A Conformational Study of 1-Bromo-2-chloroethane in Zeolites

Apr 17, 2009 - The conformational properties of 1-bromo-2-chloroethane (BCE) in several representative zeolites including silicalite-1, siliceous Y (S...
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A Conformational Study of 1-Bromo-2-chloroethane in Zeolites Haiyan Wang and Yining Huang* Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Received February 11, 2009. Revised Manuscript Received March 13, 2009 The conformational properties of 1-bromo-2-chloroethane (BCE) in several representative zeolites including silicalite-1, siliceous Y (Si-Y), Na-Y, and zeolite L have been investigated by FT-Raman spectroscopy in combination with thermogravimetric analysis. The results indicate that the conformational and dynamic behavior of BCE inside a zeoltic host is strongly influenced by several factors such as the framework topology, the Si/Al ratio, and the nature and the locations of charge-balancing cations. For siliceous zeolites (silicalite-1 and Si-Y), the anti population of BCE increases significantly relative to pure liquid upon initial loading at room temperature. Interestingly, when lowering temperature, the anti population of BCE in silicalite-1 continues to increase, but the conformational equilibrium of BCE inside Si-Y shifts to the gauche conformer. Zeolite L and Na-Y with extra-framework cations show an almost exclusive preference for the gauche mode of the BCE molecule at room temperature, and the conformational population does not change significantly at low temperatures.

Introduction Zeolite molecular sieves are microporous aluminosilicate framework materials with well-defined cage and/or channel systems with molecular dimensions. They have been extensively utilized in industry for ion-exchange, separation, catalysis, and environmental protections. Since most applications involve adsorption of guest species into the zeolitic frameworks, it is important to understand the behavior of guest molecules inside zeolite hosts. One particular type of host-guest interactions in zeolitic systems is the effect of zeolite framework on conformational properties of organic molecules adsorbed in a zeolite. Existing different conformers can significantly influence the adsorption and diffusion of the guest species and, therefore, the subsequent chemistry inside the host framework. In recent years, adsorption of halocarbons in zeolites has attracted attention due to the environmental need for developing new separation and catalytic conversion processes involving halocarbons. In this paper, we present the results of a study of the conformational property of 1-bromo-2-chloroethane (BCE) adsorbed in a series of zeolite hosts by FT-Raman spectroscopy combined with thermogravimetric analysis (TGA). Our purpose is to obtain information on how different zeolite framework structures and charge balancing cations affect the adsorptive behavior of BCE at a molecular level. The conformation of 1-bromo-2-chloroethane has been the subject of numerous investigations, for example, computational

calculations,1 vibrational spectroscopy,2 electron diffraction,3 NMR,4 dielectric studies,5 and photoelectron spectroscopy.6 These studies have shown that BCE exists in both anti and gauche forms with their relative population being different in gas and liquid phases (Figure 1).2a,2g,4a,5 Only the anti conformer exists in the crystalline solid state. Various techniques have been used to gain information on the anti-gauche energy difference.1-5 An infrared spectroscopic study showed that the energy difference between the anti and gauche conformers is 5.56 and 2.00 kJ/mol in gas and liquid phase, respectively, with the anti conformer being more stable.2b The population of the anti conformation in gas and liquid phase is 83% and 54%, respectively.2b The difference in conformational distribution between gas and liquid state is due to different dipole moments of the two conformers. The gauche conformer has a dipole moment of 2.5 D, whereas the anti conformation only possesses a very small dipole moment of 0.1 D.5 In pure liquid, intermolecular interactions help to stabilize the gauche conformer, resulting in an increase in the gauche concentration. In gas phase, the intermolecular interactions are negligent. Consequently, the conformational equilibrium shifts to the anti conformer. A Raman study indicated that the volume difference of two conformers in pure liquid is -2.0 cm3 mol-1 with the gauche conformer being smaller.2c The adsorption of a binary mixture containing BCE in zeolite ZSM-5 was recently studied,7 but no attention was paid to BCE’s conformation. Raman spectroscopy is a powerful technique for characterization of sorbate/zeolite complexes.8 This is because zeolites exhibit weak Raman signals as a result of their intrinsically small Raman

