J. Phys. Chem. C 2008, 112, 14501–14507
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UV-Raman and Fluorescence Spectroscopy of Benzene Adsorbed Inside Zeolite Pores Chao Zhang,† Paula M. Allotta,† Guang Xiong,‡ and Peter C. Stair*,†,‡ Department of Chemistry, Center for Catalysis and Surface Science and Institute of EnVironmental Catalysis, Northwestern UniVersity, EVanston, Illinois 60208, and Chemical Sciences and Engineering DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: June 21, 2008
UV-Raman spectroscopy was used to investigate the host-guest interactions of benzene molecules adsorbed within siliceous MFI. With use of 244 nm ultraviolet laser light as the excitation source, several new peaks appear in the spectra. These peaks are not expected from normal Raman scattering of liquid benzene and not seen in the FT-Raman spectra of benzene in silicalite, but appear in the UV-Raman spectra because of the Raman resonance effect. Further investigations diminish the possibility that the new peaks are due to laserinduced chemical reactions. The fluorescence spectra show the fluorescence bands of benzene with vibration progressions, which suggests that the adsorbed benzene molecules are not clustered. Considering that the dimensions of benzene molecules closely match the channel sizes of MFI, it is proposed that the symmetry of D6h-benzene in the excited state degrades to lower symmetry upon adsorption due to compression of benzene molecules inside MFI pores. In support of this result, UV-Raman spectra of toluene in MFI and benzene adsorbed in zeolite Beta were also studied. 1. Introduction Zeolites are widely used as molecular sieves for separation and catalysts for hydrocarbon reactions. During these processes, both sorbates and zeolites are believed to undergo structural changes upon absorption, thus the study of framework-sorbate interactions has been a very important subject for several decades.1,2 The benzene/MFI system is one of the more interesting systems for several reasons. During the catalytic conversion of methanol to hydrocarbons, methylbenzenes and other alkylated aromatic compounds in the zeolite’s pores have been proposed to play an essential role in the catalysis. These species are proposed to act as a “hydrocarbon-pool” for the reaction.3,4 Elucidation of benzene/framework interactions may provide insight into the influence of pore geometry on the reaction. This system is also a model for the aromatization of methane on MoO3/MFI.5 The three-dimensional channel system of MFI, with ring openings close to the size of the free benzene molecule, are consistent with a strong sorbate-framework interaction for the system. Since the benzene molecule is highly symmetric, D6h symmetry, its confinement in the MFI environment is expected to cause structural changes and a reduction in symmetry that is detectable spectroscopically. Previous studies showed that Raman spectroscopy is a very useful technique for probing the host-guest interactions of organic molecules within zeolites, because the Raman bands of sorbates and zeolitic framework do not overlap.6-10 The study of adsorbed benzene molecules within alkali metal-exchanged X and Y zeolites showed that shifts of the ring breathing mode correlate to electrostatic fields caused by cations within the channels.2 The higher the field strength at a given site, the larger is the shift in the ring breathing mode. The study of benzene adsorbed on porous glass shows that the relative intensities of * To whom correspondence should be addressed. E-mail: pstair@ northwestern.edu. † Northwestern University. ‡ Argonne National Laboratory.
