Continuous-Flow Chemistry in Undergraduate Education: Sustainable

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Continuous-Flow Chemistry in Undergraduate Education: Sustainable Conversion of Reclaimed Vegetable Oil into Biodiesel Frank A. Leibfarth,†,‡ M. Grace Russell,† David M. Langley,† Hyowon Seo,† Liam P. Kelly,† Daniel W. Carney,§ Jason K. Sello,§ and Timothy F. Jamison*,† †

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States § Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States ‡

S Supporting Information *

ABSTRACT: This laboratory experiment leverages the pedagogical value and multidisciplinary nature of biodiesel production from vegetable oil to introduce students to continuous-flow chemistry, a modern and rapidly growing approach to chemical synthesis. An interdisciplinary approach exposes students to the practical and conceptual aspects of modern continuous-flow chemistry while simultaneously reinforcing core organic chemistry techniques and investing students in issues of sustainability. Students screen reaction conditions in flow through an inquiry-guided approach and make evidence-based decisions to accomplish the sustainable conversion of waste cooking oil into biofuel. The laboratory experiment is designed to be highly modular and can be completed in two, three, five, or eight laboratory periods. By incorporating the burgeoning field of continuous-flow chemistry into the educational infrastructure, the experiments allow students to develop skills that are highly valued in the modern chemical workforce. KEYWORDS: Second-Year Undergraduate, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Esters, Fatty Acids, Organic Chemistry, Chemical Engineering, Environmental Chemistry

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biodiesel enable adoption of this state-of-the-art technology in an accessible and pedagogically useful infrastructure. The selection of biofuel production as a vehicle for an introduction to flow chemistry is based on the growing interest in and need for alternatives to fossil fuels. Indeed, biofuels are perhaps one of the most viable short-term solutions to mitigate the use of fossil fuels.9 Specifically, biodiesel is a fuel source derived from plant-based resources that can be employed as a drop-in fuel in a standard diesel engine.10 Biodiesel is the alkyl ester of simple fatty acids (usually referred to as fatty acid methyl esters, or FAMEs) and can be generated from either pristine or previously used cooking and/or vegetable oils through a transesterification process (Figure 1). Most current biodiesel production uses batch reactor technology for this process.11 Due to the intrinsic immiscibility of the reactants, vegetable oil and methanol, and the products, FAMEs and glycerol, large-scale reactors need to employ complex and/or energy-intensive mixing and purification schemes to minimize mass and heat transfer limitations and maintain a metastable emulsion. These engineering solutions increase the cost and complexity of the overall conversion process but are necessary to meet the stringent American Society for Testing and Materials (ASTM) standards for vehicle biodiesel.12

he evolution of laboratory curricula to integrate modern synthetic technology and methodology is crucial to prepare students for participation in the modern chemical workforce.1 Integrating these state-of-the-art tools into experiments that reinforce fundamental chemical concepts and expose students to emerging issues such as sustainability enables numerous educational goals to be achieved in a way that is both compelling and contemporary.2 The transesterification of vegetable oil to produce biodiesel has long been recognized as a valuable pedagogical concept for integrating sustainability and organic chemistry into hands-on laboratory experiments, as evidenced by previous reports3 and experiments4 developed for undergraduate education. Recently, this experimental concept has even been investigated for implementation into interdisciplinary high school curricula.5 We sought to leverage the success of this concept to develop an undergraduate laboratory module that would introduce students to the multidisciplinary concepts of continuous-flow chemistry. Continuous-flow chemistry is a burgeoning field in both industry and academia that relies on pumps, tubes, and connectors to conduct chemical reactions instead of flasks, beakers, and other glassware.6 Recent innovations in such methods are making strides in automating routine chemical synthesis7 and translating flow conditions to production-scale sustainable manufacturing.1,8 A set of experiments that employ continuous-flow chemistry to transform vegetable oil into © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: September 19, 2017 Revised: May 16, 2018