*Corresponding author. E-mail: [email protected]. (1) (a) Lee, H.; Su, T.; Chao, I. J. Phys. Chem. A 2004, 108, 2567–2575. (b) Trindle, C.; Crum, P.; Douglass, K. J. Phys. Chem. A 2003, 107, 6236–6242. (c) Meyer, A. Y.; Allinger, N. L. Tetrahedron 1975, 31, 1971–1978. (d) Abraham, R. J.; Cavalli, L.; Pachler, K. G. R. Mol. Phys. 1966, 11, 471–494. (e) Meyer, A. Y. J. Comput. Chem. 1981, 2, 384–391. (2) (a) Kingsbury, C.; Lee, K. J. Phys. Org. Chem. 2000, 13, 244–252. (b) Tanabe, K. Spectrochim. Acta 1972, 28, 407–424. (c) Koda, S.; Matsui, H.; Nomura, H. J. Chem. Soc., Faraday Trans. 1 1989, 85, 957–967. (d) Fujimoto, E.; Kozima, K.; Takeoka, Y. Bull. Chem. Soc. Jpn. 1971, 44, 2110–2115. (e) Powling, J.; Bernstein, H. J. J. Am. Chem. Soc. 1951, 73, 1815–1820. (f) Wilmshurst, J. K.; Bernstein, H. J. Can. J. Chem. 1957, 35, 734–736. (g) Brown, J. K.; Sheppard, N. Trans. Faraday Soc. 1952, 48, 128–137. (h) Kuratani, K.; Miyazawa, T.; Mizushima, S. J. Chem. Phys. 1953, 21, 1411–1412. (3) Huang, J.; Hedberg, K. J. Am. Chem. Soc. 1990, 112, 2070–2075.

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(4) (a) Pachler, K. G. R.; Wessels, P. L. J. Mol. Struct. 1980, 68, 145–159. (b) Abraham, R. J.; Gatti, G. J. Chem. Soc. B 1969, 8, 961–968. (c) Abraham, R. J.; Pachler, K. G. R. Mol. Phys. 1963, 7, 165–182. (5) Ghatak, A.; Das, A.; Hasan, A. J. Chem. Phys. 1973, 59, 3414–3416. (6) (a) Ames, D. L.; Turner, D. W. J. Chem. Soc., Chem. Commun. 1975, 5, 179– 180. (b) Chau, F. T.; McDowell, C. A. J. Phys. Chem. 1976, 80, 2923–2928. (7) Stoeckli, F.; Couderc, G.; Sobota, R.; Lavanchy, A. Adsorpt. Sci. Technol. 2002, 20, 189–197. (8) (a) Stair, P. C. Curr. Opin. Solid State Mater. Sci. 2001, 5, 365–369. (b) Dutta, P. K. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 215–237. (c) Bremard, C.; Bougeard, D. Adv. Mater. 1995, 7, 10–25.dLi, C.; Wu, Z. In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003; pp 423-513.

Published on Web 04/17/2009

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Article temperature range of 25-800 °C with a heating rate 10 °C/min. The flow rate of nitrogen gas is 25 mL/min.

Results and Discussion

Figure 1. Two conformers of 1-bromo-2-chloroethane.

scattering cross sections. The weak zeolitic background allows Raman signals of guest molecules to be detected. This method has been used to study the conformation of guest species in zeolites.9 Its fast time scale permits one to observe the separate peaks due to different conformers simultaneously at room temperature. In addition, the Fourier transform (FT) Raman technique employs the 1064 nm excitation from a near-infrared Nd3+:YAG laser, reducing the fluorescence background (a problem often associated with zeolites).