Raman bands from adsorbed benzene are different from those of liquid phase molecules.11 The relative intensity changes have been ascribed to interactions of benzene molecules with surface hydroxyl groups. FT-Raman studies demonstrate that the locations of adsorbed benzene within zeolite MFI at various loadings can be determined by shifts of the CH vibrations.12,13 Although information on the adsorption sites of benzene has been obtained by visible- and FT-Raman spectroscopy, detailed knowledge about possible changes in structure and symmetry of the benzene molecule is lacking. It was reported that surface-enhanced Raman scattering of benzene adsorbed on Ni and Pt particles and on cold evaporated Cu films showed symmetry-forbidden bands, resulting from a lower site symmetry of the molecules at the surface.14 Analogous spectral features have not been reported for benzene adsorbed in zeolites. Completely siliceous MFI was used in the experiments reported here to diminish the possibility of chemical reactions during adsorption and spectral acquisition. The highly stable SiO2 structure provides a very inert environment where the interaction of molecules with the pore walls is emphasized. Recent studies in several research groups have shown that UVRaman spectroscopy is a very useful technique for the structural characterization of catalysts and other solids.15-17 In this study, we present the first UV-Raman study of sorbate-framework systems including benzene/MFI, benzene/Beta and toluene/MFI. By excitation with a UV laser at 244 nm, not only the Raman spectrum but also the fluorescence spectrum is obtained for the benzene/MFI system. These measurements provide insight into the nature of guest-host interactions and the role of pore size. 2. Experimental Section 2.1. Sample Preparation. Completely siliceous MFI and zeolite Beta were kindly provided by Dr. Jeff Miller of BP. Benzene (g99.5% GC grade) and toluene (g99.5% Spectrophotometric grade) were obtained from Aldrich Chemical. The zeolites were calcined in air for 2 h (siliceous MFI at 500 °C and Beta at 350 °C) prior to the hydrocarbon adsorption to
10.1021/jp804291x CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
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Figure 1. Recirculation system for controllable aromatics loading into zeolite samples.
remove template and adsorbed water. Two methods for the introduction of benzene into the MFI structure were employed. In the first, the freshly calcined zeolite powders were cooled to room temperature in a dry nitrogen glovebag. Measured amounts of benzene or toluene liquids were dropped by syringe onto weighed powder samples in glass vials under dry nitrogen. The vials were sealed and placed in an oven for 10 h at 85 °C for benzene and 110 °C for toluene to uniformly disperse the sorbate molecules throughout the pore system.7,12 The amounts of benzene/toluene were selected so that the pore volume loading of MFI and Beta would be the same. A recirculating gas apparatus was also used to load benzene in the zeolites, as shown in Figure 1. After pretreatment of the silicalite sample in flowing oxygen at 500 °C in the fluidized bed cell, the gas flow was switched first to flowing nitrogen and then to recirculating nitrogen by a four-way valve. A calibrated volume of benzene was then injected into the system and allowed to circulate through the zeolite sample. The tubing and fluidized-bed cell were heated to 85 °C to avoid condensation of benzene. After recirculation for 3 h the 4-way valve was switched to open the loop, and the exit gas was sampled by a mass spectrometer (QMS 200 by Stanford Research Systems). No benzene was detected, which indicates that all of the benzene was loaded into the silicalite. UV-Raman spectra of the benzene/silicalite sample in the cell after recirculation were identical with the spectra collected after the conventional benzene loading method. This shows that the different loading methods do not change the interaction between the benzene molecules and zeolite framework. 2.3. Raman Measurements. For FT-Raman measurements 5-10 mg of zeolite was placed in a glass capillary tube and measured in the 180° back-scattering geometry. The spectra
Zhang et al. were recorded on a Bio-Rad spectrometer with an infrared Nd: YAG laser operating at 1064.1 nm and a liquid nitrogen cooled Ge detector. UV-Raman spectra were collected at room temperature by an ellipsoidal mirror and focused into a fiber bundle that is coupled to a Spex Triax 550 single grating spectrometer with a UV-enhanced charged-couple device (CCD) detector. A UV edge filter was used to block Rayleigh scattering. The 244.1 nm UV excitation is produced by a Lexel 95 SHG (Second Harmonic Generation) laser equipped with a nonlinear BBO crystal (Beta Barium Borate: BaB2O4), which frequency doubled visible radiation into the midultraviolet region. The laser intensity is 2 mW before passing through the quartz window of the fluidized-bed in situ cell, and the data acquisition time ranges from 5 to 30 min, depending on the loading of benzene. The details of the fluidized bed Raman cell have been described previously.16 Sample powders were placed on a porous disk inside the cell. Gas flow and mechanical vibration were adjusted to produce a stirring movement of the sample powder, which minimizes laser-induced photochemical and thermal degradation of the sample. The size of the laser beam at the sample is approximately 200 µm. A schematic diagram of the instrumentation is presented in Figure 2. 