Demonstration pubs.acs.org/jchemeduc
Esterification Reaction Utilizing Sense of Smell and Eyesight for Conversion and Catalyst Recovery Monitoring Nikki Janssens, Lik H. Wee, and Johan A. Martens* Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, KU Leuven, 3001 Heverlee, Belgium S Supporting Information *
ABSTRACT: The esterification reaction of salicylic acid with ethanol is performed in presence of dissolved 12-tungstophosphoric Brønsted−Lowry acid catalyst, a Keggintype polyoxometalate (POM). The monitoring of the reaction with smell and the recovery of the catalyst with sight is presented. Formation of the sweet-scented ester is apparent from the smell. After the reaction, the dissolved POM catalyst is recovered via encapsulation into a metal−organic framework (MOF) made of copper nitrate and benzene-1,3,5-tricarboxylic acid (BTC), Cu3(BTC)2 (also called HKUST-1). The encapsulation causes a color change from a greenish liquid to a suspension of blue catalyst-containing particles. The catalyst can be recovered by filtration, dissolved in a new batch of reactants, and reused. The simplicity of this reaction and the accessibility of the catalytic chemistry to human senses, avoiding the need of analytical tools, make this reaction suitable for a classroom demonstration of catalysis for educational purposes.
KEYWORDS: General Public, High School/Introductory Chemistry, First-Year Undergraduate/General, Second-Year Undergraduate, Demonstration, Brønsted-Lowry Acids, Catalysis, Esters
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formed during the reaction.23 Birney and Starnes described the use of the sense of smell in a laboratory experiment for identifying esters according their smell.11 This article provides a simple and relatively short reaction to demonstrate the use of a homogeneous catalyst in an esterification reaction and to observe the catalytic process by two human senses. Chemistry that students can see and smell is the chemistry they recall the best. In this demonstration, the pleasant-smelling ethyl salicylate is synthesized from salicylic acid and ethanol using a polyoxometalate (POM) catalyst, 12-tungstophosphoric acid (Scheme 1). The dissolved colorless 12-tungstophosphoric acid catalyst is recovered by encapsulation into blue recoverable metal−organic framework (MOF) particles. The recovery of a dissolved catalyst is one of the major challenges in
he importance of catalysis cannot be overestimated. Around 85% of all chemical products are produced using a catalytic process.1 Because chemistry and catalysis are inextricably linked, a practical demonstration of catalysis is an essential aspect of chemistry education.2,3 Homogeneous catalysis using a dissolved catalyst in liquid reaction medium is readily accessible.4,5 Besides demonstration of the catalytic acceleration of the conversion of reagents into products, the reaction mechanism and the dependence of reaction kinetics on reaction parameters, such as temperature, pH, catalyst, and reagent concentration, are typical elements of a demonstration.6 The use of human senses in chemistry experiments is an elegant way to verify the occurrence of a chemical reaction. Sight and smell are the two main senses a chemist may use.7−14 Eyesight is an important analytical tool in chemistry, for example, in calorimetry experiments or to determine the end point in acid−base titrations using a pH indicator or to determine the presence of a specific organic substance.15−18 The sense of smell is convenient in olfactory titrations, which uses smell to determine the end point, or in identification of organic functional groups in general, and specifically in the detection of organic ester formation.13,14,19−,22 Volatile esters are commonly used as fragrances and are contained in essential oils and pheromones. Because of the appealing smell, esters are used frequently for educational purposes, for example, Bromfield-Lee and Oliver-Hoyo studied esterification kinetics using the sense to detect the emergence of the ester aroma © XXXX American Chemical Society and Division of Chemical Education, Inc.
Scheme 1. Esterification Reaction of Salicylic Acid and Ethanol to Ethyl Salicylate
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MATERIALS 12-Tungstophosphoric acid hydrate (Sigma-Aldrich); absolute ethanol (VWR); salicylic acid (Aldrich); cupric nitrate trihydrate (Acros); benzene-1,3,5-tricarboxylic acid (ABCR); decane (Aldrich). Magnetic stir bar; hot plate stirrer; water−oil bath; clamp; glass vial (22 mL); weighing boats; pipet; pipet tips; spatula; funnel; filter papers (Whatman grade 5 circ. 90 mm, 2.5 μm).
homogeneous catalysis, and this demonstration will trigger the awareness of students to this problem. In the first part of the demonstration, homogeneous acid catalyst (Keggin-type POM) is used to accelerate formation of the ethyl salicylate ester from salicylic acid and ethanol. Ethyl salicylate is known as an artificial flavoring agent, the fragrance with an aroma similar to wintergreen.24 It is an ingredient in cosmetics, fragrances, and shampoos as well as in household cleaners and detergents. By smelling the reaction mixture, the formation of ethyl salicylate can be easily monitored. The homogeneous POM catalyst possesses elevated Brønsted− Lowry type of acidity.25 All catalytic sites are available, and catalytic activity is high due to the fact that the catalyst is homogeneous and readily dissolved. 12-Tungstophosphoric acid is a very strong acid. A major disadvantage is that the homogeneous catalyst cannot be easily separated from the products of the reaction solution.26 To solve this problem, a method of encapsulation of POM into a MOF made of copper nitrate and benzene-1,3,5-tricarboxylic acid (BTC), Cu3(BTC)2, was developed.27 MOFs are versatile porous materials; the combination of inorganic metals and organic linkers gives a large diversity of possible MOFs. With their high porosities, they are excellent for adsorption and separation of gases28 or for catalysis.29 Cu3(BTC)2 is a well-studied material that has the advantage of being easily synthesized.30 This encapsulation and formation of POM@MOF proceeds spontaneously upon addition of cupric nitrate trihydrate and benzene-1,3,5-tricarboxylic acid. The formation of POM@ MOF gives rise to a color change of the reaction medium from a greenish liquid to a blue solid. Addition of decane is needed to minimize solubility of POM@MOF, which is strongly solvent dependent,31−33 and causes the precipitation of POM@ MOF particles and decolorizing of the reaction solution. The recovered POM@MOF can be dissolved in a new batch of reactants, allowing the POM to be released for future use to catalyze another esterification reaction without significant loss of catalytic activity. A schematic overview of the encapsulation of POM into metal−organic framework is presented in Figure 1.
