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Investigation of Framework-Sorbate Interactions in Alkylcyclohexane/ZSM-5 Complexes Yining Huang* and Edward A. Havenga Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 Received February 24, 1999. In Final Form: May 13, 1999
Introduction Zeolites are microporous materials that are widely used in industry as catalysts and sorbents. The study of sorbate diffusion in zeolites has been a large area of interest for several decades.1 Recently, diffusion of several alkylcyclohexanes, in particular cyclohexane, methylcyclohexane, and trans-1,4-dimethylcyclohexane, in zeolite ZSM-5 has been investigated by several research groups.2,3 Interestingly, it was found that the diffusion coefficient of trans1,4-dimethylcyclohexane in ZSM-5 was 2 orders of magnitude larger than the diffusion coefficients of the other two sorbates. Chon and Park attributed the large diffusion coefficient of trans-1,4-dimethylcyclohexane to the possible methyl-methyl interaction resulting in a favorable orientation for diffusion through the intersection of two channel systems in the zeolite framework.3 Magalha˜es and co-workers, however, suggested that the fast diffusion of trans-1,4-dimethylcyclohexane is associated with a relatively low energy barrier for channel crossing.2 It is well-known that many zeolites, in particular ZSM5, undergo phase transitions upon adsorption of certain organic molecules.4,5 The change in framework structure induced by the sorbate molecules may well affect the diffusional processes. For this reason, we have investigated the phase transition behavior of three alkylcyclohexane/ ZSM-5 complexes by powder X-ray diffraction (XRD) and FT-Raman spectroscopy. Powder XRD has proven to be an important tool for investigating sorbate-induced phase transitions6-9 and was used in the present study to follow the structural changes in the zeolitic framework. FTRaman spectroscopy, on the other hand, is a relatively new approach. Recently, we have demonstrated that FTRaman spectroscopy is a useful technique in the investigation of sorbate-induced phase transitions in zeolites by monitoring the guest molecules, because the spectral parameters such as band frequency, splitting, and line width of the guest molecules are sensitive to the structural change in the zeolitic framework.10,11 The combination of * To whom correspondence should be addressed. (1) Chen, N. Y.; Degnan, T. F., Jr.; Smith, C. M. Molecular Transport and Reaction in Zeolites: Design and Application of Shape Selective Catalyst, VCH: New York, 1994. (2) Magalha˜es, F. D.; Laurence, R. L.; Conner, W. C. J. Phys. Chem. B 1998, 102, 2317-2324. (3) Chon, H.; Park, D. H. J. Catal. 1988, 114, 1-7. (4) Fyfe, C. A.; Kennedy, G. J.; De Schutter, C. T.; Kokotailo, G. T. J. Chem. Soc., Chem. Commun. 1984, 541-542. (5) Fyfe, C. A.; Mueller, K. T.; Kokotailo, G. T. In NMR Techniques in Catalysis; Bell, A. T., Pines, A., Eds.; Marcel Dekker Inc.: Dordrecht, The Netherlands, 1994; pp 11-67 and references therein. (6) Rohrbaugh, W. J.; Wu, E. L. In Characterization and Catalyst Development; Bradley, S. A., Gattuso, M. J., Bertolacini, R. J., Eds.; ACS Symposium Series 411; American Chemical Society: Washington, DC, 1989; pp 280-302. and references therein. (7) Wu, E. L.; Lawton, S. L.; Olson, D. H.; Rohrman, A. C., Jr.; Kokotailo, G. T. J. Phys. Chem. 1979, 83, 2777. (8) Mentzen, B. F.; Lefebvre, F. Mater. Res. Bull. 1997, 32, 813-821. (9) Mentzen, B. F. Zeoraito 1993, 10, 77-89 and references therein.
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XRD and Raman spectroscopy is particularly appropriate, since the former detects long range ordering in the host framework while the latter is sensitive to the local environment around the guest molecules. Together they provide a more complete picture of host-guest interactions in zeolitic systems. Although host-guest interactions in the cyclohexane/ZSM-5 system were previously examined,12,13 to our knowledge, phase transition behavior in trans-1,4-dimethylcylcohexane/ZSM-5 and methylcyclohexane/ZSM-5 complexes has not yet received any attention. Experimental Section Completely siliceous ZSM-5 was prepared according to a procedure described in the literature with slight modification.14 The sample crystallinity and purity were checked by powder X-ray diffraction. Cyclohexane (>99.9%), methylcyclohexane (spectro-grade), and trans-1,4-dimethylcyclohexane (>99.9%) were obtained from Aldrich Chemical Co., Eastman-Kodak Co., and Fluka Chemika, respectively. All the chemicals were used as received without further purification. Accurately weighed aliquots of freshly calcined ZSM-5 were loaded with precisely measured amounts of organic sorbates. The samples were placed in glass vials, which were then sealed and placed in an oven for 3 h at 80, 100, and 110 °C for cyclohexane, methylcyclohexane, and trans-1,4-dimethylcyclohexane, respectively, to uniformly disperse the sorbate molecules throughout the sample. All Raman spectra were recorded at room temperature on a Bruker RFS-100 FT-Raman spectrometer equipped with a Nd3+/ YAG laser operating at the wavelength 1064.1 nm and a liquid nitrogen cooled Ge detector. The laser power was typically 80 mW at the sample. The resolution used was 2 cm-1. Powder X-ray diffraction measurements were performed on a Rigaku diffractometer with graphite-monochromated Co KR radiation with the wavelength 1.7902 Å.