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DOI: 10.1021/acs.jchemed.7b00719 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

transesterification of vegetable oil in the flow reactor and purifying the crude reaction mixture by liquid−liquid extraction, respectively. The conversion of the purified material is assessed by gas chromatography (GC) utilizing a provided calibration curve. Days four and five are meant to engage students in inquiryguided research through screening reaction conditions for the transesterification of vegetable oil to simultaneously optimize reaction conversion and sustainability. The conditions chosen for screening vary catalyst choice, molar equivalents of methanol, temperature, and residence time. The last 3 days of the laboratory experiment include characterizing a sample of reclaimed vegetable oil, pretreating the oil in preparation for biodiesel production, and running the transesterification reaction on the reclaimed vegetable oil to produce biodiesel. A detailed procedure for all experiments is located within the Supporting Information.

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HAZARDS Proper personal protective equipment (PPE), including safety goggles, gloves, close-toed shoes, and a lab coat should be worn at all times. Methanol and hexanes are flammable and harmful if ingested or inhaled and should be worked with in a ventilated fume hood. Heptane can be seamlessly substituted for hexanes due to the neurotoxicity concerns of hexanes.16 Potassium hydroxide, sulfuric acid, and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) are caustic and irritants and should be handled with care. To reduce student exposure, the catalysts should be premixed with methanol prior to the laboratory period. Both vegetable oil and biodiesel are flammable and should not be exposed to an open flame. The flow reactors are under pressure (100 psi) and, while they are running, should be placed in a fume hood with the sash down. All waste should be disposed of properly, and a waste inventory sheet is provided in the notes to instructors to aid students in tracking and mitigating their waste generation.

Figure 1. Laboratory experiment utilizes the transesterification of fatty acids to teach students the utility of continuous flow. Depiction of continuous-flow system shows pumps, T-mixer, reactor coil, and back pressure regulator (BPR).

As a result of the limitations of batch chemistry for the production of biodiesel, significant research has been dedicated to the conversion of vegetable oils to FAMEs in flow reactors.13 Overall, the benefits of flow technology, specifically the superior mixing for immiscible reagents, efficient heat transfer, and portability, are well-poised to provide a practical engineering solution for this important process. Further, the small footprint and user-friendly nature of flow chemistry makes it particularly attractive for deployment in the developing world.14 Recognizing this, scientists at the Connecticut Biofuels Consortium have demonstrated a pilot-scale version of a flow reactor that continuously transforms and separates reclaimed vegetable oil into biodiesel at a rate of 1.2 L/min with >99% conversion.13 The primary pedagogical goal of this experimental course is to teach students the theoretical and practical fundamentals of continuous-flow synthesis15 through the conversion of vegetable oil into biodiesel. The secondary pedagogical goal is to engage students in considering sustainability in their evidence-based scientific decision-making. Students will simultaneously learn core organic chemistry concepts and experimental techniques, including the synthesis, purification, and identification of organic molecules. The progression from practical knowledge to hypothesis-driven experimentation contained within this laboratory experiment frames fundamental chemical principles in the modern technological context of continuous-flow chemistry and prepares students to integrate sustainability concepts into their future scientific endeavors.

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RESULTS AND DISCUSSION The laboratory course has run three times, in the Spring of 2015 through 2017, with a total of 44 undergraduates participating over the three-year period. The students were a mix of second- through fourth-year undergraduate students who had completed at least one semester of organic chemistry. Students completed the lab in groups of two with each student preparing their prelaboratory assignments and final laboratory report individually. In total, 22 separate groups of two students conducted the experiments. The data from the 30 students enrolled in the first two years, 2015 and 2016, are presented. Students are empowered to make evidence-based decisions and perform the necessary preparation in prelaboratory assignments. This lab experiment is most appropriate for students who have completed or are enrolled in the first year of an organic chemistry course. This series of experiments is further designed to be highly modular depending on a department’s specific resources and needs. The laboratory experiment can be retrofitted to be completed in two, three, five, or eight laboratory periods, as described in the Notes to Instructors. The first 3 days of the laboratory module involve setting up a flow reactor, running a transesterification reaction of pristine vegetable oil, and purifying and analyzing the reaction product. A student’s first task in a prelaboratory assignment is to choose