2.4. Fluorescence Measurements. The vibrationally resolved fluorescence spectra of adsorbed benzene were measured by using the same laser excitation and signal collection settings used for the UV-Raman measurements. The fluorescence measurements were performed at laser incident wavelengths of 244 and 257 nm. Raman and fluorescence of benzene vapor were measured by placing an open glass vial filled with benzene liquid inside a quartz cell with continuous 1 atm nitrogen flow. At room temperature, the vapor pressure is 14 kPa. 3. Results and Discussion 3.1. FT-Raman Measurements of Benzene Adsorbed in MFI. FT-Raman spectra of benzene in the liquid phase and adsorbed in siliceous MFI with loadings of 2 to 8 molecules per unit cell are shown in Figure 3. The spectrum of liquid benzene shows Raman bands at 609, 993, 1178, 1588, 1608, 2950, and 3063 cm-1. The assignments of the bands are given in Table 1.12,18 The peaks at 993 and 3063 cm-1 are assigned to the totally symmetric ring breathing and C-H symmetric stretching modes with A1g symmetry. The peaks at 609, and 1178 cm-1 are associated with ring deformation, and C-H bending with E2g symmetry. The bands at 1588 and 1608 cm-1 are assigned to the ring stretching modes with E2g symmetry, split into a doublet due to Fermi resonance. The Raman spectra of adsorbed benzene show some differences from that of liquid
Figure 2. Schematic diagram of the spectrometer used for UV-Raman and fluorescence measurements.
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Figure 3. FT-Raman spectra of liquid benzene and benzene adsorbed inside silicalite with different loading amount (mole./u.c. ) molecules of benzene per unit cell zeolite).
TABLE 1: Vibrational Frequencies of Benzene Molecules and Assignments of the FT-Raman and UV-Raman Peaks of Benzene in the Liquid Phase and Adsorbed Inside MFI symmetry A1g A2g A2u zB1u B2g B2u E1g E1u E2g
E2u
mode υ2 υ1 υ3 υ11 υ13 υ12 υ5 υ4 υ14 υ15 υ10 υ20 υ19 υ18 υ7 υ8 υ9 υ6 υ17
CH stretch ring stretch CH bend CH bend CH stretch Ring deform. CH stretch ring deform. ring stretch CH bend CH bend CH stretch ring stretch + deform. CH bend CH stretch ring stretch CH bend ring deform CH bend
calcd freq (cm-1) FT liquid (cm-1) FT adsorbed (cm-1) UV liquid (cm-1) UV adsorbed (cm-1) 3062 992 1326 673 3068 1010 995 703 1310 1150 849 3063 1486 1038 3047 1596 1178 606 975
3063 993
3069, 3073 993
993
996
1490 1588, 1608 1178 609
benzene. The 382 and 826 cm-1 peaks are assigned to Si-O-Si bending and stretching vibrations of the silicalite framework. The C-H stretching mode at 3064 cm-1 shifts upward to 3069 cm-1 and a small shoulder is detectable at 3078 cm-1. Upon adsorption, benzene has been located at three sites inside MFI: (1) the intersection of straight and zigzag channels with a diameter of 0.87 nm, (2) the midsection of the straight channels with dimensions 0.57 × 0.51 nm, and (3) the midsection of zigzag channels with a diameter of 0.54 nm.12 Since the C-H stretching mode is at the perimeter of the benzene molecule and is more sensitive to the surrounding environment, Huang et al. suggested that the frequencies of C-H stretching modes are related to different adsorption sites in MFI.12,19 In his study, peaks at 3067, 3078, and 3083 cm-1 are assigned to benzene molecules in the intersections, straight channels, and zigzag channels, respectively, and peaks at 991 and 996 cm-1 are associated with benzene molecules inside intersections and straight/zigzag channels.9 The occupation of these sites proceeds sequentially with increased benzene loading. The shift from 3063 to 3069 cm-1 seen for adsorbed benzene in the present study is
1588, 1608 1178 609
attributed to sorbate-framework interactions. The appearance of a shoulder at 3073 cm-1 suggests that benzene molecules inside MFI have occupied more than one adsorption site at a loading of 2 molecules per unit cell, i.e., a fraction are located in the straight and zigzag channels. It has also been suggested that phase transitions in MFI could be induced by the adsorption of benzene.20 At a loading of 5 molecules per unit cell a monoclinic to orthorhombic framework transition is observed. A second phase transition in MFI occurs at a loading of 7 molecules per unit cell. Molecular simulation of the adsorption of benzene proposed that the zeolite framework undergoes a transition from ORTHO (orthorhombic form with Pnma symmetry) to PARA (orthorhombic form with P212121 symmetry) above 4 molecules per unit cell.12 The occupation of channel sites (zigzag or straight) was assigned as the trigger for the phase transition, which was detected by changes in intensities and positions of MFI bands in the 350-400 cm-1 region.12 Analogous spectral changes were not observed in the present study, possibly due to the lower resolution employed for our measurements (10 vs 2 cm-1).
