Introducing education for sustainable development in the undergraduate laboratory: quantitative analysis of bioethanol fuel and its blends with gasoline by using solvatochromic dyes

Abstract

The concept of Education for Sustainable Development, ESD, has been introduced in a period where chemistry education is undergoing a major change, both in emphasis and methods of teaching. Studying an everyday problem, with an important socio-economic impact in the laboratory is a part of this approach. Presently, the students in many countries go to school in vehicles that run, at least partially, on biofuels; it is high time to let them test these fuels. The use of renewable fuels is not new: since 1931 the gasoline sold in Brazil contains 20 to 25 vol-% of bioethanol; this composition is being continually monitored. With ESD in mind, we have employed a constructivist approach in an undergraduate course, where UV-vis spectroscopy has been employed for the determination of the composition of two fuel blends, namely, bioethanol/water, and bioethanol/gasoline. The activities started by giving a three-part quiz. The first and second ones introduced the students to historical and practical aspects of the theme (biofuels). In the third part, we asked them to develop a UV-vis experiment for the determination of the composition of fuel blends. They have tested two approaches: (i) use of a solvatochromic dye, followed by determination of fuel composition from plots of the empirical fuel polarity versus its composition; (ii) use of an ethanol-soluble dye, followed by determination of the blend composition from a Beer's law plot; the former proved to be much more convenient. Their evaluation of the experiment was highly positive, because of the relevance of the problem; the (constructivist) approach employed, and the bright colors that the solvatochromic dye acquire in these fuel blends. Thus ESD can be fruitfully employed in order to motivate the students; make the laboratory “fun”, and teach them theory (solvation). The experiments reported here can also be given to undergraduate students whose major is not chemistry (engineering, pharmacy, biology, etc.). They are low-cost and safe to be introduced at high-school level.

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1. Introduction

1.1. Education for Sustainable Development (ESD)

The importance of Sustainable Development is underlined by the fact that the United Nations declared the years 2005 through 2014 the “Decade of Education for Sustainable Development”. During this decade, the goal is “to integrate the principles, values, and practices of sustainable development into all aspects of education and learning, in order to address the social, economic, cultural, and environmental problems we face in the 21st century” (UNESCO, 2005). This declaration comes in a period where chemistry education is being redesigned. Inter alia, this is due to the need of new ways of thinking the socio-economic issues that we are facing. These include the increased demand on natural resources, and the need to employ sustainable alternatives of production. In this context, chemists can play an important role by applying the concepts of Green Chemistry (Anastas and Warner, 1998) to the chain of production. This line of thinking should start at high-school and undergraduate levels, by implementing new syllabi, and introducing innovative methods that emphasize teaching through a problem-oriented approach. The latter dwells on current, socio-economically important issues, e.g., the increased use of sustainable fuels, biodegradable polymers, and green solvents. An equally important issue is the necessity of moving teaching from a teacher- to a student-centered approach. The former (classic lecturing) amounts to a simple process of information transfer, good-humoredly represented by the so-called “Nürnberger funnel” method (“discharge” the information; presume the students absorb and understand them). In variance with the latter approach, the students work in groups to solve relevant problems and are encouraged to make- and interchange suggestions for possible solutions (Eilks and Byers, 2010). With the aim of applying ESD, we have introduced in 2010, an experiment on the analysis of blends of biodiesel-, ethanol- and (petroleum-based) diesel oil (El Seoud et al., 2011). The constructivist teaching approach was a success, chemistry-majors determined the composition of mixtures of these fuels by using spectroscopy in the UV-vis region. This prompted us to increase the experiment's impact by analyzing more relevant fuel systems. The constructivist approach has been maintained, as well as the experiment's theme (biofuels) because of its socio-economic relevance in Brazil, vide infra.