Experimental Section Zeolite L (Si/Al = 3.05) with a unit cell composition of K8.9(SiO2)27.1(AlO2)8.9 was obtained from TOSOH Corp. Silicalite-1, Si-Y (Si/Al > 300), and Na-Y (Si/Al = 2.35) were obtained from Degussa and Strem Chemicals, respectively. The unit cell composition is 96SiO2, 192SiO2, and Na57.3(SiO2)134.7 (AlO2)57.3 for silicalite-1, Si-Y, and Na-Y, respectively. The identity and crystallinity of all the zeolites were checked by powder XRD. 1-Bromo-2-chloroethane (98%) was obtained from Aldrich Chemical Co. and used as received. After being dehydrated at 550 °C for 4 h, carefully weighed portions of the zeolites were transferred into Pyrex tubing under a N2 atmosphere. Addition of BCE was done through microliter syringe and quickly followed by sealing the Pyrex vessel with a flame. The samples were placed in an oven and heated at 97 °C for 3 h (∼10 °C below the boiling temperature) to uniformly disperse the sorbate molecules throughout the samples. After being slowly cooled to room temperature, the samples were transferred to a capillary tube and sealed for Raman studies. The loading level (i.e., the number of molecules per supercage or per unit cell) was confirmed by thermogravimetric analysis (TGA), and the maximum loading for a given zeolite was employed. It must be noted that the topology of the zeolite will determine the extent of loading level. No impurity peaks were detected due to thermal reactions within the zeolites during the course of the measurements. All Raman spectra were recorded on a Bruker RFS 100/S spectrometer equipped with a Nd3+:YAG laser operating at 1064.1 nm and a liquid nitrogen cooled Ge detector. The laser power was typically 100 mW at the sample, and the resolution was 2 cm-1. The typical scans were 800-1000. Low-temperature measurements were carried out by using a Bruker Eurotherm 800 series temperature control unit, which regulated the sample temperature to within (1 °C. Powder X-ray diffraction measurements were performed on a Rigaku rotating anode diffractometer with graphite-monochromated Co KR radiation with a wavelength 1.7902 A˚. TGA was performed on a Mettler Toledo TGA/ SDTA851e analyzer. TGA measurements were carried out in a (9) (a) Crawford, M. K.; Dobbs, K. D.; Smalley, R. J.; Corbin, D. R.; Maliszewskyj, N.; Udoric, T.; Cavanagh, R. R.; Ruch, J. J.; Grey, C. P. J. Phys. Chem. B 1999, 103, 1999. (b) Moissette, A.; Batonneau, Y.; Bremard, C. J. Am. Chem. Soc. 2001, 123, 123250–12334. (c) Hong, S. B.; Camblor, M. A.; Davis, M. E. J. Am. Chem. Soc. 1997, 119, 761–770. (d) Huang, Y.; Leech, J. H.; Wang, H. J. Phys. Chem. B 2003, 107, 7632–7639. (e) Huang, Y.; Leech, J. H. J. Phys. Chem. B 2003, 107, 7647–7653. (f) Wang, H.; Turner, E. A.; Huang, Y. J. Phys. Chem. B 2006, 110, 8240–8249. (g) Jaramillo, E.; Grey, C. P.; Auerbach, S. M. J. Phys. Chem. B 2001, 105, 12319–12329.

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BCE/Silicalite-1. Silicalite-1 is the completely siliceous form of zeolite ZSM-5 without extra-framework cations. It is hydrophobic and organophilic and selectively adsorbs organic molecules in the presence of water. Silicalite-1 has MFI structure which is a three-dimensional network of intersecting straight and zigzag channels. The entrance to the channels is controlled by the 10-rings with the dimensions reported as (5.3  5.6 A˚) for the straight channel and (5.1  5.5 A˚) for the zigzag channel.10 These two channels meet at a relatively open “intersection” region, having a spherical shape with a diameter of about 8.7 A˚.11 To better understand the data, the spectra of pure BCE at several temperatures were obtained (Figure 2). Based on those previously reported,2f,2g the spectral assignments are given in Table 1. The bands in the C-Cl and C-Br stretching regions are very sensitive to the conformational behavior. Since these peaks are well resolved and do not strongly couple with other vibrations, the C-Cl and C-Br stretching bands can both be used to determine the populations of the anti and gauche conformers. Since the population calculated using the C-Cl stretching bands is very close to that of the C-Br stretching modes, only the results calculated by using the C-Cl stretching modes are reported here. The intensity of a Raman band for a conformer is given by the following I i ¼ Ci σ i where Ci and σi are the concentration and the Raman scattering cross section of the i conformer, respectively.12 The anti-gauche equilibrium exists in BCE: AaG The equilibrium constant is given by the following equation: K ¼