Results and Discussion X-ray Diffraction. For each guest species, powder XRD spectra of sorbate/ZSM-5 complexes were measured as a function of loading from 1 to 4 molecules/unit cell (u.c.). The structure of calcined (unloaded) siliceous ZSM-5 at ambient conditions is monoclinic (P21/n).15 Previous studies have shown that the powder XRD pattern of ZSM-5 has two regions (2θ: 26-29° and 34-36°) that are very sensitive to the sorbate-induced structural change in the framework.6-9 Figure 1 illustrates the powder XRD profiles of ZSM-5 samples loaded with cyclohexane (CH), methylcyclohexane (MCH), and trans-1,4-dimethylcyclohexane (DMCH), respectively (all of which are at the maximum loading of 4 molecules/u.c.), in these two structure sensitive regions. The XRD pattern of calcined ZSM-5 is also included in Figure 1 for the purpose of comparison. The XRD profiles of CH/ZSM-5 complexes with the loadings 1, 2, 3, and 4 molecules/u.c. are identical and very similar to the profile of calcined ZSM-5. The only noticeable difference in the XRD pattern between CH/ ZSM-5 and unloaded ZSM-5 is that, upon adsorption, the (10) Huang, Y. J. Am. Chem. Soc. 1996, 118, 7233-7234. (11) Huang, Y.; Qiu, P. Langmuir 1999, 15, 1591-1593. (12) Mu¨ller, J. A.; Conner, W. C. J. Phys. Chem. 1993, 1451-1454. (13) Ashtekar, S.; Hastings, J. J.; Gladden, L. F. J. Chem. Soc., Faraday Trans. 1998, 94, 1157-1161. (14) Collection of Verified Zeolite Synthesis; Robson, H., Ed.; Microporous Mesoporous Mater. 1998, 22, 628-629. (15) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Zeolites 1990, 10, 235-242.
10.1021/la990212d CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
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Figure 1. Powder XRD patterns of ZSM-5 in the 2θ regions of 26-29.3° and 33.6-34.6° with various loadings of alkylcyclohexanes. The intensities of the reflections in the 2θ region of 33.6-34.6° were magnified four times relative to those in the 2θ region of 26-29.3°. The doublet to singlet changes in the two regions (see text) are indicated by the arrows (V).
reflections shifted slightly toward lower 2θ values, presumably due to the slight expansion of the unit cell. The similarity in powder XRD profiles between unloaded ZSM-5 and CH/ZSM-5 complexes suggests that adsorption of CH does not induce any structural change in the zeolite framework. This result is consistent with that reported by Mu¨ller and Conner.12 For the MCH/ZSM-5 system, the same result was obtained; that is, no sorbate-induced phase transition was detected even at the maximum loading of 4 molecules/u.c. However, adsorption of DMCH in ZSM-5 results in distinct changes in the XRD profile. For example, in the spectrum of unloaded ZSM-5, two doublets were observed in the 2θ regions 28-29° and 33.5-34.5°. However, both doublets emerged as two singlets upon adsorption of DMCH (Figure 1). Previous work has shown that the above-mentioned changes (i.e. the loss of the doublets at ∼28.5° and 34.1°) are characteristic of a phase transformation in the framework of ZSM-5 from monoclinic to orthorhombic symmetry.6-9,16 These changes in the XRD pattern have been widely accepted as the indication of phase transformation in ZSM-5. Since the above-mentioned changes were clearly observed in the DMCH/ZSM-5 complex, we conclude with confidence that adsorption of DMCH results in a phase transition in ZSM-5 from monoclinic to orthorhombic symmetry. The fact that the changes start at a loading as low as 1 molecule/u.c. (Figure 1) indicates that the framework begins to undergo the (16) Hay, D. G.; Jaeger, H. J. Chem. Soc., Chem. Commun. 1984, 1433.
