In the Laboratory
Study of Molecular-Shape Selectivity of Zeolites by Gas Chromatography Pei-Yu Chao, Yao-Yuan Chuang, Grace Hsiuying Ho, Shiow-Huey Chuang, and Tseng-Chang Tsai* Department of Applied Chemistry, National University of Kaohsiung, Nanzi, Kaohsiung 81148, Taiwan; *
[email protected] Chi-Young Lee Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Shang-Tien Tsai Department of Chemistry and Biochemistry, National Chung Cheng University, Min-Hsiung, Chiayi 62102, Taiwan Jun-Fu Huang National Science and Technology Museum, Kaohsiung 80765, Taiwan
Zeolite is a family of porous crystalline aluminosilicate materials consisting of tetrahedral silicon and alumina. It is known for many applications such as adsorbents, ion exchange materials, catalysts, electronic materials, as well as abatement of environmental pollutant and nuclear waste (1–3). There have been stimulating articles in this Journal describing the applications of zeolites in chemistry courses (4–11). Here we present a laboratory experiment focusing on the shape selectivity of zeolite.
Zeolite name 8
7
ECR-34 ALPO-8 Y ALPO-5 L
Zeolite type
Molecule
>12-membered rings
12-membered rings
Size / Å
beta MOR SSZ-53 2,3-dimethylbutane
6
ITQ-24 ZSM-12 5
ZSM-5
10-membered rings
Experiment 2-methylpentane
4
ZSM-23 5A FER MCM-22 4A ERI
8-membered rings
n-hexane
3A 3
Zeolite pore size
Molecule outer diameter
Figure 1. Plot of adsorbing molecular sizes versus pore opening sizes of zeolites.
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According to the configuration of pores (Figure 1), zeolites can be divided into four major classes; namely, 8-membered oxygen rings (8-MR), 10-MR, 12-MR, and larger than 12MR with pore opening in the ranges of 3.1–4.5 Å, 4.5–5.5 Å, 5.5–8.0 Å, and larger than 7.5 Å, respectively (1, 2). Each zeolite has a unique pore structure with a specific diameter of cavity and pore opening comparable to the molecular sizes of many organic molecules and small inorganic species. Therefore, zeolite bears the nickname a “molecular sieve”. Weisz and Frilette coined a term “shape-selective catalysis” to describe the unique catalysis behavior of zeolites in terms of the molecular sizes of reactants, products, and reaction transition-state complexes (12). Many intriguing shape-selective catalysis reactions are practiced in industry (13, 14). Zeolites also have sensitive “cut off ” adsorption behavior in terms of molecular size of adsorbate, which show a characteristic molecular-shape selective adsorption. Sharma recently demonstrated the applicability of assembling versatile well-defined lego-block models to represent the structural network of some newly discovered advanced materials (15). We present a hands-on experiment to help students understand the molecular-size effect of zeolites through a simple sorption experiment. The experiment can be incorporated into undergraduate chemistry laboratory programs for analytical or physical chemistry. It should be useful for students to realize the molecular behavior in the subnano regime.
This experiment demonstrates the molecular-shape-selectivity behavior of zeolites through a simple gas chromatography (GC) setup by using hexane isomers as adsorbates and zeolites as adsorbents. Some representative zeolite materials are shown in Figure 1. The molecular size of the hexane isomer increases by about 0.8 Å for every additional methyl group in a branch chain with the molecular sizes of 4.1 × 4.1 × 9.0 Å; 4.1 × 4.8 × 7.9 Å; 5.0 × 6.0 × 7.0 Å for n-hexane, 2-methylpentane, and 2,3-dimethylbutane, respectively. The pore openings of various 8-MR and 10-MR zeolites can be discriminated by the n-hexane and 2-methylpentane isomers, respectively. Therefore, adsorbate–adsorbent pairs for the sorption protocol can be chosen from a variety of zeolites and hexane isomers.
