Chemistry Everyday for Everyone
Secondary School Chemistry
Experiments with Zeolites at the Secondary-School Level: Experience from The Netherlands Eric N. Coker*† and Pamela J. Davis Laboratory of Organic Chemistry and Catalysis, Faculty of Chemical Technology and Materials Science, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft, The Netherlands; *
[email protected] Aonne Kerkstra Interconfessioneel Westland College, PO Box 114, NL-2670 Naaldwijk, The Netherlands Herman van Bekkum Laboratory of Organic Chemistry and Catalysis, Faculty of Chemical Technology and Materials Science, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft, The Netherlands
Almost every brand of laundry detergent on the European market contains 15 to 30% zeolite by weight. Numerous other household products contain or were manufactured with the help of zeolites, and the petrochemical industry relies heavily on them as catalytic and separative media in the production of gasoline. In the synthesis of fine chemicals, zeolites seem destined to play an important role as clean and recyclable catalysts. However, despite their importance, zeolites are virtually unknown in the secondary-school classroom. This article describes a series of experiments demonstrating some of the properties of zeolites. It has been well received by secondary schools in The Netherlands. Zeolite structures and properties have been described elsewhere (1–3); thus only a brief review of some areas of particular relevance to the experiments described in this article will be given here. Zeolite structures may be described as being formed by linking together tetrahedral SiO4 and AlO4 units at their corners. They have the empirical formula [(SiO2)(AlO2)x]Mx/nn+?wH2O, where M is an n-valent cation, usually alkali or alkaline earth. The entities within the square brackets comprise the framework, and the remaining species occupy the pores. Figure 1 represents the framework structure of zeolites A and Y. While SiO2 is electroneutral, the formal charge on an AlO2 unit is {1; thus for each aluminum atom present in the structure, a counterbalancing cation is required to preserve electroneutrality. These cations, as well as the water molecules, reside in the channels and cavities of the structures and are free to migrate. When a zeolite sample is placed in an electrolyte solution, exchange of ions may occur between the zeolite and fluid phases. Building models of zeolite structures is a valuable way to gain an understanding of their function; examples of how to do this may be found in the literature (4, 5). The microporous nature of zeolites (meaning their pores are less than 2 nm in diameter) allows them to adsorb molecules and ions equal in size to or smaller than the pore openings, while excluding those that are larger. For this reason, zeolites have earned the name “molecular sieves”. Figure 2 illustrates the three major types of pore structure
Figure 1. The structure of zeolite A (top) and zeolite Y (bottom). Each corner represents a silicon or aluminum atom; oxygen atoms lie close to the midpoint of each straight line.
Figure 2. Three major pore structures found in zeolites. Top: one-dimensional (parallel channels); middle: two- or three-dimensional (crossing channels); bottom: three-dimensional (interconnected cages).
† Current address: BP Amoco Chemicals Ltd., Poplar House, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LL, UK.
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Chemistry Everyday for Everyone Table 1. Pore Characteristics of Some Common Zeolites Zeolite Type
Figure 3. Molecular sieving occurs when n -octane and isooctane attempt to enter zeolite A. The straight-chain n-octane is able to pass through the 0.41-nm pore opening, whereas its branched isomer is too bulky to fit.
that occur in zeolites. Each type of zeolite possesses a welldefined crystalline structure, with pores of certain distinct sizes. Table 1 lists the pore sizes for a few common zeolites, while Figure 3 illustrates the concept of molecular sieving: n-octane is able to pass through the 0.41-nm pore opening of zeolite A, whereas its isomer 2,2,4-trimethylpentane (“isooctane”) is not. For a given application, one may choose a zeolite with the desired properties and pore size. By replacing the exchangeable cations by protons, acidic zeolites may be generated and used as catalysts. Owing to the selective adsorption of molecules described above, the zeolites may act as highly selective catalysts; only molecules small enough to enter the pores will be able to react with the catalytic sites inside the structure. The most important applications of zeolites are arguably in the petrochemical industry. Zeolite Y is used extensively in catalytic cracking processes, while mordenite catalyzes the isomerization of straight-chain alkanes to branched ones, which increases the octane number of gasoline. The largest use of zeolites, in terms of volume, is in laundry detergents, where they act as water-softeners, superseding the environmentally damaging sodium tripolyphosphate (Na5P3O10). Sodium zeolite A is most commonly used in detergents, where it softens the water by removing Ca2+ and Mg2+ ions and replacing them with Na+ ions. Recently, zeolite MAP has been introduced into detergent formulations. It is claimed to possess better ion-exchange properties with respect to Ca2+ and Mg2+ than zeolite A (6 ). Only a few examples exist in the literature of experiments involving zeolites that are suitable for students of secondary schools (2, 7–9) or universities (3,10–13). The experiments described in this article demonstrate the water-softening capability of detergents containing zeolites, giving students the opportunity to compare the performance of a variety of commercially available laundry detergents. Catalytic properties are also demonstrated, by the production of an ester in the presence of an acidic zeolite. Experiments with Zeolites
Determination of the Hardness of Tap Water Tap water contains variable quantities of Ca2+, Mg2+, and anions such as HCO3{. The concentration of these ions de1418
Pore Diameter/nma
O Atoms Defining Pore (No.)b
Zeolite A
0.41
8
Zeolite Y
0.74
12
Mordenite
0.65 × 0.70 0.26 × 0.57
12 8
ZSM-5
0.53 × 0.56 0.51 × 0.55
10 10
aWhere two dimensions are given, these are the minimum and maximum diameters of noncircular pores. bEqual to the number of (Si + Al) atoms defining the pore opening.