Notes
structural change as soon as the first DMCH molecule enters the channel. To further confirm the finding, we also measured the XRD pattern of p-dichlorobenzene/ ZSM-5 with a loading of 2 molecules/u.c. (not shown). This particular system was chosen as reference because the exact structure of this complex has been recently determined unambiguously from single-crystal X-ray diffraction measurements17 and belongs to the orthorhombic system. The fact that the powder XRD patterns for p-dichlorobenzene/ZSM-5 and DMCH/ZSM-5 complexes look very similar suggests that the structure of DMCH/ZSM-5 complex is indeed orthorhombic. Previous single-crystal X-ray diffraction studies17-19 have shown that the framework of ZSM-5 can adopt two different orthorhombic forms: Pnma and P212121 depending on the nature and the loading level of the sorbate. For example, the single-crystal studies by van Koningsveld et al. have shown that loading ZSM-5 with 2 p-dichlorobenzene molecules/u.c. causes a phase transition from monoclinic to orthorhombic (space group Pnma).17 Further loading of p-dichlorobenzene to 8 molecules/u.c. results in the second orthorhombic phase but with a different space group, P212121.18 Although a phase transition from monoclinic to orthorhombic can be identified unambiguously from the changes in powder XRD patterns, it is rather difficult to differentiate the two possible space groups of orthorhombic phase from powder XRD data alone. Recently, Mentzen et al. examined benzene/ZSM-5 complexes with various loadings by powder XRD and 29Si MAS NMR.8 Using energy minimization calculations and a Rietveld refinement method, they were able to determine the space group symmetry of the benzene/ZSM-5 complex with a loading of 6 molecules/u.c. to be P212121. Careful inspection of the data reveals that the powder XRD pattern of DMCH/ ZSM-5 at a loading of 4 molecules/u.c. is almost identical to that of benzene/ZSM-5 at a loading of 6 molecules/u.c. If the structural assignment of Mentzen et al. is accepted, it may be tentatively suggested that the structure of ZSM-5 loaded with DMCH belongs to the orthorhombic phase with a space group of P212121. It is interesting to notice that MCH does not induce any phase change in the framework structure of ZSM-5. The difference between MCH and DMCH is the extra methyl group on the cyclohexane ring. It seems that the nature of the host-guest interaction in these systems is mainly based on the exact size and shape of the organic molecule. Thus, the behavior of the organic molecules inside the zeolite framework is of particular interest. For this reason, we also used FT-Raman spectroscopy to probe the behavior of the guest molecules inside the zeolitic framework. FT-Raman Spectroscopy. The FT-Raman spectra of CH, MCH, and DMCH adsorbed on ZSM-5 were measured as a function of loading from 1.5 to 4 molecules/u.c. Assignments of the vibrational modes of cyclohexane and the alkyl derivatives were based on those previously reported.20,21 The spectra of the pure liquids were also obtained to assist in interpreting the data. For the DMCH/ZSM-5 system, within the entire loading range of 1.5-4 molecules/u.c., the Raman spectra of DMCH adsorbed on zeolite were independent of the coverage, (17) van Koningsveld, H.; Jansen, J. C.; de Man, A. J. M. Acta Crystallogr. 1996, B52, 131-139. (18) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Acta Crystallogr. 1996, B52, 140-144. (19) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1989, B45, 423-431. (20) Gardiner, D. J.; Walker, N. A.; Dare-Edwards, M. P. Spectrochim. Acta 1987, 43A, 21-34. (21) Sterin, Kh. E.; Zhirzin, G. N.; Aleksanyan, V. T. Raman Spectra of Hydrocarbons; Permagon Press: Oxford, 1980.
Notes
Figure 2. FT-Raman spectra of pure DMCH and DMCH adsorbed in ZSM-5 in the C-H stretching region. The band indicated by the arrow (V) is the new peak that appears upon adsorption (see text).