Journal of Chemical Education • Vol. 85 No. 11 November 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
Results In the present context, zeolite A (8-MR zeolite), ZSM-5 (10-MR zeolite), and Y (12-MR zeolite) in hydrogen form are used. The sorption tests of n-hexane over zeolite A (zeolite 3A, 4A, and 5A; each with slightly different pore size; see Figure 1) were conducted in the presence of flowing nitrogen gas starting with an adsorption measurement at a pressure of 345 kPa followed by a temperature programmed desorption (TPD) test by heating the zeolite column to 503 K at atmospheric pressure. As shown in Figure 2, while the n-hexane adsorption over zeolite 5A does not exhibit the FID peak, the other adsorption tests over zeolite 3A and 4A generate GC FID peaks. The TPD spectra shown in Figure 3 reveal that only the n-hexane adsorption of zeolite 5A (showing no FID peaks in Figure 2) generates an FID peak. In comparison, all the adsorption tests of 2-methylpentane over zeolite 3A, 4A, and 5A show FID peaks (Figure 4), thus there should be no FID peaks in the TPD tests up to 503 K (not shown). The adsorption spectra of three hexane isomers on zeolite Y are shown in Figure 5. No FID peak is observed, indicating that the three hexane isomers are adsorbed inside zeolite Y. Afterwards, TPD tests are carried out by heating the GC oven to
T/K
310 308 306
0
1
2
3
Retention Time / min
FID Signal
Figure 2. GC spectra of n-hexane adsorption over zeolite 3A, 4A, and 5A (308 K isothermal adsorption, 345 kPa pressure, 0.5 μL injection).
T/K
Zeolite dust is irritating and should not be inhaled. Gloves and mask should be used to avoid contact and inhalation of the fine powders when loading the column. Hexane isomers are flammable and should be kept away from fire. Special care should be taken not to contact hot parts such as injector, oven, and detector of the GC instrument. Hexane isomers are harmful if inhaled; cause irritation to skin, eyes, and respiratory tract; and affect the central and peripheral nervous systems.
4A 5A
5A 3A 4A
450 300 0
10
20
30
40
50
60
70
Retention Time / min Figure 3. Temperature program desorption GC spectra of n-hexane over zeolite 3A, 4A, and 5A (308 K → 503 K at 5 °C/min., atmospheric pressure).
FID Signal
Hazards
3A
5A 4A 3A 310
T/K
The experimental setup is a GC system equipped with a flame ion detector (FID). Nitrogen is used as the carrier gas. A similar setup has already been reported by Choudhary et al. (16) and Moura et al. (17). A 1/8 in. stainless steel tube packed with 25 mg zeolite is used as the GC column. The zeolite column is dried at 573 K for 3 hours in situ to desorb any adsorbed substances, particularly water. Then, it is cooled down to 308 K for the adsorption measurement. Adsorption and desorption of the hexane isomers are monitored from the FID signals. Each adsorption measurement is conducted by alternately injecting various hexane isomers into a zeolite column. The hexane isomers will be adsorbed only if their molecular sizes are smaller than the pore opening of zeolite. The larger isomers are excluded if their molecular sizes are larger than the pore opening of zeolite pore. Therefore, for a given zeolite sample, the sequence of adsorption tests is preferentially in the order of 2,3-dimethylbutane, 2-methylpentane, and n-hexane. If adsorption occurs, as monitored from FID signals, the zeolite sample needs to be regenerated by heating the zeolite column to 573 K to desorb any adsorbed hydrocarbon for the consecutive adsorption test.
FID Signal
In the Laboratory
308 306 0
1
2
Retention Time / min
3
Figure 4. GC spectra of 2-methylpentane adsorption over zeolite 3A, 4A, and 5A (308 K isothermal adsorption, 345 kPa pressure, 0.5 μL injection).
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 85 No. 11 November 2008 • Journal of Chemical Education
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FID Signal
FID Signal
In the Laboratory
dimethylbutane methylpentane n-hexane
n-hexane methylpentane 310
T/K
310
T/K
dimethylbutane
308 306 0
1
2
308 306 0
3
1
Retention Time / min Figure 5. GC spectra of adsorption of hexane isomers over zeolite Y (308 K isothermal adsorption, atmospheric pressure, 0.5 μL injection).