1.2. Background of the experiment: an ESD approach to biofuels and solvatochromism

1.2.1. Biofuels. Bioethanol (hereafter designated as ethanol) and other biofuels (in particular biodiesel) are here to stay! From the socio-economic point of view, ethanol and its mixtures with gasoline are much more relevant because: (i) the content of biodiesel in diesel oil in Brazil is meager, currently at 5 vol-%; (ii) the use of ethanol–diesel oil blends is associated with a series of technical problems, including phase-separation – especially at low temperatures – and decreasing the cetane number; (iii) the use of “gasohol” (gasoline + alcohol) has expanded to many countries (AIT/FIA Information Centre (OTA), 2007; Ethanol.org, State Legislation, 2008); (iv) a major issue, however, is that the use of ethanol is an important part of the history of the Brazilian car industry. The first car to run on 70 vol-% ethanol was tested in 1925 (Marcolin, 2008). Since 1931 the gasoline sold contains ethanol that has been employed, in part, in order to boost the octane rating with concomitant decrease in the use of anti-knocking additives (the octane number of ethanol is almost equal to that of isooctane, the “reference” fuel employed for octane rating). Mass-produced passenger cars have been running on (hydrated) ethanol for the last 32 years (Marcolin, 2008), see Fig. 1; since 2003 the cars sold in Brazil are flexible-fuel ones, i.e., they run on gasoline, ethanol, or any mixture of both (Lemos, 2007).

Fig. 1 The first car to run on 70 vol-% ethanol was tested in Rio de Janeiro in 1925 (A); First vehicle that runs on hydrated ethanol, mass produced in 1978 (B) (Marcolin, 2008).

1.2.2. Education for Sustainable Development. For the reasons listed above, it is not surprising that several articles appeared in science education journals for introducing ESD in the undergraduate laboratory. The environmental/sustainability issues have been a main concern. The following are some representative examples. Bullock showed that fermentation is an important anaerobic process to obtain ethanol from sugars and yeasts or bacteria (Bullock, 2002). Lancaster described the use of corn to produce detergents, fuels, biodegradable lubricants, and solvents (Lancaster, 2002). King reported that waste, ground coffee provides an inexpensive, environmentally friendly source of biodiesel fuel (King, 2010). Oliver-Hoyo and Pinto described how students employed simple chemical concepts, e.g., stoichiometry, density, and combustion reactions in order to calculate the emission rates of CO₂, and to compare the CO₂ emission to the fuel consumption for gasoline and diesel engines (Oliver-Hoyo and Pinto, 2008). Apelian introduced a course on sustainable development with the general aim of letting the students “grapple real and messy problems of our world”. (Apelian, 2010). Pietro described an undergraduate activity where freshmen chemistry students calculated the “corn-area-per-car” ratio using the first law of thermodynamics (Pietro, 2009). Problem-oriented lesson plans based on the use of ethanol as alternative fuel and energy source that specifically focus on handling the scientific and technological issues within society have been presented by Feierabend and Eilks (Feierabend and Eilks, 2011).

1.2.3. Solvatochromism. The term solvatochromism describes the change in position (sometimes also the intensity and shape) of a spectroscopic band, absorption or emission, of a dissolved compound caused by a change in polarity of the medium (Reichardt, 1994, 2008; Reichardt and Welton, 2010). A hypsochromic band shift with increasing solvent polarity is called negative solvatochromism, the corresponding bathochromic band shift is called positive solvatochromism. These shifts are caused by differential solvation of the electronic ground- and first excited state of solvatochromic dyes, thus changing the energy gap between the two states. Better solvation of a more dipolar ground state, relative to the less dipolar excited state leads, with increasing solvent polarity, to negative solvatochromism; the converse produces positive solvatochromism.

Provided that the absorption band of a solvatochromic dye is in the visible region, and the polarities of the components of the medium differ to some extent, then the dye solutions exhibit different colors which can be (hopefully) distinguished with the naked eye, and be correlated with the mixture composition. Although the approach of determining the composition of fuel-blends by solvatochromic dyes has been employed some years ago by Machado (Budag et al., 2007), and more recently by this group to mixtures of ethanol and biodiesel (El Seoud et al., 2011), it is in fact a recurring idea. It was introduced first by W. John who employed oxy-phenazine dyes for the analysis of fuels. An account of this work was published by K. Dimroth after John's death (John, 1947). Thus, there is a historical aspect to this application; it was first suggested during the Second World War!

1.3. The pre-lab activity

Although an important goal of the laboratory is to learn a practical skill (how can something be done?) and to organize, analyze and present the data obtained (what do these results mean?), the experiment should be linked to theory (Boud et al., 1980). Most importantly, however, the experiment should address, where possible, a real problem with an important socio-economic impact. These criteria are met in an experiment on the analysis of fuel blends that we gave in the UV-vis part of a spectroscopy course (Chem. 2144; fifth semester, 60 chemistry-major students). This analysis is important to consumers (the students) because fuel adulteration is a potential problem; typically by adding water to ethanol.