CG IG =σG IG σ A ¼ ¼ CA IA =σA IA σ G

It is very difficult to directly measure σ of each conformer. Instead, the relative anti/gauche Raman cross-section ratio (σA/ σG) was often used, and this ratio can be calculated.13 For BCE, the value of σA/σG was estimated to be 6.0 by Gaussian calculations using the DFT method and used to calculate the conformational population. From the integration of the intensities of the 726 and 666 cm-1 bands associated with the C-Cl stretching modes (Table 1), the population of the anti conformer of pure liquid BCE is estimated at 52.5% at room temperature, which is in good agreement with the value of 54% reported in refs 2f and 2g. The solid Raman spectrum (Figure 2C) indicates that only the anti conformer exists in the solids as previously reported.2g FT-Raman spectra of BCE in silicalite-1 with 8 molecules/ unit cell (u.c.) are shown in Figure 3. When BCE is adsorbed in (10) Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of Zeolite Framework Types, 5th ed.; Elsevier: Amsterdam, 2001. (11) Xiao, J.; Wei, J. Chem. Eng. Sci. 1992, 47, 1123–1141. (12) Kato, M.; Abe, I.; Taniguchi, Y. J. Chem. Phys. 1999, 110, 11982–11986. (13) Melendez-Pagan, Y.; Taylor, B. E.; Ben-Amotz, D. J. Phys. Chem. 2001, 105, 520–526.

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Figure 2. FT-Raman spectra of pure BCE at 298 K (A), 263 K (B), and 253 K (C). The intensities are not scaled.

Table 1. Raman Frequencies and Assignments of the BCE Adsorbed in Various Zeolites BCE liquid

BCE in silicalite-1

BCE in zeolite L

BCE in Si-Y

BCE in Na-Y

assignmenta

3007 2971 2962 2950 1446 1439 1428 1423 1299 1286 1261 1258 1205 1188 1127

3015 2981 2962 2946 1450 1444 1434 1423 1301 1285 1262 1258 1202 1188 1127

3016

3013

1430 1424 1301 1291 1264 1258 1205 1194 1126

3011 2975 2967 2951 1446 1439 1425 1424 1302 1286 1262 1258 1204 1187 1126

1445 1434 1426 1425 1300 1290 1264 1258 1209 1192 1125

1053 1022 958

1054 1027 958

1054 1026 958

1053 1022 958

1052 1022 958

923

923

922

922

924

857

857

854

859

856

726 666 632 569 386 251 248 209

732 672 634 573 384 251 248 209

720 652 560 386 245 244 208

730 668 632 571 385 250 248 205

711 654 618 561 386 248 247 205

CH stretch (A, G) CH stretch (A, G) CH stretch (A, G) CH stretch (A, G) CH2 bend (A) CH2 bend (A) CH2 bend (G) CH2 bend (G) CH2 wag (G) CH2 wag (A) CH2 wag (G) CH2 twist (A) CH2 wag (A) CH2 twist (G) CH2 twist (G) CH2 twist (A) CC stretch (A) CC stretch (G) CH2 rock (A) CCl stretch + CCCl bend (A) CH2 rock (G) CCl stretch + CCCl bend (A) CH2 rock (G) CH2 rock (A) CCl stretch (A) CCl stretch (G) CBr stretch (A) CBr stretch (G) CCCl bend (G) CCCl bend (A) CCBr bend (G) CCBr bend (A)

a

2969

2967

Assignments were taken from refs 2f and 2g.

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Figure 3. FT-Raman spectra of BCE liquid at 298 K (A). BCE/silicalite-1 at 298 K (B), 253 K (C), 193 K (D), and 153 K (E). The loading is 8 molecules/u.c. Asterisk denotes zeolite peaks. The intensities are not scaled.