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OVERVIEW OF LABORATORY EXPERIMENT The laboratory experiment is written to be completed in eight 4 h laboratory periods and tasks students with building their own flow reactor, performing a transesterification in their reactor, screening and optimizing the conditions of the reaction, and applying their chosen reaction conditions to the production of biodiesel from reclaimed vegetable oil. The first day of the laboratory experiment familiarizes students with the equipment of flow chemistry by building and testing the flow setup to ensure normal performance. Days two and three of the laboratory experiment focus on running a B




a flow rate for the triglyceride (between 40 and 60 μL/min) and calculate the necessary flow rate of methanol to provide a 50 mol equiv excess in the reaction. This exercise encourages students to begin to think of stoichiometry and reaction time in terms of flow rate and reactor volume. Each of the 30 students correctly calculated a reactor volume in their assignments. The reaction conditions for day 2 are meant to be a control experiment that provides complete conversion of the triglyceride to biodiesel. Students conduct the flow reaction on day 2 and purify and analyze the results of the reaction by gas chromatography on day 3. Of the 15 groups that conducted the experiment, 12 groups obtained >95% conversion of their triglyceride to biodiesel as assessed by GC. A failure to rigorously remove solvent from the product after extraction was found to be the major source of error within the remaining three groups. This detail of the procedure should be emphasized prior to purification, and access to a vacuum line to remove residual solvent after rotary evaporation was found to be beneficial. The next section of the laboratory experiment involves leveraging student’s recently established experience with flow chemistry and transesterification catalysis to screen reaction conditions. Each group was given a unique set of conditions and asked again to run a flow reaction with pristine vegetable oil. The reaction conditions that students screened were designed to optimize multiple variables that contribute to the overall sustainability of biodiesel production. The prelaboratory lecture on day 4 focused on the 12 Principles of Green Chemistry17 and how reactions can be simultaneously optimized to become both higher-yielding and more sustainable. To supplement the lecture, students were asked to use the web-based portal provided by the American Chemical Society and correlate their experiments with as many of the 12 Principles of Green Chemistry as possible.17b In the students’ experiments, equivalents of methanol were varied to minimize waste and solvent usage, and catalyst loading was varied to reduce derivatives. Catalyst identity was varied to design safer processes with fewer byproducts, and temperature and reaction time were varied to design for energy efficiency. Overall, this reaction screening process is successful in directly involving students in the critical analysis of 8 of the 12 Principles of Green Chemistry (prevention, less hazardous chemical synthesis, design with safer chemicals, safer solvents and auxiliaries, design for energy efficiency, use of renewable feedstocks, catalysis, and design for degradation). Furthermore, the catalysts screened were chosen to give students a broad perspective on the mechanisms of transesterification catalysis and how catalyst choice relates to reaction outcome. The catalysts included a nucleophilic base (potassium hydroxide), a strong Brønsted acid (sulfuric acid), and a basic organocatalyst (TBD). Instructors focused the day 5 prelaboratory discussion on the mechanism of TBD-catalyzed transesterification because two possible mechanistic hypotheses have recently been proposed.18 This discussion engaged students in modern scientific literature and reinforced core concepts in organic chemistry. A detailed analysis of the mechanism of TBDcatalyzed transesterification is included in the Notes to Instructors in the Supporting Information to facilitate the prelaboratory lecture. Of the 15 groups that performed the experiments, 14 observed results similar to those from experiments previously run in a professional laboratory setting (Table 1). The only anomalous result (entry 4) was due to a defective glass syringe

Table 1. Conditions and Results of Reaction Screening in Continuous Flowa Entry

Catalyst

Catalyst Loading (%)

Temp (°C)

Time (min)

MeOH (equiv)

