In the Laboratory
Experiments for the Undergraduate Laboratory That Illustrate the Size-Exclusion Properties of Zeolite Molecular Sieves Jason Cooke* and Eric J. Henderson Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada; *
[email protected] Zeolites are a diverse class of materials with useful proper‑ ties that have attracted interest from the chemical education community (1, 2). They are a special class of naturally occur‑ ring or chemically synthesized framework aluminosilicates that are based on linked SiO4 and AlO4 tetrahedra, have a general x‒ formula Mn+ x/n[AlxSiyO2x+2y] ⋅zH2O, and have structures that contain open pores and channels (3). The resulting “molecular sieves” are so‑named because the zeolite is able to selectively adsorb molecules or cations that are small enough to fit into the channels or pores while excluding those that are too large, much in the same way that a mechanical sieve allows particles of a certain size to pass through its grating while retaining those of a larger size. Despite this useful ability, there are few experiments for the undergraduate laboratory that illustrate this property (1a, 1g, 1k). Rather, most existing studies demonstrate the im‑ portant ability of zeolites to selectively exchange cations and to act as drying agents by adsorbing water (1c, 1e–g, 2). We have developed a series of experiments that demonstrate the ability of zeolites to differentiate molecules on the basis of their size, thereby illustrating the usual “molecular sieve” description. Our goal was also to utilize equipment and techniques that would commonly be encountered in undergraduate laboratories so as to allow the experiments to be easily incorporated into existing programs. Several zeolites have common names that describe the ap‑ proximate pore diameter and thereby provide a rough guideline for the largest diameter molecule that might enter. Zeolite 4A, with an approximate pore diameter of 4 Å, is one of the more commonly encountered molecular sieves and has the simple empirical formula NaAlSiO4 in its fully dehydrated state. If a portion of the sodium counterions in zeolite 4A are exchanged for potassium, the central aperture collapses to a diameter of ap‑ proximately 3 Å (and is thus referred to as zeolite 3A) whereas if a portion are replaced with calcium ions, the aperture expands to approximately 5 Å (zeolite 5A) (4, 5). Zeolites can be designed with larger pore sizes; one such commercially available example is zeolite 13X, which has a central cavity opening that is ap‑ proximately 7.4 Å wide (1g, 1l, 2a). Apart from van der Waals forces, the primary forces in‑ volved in retaining small molecules within the zeolite are associ‑ ation with cations (through Lewis acid–Lewis base interactions) and, if available, through hydrogen bonding with the framework oxygen atoms (4). When it comes to approximating the size of molecule that will enter a pore of a particular diameter, there has been some debate as to the most appropriate measurement to use (6). Experimentally, the determination of whether a molecule is small enough to be admitted through the pore aperture and to be adsorbed within the extended channel network can be made by measuring the change in concentration of the species when it is exposed to the zeolite or by measuring a gain in mass of the solid zeolite (1g, 6). However, when carrying out such experi‑ ments, the potential for small changes from surface adsorption must not be neglected. 606