the framework, the intensities of the peaks due to the anti conformer increase. As mentioned earlier, the intensity of a Raman band is related to the population of a particular conformation. Therefore, the differences in the intensity indicate the changes in conformational distribution after adsorption. In the C-Cl stretching region, the 732 cm-1 band due to the anti conformer increases in the intensity relative to the band at 672 cm-1 associated with the gauche conformer. Similar changes also occur in the C-Br stretching region. There is an increase in the intensity of the 634 cm-1 band corresponding to the anti conformer. The intensity of the band at 573 cm-1 belonging to the gauche conformer decreases upon loading. The populations of different conformers were estimated using the integrated intensities of the 732 and 672 cm-1 in the C-Cl stretching region. The result indicates that the concentrations of the anti and gauche conformer are 84.4% and 15.6%, respectively, at room temperature. Compared to pure liquid BCE, the population of the anti conformer significantly increases as BCE is loaded in silicalite-1. For pure BCE, the population of the anti conformer in gas phase is reported to be 83%.2b The result indicates that the relative concentration of the anti conformer of BCE in silicalite-1 is similar to that of BCE in gas phase. There are several reasons for the observed change in conformation: (1) In liquid, the interaction among the BCE molecules helps to stabilize the gauche conformer with a larger molecular dipole moment. In silicalite-1 framework, the guest molecules are distributed among three adsorption sites:14 the straight channel, the zigzag channel, and the channel intersec(14) (a) Long, Y. C.; Jiang, H. W.; Zeng, H. Langmuir 1997, 13, 4094–4101. (b) Huang, Y.; Wang, H. Langmuir 2003, 19, 9706–9713. (c) Huang, Y.; Havenga, E. A. Langmuir 1999, 15, 6605–6608.

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tion (Scheme 1A). At these adsorption sites, the small BCE molecules are relatively isolated from each other. Thus, the intermolecular interactions of BCE molecules are likely very small. Consequently, the population of the anti conformer increases significantly. (2) The pore sizes of the straight and the zigzag channel are both comparable to the size of BCE. Thus, the tight fit of the BCE molecules in the framework leads to an increase in the anti conformer with a more linear shape. (3) Since silicalite-1 is completely siliceous and free of cations, the host-guest interaction is mainly van der Waals in nature. The strong cation-dipole interaction which would stabilize the gauche conformer is absent. The DTG curve (Figure 4A) contains a broad peak due to weight loss centered at 177 °C. To better understand the result, the DTG data of BCE in siliceous Y (Si-Y) is also measured (Figure 4B). There is a single narrow peak in the DTG curve of the BCE/Si-Y system. Since the frameworks of silicalite-1 and Si-Y are both siliceous without cations, the host-guest interactions in both zeolites are very similar and mainly van der Waals in nature. The framework of silicalite-1 has three different adsorption sites, whereas there is only one adsorption site (the supercage) in Si-Y. Therefore, the desorption of BCE from SiY gives only a single sharp peak at a lower temperature. The broad envelope observed in the DTG curve of BCE/silicalite-1 is most likely due to several overlapping peaks corresponding to the BCE molecules desorbed from the three adsorption sites with similar energies. As the temperature of the BCE/silicalite-1 system is cooled down, a shift in the conformational equilibrium was observed. The intensities of the bands due to the anti conformer increase upon lowering temperature. In the C-Cl stretching region, there is an increase in the intensity of the band at 732 cm-1 due to the DOI: 10.1021/la9002675

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Scheme 1. Illustrations of the Locations of BCE Molecules in the Framework of (A) Silicalite-1, (B) in the 12-Ring Channel of Zeolite L, (C) in the Supercage of Si-Y, and (D) Frozen Conformation of BCE in Na-Y via Interactions with Na+

Figure 4. TGA curves of (A) BCE/silicalite-1 (8 molecules/u.c.), (B) BCE/Si-Y (3 molecules/s.c.), (C) BCE/zeolite L (2 molecules/u.c.), and (D) BCE/Na-Y (3 molecules/s.c.).