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Zhang et al.
Figure 4. UV-Raman spectra of silicalite, liquid benzene, and benzene adsorbed inside silicalite with different benzene loading.
3.2. UV-Raman Measurements of Benzene Adsorbed in MFI. UV-Raman spectra of benzene adsorbed in MFI with loadings from 2 to 8 molecules per unit cell are shown in Figure 4. Interestingly, besides the main 996 cm-1 peak, three pronounced new peaks at 1081, 1490, and 1648 cm-1 and a number of small additional peaks between 1100 and 1470 cm-1 were observed at all loadings. These peaks are absent from the UV-Raman spectra of liquid benzene and the FT-Raman spectra of MFI-loaded samples. Experiments were conducted to rule out the possibility that the new peaks are due to laser-induced photochemistry. Benzene loaded in silicalite was exposed to 4 mW of UV light for 3 h to produce photochemistry. The 4-way valve in Figure 1 was rotated for recirculation, the aromatics trap was cooled to liquid nitrogen temperature, and the cell was heated to 250 °C. After 1 h the 4-way valve was opened, and the aromatics trap was warmed to room temperature. Only benzene was detected by mass spectrometry. Following this procedure temperatureprogrammed oxidation was used to check for the formation of nonvolatile carbonaceous species in the zeolite. The sample in the cell was heated at the rate of 10 °C per minute to 600 °C in 5% oxygen flow (balance nitrogen) and held at 600 °C for 1 h. No carbon monoxide or carbon dioxide was detected either by mass spectrometry or by gas chromatography(Agilent 3000A Micro GC), proving that no coke species were formed inside the silicalite pores after the UV laser exposure. From these results and considering the fact that the MFI support used is completely siliceous with no acid sites, there is no evidence that benzene molecules undergo any chemical changes during the adsorption and UV laser exposure. The new peaks must be attributed to benzene molecules inside the silicalite pores. UV-Raman spectra of the benzene/MFI loading of 6 molecules per unit cell, benzene vapor, and liquid benzene are compared in Figure 5. The broad bands in the benzene vapor spectrum are due to fluorescence. The intensity and width of the small bands between 1100 and 1470 cm-1 observed for benzene/MFI are also consistent with fluorescence. However, the peak positions, widths, and intensities of the three narrow peaks at 1081, 1490, and 1648 cm-1 are not consistent with fluorescence. Possible assignments of these new peaks are given in Table 1. The peak at 1081 cm-1 can be assigned to the combination of A2u (673 cm-1) and E2u (410 cm-1) modes. The peak at 1490 cm-1 is very close to the Raman shift of the E1u
Figure 5. UV-Raman spectra of benzene/MFI of 6 molecules per unit cell loading, benzene vapor, and liquid benzene.