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DEMONSTRATION This demonstration is divided into three parts. First, esters are prepared using fresh POM. The second part is the encapsulation of POM catalyst through formation of POM@ MOF particles and their separation from the reaction medium. The third part is concerned with the reuse of the catalyst in a subsequent reaction. Prior to a lecture, an instructor should weigh out salicylic acid, POM, cupric nitrate trihydrate, and BTC in separate weighing boats. Synthesis of Ethyl Salicylate
In a dry 22-mL vial equipped with a magnetic stir bar, ethanol (18 mmol, 1.05 mL) is added as the first compound followed by salicylic acid (3.6 mmol, 0.5 g). Because of the limited solubility of salicylic acid in ethanol in a 1:5 molar ratio, salicylic acid powder is still noticeable in the liquid. After addition of 12tungstophosphoric acid (0.017 mmol, 50 mg), the vial is closed and inserted into a water bath at 85 °C. Salicylic acid and POM are now dissolved in the ethanol solution, and a yellow color is observed (Figure 2A). After 10 min stirring, the vial is taken out
Figure 2. Overview of color changes in the demonstration. (A) Reaction of salicylic acid and ethanol with POM; (B) after addition of cupric nitrate hydrate and BTC; (C) after addition of decane; (D) after 30 min stirring at room temperature.
of the bath. Before opening, the vial is cooled under running water. The typical wintergreen odor of ethyl salicylate is apparent. Students are instructed to smell chemicals by wafting. To validate the product by sight and odor, a blank sample is prepared without POM. This sample does not display the color and aroma of the desired product. Although the esterification of salicylic acid takes a few hours, after 10 min of reaction, a reasonable yield is obtained, sufficient to notice the smell. The formation of the ester product can be confirmed using gas chromatography−mass spectrometry (GC−MS) (See the Supporting Information). Recovery of the POM through Encapsulation in POM@MOF Particles
At the end of the reaction, the POM catalyst is recovered by encapsulation into a MOF. For this, Cu(NO3)2·3H2O (0.02 mmol, 50 mg) and BTC (0.11 mmol, 22 mg) are added to the reaction mixture; BTC acts as the linker (Figure 2B). To reduce the POM@MOF solubility, decane (approximately 15 mL) is added. The solution has a green color due to the
Figure 1. Schematic presentation of POM catalyst recovery and reuse via encapsulation into a MOF. B
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Flemish Government (Methusalem Funding) and the Belgian Government (IAP-PAI networking).
solvated tetraaquo copper(II) ions (Figure 2C). After approximately 30 min, the solution turns blue due to the formation of Cu3(BTC)2 particles (Figure 2D). The POM@MOF particles are gravity filtered using filter paper and a funnel; the colorless filtrate consists of decane with a trace of ethanol, salicylic acid, and ethyl salicylate. The filtrate can be reused in a new esterification cycle. The POM@MOF, which also contains some undissolved salicylic acid, can be airdried and stored for later use.
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The esterification can be repeated using the recovered POM@ MOF. It will dissolve readily upon addition of a mixture of salicylic acid and ethanol. The same quantities as above can be used. The color of the mixture will be green. As before, the wintergreen smell of the ethyl salicylate ester can be detected after about 10 min stirring and heating in a water bath at 85 °C. The POM@MOF is recovered by cooling the reaction mixture to room temperature and addition of decane (15 mL) followed by filtration. A blue-green, rather than a blue, color of the recovered encapsulated catalyst is due to some salicylic acid adhering to it. This POM@MOF material can be repeatedly reused and recovered in new cycles. X-ray diffraction (see the Supporting Information) shows that the POM@MOF is crystalline.
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HAZARDS Use gloves when handling 12-tungstophosphoric acid, salicylic acid, and cupric nitrate trihydrate because they can cause burns and are irritants in terms of skin contact and eye contact. Work in a hood to avoid breathing their dusts. During the procedure, eye protection and gloves should be used. In case of direct contact, flush contaminated areas with water. Ethanol and decane are flammable and should not be handled near open flames.
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SUMMARY This demonstration successfully illustrates the use of the sense of smell to monitor the selective formation of ethyl salicylate ester (Scheme 1) and the use of eyesight in the homogeneous POM catalyst recovery via encapsulation into metal−organic frameworks Cu3(BTC)2. The duration of this demonstration is ∼1 h. This easy and simple demonstration does not require expensive analytical tools, such as gas chromatography−mass spectrometry and X-ray diffraction instruments, but these additional measurements could be useful to confirm the observations. Also, FTIR could be used to confirm formation of the product in a laboratory setting if the instrumentation is available to students. ASSOCIATED CONTENT
S Supporting Information *
GC-MS of reaction products; XRD patterns of POM@MOF material. This material is available via the Internet at http:// pubs.acs.org.
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Reuse of POM@MOF
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AUTHOR INFORMATION
Corresponding Author
*J. A. Martens. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. C
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