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anti conformer relative to that of the gauche band at 672 cm-1. In the C-Br stretching region, the 634 cm-1 peak belonging to the anti conformer increases in the intensity, whereas the intensity of the 573 cm-1 gauche band decreases. This change in conformational population is due to the shrinking of the zeolite framework at lower temperature. The smaller channel size at low temperatures favors the anti conformer. Using the Raman intensity ratio of the anti to gauche conformer in the CCl stretching region at different temperatures, the van’t Hoff plot for the anti-gauche equilibrium within silicalite-1 framework was obtained (Figure 5). The anti-gauche enthalpy difference is determined to be 1.64 ( 0.27 kJ/mol. BCE/Zeolite L. The next framework that was examined is zeolite L with LTL structure. Zeolite L has an one-dimensional, 12-membered ring channel system which is characterized by a 7.1 A˚ diameter pore opening.15 The largest free diameter of the channel is about 13 A˚, depending on the charge-compensating cations.15 FT-Raman spectra of BCE in zeolite L with a loading of 2 molecules/u.c. are shown in Figure 6. When BCE is adsorbed in the framework of zeolite L, the change in the spectrum is different from that of BCE/silicalite-1. A sharp increase in the intensities for all the gauche modes occurs relative to their anti counterparts. In the CH2 bending region, the peaks at 1424 and 1430 cm-1 due to the gauche conformer increase in their intensities relative to their anti counterparts. There is a significant increase in the intensities of the bands at 1194, 922, and 386 cm-1 corresponding to the gauche conformer in the CH2 twisting, CH2 rocking, and C-C-Cl bending regions, respectively. Especially in the C-Cl and C-Br stretching regions, the strong peaks at 726 and 632 cm-1 due to the anti conformation in the spectrum of pure liquid BCE become very weak upon loading. The 652 and 560 cm-1 bands associated with the gauche conformer are significantly more intense than their anti counterparts. Integration of the intensities of the bands at 720 and 652 cm-1 in the C-Cl stretching region indicates that the relative population of the anti and gauche conformer is 4.7% and 95.3%, respectively. In general, a more polar molecule will be more strongly and selectively adsorbed in a zeolite with a large cation charge density.16 In zeolite L, the guest molecules are located in the 12-ring channel and interact strongly with K+ ions. The gauche conformer with a larger dipole moment is stabilized by the electric field produced by the K+ ions, resulting in an increase in the gauche population. Early work showed that for several related halo-substituted ethane molecules the barrier to C-C bond rotation in gas phase has the order CH2Cl-CH2Cl < CH2Cl-CH2Br < CH2BrCH2Br.17 Although there are a number of factors, the nonbonding interactions contribute at least partially to the trend for isolated molecules. For BCE adsorbed in the zeolite with chargebalancing cations such as zeolite L, the interactions between halogen atoms and cations are expected to significantly affect the barrier to rotation and hence conformational behavior. Indeed, the existence of cation-halogen interaction is evidenced from the observation that the bands in the C-Cl and C-Br stretching regions shift to the lower energies upon adsorption. (15) (a) Barrer, R. M.; Villiger, H. Z. Kristallogr. 1969, 128, 352–370. (b) Silbernagel, B. G.; Garcia, A. R.; Newsam, J. M. Colloids Surf., A 1993, 72, 71–80. (c) Newsam, J. M. J. Phys. Chem. 1989, 93, 7689–7694. (16) Sircar, S.; Myers, A. L. In Handbook of Zeolite Science and Technology; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003; pp 1063-1104. (17) Lowe, J. P. In Progress in Physical Organic Chemistry; Streitwiser, A. Jr., Taft, R. W., Eds.; Interscience Publishers: New York, 1968; Vol. 6, pp 1-80.