carbon band (1486 cm-1), but this band is symmetry forbidden in the D6h point group. An alternative assignment would be the combination of E1u (1038 cm-1) and E2u (410 cm-1) modes. The peak at 1648 cm-1 can be assigned to a combination of A2u (673 cm-1) and E2u (975 cm-1) modes. One difficulty with these assignments is the observed intensities of these bands which are comparable to the A1g ring breathing mode. The combination bands are expected to be very weak in Raman spectroscopy. An explanation for the observation of combinations of nontotally symmetric vibrations in UV-Raman spectra is the resonance Raman scattering. Experiments and theory have demonstrated three primary mechanisms for resonance enhancement, identified as A-term, B-term, and C-term resonance.21,22 For A-term resonance enhancement, only totally symmetric modes are enhanced, and either the frequency ω, or the equilibrium normal coordinate, Q0, of the vibration must change between the ground and excited electronic states. For B-term resonance enhancement, the vibration must be vibronically active for coupling the excited electronic state to other excited electronic states. Under these conditions, both the totally symmetric (A-term) and nontotally symmetric (B-term) fundamental transitions can be resonance enhanced with excitation in a dipole-allowed electronic transition. However, these resonance effects would not enhance the observed combination bands. Moreover, excitation at the 244 nm wavelength used in the present study does not excite a dipole-allowed electronic transition. When the laser is in resonance with a dipoleforbidden, vibronically coupled electronic transition, the A- and B-terms are zero, and resonant intensity is produced by the C-term. This term gives rise to enhancement of totally symmetric vibrations as well as overtones and combinations of nontotally symmetric modes.23 There are four main electronic transitions for benzene molecules at 180, 210, 260, and 340 nm, observed in the electronic absorption of benzene in acetonitrile solution.23,24 The absorption band at 180 nm is assigned to dipole-allowed transition to the E1u state. The other two absorption bands at 260 and 210 nm are attributed to low lying π-π* electronic transitions to the vibronically coupled B2u and B1u states, respectively, which are dipole forbidden. The 340 nm band is assigned to the E2g state. The 244 nm laser line is in resonance
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TABLE 2: The Resolution of Symmetry Species of D6h Point Group into Lower Symmetry Point Groups and Their Possible Dipole-Forbidden Vibronically Coupled Transitions
Figure 6. UV-Raman spectra of liquid toluene and toluene adsorbed inside silicalite at loadings of 2, 4, 6, and 8 molecules per unit cell.
with vibrationally excited states of the B2u band.25 Transitions to the B2u excited electronic state can be promoted by E2g vibrations with an admixture of E1u excited states, thus inducing in-plane polarized intensity.26,27 The resonance Raman spectra of benzene have been reported with use of laser excitation at 212.8, 231-232, and 265 nm.23,25,27,28 In agreement with symmetry predictions, the overtones and combinations of the E2g modes, which are responsible for vibronic coupling to the B1u and B2u transition, are prominent in the spectra. However in the UV-Raman spectra with 244 nm excitation only combinations of A2u, E1u, and E2u modes are observed, which suggests a different symmetry for vibronic coupling between the A1g ground electronic state and the low-energy electronic excited states of benzene. The FT-Raman spectra in the present work indicated that benzene molecules are located both in the channel intersections and in the straight/zigzag channels of the MFI under our loading conditions. The fact that the three new bands were not observed in the FT-Raman spectra of adsorbed benzene, nor in the UVRaman spectra of liquid benzene, but only in UV-Raman resonance spectra of adsorbed benzene implies that the appearance of new combinations must be associated with interactions of benzene molecules in the B2u electronic excited state and the MFI framework. Considering that the characteristic size of the ground state benzene molecule (0.58 nm) and the diameter of the MFI channels (0.57 × 0.51 and 0.54 nm)29 are very close and that the benzene ring is expanded in the electronically excited B2u state,30 it seems very likely that in the electronic excited state, interaction with the pore walls would cause benzene to undergo structural distortion and lower the symmetry from D6h. Table 2 shows the resolution of symmetry species from the D6h point group (free benzene molecule) into lower symmetry point groups and their possible dipole-forbidden vibronically coupled transitions. For combinations of vibrations observed to be involved in vibronic coupling between the ground and electronically excited states of benzene, the point group in the