FID Signal
2
FID Signal
dimethylbutane
Figure 7. GC spectra of adsorption of hexane isomers over ZSM-5 (308 K isothermal adsorption, atmospheric pressure, 0.5 μL injection).
methylpentane
n-hexane methylpentane dimethylbutane
n-hexane 600
600
T/K
T/K
3
Retention Time / min
450 300 0
10
20
30
40
50
60
70
Retention Time / min
450 300 0
10
20
30
40
50
Retention Time / min
60
70
Figure 6. Temperature program desorption GC spectra of hexane isomers over zeolite Y (308 K → 573 K at 5 °C/min., atmospheric pressure).
Figure 8. Temperature program desorption GC spectra of hexane isomers over ZSM-5 (308 K → 573 K at 5 °C/min., atmospheric pressure).
573 K. As shown in Figure 6, the three pre-adsorbed tests show FID peaks but they appear at different peak temperatures in the TPD tests. These TPD peaks confirm the adsorption of hexane isomers inside zeolite Y. In the adsorption tests of zeolite ZSM-5 (Figure 7), only 2,3-dimethylbutane sorption test generates a GC peak but not the n-hexane and 2-methylpentane sorption test, indicating that n-hexane and 2-methylpentane are adsorbed inside ZSM-5 pores. Afterwards, TPD tests (Figure 8) reveals that only pre-adsorbed samples of n-hexane and 2-methylpentane show FID peaks.
Figure 1, the pore opening of 3A and 4A are too small, prohibiting the entrance of all hexane isomers. Therefore, zeolite 3A and 4A do not have the capacity to absorb hexane isomers (as shown in Figure 2). On the other hand, the pore opening of zeolite 5A estimated from crystallography is 4.1 × 4.1 Å (1, 2), which is in between 4.1 × 4.8 Å (2-methylpentane) and 4.1 × 4.1 Å (n-hexane). Note that zeolite 5A has no adsorption capacity at atmospheric pressure (not shown) and shows a marginal capacity only at an elevated pressure, that is, 345 kPa. In comparison, 10-MR and 12-MR zeolites have much larger pore openings; thus, their adsorption measurement can be easily conducted at atmospheric pressure. Since zeolite Y can adsorb all hexane isomers (Figure 5), the pore opening size of zeolite Y should be larger than the bulkiest isomer 2,3-dimethylbutane, which is estimated as 5.0 × 6.0 × 7.0 Å. The observation agrees with the pore opening derived by a crystallographic modeling of zeolite Y as 7.4 Å (2). As for ZSM-5, it can adsorb n-hexane and 2-methylpentane but not 2,3-dimethylbutane (Figure 7), the pore opening size of ZSM-5 should be in between the molecular size of 2,3-dimethylbutane and 2-methylpentane. The crystallographic modeling estimates the pore openings of zeolite ZSM-5 as 5.1 × 5.5 Å and 5.3 × 5.6 Å (2).
Discussion In the sorption tests, if a hexane isomer is adsorbed by zeolite, there will not be a FID peak and a FID peak will appear in the TPD test and vice versa. Indeed, as shown in all sorption tests, an FID peak appears in the TPD test only if there is no FID peak in the adsorption test. Clearly, the adsorption and desorption of hexane isomers on zeolites are reversible in the proposed GC protocol. According to the molecular-shape selectivity, the zeolitic pore opening should be larger than the molecular size of the hexane isomer to allow adsorption and desorption. Referring to 1560
Journal of Chemical Education • Vol. 85 No. 11 November 2008 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Laboratory
It can be concluded that the desorption temperature decreases with increasing pore opening of zeolite and the molecular size of hexane isomer. The desorption temperature reflects the strength of interaction between adsorbate (hexane isomers) and adsorbent (zeolite) (18, 19). Gribov et al. concluded that subject to adsorption, n-hexane is confined inside zeolite pores, and the smaller the zeolite pores, the stronger the interaction between n-hexane and the zeolite pores (18). It should be noted that the crystal size of zeolite sample could have a significant effect on the sorption properties of zeolite. Use of a sample with