termines the hardness of the water. Calcium is usually the dominant species. As a prelude to the experiments involving water-softening by zeolites, it is necessary to know the hardness of the local tap water, so that the effectiveness of the zeolite may be assessed. This is easily determined through a complexometric titration with ethylenediaminetetraacetic acid (EDTA) in the presence of an indicator. Since complexometric titrations with EDTA are quantitative only in a basic environment (under which conditions EDTA is fully deprotonated), a buffer solution must be prepared. A buffer operating at a pH of about 10.5 may be prepared by carefully adding 68.2 mL of hydrochloric acid (37%) to 31.8 mL of 2-aminoethanol in a fume hood. To ensure that the color change of the Eriochrome black T indicator is clearly visible, it is often necessary to add a small quantity of (MgEDTA)2{ complex to the buffer to give approximately 0.0025 M (MgEDTA)2{. Measure 50 mL of tap water using a measuring cylinder or graduated pipet, and place it in an Erlenmeyer flask. To this, add 4 mL of the buffer solution and swirl gently to mix the contents. then add a small amount of Eriochrome black T indicator from the tip of a spatula, and again swirl the flask gently. Titrate this solution against 0.01 M EDTA from a buret until the color of the indicator changes from red to clear blue. From the amount of EDTA solution used, and given that the molar stoichiometry of the M2+–EDTA complex is 1:1, the hardness of the water may be calculated.
Determination of the Ability of Zeolite A to Bind Ca2+ Zeolite Na-A may be purchased for a reasonable price from a number of chemical suppliers, or it may be synthesized following a simple and quick procedure, as reported by Lowe (2). Alternatively, the zeolite may be extracted from laundry detergent as described by Smoot and Lindquist (3), since it is the only insoluble component, or it may be received as a gift from a bulk zeolite manufacturer.1 In this experiment, zeolite Na-A is placed in contact with an excess of Ca2+ ions, and the amount of calcium remaining in solution after ion exchange with the zeolite is determined by a complexometric titration similar to that in the first experiment. A weighed amount of zeolite Na-A (approximately 200 mg) is placed in a 100-mL beaker and 75 mL of 0.01 M CaCl2 is added from a graduated cylinder or pipet. After a few minutes of stirring, the suspension may be filtered to remove the zeolite. A 25-mL portion of the filtrate is transferred to an Erlenmeyer flask, to which is added 4 mL of buffer solution and a small quantity of Eriochrome black T indicator,
Journal of Chemical Education • Vol. 76 No. 10 October 1999 • JChemEd.chem.wisc.edu
Chemistry Everyday for Everyone
with swirling. The calcium in solution is then titrated against 0.01 M EDTA solution until the indicator changes from red to clear blue. A duplicate determination should be carried out on a second 25-mL portion of the filtrate. From this experiment, the amount of calcium that is bound per gram of zeolite may be calculated. This result will be used in the calculations associated with the following experiment.