excited state must change to D3d (a2u, e1u, and e2u are resolved into a1g and eg) or D2h (a2u, e1u, and e2u are resolved into b1u, b2u + b3u, and au + b1u) in the adsorbed state. The D3d point group corresponds to distortion of the benzene molecule from planar to chair conformation. The D2h point group corresponds to an in-plane squeezing of the hexagonal ring in one direction. 3.3. UV-Raman Studies of Toluene Adsorbed Inside MFI. For a molecule with lower symmetry, e.g., toluene in the C2V point group, a symmetry change that accompanies guest-host interactions upon adsorption or electronic excitation will not be as dramatic. This is confirmed in the UV-Raman spectra of adsorbed toluene shown in Figure 6 where only fundamental vibrations are observed, using the assignments reported in ref 2. Interestingly, the modes which involve in-plane C-C ring vibrations at 1005, 1032, 1174, and 1615 cm-1 are prominent in the adsorbed state, whereas the in-plane aromatic C-H bending vibration at 1212 cm-1 is quite weak. This may be an indication of interactions between the ring hydrogens and the pore walls. Vibrational motions that involve primarily ring deformation, C-H bending, and C-CH3 stretching at 786 and 1380 cm-1 are relatively weak. For A-term resonance enhancement of vibrations the equilibrium normal coordinate in the electronically excited state must shift with respect to the ground state. Calculations based on fluorescence excitation spectra provide accurate values of bond length changes for toluene between the ground state and first excited, S1 state.31 In the excited state of toluene the C-C bonds parallel to the figure axis (C2C3 and C5C6) are shorter than the “perpendicular” C-C bonds (C1C2, etc.), but the C-CH3 bond length is essentially unchanged. Thus, the prominent Raman bands involve vibrations that are expected to be resonance enhanced based on ring bond length and bond angle changes. 3.4. UV-Raman Studies of Benzene Adsorbed Inside Beta. The zeolite Beta framework has a larger pore diameter (0.66 nm) than MFI,29 which provides enough space for benzene molecules to fit inside the channels without distortion in either the ground or the B2u excited electronic states. The UV-Raman spectra of benzene adsorbed in Beta are shown in Figure 7. The 991 cm-1 peak due to the ring breathing vibration is strong. Note that the strong peaks at 1075 and 1483 cm-1 observed for benzene adsorbed in silicalite are absent, consistent with no changes in the point group in either the ground or excited B2u state. The 811 and 1194 cm-1 peaks are due to fluorescence.
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Zhang et al. TABLE 3: Assignments of the Fluorescence Peaks of Benzene Adsorbed Inside MFI with (a) 244 nm and (b) 257 nm UV Laser As Incident Lighta (a) 244 nm freq (cm-1) intensity 38562 38485 38417 38332 38263 38179 38079 38037 37935 37822 37779 37627 37503
Figure 7. UV-Raman spectra of benzene adsorbed inside Beta zeolite at loadings of 1, 2, and 4 molecules per unit cell.
s w s w s w s w s w s ss ss
(b) 257 nm
assignment 1
0
60 (A0 ) 612(C00) 6011611(A01) 6121611(C01) 6011622(A02) 6121622(C02) 6011633(A03) 6121633(C03) 6011644(A04) 6121644(C04), 6011655(A05) 601110(A-10) 610(B00)
freq (cm-1) intensity
assignment 1
37623 37489 37403 37336 37228 37174 37084 37017 36926 36861
w ss w ss w s w w ww ww
60 110(A-10) 610(B00) 621(D00) 6101611(B01) 6211611(D01) 6101622(B02) 6211622(D02) 6101633(B03) 6211633(D03) 6101644(B04)
36652
ww
6211644(D04)
36519 36422 36360 36262 36198 36108
ss w ss w ss w
610110(B-10) 621110(D-10) 6101101611(B-11) 6211101611(D-11) 6101101622(B-12) 6211101622(D-12)
a
Intensities are represented by a series of qualitative indications ranging form ww ) very weak, etc. to ss ) very strong.
Figure 8. Fluorescence of benzene adsorbed inside silicalite excited by 244 and 257 nm UV laser wavelengths.