Selective Adsorption of Alcohols Monitored by IR Spectroscopy One well-studied property of zeolites is their ability to adsorb alcohols. For example, zeolite 4A is known to adsorb methanol and ethanol but to exclude 2-methyl-2-propanol (tertbutanol); on the other hand, a zeolite with a larger pore size, such as zeolite 13X, adsorbs all three of the aforementioned al‑ cohols (4). Alcohols have a characteristic signature in IR spectra, specifically a strong OH peak that is typically observed between 3650–3200 cm‒1 (7). Our plan was thus to study the IR spectra of alcohol solutions before and after treatment with zeolites of differing pore sizes. Dichloromethane was chosen as the solvent owing to its relative transparency to IR radiation in the region where the alcohol OH band absorbs strongly. Two series of alcohols of regularly increasing size were screened against four zeolites of differing pore aperture; the se‑ lected alcohols were (CH3)xCH3−xOH and (C6H5)xCH3−xOH and the zeolites were commercially available 3A, 4A, 5A, and 13X molecular sieves. The IR spectra of the 0.2 M alcohol solutions in CH2Cl2 were recorded in absorbance mode prior to exposure to oven-dried (and subsequently cooled) zeolite pellets and again following 1 hour of mild shaking in a sealed vial on an orbital shaker; full data are summarized in the online material accompanying this article. Pellet zeolites were utilized, and mechanical or magnetic stirring was not used so as to avoid powdering the pellets. If a suitable shaker is not available, the adsorption of alcohols does proceed by diffusion, albeit at slower rates; roughly 2–3 hours of standing is required to observe the same results. To ensure that no concentration changes occurred simply as a result of the shaking process, a sealed vial contain‑ ing just the alcohol solution received the same treatment; the OH absorbance of the reference sample showed no measurable change. The most dramatic changes were observed for the cases where the alcohol was much smaller than the zeolite pore size and the OH stretch disappeared from the IR spectra, such as the combination of methanol and zeolites 4A, 5A, and 13X. Conversely, only small changes in the OH absorbance were ob‑ served when the alcohol was too large to enter the zeolite pore, such as with 2-methyl-2-propanol and zeolite 3A, 4A, and 5A or triphenylmethanol with all four zeolites studied. Invariably, there is a small decrease in absorbance even if the alcohol is too large to enter the pores; this is attributed to surface adsorption. Additional experiments were conducted to discover the repro‑ ducibility of the absorbance measurements, and it was found that, for a single operator conducting repeat trials, absorbance measurements were reproducible to ±2%. One anomalous result was observed when diphenylmetha‑ nol, (C6H5)2CHOH, was exposed to zeolite 5A. Even though the exclusion of the smaller phenylmethanol, (C6H5)CH2OH, suggests that diphenylmethanol should be too large to enter the pores of zeolite 5A, the OH absorbance nevertheless decreased
Journal of Chemical Education • Vol. 86 No. 5 May 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory Table 1. IR Data for Alcohol Adsorption by Zeolites
A CH3OH/zeolite 4A
Absorbancea ν/cm–1
CH3OH
3A
CH3OH CH3OH
Initial
1h Shake
Overnight Shake
3625
0.41
0.35
0.18
4A
3625
0.41
0.00
—
13X
3625
0.41
0.00
—
(CH3)3COH
3A
3600
0.40
0.39
0.39
(CH3)3COH
4A
3600
0.40
0.37
0.38
(CH3)3COH
13X
3600
0.40
0.08
0.00b
(C6H5)3COH
3A
3590
0.41
0.39
0.39
(C6H5)3COH
4A
3590
0.41
0.39
0.40
(C6H5)3COH 13X
3590
0.41
0.36
0.36
aCarried
out in 3 dram plastic-capped vials containing 2 mL 0.1 M ROH in CH2Cl2 over 0.3–0.4 g zeolite pellets. bThis result is also obtained after 3–4 h shaking.