crystal size of 0.1 μm ~ 1 μm is recommended. The external surface area increases dramatically with decreasing crystal size of zeolite sample. The shape-selectivity principle might not be applicable for nano-crystal zeolite samples. The implementation of the proposed experiment in laboratory courses depends on the experimental time schedule. The experiment including the sorption tests of hexane isomers on zeolite A, ZSM-5, and Y requires approximately 8 hours. It can be conducted as a team project of six students. More zeolite structures are available at the Web site of the International Zeolite Association (20). Alternately, with the versatile zeolite types (see Figure 1), students can construct their own adsorbate–adsorbent pairs for a sorption protocol within some preset studying time period. The simplest sorption protocol is zeolite A (3A, 4A, and 5A). The synthesis experiment (not discussed in the present text) and sorption experiment of zeolite A have been checked by the students of the second-year course in physical chemistry. The data shown in Figures 2–4 were collected within three hours. Conclusion Shape selectivity is one of the most intriguing properties of zeolites. A hands-on sorption experiment has been devised as an aid to teach the shape selectivity of zeolites. The experiment uses a simple gas chromatography setup using a zeolite column. In the proposed experiment, hexane isomers and zeolites are used as the adsorbate and adsorbent, respectively. The adsorption and the temperature programmed desorption test of various hydrocarbons on zeolites can be monitored directly through FID signals of GC. By doing so, the adsorption of hydrocarbon isomers on various zeolite framework structures can be realized. By measuring the sorption behavior of hexane isomers in various zeolites with different pore openings, students can observe the shape-selectivity principle of zeolites. Acknowledgment The financial support of this work by the Ministry of Education, Republic of China (Basic Science Course Improvement
Program; Nanotechnology Human Resource Development Program), is gratefully acknowledged. Literature Cited 1. Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape Selective Catalysis in Industrial Application, 2nd ed.; Marcel Dekker, Inc.: New York, 1996. 2. Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, 3rd ed.; International Zeolite Association, Butterworth-Heinemann: Boston, 1992. 3. Whyte, T. E.; Dalla Betta, R. A. Catal. Rev.-Sci. Eng. 1982, 24, 567–598. 4. Huang, Y. Y. J. Chem. Educ. 1980, 57, 112–113. 5. Bibby, D. M.; Copperthwalte, R. G.; Hutchings, G. J.; Johnston, P.; Orchard, S. W. J. Chem. Educ. 1986, 63, 634–637. 6. Blatter, F.; Schumacher, E. J. Chem. Educ. 1990, 67, 519–521. 7. Balkus, K. J., Jr.; Ly, K. T. J. Chem. Educ. 1991, 68, 875–877. 8. Smoot, A. L.; Lindquist, D. A. J. Chem. Educ. 1997, 74, 569–570. 9. Coker, E. N.; Davis, P. J.; Kerkstra, A.; van Bekkum, H. J. Chem. Educ. 1999, 76, 1417–1419. 10. Pietraß, T. J. Chem. Educ. 2002, 79, 492–493. 11. Belver, C.; Vicente, M. A. J. Chem. Educ. 2006, 83, 1541–1542. 12. Weisz, P. B.; Frilette, V. J. J. Phys. Chem. 1960, 64, 382. 13. Tsai, T. C.; Liu, S. B.; Wang, I. Appl. Catal. A: Gen. 1999, 181, 355–398. 14. Chen, N. Y.; Garwood, W. E. Ind. Eng. Chem. Pro. Des. Dev. 1978, 17 (4), 513–518. 15. Sharma, C. V. K. J. Chem. Educ. 2001, 78, 617–622. 16. Choudhary, V. R.; Mantri, K. Langmuir 2000, 16, 8024–8030. 17. Moura, F. C. C.; Pinto, F. G.; dos Santos, E. N.; do Amaral, L. O. F.; Lago, R .M. J. Chem. Edu. 2006, 83, 417–420. 18. Gribov, E. N.; Sastre, G.; Corma, A. J. Phys. Chem. B 2005, 109, 23794–23803. 19. Bougeard, D.; Smirnova, K. S. Phys. Chem. Chem. Phys. 2007, 9, 226–245. 20. International Zeolite Association. Database of zeolite structures. http://www.iza-structure.org/databases/ (accessed July 2008).
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