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13

KOH KOH KOH KOH KOH TBD TBD TBD TBD TBD H2SO4 H2SO4 H2SO4

5 5 5 5 10 5 5 5 10 10 5 5 5

100 100 50 100 50 100 50 100 100 50 100 50 100

10 10 10 5 10 10 10 5 5 10 10 10 5

9 6 9 9 9 9 9 9 9 9 9 9 9

96b >99b 73 2.7c 93 >99 >99 97b 95 82 7.5 27 2.1

a

For all entries, the methanol and catalyst were loaded into a stainless steel syringe, and the vegetable oil was contained in a glass syringe. The reagents were introduced via syringe pumps to a T-mixer and pumped through a reactor at the designated time and temperature. b These results are an average of two runs conducted during two separate years of the laboratory experiment. cThe low conversion for this condition was found to be the result of a mechanical failure of a syringe during the reaction.

that compromised the expected stoichiometry of the reaction. The success of students in conducting these reactions demonstrates the skills they developed through days one through three of the lab while providing them with a wealth of data for analysis. In preparation for making biodiesel from reclaimed vegetable oil, samples of used cooking oil were obtained from three local restaurants and students performed a colorimetric titration approved by the ASTM to quantify the amount of free fatty acid that had accumulated in the oil during regular use.19 Used vegetable oil is recommended to have a free fatty acid content of below 2% before transformation into biodiesel.20 To prepare the material for transesterification, students performed a Fischer esterification,21 purified the resulting material, and retitrated it to confirm a reduced acid content (Table 2). Free Table 2. Student Results for the Colorimetric Titration of Used Vegetable Oil before and after Acid-Catalyzed Esterificationa Entry

Sample

Initial Acid Content (mol %)

Final Acid Content (mol %)

1 2 3

Restaurant A Restaurant B Restaurant C

19 ± 3 15 ± 3 1.3 ± 0.2

1.5 ± 0.8 2.3 ± 0.3 0.76 ± 0.16

a

The reported methods provided consistent student results as evidenced by the small standard deviations.

fatty acid content varied significantly between restaurants, but in all cases the methods described in the laboratory manual proved robust for quantifying and reducing the free fatty acid content of reclaimed vegetable oil near the necessary 2% in preparation for its conversion to biodiesel. The final section of the laboratory experiment tasks students with making evidence-based decisions in choosing a set of reaction conditions from the screening results to transform reclaimed vegetable oil from local restaurants into biodiesel. C




including how to design and build a flow reactor, how to use flow rate, reagent concentration, and reactor length to control stoichiometry and reaction time, and how to translate a batch reaction to flow. The 30 students succeeded in running numerous reactions in continuous flow, modifying their system design for various reaction conditions, and conceptualizing the benefits of flow technology in synthetic chemistry. The secondary goal of engaging students in considering sustainability in their evidence-based scientific decision-making was accomplished through reaction optimization and application to a sample of used vegetable oil. The students were assessed on their application of the Principles of Green Chemistry in choosing optimal conditions for the application to used vegetable oil. The progression of the laboratory experiment empowers students to make decisions about their experimental conditions and illustrates the translation of their investigations into a real-world application, the conversion of reclaimed vegetable oil into biodiesel. The modular design of the laboratory experiment, its broad utility, and the interdisciplinary topics covered make it a valuable tool to train students to tackle modern challenges in the chemical sciences.

