11406
J. Phys. Chem. B 2006, 110, 11406-11414
Adsorption and Photophysics of Fullerene C60 at Liquid-Zeolite Particle Interfaces: Unusually High Affinity for Hydrophobic, Ultrastabilized Zeolite Y Eric H. Ellison* Department of Chemistry and Biochemistry, The UniVersity of Mississippi, UniVersity, Mississippi 38677 ReceiVed: March 14, 2006; In Final Form: April 24, 2006
Adsorption of fullerene C60 from solution to the external surface of zeolite particles has been investigated. The most intriguing result of this study was the nature of C60 adsorption to ultrastablized zeolite Y (or USY). Two commercial samples of USY were tested: CBV780 (Y780) and CBV901 (Y901). Y901 was shown in previous reports to be more hydrophobic than Y780. Higher affinity of C60 for Y901 was found relative to Y780 in a variety of hydrocarbon solvents, including toluene and cyclohexane. In these same solvents, weak or no affinity for Y901 of typical arenes such as naphthalene or pyrene was observed. In toluene, adsorption isotherms for C60 gave dissociation constants (and values of saturation binding) ) 0.5 µM (5.8 µmol g-1) and 8 µM (1.4 µmol g-1) for Y901 and Y780, respectively. C60 was estimated to cover nearly one-half of the estimated external surface area of Y901 particles at saturation. Significant adsorption of C60 to the ionic zeolites NaX, NaY, and KL was observed in cyclohexane but not in toluene, consistent with the π-cation effect as a driving force for adsorption to these materials. The main driving force for C60 adsorption to Y901 is postulated to involve the interaction of C60 with lone pair electrons of framework oxygen atoms of the 12-ring entry aperture to the supercage. In the 12-ring site, C60 is located in halfsupercage bowls on the exterior particle surface. The adsorptive interaction on Y901 relies on the spherical shape of C60 and the hydrophobicity of the zeolite surface. On ionic zeolites, the presence of specific adsorption sites such as exchangeable cations and hydroxyl groups hinder the special positioning necessary for C60 interaction with the 12-ring site. The ground-state and triplet-state absorption spectrum of adsorbed C60 was solution-like on all zeolites. Quenching of the C60 triplet state was examined by using transient absorption spectroscopy. Rate constants for quenching by rubrene, ferrocene, and O2 at the Y901-toluene interface were 18, 9, and 3 times lower, respectively, relative to rate constants in solution. These differences point out that the approach of molecular quenchers to C60 at the interface is more hindered for larger molecules, an expected result for C60 located in half-supercage bowls. The high affinity of fullerenes for hydrophobic zeolite Y provides a strategy for organizing fullerenes at interfaces and for studies of fullerene photochemistry.
Introduction The association of fullerene C60 with zeolites X and Y has been explored by a number of research groups, mostly within the past decade.1-13 The impetus for such studies relates to the potential for isolating or organizing C60 in zeolite voids or cage structures. Zeolite-C60 hybrids may eventually be used as catalysts or as components of photonic or electronic devices with supramolecular functionality. The majority of studies on zeolite-60 hybrids have utilized zeolites X and Y, which are synthetic forms of the naturally occurring mineral faujasite. Details about the structure and properties of faujasite and many other zeolites can be found in Breck’s book14 and in more recent compilations.15 The interest in faujasite stems from its relatively large cage (or supercage) diameter of 13 Å, which is large enough to accommodate C60. Most zeolites do not exhibit such a large cage structure. Although many types of arenes can be placed in the supercage of faujasite by adsorption from solution, this is not possible for C60. The crystalline diameter of C60 is 7.9 Å, which is larger than the 7.4-Å diameter of the pore opening (or 12-ring entry aperture) to the supercage. Therefore, migration of C60 from * Tel: (662) 915-7875. Fax: (662) 915-7300. E-mail: eellison@ olemiss.edu.