before treatment
0.4
Absorbance
Zeolite
0.2
after treatment 0.0 3800
3600
Wavenumber / cm∙1
3400
B (CH3)3COH/zeolite 4A 0.4
Absorbance
Alcohol
before treatment after treatment
0.2
0.0 3800
to zero over the 1 hour shaking period. In this case alone for the combinations studied, the zeolite promoted conversion of the alcohol to its corresponding ether, (C6H5)2CHOCH(C6H5)2, in a net dehydration reaction; this interesting transformation is discussed further in the accompanying article (8). On the basis of the screening results described above, a selection of three alcohols of easily recognizable size variation, R3COH where R = H, CH3, C6H5, was made to demonstrate the size-exclusion abilities of zeolite 3A, 4A, and 13X. Each student in a class of 50 was assigned an alcohol–zeolite combina‑ tion to study, and the results were compiled for the class so that each student could independently evaluate the data collected for all nine combinations. The alcohol solutions were provided from common stock bottles, and the starting concentration was changed to 0.1 M. Generally, the results mimicked the screen‑ ing results, with zeolite 3A excluding all three alcohols and each step larger in zeolite pore size accepting the next largest alcohol, with triphenylmethanol being the only alcohol that was excluded by all three zeolites studied (Table 1). Representative IR spectra in the OH stretching region are shown in Figure 1. The case of 2-methyl-2-propanol and zeolite 13X is worthy of mention; although the alcohol is able to enter the zeolite chan‑ nels, the rate of adsorption slows near the end of the 1 hour shaking period so that 3–4 hours is needed to observe the OH absorbance decrease to zero in the IR spectrum. This observation is qualitatively attributed to the channels becoming congested with adsorbed molecules that slow the migration of molecules entering from solution. For cases where the alcohol appeared to be excluded by the zeolite, the shaking can be continued overnight to demonstrate that longer treatment does not result in adsorption. When this is done, an interesting result is that the combination of methanol and zeolite 3A does produce a reproducible decrease in alcohol concentration following overnight treatment without detectable formation of other products. In a study where zeolite 3A was used to dehydrate methanol (9), it was noted that methanol adsorption was both quite rapid and competitive with that
3600
Wavenumber / cm∙1
3400
Figure 1. Representative FT-IR spectra recorded before and after treating 2 mL 0.1 M ROH in CH2Cl2 with 0.3–0.4 g zeolite pellets for 1 h (gentle shaking in a capped vial): (A) CH3OH over zeolite 4A and (B) (CH3)3COH over zeolite 4A.
of water on the surface of powdered zeolite 3A. Dehydration was more effective when pellet 3A molecular sieves were used because the smaller water molecules were able to enter the pores more easily, but it was still felt that methanol could compete to some extent and enter the pores owing to its small size (9). Conversely, zeolites 4A and 5A were found to be ineffective for drying methanol because the alcohol was able to freely enter into the larger pores of these zeolites along with the dissolved water. Although certain zeolites are known to catalyze the conversion of methanol to gasoline (10), these processes typically occur at high temperature and with zeolites of relatively large pore size. When the overnight exposure of methanol to zeolite 3A was repeated in CD2Cl2, only extremely dry methanol was observed in the 1H NMR spectrum: δ 3.42 (d, 3JHH = 5.6 Hz, 3H, CH3) and δ 0.99 (q, 3JHH = 5.6 Hz, 1H, OH). The main deviations between the screening experiments and those conducted by the undergraduates were that some students observed minor absorbance increases in cases where the alcohol was not adsorbed and that a greater variation was also observed in the reproducibility of absorbances for repeat mea‑ surements. In the case of the former observation, evaporation of dichloromethane during the sampling process was strongly sus‑ pected while the latter result was ascribed to a lack of experience with the solution IR method. In both cases, increased practice and familiarity with the technique resulted in more uniform results. To minimize the problem of solvent evaporation, higher boiling chloroform was also investigated as a potential solvent. Initial trials were frustrated by paradoxical results that showed
