were not the focus of this project. Kinetics parameters such as turnover number could be
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RESULTS AND DISCUSSION Gas chromatography with mass spectrometric detection is an excellent method for analysis of the cyclooctene oxidation reaction. Several controls are run by the students to ensure the catalytic mediation of the oxidation. The stock cyclooctene is tested to determine amount of oxidation, a control is also conducted using the cyclooctene mixed with hydrogen peroxide (no metalloporphyrin), and finally, a mixture of cyclooctene and the metalloporphyrin (no peroxide) is tested. Minimal levels of cylcooctene oxide (∼1 mM) are observed in any of the controls (Table 1 in the Supporting Information). A typical chromatogram obtained prior to oxidation is shown in Figure 2 273
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Figure 5. Decomposition of the ClFeTPPF5 during the oxidation of cyclooctene monitored using UV−vis. The Soret bands at 0, 1, and 3 min, respectively, are 401 nm.
In particular, the problems of the solvent system are discussed in relation to green chemistry principle 5, safer solvent, and auxiliaries. Principle 9, catalysis, is highlighted by the use of the metalloporphyrin catalysts. After the data were collected and the students observed that the simple (and easy to synthesize) ClFeTPP catalyst did not produce the desired results, the need to improve a first-generation catalyst was discussed. The merits of the functional catalyst, ClFeTPPF5, that contained a number of halogens was brought to the attention of the students during the discussion.
Figure 3. GC−MS response for cyclooctene after fluoroporphyrin oxidation. The inset on the right shows the magnified peak for the decane internal standard at 3.33 min.
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CONCLUSIONS
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ASSOCIATED CONTENT
The experiment presented here is an excellent tool to start the conversation of green chemistry in an inorganic laboratory setting. By using hydrogen peroxide as the oxidant, no hazardous waste is produced, exemplifying green chemistry principle 1. The instructor presents principle 5, safer solvents, and uses it as the beginning of a discussion about the problems inherent in designing a green chemistry reaction, in this case, the solvent system. The opportunity to discuss green chemistry principle 9, catalysis, and the need to balance the robustness of the catalyst with green chemistry principles is also provided. In the experiment, whereas the ClFeTPP is probably “greener”, the students are faced with the reality that it does not work, requiring a practicing chemist to continue searching for green catalysts. Overall, the experiment provides instructors with an opprtunity to teach several green chemistry principles so that the students can take green chemistry knowledge into the workforce and apply it beyond the classroom. These chemists will then have the tools and knowledge to not only formulate better consumer goods, but to do so in a manner that does not harm but helps to sustain the earth.
Figure 4. After final addition of H2O2 to the cyclooctene and ClFeTPP reaction mixture, the decomposition of the iron porphyrin was monitored by UV−vis. Soret at 0 min = 403 nm.
calculated in a future project or as an additional lesson in tandem with the experiment presented here. As shown in Figures 4 and 5, the ClFeTPP degrades very rapidly, mostly destroyed after 1 min, which explains the minimal conversion of cyclooctene to cyclooctene oxide using this catalyst.18 The ClFeTPPF5 catalyst survives for a longer period of time, allowing for more oxidation of the substrate. The stability of the perhalogenated catalyst is due to the electron withdrawing effects of the five fluorines on each phenyl in the metalloporphyrin that changes the redox potential of the Fe(IV)/Fe(III) couple as compared to the ClFeTPP in the same solvent system.10 Before, during, and after the activity, a discussion with the students is maintained regarding the “greenness” of the process.
S Supporting Information *
Student handout; table showing the levels of cylcooctene oxide produced; calibration curves for the cyclooctene oxide. This material is available via the Internet at http://pubs.acs.org. 274
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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REFERENCES
(1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: London, 2000. Going Green: Integrating Green Chemistry into the Curriculum; Print Parent, K., Kirchhoff, M., Eds.; American Chemical Society: Washington, DC, 2004. (2) Conley, B. L.; Tenn, W. J. III; Young, K. J. H.; Ganesh, S. K.; Meier, S. K.; Ziatdinov, V. R.; Mironov, O.; Oxgaard, J.; Gonzales, J.; Goddard, W. A. III; Periana, R. A J. Mol. Catal. A: Chem. 2006, 251, 8−23. (3) Katritzky, A. R.; Cai, C.; Collins, M. D.; Scriven, E. F. V.; Singh, S. K.; Barnhardt, E. K. J. Chem. Educ. 2006, 83, 634−636. (4) Martin, E.; Kellen-Yuen, C. J. Chem. Educ. 2007, 84, 2004−2006. (5) Cook, A. G. J. Chem. Educ. 2007, 84, 1477−1479. (6) Montes, I.; Sanabria, D.; Garcia, M.; Castro, J.; Fajardo, J. J. Chem. Educ. 2006, 83, 628−631. (7) Murphree, S. S.; Kappe, C. O. J. Chem. Educ. 2009, 86, 227. (8) Miller, M. T.; Leadbeater, N. E. Chem. Educator 2009, 14, 98− 104. (9) Doxsee, K.; Hutchinson, J. Green Organic Chemistry: Strategies, Tools, and Laboratory Experiments; Brooks Cole: New York, 2003. Mohrig, J. R.; Hammond, C. N.; Schatz, P. F. Techniques in Organic Chemistry, 2nd ed.; W.H. Freeman: New York, 2006. (10) “Twelve Principles of Green Chemistry” http://www.epa.gov/ sciencematters/june2011/principles.htm (accessed Nov 2011). (11) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251−259. (12) Lane, B. S.; Burgess, K. Chem. Rev. 2003, 103 (7), 2457−2473. (13) Groves, J. T.; Nemo, T. E.; Myers, R. S. J. Am. Chem. Soc. 1979, 101, 1032−1033. (14) Nam, W.; Goh, Y. M.; Lee, Y. J.; Lim, M. H.; Kim, C. Inorg. Chem. 1999, 38, 3238−3240. (15) Nam, W.; Oh, S.-Y.; Sun, Y. J.; Kim, J.; Kim, W.-K.; Woo, S. K.; Shin, W. J. Org. Chem. 2003, 68, 7903−7906. (16) Cunningham, I. D.; Danks, T. N.; Hay, J. N.; Hamerton, I.; Gunathilagan, S.; Janczak, C. J. Mol. Catal. A: Chem. 2002, 185, 25− 31b. (17) The addition of H2O2 generates the active catalyst, a highly oxidized iron(IV) radical cation porphyrin. However, the active catalyst also readily decomposes, resulting in the loss of overall reactivity. (18) The porphyrin acts as the catalyst; however, it is destroyed, not due to reaction of interest, but because of the highly reactive nature of the active catalyst, an iron(IV) radical cation porphyrin.
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