the zeolite particle exterior and into the supercage does not occur at room temperature, and elevated temperatures are needed to soften the zeolite framework and place C60 in the supercage. Previous studies having the goal of investigating C60 in the supercage have most often heated a mixture of solid C60 and dehydrated zeolite powder for up to several days under vacuum at temperatures of 400-650 °C, followed by extraction of the remaining C60 in a solubilizing solvent such as benzene or toluene. Surprisingly, there is no published information concerning the adsorption of C60 from solution to the external particle surface of zeolites. The current study addresses this topic. Recently, we found that C60 dissolved in toluene showed high affinity for a sample of ultrastabilized zeolite Y (USY). This result is intriguing considering that C60 does not adsorb from toluene to zeolites X and Y and that typical arenes such as pyrene or naphthalene do not adsorb from hydrocarbon solvents to USY. USY has a pore and framework structure virtually identical to that of zeolites X and Y, but is much lower in ionicity as a result of chemical treatments designed to remove aluminum from the framework. Recent efforts by zeolite chemists have focused on increasing the hydrophobicity of USY through novel hydrothermal treatments.16 The resulting materials show low affinity for water and high affinity for nonpolar
10.1021/jp061577r CCC: $33.50 © 2006 American Chemical Society Published on Web 05/23/2006
Fullerene Adsorption on Zeolites
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11407
adsorbates such as hydrocarbon solvents.17-19 Hydrophobicity indices have been evaluated and compared for various Y zeolites.18 Although the acidity of hydrophobic USY is much lower than that of typical ultrastabilized H-Y zeolites, they do exhibit Bro¨nsted acidity and considerable hydroxyl content. The following report describes the association of C60 with the external surface of zeolite particles. C60 adsorption isotherms on a variety of materials and in various solvents in which C60 is soluble are described. The absorption spectrum of adsorbed C60 in the ground state and triplet state is also described, as well as the behavior and quenching of triplet C60 (3C60) at interfaces. In addition to these studies, an attempt was made to place small amounts of C60 in the supercage with the goal being to compare the spectroscopic features of C60 in the supercage with that of externally located C60. The results of this study show affinity of both C60 and C70 for hydrophobic USY in a variety of solvents that is much higher than that typically observed for other arenes such as pyrene or anthracene. The affinity is such that significant amounts of fullerene can be placed on the USY surface with a negligible fraction located in the bathing solvent. Therefore, the behavior of fullerenes can be examined exclusively on the surface of USY particles bathed in a variety of solvents, including polar solvents such as methanol. The adsorbed C60 exhibits spectroscopic features that are solution-like and is highly accessible to triplet quenchers dissolved in the solvent bathing the zeolite. Although this study shows that C60 can be both immobilized and organized at a liquid-solid interface, the driving force for adsorption is less clear. Some possibilities are discussed. Experimental Section USY samples that were tested include CBV901 and CBV780 from Zeolyst International. These materials have SiO2/Al2O3 ) 83 and 80, respectively. CBV901 has recently been described as more hydrophobic than CBV780.18 According to the manufacturer, CBV901 is prepared by exposing CBV780 to steam under what is referred to as “turbulent, fluidized conditions at 650-1000 °C”.16 A sodium-exchanged sample of zeolite Y (CBV100) having SiO2/Al2O3 ) 5.1 was also obtained from Zeolyst. The Zeolyst materials are abbreviated in the text as Y901, Y780, and Y100. Samples of sodium-exchanged zeolites A (NaA), X (NaX), and Y (NaY) having SiO2/Al2O3 ) 1.0, 1.4, and 2.5, respectively, and Davisil silica gels having nominal pore diameters of 22 Å (Si-22) and 500 Å (Si-500) were all obtained from Aldrich. Potassium-exchanged zeolite L (KL) having SiO2/Al2O3 ) 6.0 and silicalite (ZSM-5) having SiO2/ Al2O3 ) 60 were obtained from Tosoh Corporation and used as received. Siliceous MCM-41 (Si-MCM-41) having a pore size of 32 Å was obtained as a gift from Dr. Michael Wark. Fullerenes C60 and C70, rubrene, and ferrocene were obtained from Aldrich and used as received. Pyrene was purified by column chromatography. Solvents including toluene, cyclohexane, methanol, 1,2-dichlorobenzene (1,2-DCB), and 1-methylnaphthalene (1-MNP) were of the highest purity available. O2 concentrations in solvents were achieved by bubbling with N2/ O2 mixtures or pure O2. Adsorption isotherms of C60 on zeolite powders were determined from measurements of concentration depletion of C60 solutions by quantitative absorption spectroscopy. Sample equilibration was carried out by agitation of slurried samples for at least 2 h in capped glass centrifuge tubes on an orbital shaker. Prior to the equilibration step, the solid was activated in the centrifuge tube and allowed to cool for no more than 10 min in a desiccator. All zeolites and MCM-41 were activated
Figure 1. Adsorption isotherms for C60. In a few cases, the reproducibility of measurements is shown. See the text for details.