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In the Laboratory
negative absorbances in the OH region, until it was realized that chloroform is typically stabilized with 1% ethanol. Thus, pre-treatment of the chloroform with zeolite 4A, 5A, or 13X was needed to first remove the ethanol stabilizer. When this was done, all subsequent experiments followed the same pattern as the previous studies conducted in dichloromethane. Demonstration of Zeolite Pore Size Differences by Solvent Displacement of Adsorbed Nitrogen Gas Zeolites are known to adsorb gases (1d, 1l, 4), and this property is exploited commercially in applications such as oxy‑ gen generators. For example, dehydrated zeolite 4A will contain about 1% of its mass as nearly pure nitrogen at room tempera‑ ture; this equates to roughly 8.5 mL of adsorbed nitrogen gas per gram of zeolite (11). This is strictly not a size effect, for although dioxygen has a slightly larger diameter, dinitrogen has a signifi‑ cant quadrupole moment that causes it to be selectively adsorbed into the polar environment of the zeolite channels (4, 11). In an oxygen generator, a bed of zeolite therefore adsorbs the nitrogen from the air and expels an enriched oxygen stream. One result of the aforementioned effect is that when the oven-dried zeolite 5A or 13X pellets are added to dichlo‑ romethane solutions during the IR-monitoring experiments described above, vigorous bubbling is observed that quickly subsides; conversely, no bubbles are seen from the surface of zeolite 3A or 4A pellets. When chloroform is used, bubbling is seen for the 13X pellets but not for 3A, 4A, or 5A. The bubbling is thus due to the solvent entering the zeolite pores and displac‑ ing the adsorbed nitrogen gas, with dichloromethane being small enough to enter the pores of zeolite 5A and 13X but being too large to enter those of zeolite 3A or 4A. Similarly, chloroform is large enough to be excluded from all but the pores of zeolite 13X. To further test this premise, the smaller and more flexible solvent hexane was similarly placed in contact with all four types of zeolite pellets, with bubbles now being observed for 4A, 5A, and 13X pellets but not for 3A. Water can be used to demonstrate that a small molecule can enter the pores of zeolite 3A, but the gas bubbles evolved are much smaller and form more slowly than with the other solvent–zeolite combinations. When a reasonably large mass of zeolite pellets are used and outgassing is observed, the solvent is noticeably much warmer, thus demon‑ strating that the release of the gas is exothermic. Unfortunately, using alcohols to demonstrate this effect to parallel the results obtained from the IR study is not as satisfying. Typically, rather than a spectacular outgassing of the molecular sieves for an ap‑ propriate matching of zeolite and alcohol, a much longer lasting stream of tiny bubbles is observed. It is therefore difficult to draw the same qualitative conclusions as are available from the IR spectroscopy study. The evolution of gas can be quantified by using a simple apparatus to measure the volume of the gas that is produced, such as a water column held inside an inverted graduated cyl‑ inder (12). For example, when 5 mL of hexane is added to 3 g of zeolite 4A pellets, 22–23 mL of gas is collected within about 30 seconds. This compares favorably with the theoretical value of 24 mL, after correcting for the approximately 5% mass of the pellets that is inert binder (and is thus incapable of adsorbing gas) (6). If water is used, the process is much slower with smaller bubbles being observed, taking about 10 minutes to evolve 18–19 mL gas under the same conditions. 608