The activities were started by dividing the students into 8 groups; we asked them to read about biofuels; a week later we gave them a quiz that is divided into three parts. The objective of the first part was to evaluate their knowledge of the history of using ethanol as fuel, and the current use of fuel blends. After the first part, they discussed the consequences of fuel adulteration, and the importance of developing techniques that allow easy evaluation of fuel purity/quality. The second part was on the specifications and production of fuel blends. In part three, we asked them to suggest a simple UV-vis experiment in order to obtain information on the alcohol contents of hydrated ethanol and gasoline–ethanol blends (a copy of this quiz can be obtained from the correspondence author). Their answers were discussed in the class; we introduced the concept of solvatochromism; they went to the laboratory and tested the different procedures.

2. Experimental

2.1. Synthesis of the solvatochromic dye and purification of the fuels

The dye employed, 2,6-bis[4-(tert-butyl)phenyl]-4-{2,4,6-tris[4-(tert-butyl)phenyl]-pyridinium-1-yl}phenolate, (t-Bu)₅RB, molecular structure shown in Fig. 2, has been synthesized as given elsewhere (Reichardt and Harbusch–Görnert, 1983; Reichardt, 1994; Reichardt et al., 1998). Its UV-vis spectrum is sensitive to acids and oxidizing agents. Before the laboratory period, the staff did the following treatments of commercial products: “Anhydrous” ethanol was further dried with activated molecular sieves type 3 Å. Commercial gasohol, 500 mL, was washed, with 100 mL of an aqueous solution containing 10 mass-% NaCl, then with 1 mass-% HCl (in order to remove ethanol and the octane-number additive tert-butyl methyl ether), and finally with water. The fuel was stirred for 2 h with 5 g of activated charcoal and 10 g of anhydrous MgSO₄, filtered, and passed through a column containing 25 g of silica gel used in flash chromatography. Aliquots, each 0.2 mL of a 1.0 × 10⁻³ mol L⁻¹ solution of (t-Bu)₅RB in acetone, were pipetted into 2 mL volumetric tubes, followed by solvent evaporation under reduced pressure. The tubes were stoppered and later they were given to the students.

Fig. 2 Molecular structure of the electronic ground (A) and first excited state (B) of the penta-tert-butyl-substituted pyridinium N-phenolate betaine dye (t-Bu)₅RB.

2.2. UV-vis determination of the dependence of E_T[(t-Bu)₅RB] on fuel composition

The following steps were carried out by the students in a four-hour laboratory period:

2.2.1. Preparation of the dye solutions. Eleven solutions of (t-Bu)₅RB in ethanol/water mixtures (90 to 100 vol-% ethanol) and eighteen solutions of (t-Bu)₅RB in anhydrous ethanol/gasoline (alcohol-free) mixtures (10 to 100 vol-% ethanol) were prepared in 10 mL volumetric flasks.

2.2.2. Determination of λ_max of the solvatochromic dye. The UV-vis spectra were measured with a Shimadzu UV-2550 spectrophotometer, equipped with a thermostatted cell-holder. The stoppered cells with 1 cm path-length, containing the dye solution, were thermostatted at 25 °C for 10 min and then the spectra were recorded with a scanning rate of 140 nm min⁻¹ in the range of 360 to 900 nm. Values of λ_max were calculated from the first derivative of the absorption spectra.

2.3. Treatment of residues

All solvents and spent fuels were collected and sent to the residue treatment unit of Institute of Chemistry, USP, for proper disposal.

3. Results and discussion

3.1. Results of the quiz

The answers of the students to the three parts of the quiz were as follows: The first part showed that all students knew when Brazil started producing cars that run on hydrated ethanol (1978); what the terms E10 and E25 mean (gasohol with 10 and 25 vol-% ethanol, respectively); one group was aware of the fact that ethanol has been used as additive for gasoline since 1931.

The second part showed that three groups knew that cars in Brazil run on 92.6–93.8 vol-% ethanol (Brazilian legislation); one group knew the boiling point and composition of the azeotropic ethanol/water mixture. The methods that they suggested for the industrial production of anhydrous ethanol included azeotropic distillation with a third component (toluene; one group), treatment with a drying agent (CaCl₂, MgSO₄, CaO, H₂SO₄; five groups) and treatment with activated molecular sieves (two groups). Before handing out the third quiz sheet, we briefly discussed the use of UV-vis spectroscopy for the quantitative analysis of fuels. We indicated that, in principle, this can be done directly, by measuring the absorbance of the fuel itself, or by using an indicator dye.