Students were evaluated on their understanding of sustainability concepts by the choice and justification for the final conditions they chose. Of the 15 groups that completed the laboratory experiment, 13 of the groups (86%) gave explanations of their choice grounded in the principles of green chemistry. The remaining groups were simply focused on maximizing reaction conversion. Improving this will require a more explicit focus on a sustainable chemical process in the prelab lectures rather than the numerical value of the outcome. Of the 15 groups, three different reaction conditions were chosen over the first two years (entries 1, 2, and 8 in Table 1). Overall, students prioritized short reaction times (5 min) and low catalyst loadings (5%) over low methanol content. Although TBD provided marginally better catalytic activity than KOH, its increased cost led many students to determine that KOH was a better alternative for a large-scale process. Considering the principles of green chemistry, students generally chose to prioritize energy efficiency and reduce derivatives over minimizing waste. Students justified these evaluations in their final laboratory reports based on the diminished conversions they observed with lower methanol content and ease of methanol removal from the final product through liquid−liquid extraction. Students reported that their perspective was influenced by a prelaboratory demonstration of the Zaiput Flow Technologies in-line, liquid−liquid separation technology.22 Students stated that this demonstration, conducted on day 6 of the laboratory experiment, provided a low-energy and green method to remove methanol and glycerol and could increase the sustainable nature of methanol and glycerol removal in continuous biodiesel production. Instructions on the optional demonstration can be found in the Notes to Instructors. Overall, students observed a similar conversion of used vegetable oil to previous experiments with new vegetable oil. Quantitative yields were not measured because the vegetable oil was collected from multiple sources, but 1H NMR spectroscopy was used to measure the conversion of the fatty acids. In the process of converting reclaimed vegetable oil to biodiesel in flow, students are guided to consider opportunities related to scale-up and continuous production. Using the conversions and yields they obtained, students calculate the amount of biodiesel that their flow instruments could produce in 1 day, 1 month, and 1 year of continuous production and report results in their final laboratory reports. Imagining moving a flow-based technology out of the lab along with considerations of reaction conditions, experimental results, and factors that influence both the quality and sustainability of the overall process provided students with a broad perspective on decision-making for sustainable and scalable chemistry. Students were asked to consider factors for scale-up, including improvements in pumping technology, mixing in large volumes, and continuous purification, and include them in their final laboratory reports. Students were given a manuscript describing the conversion of reclaimed vegetable oil into biodiesel at a rate of 1.2 L/min to enhance this discussion.13

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00719. Student handout consisting of the full laboratory manual with background on continuous-flow chemistry and biodiesel production and detailed experimental procedures, Notes to Instructors including a list of all equipment and chemicals required along with additional notes for prelaboratory discussions, sample characterization data including GC and 1H NMR spectra, sample questions for an oral quiz, and guidelines about completing a final laboratory report and waste inventory sheet (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Frank A. Leibfarth: 0000-0001-7737-0331 Jason K. Sello: 0000-0001-6263-7902 Timothy F. Jamison: 0000-0002-8601-7799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Mariusz Twardowski for his help, advice, and guidance on making and implementing a new laboratory module. Further, Rick L. Danheiser is acknowledged for his mentorship, and Kelley Danahy and Saki Ichikawa are thanked for their time and aid in beta-testing the laboratory experiment. F.A.L. would like to thank the NSF SEES Postdoctoral Fellowship (CHE-1314022) for funding. J.K.S. is the recipient of a NSF CAREER Award (MCB1053319).

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CONCLUSIONS A laboratory experiment was developed that incorporates modern techniques of continuous-flow chemistry into the established pedagogical structure of the conversion of vegetable oil into biodiesel. The three years of its implementation have been successful in the primary pedagogical goal of educating students in the practical aspects of continuous-flow chemistry, D