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The frequencies of the bands at 666 cm-1 [ν(C-Cl)] and 569 cm-1 [ν(C-Br)] due to the gauche conformer shift by 14 and 9 wavenumbers to lower energies relative to the same modes in the Raman spectrum of pure liquid BCE. The low-frequency shifts are due to the strong interaction of halogen atoms with K+ ions because the C-X 3 3 3 K+ (X = Cl, Br) interaction weakens the C-X bonds. The fact that the magnitude of the shift for CCl stretching vibration is larger than that for C-Br stretching mode implies that the chlorine atom in BCE interacts more strongly with K+ in the zeolite. It is also intriguing to note that the ν(C-Cl) low-frequency shift for the anti isomer is only 6 cm1 (Table 1). The implication is that the interaction of potassium ion with chlorine in anti conformer is significantly weaker than that in the gauche isomer, leading to the very small anti population. Zeolite L (Si/Al = 3.05) under examination contains 8.9 K+ per unit cell residing in four cation positions (site A, B, C, and D).18 But, only the K+ ions located at the site D occupying the wall of 12-ring channel can directly interact with the halogen atoms of BCE. The DTG curve consists of a broad asymmetric peak at 243 °C with an obvious low-temperature shoulder (Figure 4C), indicating the existence of two adsorption sites. There are 6 D sites in the unit cell, and they are only occupied partially by 4 K+ ion.18 The DTA data suggest that the 4 K+ ions must be unevenly distributed among 6 sites, resulting in two types of adsorption sites (I and II) with slightly different energies. This situation is illustrated in Scheme 1B. The high desorption temperature indicates that the interaction of BCE with cation in zeolite L is strong. When the temperature of the BCE/zeolite L system decreases, the intensity of the bands due to the anti conformer is too low to be measured accurately. Thus, the concentrations of different conformers were not determined. BCE/Faujasite. We also examined the adsorption of BCE in the zeolites with the faujasite framework, which include siliceous Y (Si-Y) and Na-Y. Both have the identical (FAU) framework topology but different Si/Al ratios (300 and 2.35 for Si-Y and Na-Y). The FAU structure has two different types of cages. The smaller sodalite cages (or β-cages) are linked to one another via double 6-rings, forming the large spherical voids in this structure. These large voids are referred to as supercages. The supercage has a diameter of 12.5 A˚. The guest molecule can access the supercage by passing through a 12-ring pore window with a diameter of 7.4 A˚. The β-cage is normally not accessible for most organic molecules including BCE because 6-ring window is much too small. Raman spectra of BCE/Si-Y at a loading level of 3 molecules/s.c. are shown in Figure 7. As BCE is adsorbed in the SiY, an increase in the relative intensity of the anti mode and a concomitant decrease in the gauche mode at room temperature are observed (Figure 7B). In the C-Cl stretching region, the intensity of the band at 730 cm-1 due to the anti conformer increases at the expense of the 668 cm-1 peak corresponding to the gauche conformation. Similar changes are also seen for the C-Br stretching vibrations. The population of the anti and gauche conformer in the BCE/Si-Y system is calculated to be 70.5% and 29.5%, respectively. Compared to the pure liquid BCE (52.5% of the anti conformer), the population of the anti conformer increases substantially upon loading. Si-Y possesses no extra-framework cations. Its nonpolar siliceous framework favors the almost nonpolar anti conformer. (18) (a) Newell, P. A.; Rees, L. V. C. Zeolites 1983, 3, 28–36. (b) Zhu, J.; Huang, Y. J. Phys. Chem. C 2008, 112, 14241–14246.

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Figure 5. van’t Hoff plots for the anti-gauche equilibrium of BCE/zeolites.

Figure 6. FT-Raman spectra of BCE liquid at 298 K (A). BCE/zeolite L at 298 K (B), at 253 K (C), at 193 K (D), and at 153 K (E). The loading is 2 molecules/u.c. Asterisk denotes zeolite peaks. The intensities are not scaled.

On the other hand, on average, there are 3 molecules per supercage. Thus, the sorbate molecules can interact with each other (Scheme 1C). The intermolecular interactions between BCE molecules stabilize the gauche conformer with a larger dipole moment. The above-mentioned sorbent-sorbate and sorbate-sorbate interactions have an opposing effect on conformational equilibrium. It seems that the host-guest interaction is stronger than the intermolecular interaction. Consequently, upon loading, the population of the anti conformer increases. As the temperature of BCE/Si-Y complex is cooled down, there are changes in the population of different conformers. Specifically, the relative intensities of the Raman bands due to the gauche conformer increase relative to those of the anti conformer. The bands at 1187 and 385 cm-1 associated with 8048 DOI: 10.1021/la9002675

the gauche conformer in the CH2 twisting and C-C-Cl bending region increase in their intensities upon lowering temperature. In the C-Cl stretching region, the peak at 668 cm-1 due to the gauche conformer gains intensity. There is also an increase in the intensity of the 571 cm-1 band corresponding to the gauche conformer in the C-Br stretching region when the temperature decreases. It seems that the intermolecular interaction favoring the gauche conformer becomes stronger at low temperatures, resulting from a slight reduction in supercage volume due to the lattice shrinkage. The smaller cage also forces the BCE molecules to adopt the gauche conformer with a smaller molecular volume. Figure 5 shows the van’t Hoff plot for the anti-gauche equilibrium of BCE within Si-Y framework. The enthalpy change in the transformation of the anti to the gauche conformer is estimated to be -2.55 ( 0.31 kJ/mol. Langmuir 2009, 25(14), 8042–8050

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Figure 7. FT-Raman spectra of BCE liquid at 298 K (A). BCE/Si-Y at 298 K (B), at 253 K (C), at 193 K (D), and at 153 K (E). The loading is 3 molecules/s.c. Asterisk denotes zeolite peaks. The intensities are not scaled.