The answers of the quiz showed that the students are aware of the historic background of the use of ethanol as fuel in Brazil. Except for high costs, most of their answers regarding the industrial production of anhydrous ethanol were correct. For determining fuel composition, they realized that measuring the fuel absorbance directly will not work because commercial gasoline has some color, hence its absorbance depends on the crude oil employed and the distillation conditions in the refinery. Instead, they suggested the use of a dye that is soluble in one component only, ethanol. The sequence that they suggested was: Add excess solid dye to the fuel mixture; dissolve; filter (the remaining dye); measure the absorbance of the filtrate; determine the composition from a Beer's law plot.

3.2. Linking the experiment to theory: solvatochromism

We have introduced the phenomenon of solvatochromism (dependence of solute color on solvent polarity); explained why this property is sensitively dependent on mixture composition, and then we introduced the indicator (t-Bu)₅RB; see Fig. 2. The reason for the observed negative solvatochromism was briefly mentioned; the equation for calculating the corresponding transition energy was given: E_T[(t-Bu)₅RB] (E_T/kcal mol⁻¹ = 28591.5/λ_max in nm). (Reichardt, 2005, 2008; El Seoud, 2007, 2009; Reichardt and Welton, 2010). After this discussion they went to the laboratory; Fig. 3 shows the students in “action”.

Fig. 3 A group of students using the UV-vis spectrophotometer.

3.3. The results obtained

Fig. 4 and 5 show the results obtained, namely the dependence of E_T[(t-Bu)₅RB] on the composition of authentic and unknown samples of ethanol/water and ethanol/gasoline mixtures, respectively.

Fig. 4 Analytical calibration curve () and unknown samples () measured for ethanol/water mixtures. The straight line is drawn in order to guide the eye. The volume fraction of ethanol is 90 to 100%. In this, and in Fig. 5, the polynomial fitting was made with a commercial software (Microcal Origin) that minimizes the nonlinear correlation coefficient, r².

Fig. 5 Analytical calibration curve () and unknown samples () measured for ethanol/gasoline mixtures. The straight line is drawn in order to guide the eye. The volume fraction of anhydrous ethanol is 10 to 100%. The data point of pure gasoline, was omitted because the calculated E_T is much lower than what would be expected from this curve. This is due to strong preferential solvation by ethanol, as discussed elsewhere (El Seoud, 2009).

This dependence is not linear because of preferential solvation of the dye by one component of the binary mixture (Marcus, 2002; (Reichardt, 2005, 2008; El Seoud, 2007, 2009; Silva et al., 2009; Reichardt and Welton, 2010). In case of (t-Bu)₅RB, the electronic ground state is highly dipolar and has a strong hydrogen-bond accepting (HBA) phenolate oxygen. Therefore, it is preferentially solvated by the more polar, hydrogen-bond donating (HBD) solvent ethanol, as depicted in Fig. 6. The students' results, including the determination of the composition of unknown samples, were excellent, in view of the fact that this was their first experience with a UV-vis spectrophotometer and derivative spectra.

Fig. 6 Schematic representation of the possibilities for the solvation of a dye (yellow), by an equimolar mixture of ethanol (blue) and gasoline (red). Ideal solvation: composition of the solvation shell corresponds to that of the bulk mixture; preferential solvation: the solvation shell is enriched in ethanol.

After the laboratory work, the groups reported their results in a seminar. They exchanged their experimental data in order to construct a full analytical calibration curve (each group measured half of the samples) and determined the composition of one unknown sample. They discussed the results; the concepts involved, and decided to test the approach that they have suggested, i.e., to use a dye that is soluble in ethanol. Two groups went again to the laboratory, did the experiment, and reported the results obtained. Their first choice was methylene blue. This dye is extremely soluble in ethanol, insoluble in gasoline, but becomes soluble in the presence of a small volume of ethanol, e.g., 10 vol-%. The blue color developed was very intense; large amounts of solid dye should be added; several dilutions of the filtrate were required in order to give absorbance of ca. 1 (in 1 cm path-length cell). This dye was not further pursued because of the labor involved. Two additional compounds were then tested, Congo red and Indigo blue; both are very slightly soluble in gasoline and soluble in ethanol. The values of λ_max of the former indicator increased from 469 to 500 nm as a function of increasing the volume fraction of ethanol from 20 to 90%. A Beer's law-type plot constructed from absorbance (A) at λ_maxversus ethanol vol-% is not linear, but is described by an exponential growth curve (A = 0.1076 × e^{0.0514 Ethanol vol-%}; r² = 0.995, n = 9 samples). A similar curve (with r² = 0.959) was obtained when (A) at λ, arbitrarily fixed at 485 nm, was used instead. Interestingly, their results showed that the dependence of λ_max on ethanol vol-% is reasonably linear (λ_max = 462.24 + 0.405 ethanol vol-%; r² = 0.823) and not curved, as expected for preferential solvation. Indigo blue was not convenient. The dye absorbance was low (A = 0.126 to 0.49, 1 cm path-length cell); plots of (A) or λ_maxversus ethanol vol-% showed scattering (A = 0.157 + 0.0038 × ethanol vol-%; r² = 0.942; λ_max = 560 − 0.152 × ethanol vol-%; r² = 0.741).