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(13) Boucher, M. B.; Weed, C.; Leadbeater, N. E.; Wilhite, B. A.; Stuart, J. D.; Parnas, R. S. Pilot Scale Two-phase Continuous Flow Biodiesel Production via Novel Laminar Flow Reactor. Energy Fuels 2009, 23 (5), 2750−2756. (14) (a) Snead, D. R.; Jamison, T. F. A Three Minute Synthesis and Purification of Ibuprofen: Pushing the Limits of Continuous-Flow Processing. Angew. Chem., Int. Ed. 2015, 54 (3), 983−987. (b) Lévesque, F.; Seeberger, P. H. Continuous-Flow Synthesis of the Anti-Malaria Drug Artemisinin. Angew. Chem., Int. Ed. 2012, 51 (7), 1706−1709. (15) Examples of laboratory experiments incorporating continuousflow chemistry: (a) Simeonov, S. P.; Afonso, C. A. M. Batch and Flow Synthesis of 5-hydroxymethylfurfural (HMF) from Fructose as a Bioplatform Intermediate: An Experiment for the Organic or Analytical Laboratory. J. Chem. Educ. 2013, 90 (10), 1373−1375. (b) König, B.; Kreitmeier, P.; Hilgers, P.; Wirth, T. Flow Chemistry in Undergraduate Organic Chemistry Education. J. Chem. Educ. 2013, 90 (7), 934−936. (c) Tundo, P.; Rosamilia, A. E.; Aricò, F. Methylation of 2-Naphthol using Dimethyl Carbonate under Continuous-Flow Gas-Phase Conditions. J. Chem. Educ. 2010, 87 (11), 1233−1235. (d) Feng, Z. V.; Edelman, K. R.; Swanson, B. P. Student-Fabricated Microfluidic Devices as Flow Reactors for Organic and Inorganic Synthesis. J. Chem. Educ. 2015, 92 (4), 723−727. (16) Takeuchi, Y.; Ono, Y.; Hisanaga, N.; Kitoh, J.; Sugiura, Y. A Comparative Study on the Neurotoxicity of n-Pentane, n-Hexane, and n-Heptane in the Rat. Br. J. Ind. Med. 1980, 37 (3), 241−247. (17) (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; p 30. (b) American Chemical Society. Principles of Green Chemistry; https://www.acs.org/ content/acs/en/greenchemistry/what-is-green-chemistry/principles/ 12-principles-of-green-chemistry.html (accessed April 4th, 2018). (18) (a) Chuma, A.; Horn, H. W.; Swope, W. C.; Pratt, R. C.; Zhang, L.; Lohmeijer, B. G. G.; Wade, C. G.; Waymouth, R. M.; Hedrick, J. L.; Rice, J. E. The Reaction Mechanism for the Organocatalytic RingOpening Polymerization of l-Lactide Using a Guanidine-Based Catalyst: Hydrogen-Bonded or Covalently Bound? J. Am. Chem. Soc. 2008, 130 (21), 6749−6754. (b) Kiesewetter, M. K.; Scholten, M. D.; Kirn, N.; Weber, R. L.; Hedrick, J. L.; Waymouth, R. M. Cyclic Guanidine Organic Catalysts: What Is Magic About Triazabicyclodecene? J. Org. Chem. 2009, 74 (24), 9490−9496. (c) Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A. P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Guanidine and Amidine Organocatalysts for Ring-opening Polymerization of Cyclic Esters. Macromolecules 2006, 39 (25), 8574−8583. (19) Standard Method for Acid and Base Number by Color-Indicator Titration, D974-14; ASTM International: West Conshohocken, PA, 2014. (20) Hayyan, A.; Alam, M. Z.; Mirghani, M. E. S.; Kabbashi, N.; Hakimi, N. I. N. M.; Siran, Y. M.; Tahiruddin, S. Reduction of High Content of Free Fatty Acid in Sludge Palm Oil via Acid Catalyst for Biodiesel Production. Fuel Process. Technol. 2011, 92 (5), 920−924. (21) Boucher, M. B.; Unker, S. A.; Hawley, K. R.; Wilhite, B. A.; Stuart, J. D.; Parnas, R. S. Variables Affecting Homogeneous Acid Catalyst Recoverability and Reuse After Esterification of Concentrated omega-9 Polyunsaturated Fatty Acids in Vegetable Oil Triglycerides. Green Chem. 2008, 10 (12), 1331−1336. (22) Adamo, A.; Heider, P. L.; Weeranoppanant, N.; Jensen, K. F. Membrane-based, Liquid−Liquid Separator with Integrated Pressure Control. Ind. Eng. Chem. Res. 2013, 52 (31), 10802−10808.

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Continuous-Flow Chemistry in Undergraduate Education: Sustainable

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