The peak at 1643 cm-1 is assigned to benzene that has reacted on the acid sites of the zeolite. 3.5. Fluorescence of Benzene Adsorbed Inside MFI. The assignment of peaks in benzene fluorescence with excitation at 253 nm has been extensively reported in the literature.27,30,32-35 Figure 8 compares the fluorescence spectra of benzene in siliceous MFI at 244 and 257 nm excitation. Detailed fluorescence band assignments that follow refs 32-40 are presented in Table 3. Band assignments use the notation introduced by Callomen, Dunn, and Mills.30 The fluorescence spectrum excited at 257 nm is essentially identical in terms of both peak positions and intensities to that reported in Garforth and Ingold,32-34 as well as Atkinson and Parmenter.35 The higher energy fluorescence spectrum, produced by 244 nm excitation, has not been reported previously, although the band assignments follow those given by Garforth and Ingold.34 Figure 9 shows the fluorescence bands of benzene in liquid and vapor and adsorbed in siliceous MFI, excited at 244 nm. Noticeable differences are observed between liquid benzene and benzene in the vapor or adsorbed state. The fluorescence of
Figure 9. Fluorescence spectra of benzene in the liquid, vapor phase, and within silicalite at a loading of 6 molecules per unit cell.
liquid benzene produces a single, very broad band with no vibrational resolution while adsorbed and vapor phase benzene exhibit strong vibrational progressions. The width of the vibrational progressions has been shown to correlate strongly with the degree of clustering and depends on concentration and total pressure.36 The fluorescence spectrum of adsorbed benzene is similar to that of gas phase benzene at 0.1 Torr in 100 Torr of isopentane.40 This result indicates that the benzene molecules within MFI are present as isolated monomers with minimal benzene-benzene interactions. As shown in Figure 9, the relative intensity fluorescence progressions of ν6 versus ν16 are noticeably different for benzene molecules inside MFI and in the vapor phase. The higher energy excitations from ν6 are much weaker for adsorbed benzene. The intensity decrease is due to the collisional perturbation and quenching of the excited states.41
Benzene Adsorbed Inside Zeolite Pores 4. Conclusions In this study, we present the first UV-Raman measurements of sorbate-framework systems using benzene adsorbed in zeolites. Benzene/MFI and benzene/Beta systems were studied with FT-Raman, UV-Raman, and fluorescence spectroscopy. The FT-Raman spectra confirms that, under our loading conditions, benzene molecules inside MFI have occupied more than one adsorption site even at a very low loading of 2 molecules per unit cell. The fluorescence spectra, showing vibration progressions assigned to benzene monomer, are observed for adsorbed benzene. Compared with the fluorescence bands of liquid benzene and benzene vapor, the spectra give strong evidence that the molecules are located inside the pores and are highly isolated. Combinations of nontotally symmetric fundamental vibrations appeared in UV-Raman spectra of benzene adsorbed inside MFI with different loadings. These bands were not observed in FTRaman spectra of benzene/MFI or in UV-Raman spectra of benzene/Beta. The appearance of these combination bands can be explained by the C-term resonance Raman excitation into a lower symmetry (D3d or D2h) electronically excited state produced by the interaction with the MFI pore walls. Acknowledgment. Financial support of this work was provided by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-FG0297ER14789. References and Notes (1) Gies, H. Analysis of the Guest-Molecule Host-Framework Interaction in Zeolites with Nmr-Spectroscopy and X-Ray-Diffraction. In. AdV. Zeolite Sci. Appl. 1994, 295. (2) Freeman, J. J.; Unland, M. L. J. Catal. 1978, 54, 183. (3) Guisnet, M.; Magnoux, P.; Moljord, K. Abstr. Pap. Am. Chem. Soc. 1995, 210, 34. (4) Haw, J. F.; Song, W. G.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317. (5) Derouane-Abd Hamid, S. B.; Anderson, J. R.; Schmidt, I.; Bouchy, C.; Jacobsen, C. J. H.; Derouane, E. G. Catal. Today 2000, 63, 461. (6) Huang, Y. N. J. Am. Chem. Soc. 1996, 118, 7233. (7) Huang, Y. N.; Havenga, E. A. Chem. Mater. 2001, 13, 738. (8) Ashtekar, S.; McLeod, A. S.; Mantle, M. D.; Barrie, P. J.; Gladden, L. F. J. Phys. Chem. B 2000, 104, 5281.
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