implying that the DMCH molecules access identical sites in the framework, presumably at the intersections of the straight and sinusoidal channels. The spectra of encaged DMCH differ from that of pure DMCH liquid only in the C-H stretching region. The reason that the C-H stretching vibrations of the guest molecule are more sensitive to its surrounding environment is because the C-H bonds are located on the outside of the sorbate molecule and therefore the vibrational modes arising from the stretching motions are most affected by the change in the surrounding environment. Figure 2 shows the Raman spectra of pure DMCH liquid and DMCH incorporated into the zeolite at a loading of 4 molecules/u.c. in the ν(CH) region. Upon adsorption, asymmetric and symmetric C-H stretching modes of methylene (CH2) groups positioned at 2910 and 2844 cm-1 in the spectrum of pure DMCH liquid shifted by 5 and 4 wavenumbers to 2915 and 2848 cm-1, respectively. The changes in frequency are the result of the close match between the channel size and the molecular dimension. The tight fit of the DMCH molecule within the framework causes the perturbation of the methylene C-H stretching vibrations. Thus, the observed shift toward higher energies may be attributed to the restriction of the C-H stretching motions from the surrounding framework. The frequency shift values for ν(CH2) modes for methylene units are comparable to those for ring ν(CH) modes of p-dichlorobenzene adsorbed in ZSM-5 at a loading of 4 molecules/ u.c.11 The band centered at 2950 cm-1 also due to the asymmetric C-H stretching vibration of methylene groups split into a doublet (a band positioned at 2946 cm-1 with a weak shoulder at 2952 cm-1). The behavior of the band at 2844 cm-1 (the symmetric C-H stretching mode of
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methylene groups) in the spectrum of pure DMCH liquid is also worth noticing. Figure 2 shows that, in the spectrum of pure DMCH liquid, this band is rather narrow and exhibits the highest relative intensity compared to those of all other ν(CH) vibrations. Upon adsorption, the peak became quite broad and the shape asymmetric. Its relative intensity was also reduced significantly. It appears that this band also split into two components. All these results indicate that there are relatively strong interactions between the zeolitic framework and C-H bonds of methylene groups. Interestingly, there seems to be little interaction between the methyl groups and the framework, since the C-H stretching modes for methyl groups (the bands at 2926 and 2868 cm-1) are much less affected upon adsorption. This result implies that the long axis (connecting two methyl groups) of DMCH is oriented along the channel axis and, consequently, the methyl C-H stretching vibrations are less restricted by the framework. Upon adsorption, a new band appeared in the C-H asymmetric stretching region for methylene groups at 2976 cm-1 (Figure 2). It is worth mentioning that this new band has the highest frequency among all other ν(CH) vibrations. It is well-known that the C-H asymmetric stretching vibrations for CH2 units, νas(CH2), in small cycloalkanes (six-membered ring or smaller) are very sensitive to the ring size and, therefore, the strain of the ring.22 As the size of the ring decreases from a six-membered to a threemembered ring, νas(CH2) steadily increases to higher wavenumbers due to the increasing strain of the ring.22 For instance, the νas(CH2) of cyclopropane is even as high as the νas(CH) of aromatic compounds. Thus, the appearance of a new methylene ν(CH2) mode at rather high frequency infers that a part of the cyclohexane ring may experience larger strain. At this point, a few words can be said about the location of DMCH inside the framework. Very recently, Magalha˜es et al. have suggested that the DMCH ring is positioned at the intersection of the straight and sinusoidal channels, with the methyl groups being located inside the channels and the cyclohexane ring lying in the intersection.2 Their argument was based on the facts that the kinetic diameter of DMCH was greater than 9 Å (comparing the zeolite pores being ∼5.5 Å) and that the measured maximum adsorption capacity of DMCH was 4 molecules per unit cell, corresponding to 1 molecule per intersection. Our vibrational data support their argument and also suggest that a part of the cyclohexane ring may actually be inserted in the channel. This suggestion is consistent with the observation of the new ν(CH2) band at high frequency, since, compared to CH2 groups lying in the intersections, the asymmetric C-H stretching motions arising from the CH2 as part of the ring inserted in the channel should be much more restricted by the wall of the channel, leading to a much higher stretching frequency. For the CH/ZSM-5 and MCH/ZSM-5 systems, the Raman spectra of the sorbates adsorbed on zeolite exhibit little difference from those of the pure liquids; that is, no new bands appeared and no splitting occurred upon adsorption, suggesting that the interactions between guest molecules and the framework are weaker. The weaker interactions may result from the sizes of CH and MCH being smaller than that of DMCH and explain why adsorption of these molecules does not induce any structural change in the zeolitic framework. (22) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; Wiley and Sons: New York, 1974.
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Summary The present study investigates the possible phase transitions in ZSM-5 induced by several alkylcyclohexanes. Using powder X-ray diffraction and FT-Raman spectroscopy, we found that the framework structure of ZSM-5 undergoes a phase transition induced by sorbed trans-1,4-dimethylcyclohexane. However, adsorption of cyclohexane or methylcyclohexane does not cause any structural change in the zeolitic framework. The diffusion of cyclohexane, methylcyclohexane, and trans-1,4-dimethylcyclohexane in ZSM-5 had been previously studied. We hope that the results of the present
Notes
study may shed some light on why the diffusion coefficient of DMCH is very different from those of MCH and CH in ZSM-5 and stimulate more studies on these interesting systems. Acknowledgment. Y.H. acknowledges the financial assistance of the Natural Science and Engineering Research Council of Canada. We wish to thank Dr. M. Baker (University of Guelph) for access to an FT-Raman spectrometer. LA990212D