Hazards Liquid alcohols are flammable (burning with invisible flames) and can be toxic by ingestion; appropriate care must therefore be taken during the preparation of the solutions to exclude ignition sources. Phenylmethanol, diphenylmethanol, and triphenylmethanol are irritants. Hexane is a flammable liq‑ uid. Dichloromethane and chloroform are not flammable, but both are suspected carcinogens and should therefore be handled with adequate ventilation, preferably in a fume hood. Zeolites are desiccants that may cause skin irritation. Using powdered zeolites is not recommended, but if they are utilized, appropriate precautions must be taken to avoid inhalation of the dust. Pellet zeolites typically do not pose such a risk. Summary Experiments have been presented that demonstrate the sizeexclusion properties of zeolites, thereby illustrating the reason for naming zeolites “molecular sieves”. If an IR spectrometer is available, the adsorption or exclusion of alcohols of varying sizes from dichloromethane or chloroform solutions can be readily demonstrated by monitoring changes in the intensity of the alcohol OH band in the IR spectra. It is also possible to qualita‑ tively illustrate whether solvents of varying sizes are able to enter the pores of different zeolites by observing the displacement of adsorbed nitrogen gas from within the pores (or lack thereof ). The volume of evolved gas is considerable and can be measured if desired. The experiments were designed to use equipment that is commonly available in undergraduate synthetic laboratories and therefore should be easily incorporated into existing curricula. Acknowledgments The Department of Chemistry at the University of Alberta is thanked for providing the funding for the project. The efforts of the numerous undergraduate students who have successfully completed versions of the experiment are gratefully acknowl‑ edged, as are the helpful suggestions of the graduate teaching assistants for the course. Literature Cited 1. (a) Chao, P.-Y.; Chuang, Y.-Y.; Ho, G. H.; Chuang, S.-H.; Tsai, T.-C.; Lee, C.-L.; Tsai, S.-T.; Huang, J.-F. J. Chem. Educ. 2008, 85, 1558–1561. (b) Belver, C.; Vicente, M. A. J. Chem. Educ. 2006, 83, 1541–1542. (c) Williams, D. J.; Huck, B. E.; Wilkinson, A. P. Chem. Educator 2002, 7, 33–36. (d) Pietraß, T. J. Chem. Educ. 2002, 79, 492–493. (e) Coker, E. N.; Davis, P. J.; Kerkstra, A.; van Bekkum, H. J. Chem. Educ. 1999, 76, 1417–1419. (f ) Smoot, A. L.; Lindquist, D. A. J. Chem. Educ. 1997, 74, 569–570. (g ) Balkus, K. J., Jr.; Ly, Kieu T. J. Chem. Educ. 1991, 68, 875–877. (h) Blatter, F.; Schumacher, E. J. Chem. Educ. 1990, 67, 519–521. (i) Bibby, D. M.; Copperthwaite, R. G.; Hutch‑ ings, G. J.; Johnston, P.; Orchard, S. W. J. Chem. Educ. 1986, 63, 634–637. (j) Copperthwaite, R. G.; Hutchings, G. J.; van der Riet, M. J. Chem. Educ. 1986, 63, 632–634. (k) Huang, Y.-Y. J. Chem. Educ. 1980, 57, 112–113. (l) Breck, D. W. J. Chem. Educ. 1964, 41, 678–689. 2. (a) Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, 3rd ed.; University Science
Journal of Chemical Education • Vol. 86 No. 5 May 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
3. 4. 5. 6. 7. 8. 9. 10.
Books: Sausalito, CA, 1999; pp 37–43. (b) Tanaka, J.; Suib, L. Experimental Methods in Inorganic Chemistry; Prentice Hall: Upper Saddle River, NJ, 1999; pp 305–311. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity, 4th ed.; HarperCollins: New York, 1993; pp 2–7, 742–748. Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978; pp 7–13. Eberly, P. E. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph Series 171; American Chemical Society: Wash‑ ington, DC, 1976; pp 392–433. Breck, D. W. Zeolite Molecular Sieves; Wiley-Interscience: New York, 1974; pp 598, 634. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991; pp 110–111. Cooke, J.; Henderson, E. J.; Lightbody, O. C. J. Chem. Educ. 2009, 86, 610–612. Burfield, D. R.; Smithers, R. H. J. Org. Chem. 1983, 48, 2420– 2422. Amamedía, M. A.; Borau, V.; Jiménez, C.; Marinas, J. M.; Roldán, R.; Romero, F. J.; Urbano, F. J. Chem. Lett. 2002, 672–673 and references therein.
11. Peterson, D. In Adsorption and Ion Exchange with Synthetic Zeolites: Principles and Practice; Flank, W. H., Ed.; ACS Symposium Series 135; American Chemical Society: Washington, DC, 1980; pp 107–121. 12. Bochmann, M. In Inorganic Experiments; Woolins, J. D., Ed.; VCH: New York, 1994; pp 28–30.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/May/abs606.html Abstract and keywords Full text (PDF) Links to cited JCE articles Supplement Instructions for the students Notes for the instructor JCE Featured Molecules for May 2009 (see p 656 for details) Structures of some of the molecules discussed in this article are available in fully manipulable Jmol format in the JCE Digital Library at http://www.JCE.DivCHED.org/JCEWWW/Features/ MonthlyMolecules/2009/May/.
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