Figure 8. FT-Raman spectra of BCE liquid at 298 K (A), BCE/Na-Y at 298 K (B), at 253 K (C), at 193 K (D), and at 153 K (E). The loading is 3 molecules/s.c. Asterisk denotes zeolite peaks. The intensities are not scaled.

Langmuir 2009, 25(14), 8042–8050

DOI: 10.1021/la9002675

8049

Article

To study the effect of the extra-framework cations in the FAU framework on BCE conformation, the Raman spectra of BCE/ Na-Y with 3 molecules/s.c. are obtained (Figure 8). Na-Y has the same framework as Si-Y but contains extra-framework Na+ ions. Upon loading, a significant increase in the intensities of the bands due to the gauche conformer is seen (Figure 8B) at room temperature. In the C-Cl stretching region, the gauche band at 654 cm-1 increases and the peak at 711 cm-1 due to the anti conformation decrease upon loading. The similar changes also occur in the C-Br stretching region. In the C-C-Cl bending region, the 386 cm-1 peak due to the gauche conformer appeares as a weak band in the spectrum of pure liquid BCE significantly increases its intensity relative to its anti conformer counterpart (248 cm-1). The results suggest Na-Y favors the gauche conformer. The concentration of the anti and gauche conformer is 15.2% and 84.8%, respectively. There is a broad peak at 214 °C in the DTG curve of BCE/ Na-Y (Figure 4D). The broad weight loss peak and the higher desorption temperature indicate a strong host-guest interaction. This is because the Na-Y examined consists of 57 sodium cations in every unit cell. The polar gauche conformer is strongly adsorbed in the polar framework of Na-Y due to a strong interaction with the electric field produced by the cations. In the C-Cl and C-Br stretching regions, the frequencies of the bands shift when BCE is loaded in the Na-Y. The bands at 711, 654, 618, and 561 cm-1 shift by 15, 12, 14, and 8 wavenumbers to lower energies compared to those in the spectrum of pure liquid BCE (Table 1) due to the sorbatecation interactions. As the temperature of BCE/Na-Y system decreases, there is little change in the intensities of the bands (Figure 8), implying the BCE molecules are almost static in the supercage of Na-Y. Similar situation has been reported for several halocarbons in zeolite Y.9d,9e This is because upon adosption, the conformation of BCE can be frozen by the interactions with the extra frame(19) Moissette, A.; Bremard, C. Microporous Meoporous Mater. 2001, 47 345–357.

8050 DOI: 10.1021/la9002675

Wang and Huang

work Na+ ions in the supercage (Scheme 1D). These interactions also lock the guest molecules into various orientations varying from cage to cage, resulting in a static disorder. A previous work also showed that Na+ in zeolites can hinder the dynamic process of guest molecules.19

Summary The conformational properties of 1-bromo-2-chloroethane adsorbed in a series of zeolites have been studied. For the BCE/ silicalite-1 system, the population of the anti conformer significantly increases upon adsorption. This is because the nonpolar framework and the narrow pore channels of silicalite-1 prefer the anti conformer. As the temperature of BCE/silicalite-1 decreases, the population of the anti conformer further increases. There is also an increase in the population of the anti conformer for the BCE/Si-Y system due to siliceous nature of the framework. However, low temperature favors the gauche conformer due to an increase in the intermolecular interaction and the reduced size of the supercage. Zeolite L and Na-Y with extra-framework cations show an almost exclusive preference for the gauche configuration. The main reason is that the stronger interactions between the larger dipole moment of the gauche conformer and the electric field yielded by the extra-framework cations stabilize the gauche conformer. The bands in the C-Cl and C-Br stretching regions have a frequency shift to lower energies due to the interaction between halogen atom and cation. As the temperature of BCE/ zeolite L and BCE/Na-Y is cooled down, there is no significant change in the anti-gauche equilibrium. Acknowledgment. Y.H. acknowledges the financial assistance from Natural Science and Engineering Research Council of Canada for a research grant and the award of an FT-Raman spectrometer. Funding from Canada Research Chair and Premier’s Research Excellence Award programs is also gratefully acknowledged. We thank Dr. E. A. Turner for obtaining TGA data.

Langmuir 2009, 25(14), 8042–8050