Investigation of Laundry Detergents Here, the preceding experiment is repeated using commercial laundry detergents containing zeolites in place of the pure zeolite powders. By choosing a variety of commercially available detergents, students can survey the water-softening capabilities of different brands. Of course, the brand used by the student’s parents at home is included. The first step is to “dissolve” a 1-g portion of the laundry detergent in 100 mL of CaCl2 solution (0.01M). After stirring for a few minutes, the suspension is allowed to stand until the next laboratory session so that the solids (zeolite) may settle to the bottom of the vessel. Filtration of these suspensions is often unsuccessful. When all solids have settled, 25 mL of the supernatant is measured into an Erlenmeyer flask and 4 mL of buffer and a small quantity of Eriochrome black T indicator are added as before. The calcium in solution is then titrated with 0.01 M EDTA, the end point being when the indicator changes color from red to clear blue. The determination may be repeated on a second 25-mL aliquot of the supernatant. Each student or group of students may be given a different brand of detergent. The amount of calcium bound by one gram of laundry detergent can be calculated and, given the result of the previous experiment, the weight percentage of zeolite A in the washing powder may be determined. Zeolite Hydrogen-Y as a Catalyst for the Preparation of an Ester An ester is formed from a carboxylic acid and an alcohol. In this experiment, pentyl ethanoate (amyl acetate), a common flavoring used in sweets, will be prepared from ethanoic acid and 1-pentanol. The catalytic properties and the recyclable nature of zeolite catalysts will be shown by performing two reactions: (i) a conventional esterification reaction using sulfuric acid as catalyst, and (ii) the same reaction catalyzed by a zeolite. Concentrated sulfuric acid used to be the catalyst of choice for such reactions in school chemistry. Zeolites and other solid acid catalysts have the advantages of being easily removed after reaction by filtration and being nonirritating to the skin (in fact, zeolites may be ingested without harm). In the zeolite-catalyzed experiment, the catalyst is removed from the products, regenerated, and then reused, whereas the sulfuric acid is waste material. Reuse of the zeolite and neutralized sulfuric acid waste will illustrate to the student modern and conventional techniques, respectively. Zeolite H-Y is usually prepared from zeolite NH4-Y by removal of ammonia at elevated temperature. Before the
esterification experiment begins, the zeolite catalyst must be activated, that is, heated to around 300 °C for 2 hours, to drive off water, ammonia, and other adsorbed species. The resultant catalyst will be in its hydrogen (acidic) form, and its pores will be free of adsorbed molecules. It should be noted that exposure of the activated zeolite to air will result in re-adsorption of water from the atmosphere, but this will not interfere too much with the experiment. Introduce 15 mL of ethanoic acid and 15 mL of 1pentanol into each of two round-bottomed flasks. To one of these, 2 g of activated zeolite H-Y is added, and to the other, 0.5 mL of concentrated sulfuric acid. Both flasks are then fitted with reflux condensers, and are boiled for 1 hour using an electric heating mantle. After cooling, the zeolite is collected by filtration, washed with water and dried, and following reactivation at 450 °C, it may be reused in a subsequent experiment. The contents of the flasks are then poured into beakers containing 300 mL of cold water. Phase separation occurs; the remaining acid and alcohol dissolve in the water, while the ester is immiscible. A separatory funnel is used to separate the organic and aqueous layers. To purify the ester and remove any traces of alcohol or acid, the organic layer may be washed several times with 25-mL aliquots of distilled water. Acknowledgment We thank A. J. Hoefnagel (Delft University) for assistance in developing the esterification experiment. Note 1. In The Netherlands, two companies felt that these experiments would bring chemistry in a natural way closer to modern chemistry and chemical materials, and they generously provided the secondary school teachers with 2-kg samples of zeolites. Thus the detergent company Unilever provided the Na-A samples and catalyst manufacturer Akzo Nobel Chemicals provided the NH4-Y samples.
Literature Cited 1. For a general introduction to zeolites, see Dyer, A. An Introduction to Zeolite Molecular Sieves; Wiley: Chichester, 1988. 2. Lowe, B. M. Educ. Chem. 1992, 29, 15–18. 3. Smoot, A. L; Lindquist, D. A. J. Chem. Educ. 1997, 74, 569–570. 4. Walton, A. Educ. Chem. 1972, 9, 146–147, 149. 5. Huang, Y. J. Chem. Educ. 1980, 57, 112–113. 6. Adams, C. J.; Araya, A.; Carr, S. W.; Chapple, A. P.; Franklin, K. R.; Graham, P.; Minihan, A. R.; Osinga, T. J.; Stuart, J. A. Stud. Surf. Sci. Catal. 1997, 105, 1667–1674. 7. Dietrich, V. Prax. Naturwiss., Chem. 1996, 45, 17–25. 8. Scheller, R.; Just, E. Prax. Naturwiss., Chem. 1995, 44, 23–31. 9. Peter, R. Prax. Naturwiss., Chem. 1989, 38, 41–44. 10. Balkus, K. J.; Ly, K. T. J. Chem. Educ. 1991, 68, 875–877. 11. Blatter, F.; Schumacher, E. J. Chem. Educ. 1990, 67, 519–521. 12. Copperthwaite, R. G.; Hutchings, G. J.; van der Riet, M. J. Chem. Educ. 1986, 63, 632–634. 13. Bibby, D. M.; Copperthwaite, R. G.; Hutchings, G. J.; Johnston, P.; Orchard, S. W. J. Chem. Educ. 1986, 63, 634–637.
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