3.4. Evaluation

3.4.1. The experiment. The most fascinating aspect of this experiment is the color change as a function of fuel composition, as shown in Fig. 7 and Fig. 8. This connects an abstract phenomenon such as solvation to a real situation (the fuel of our cars) in a vivid way. The experiment introduces a novel approach for quantitative analysis: one determines the dependence of λ_max, not the absorbance, on the solution composition. We have employed (t-Bu)₅RB because of its appreciable solvatochromism [Δλ = λ_max(gasoline) − λ_max (ethanol) = 70 nm!] and sufficient solubility in apolar media.

Fig. 7 The continuous color variation of solutions of (t-Bu)₅RB in binary gasoline/ethanol mixtures from pure gasoline to pure ethanol; E refers to the volume percentage of ethanol in the mixture.

Fig. 8 The continuous color variation of solutions of (t-Bu)₅RB in binary ethanol/water mixtures from pure ethanol to 90% water in ethanol; W refers to the volume percentage of water in the mixture.

3.4.2. The students' reaction. Because the students' are not used to this (constructivist) approach, they reacted initially with some reserve to the idea of asking them to make the decision. Once the activities were started, they realized that it is “fun” to be in charge. The students' post-experiment evaluation showed that they have enjoyed several aspects of our approach: the way that we introduced the subject; the connection to a real problem; the visual vividness of the colors, and that the results explain the apparently abstract phenomenon of solvation. They have pointed out that this approach is challenging because they were not instructed on how to proceed; they had to find out. Although they are used to write reports on their experiments, they found the (alternative) presentation of a seminar “nice and fruitful”. A few of their comments reflect how they reacted to this experiment: “We thought that the theme was very interesting and practical, showing applications of spectroscopy in an everyday situation. The approach employed was much more interesting since we had to find out how to do everything”; “I really liked the way the experiment was run; the quiz and the group work helped”; “this method holds your attention and induces you to think of a solution. I really liked discussing the material with my team-mates and I now think more about bioethanol”.

3.4.3. Final comments. Although preparing for the experiment is time-consuming, the outcome is gratifying because of the introduction of ESD; the (constructivist) methodology employed, and the students' positive reaction. The labor involved, however, can be reduced, e.g., by using: model fuel instead of commercial gasoline (we have successfully experimented a mixture of equal volumes of isooctane, decalines, and xylenes); the commercially available dye (2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate), RB, or some other easily synthesized solvatochromic compounds, (Hubert et al., 1995; Budag et al., 2007) instead of (t-Bu)₅RB. These indicators can be employed, provided that the blend contains > 5 vol-% ethanol.

4. Conclusions

A constructivist approach has been employed in order to introduce an ESD experiment on fuel analysis; the proposal was tested with 60 chemistry-major students in their fifth-semester. The theme chosen, blends of fuels, has proved interesting because of the long history of using ethanol as fuel in Brazil. The quiz given has been effective in bringing this important socio-economic subject into focus; in involving the students in the planning of the experiment, and in testing dyes whose responses to fuel composition have distinct origins (solvatochromism and simple solubilization, for (t-Bu)₅RB and Congo red, respectively). We feel that the outcome of this ESD experiment is highly positive: the students acquired experimental skills; they enjoyed the constructivist approach and the nice, vivid colors of the dye in different blends; they determined the properties of mixtures by measuring the spectral response of an appropriate dye; they learned theory (solvation), and some history. This experiment is safe, low cost, and requires readily available apparatus. It can be employed with students of different backgrounds, including undergraduates whose major is not chemistry, and high-school students.

Acknowledgements

We thank FAPESP (State of São Paulo Research Foundation) and the CNPq (National Council for Scientific Technological Research) for financial support and fellowships, Cezar Guizzo for his help; the students of Chem. 2144 for their enthusiasm.

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Footnote

† This article is part of a themed issue on sustainable development and green chemistry in chemistry education.

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