in air at 550 °C. Equilibrated samples were centrifuged and the C60 concentration in the supernatant was analyzed by the use of standard curves. The apparatus used for time-resolved measurements of fluorescence and optical density was described in a previous report.20 All absorbance measurements in zeolite samples were made in the diffuse transmittance mode. Pulsed laser excitation of C60 was achieved by using 5-ns pulses at 355 nm, or 0.5-ns pulses at 337 nm. Decay profiles were fit by use of a NLLS algorithm. Results and Discussion Isotherms for the adsorption of C60 from cyclohexane to USY and NaY are shown in the upper graph of Figure 1. Although not shown, double reciprocal plots were nonlinear for all materials except Y100. Therefore, an energetically homogeneous distribution of adsorption sites, or a single-site Langmuir model with a specified dissociation equilibrium constant for binding (KD), cannot necessarily be assumed for these materials. Although a significant portion of the isotherm could be measured, the limited solubility of C60 in cyclohexane prevented a direct measurement of the amount adsorbed at saturation. Nevertheless, the isotherms reveal tighter binding on Y901 as
11408 J. Phys. Chem. B, Vol. 110, No. 23, 2006 compared to the other materials. More on the quantitative aspects of adsorption are discussed below. Adsorption of C60 from toluene was also examined. For the ionic materials NaX, NaY, and Y100, adsorption was too weak to be measured by the depletion method. This is expected based on the π-cation effect as a driving force for adsorption21,22 because toluene competes with C60 for zeolite cations behaving as adsorption sites. In contrast to these results, C60 showed high affinity for Y901 in toluene. Isotherms for Y780 and Y901 are shown in the lower graphs of Figure 1. The solubility of C60 in toluene was such that direct measurements of the amount of C60 adsorbed at saturation could be measured. These values were 5.8 µmol g-1 and 1.4 µmol g-1 for Y901 and Y780, respectively. KD was estimated from the C60 concentration at 50% saturation to be 0.5 µM and 8 µM for Y901 and Y780, respectively. With the diameter of C60 taken as 0.79 nm, the surface area covered by C60 on Y901 at saturation is estimated to be 1.7 m2 g-1. This value can be compared with a calculation of the external surface area of zeolite particles. By using a nominal particle diameter (dp) of 1.5 µm for 1-2 µm spherical particles, and an estimate of the density (Fp) of dehydrated Y901 particles or crystallites as 1 g mL-1,23 the external surface area (Sex) is calculated to be 4.0 m2 g-1 for a smooth sphere (Sex ) 6/Fpdp).24 Errors in crystal density, surface topology, and particle morphology should contribute no more than a factor of 2 in error. This comparison points out that the external surface of Y901 is highly covered by C60 at saturation. The data in toluene clearly show that the treatment designed to increase the hydrophobicity of USY from Y780 to Y901 has a significant impact on binding of C60 to USY. This is discussed in more detail below. The data also show that the main driving force for adsorption to USY differs from that to Y100 and NaY because C60 does not adsorb to either Y100 or NaY from toluene. The affinity of C60 for Y100 and NaY in alkane solvents such as cyclohexane resembles that observed for typical arenes such as pyrene or anthracene, which adsorb to these materials from cyclohexane but not toluene. Of further importance is that arenes generally show weak affinity for USY from cyclohexane. For example, we find that pyrene adsorption isotherms on USY cannot be measured in cyclohexane by the depletion method because of weak adsorption. Therefore, the adsorption of C60 to USY is unique with respect to arene adsorption on zeolites in both cyclohexane and toluene. Adsorption of C60 from cyclohexane to other porous solids including silica gel, siliceous MCM-41, and the zeolites NaX, KL, ZSM-5, and NaA was also examined in a qualitative way by the depletion method. In this case, 0.10 g of activated (dehydrated) powder was equilibrated for 2 h with 5 mL of a 45 µM solution of C60 in a given solvent. Table 1 (top section) compares the percent depletion from solutions in cyclohexane for various materials. The results provide a qualitative assessment of whether C60 adsorbs to the surface. Of the zeolites tested, C60 showed significant adsorption on USY, NaY, and KL and relatively lower levels of adsorption on NaX, ZSM-5, and NaA. Negligible depletion was observed for silica gels. Samples of silica gel were activated at 600 °C under vacuum and compared with those activated at 150 °C. Activation of silica gel at 600 °C is known to irreversibly produce a hydrophobic surface25,26 that weakens the adsorption of arenes from cyclohexane. Evidently, the surface hydrophobicity created by the activation treatment has no effect on the adsorption of C60 to silica gel. Unlike the results for silica gel, adsorption to SiMCM-41 was measurable. However, the amount adsorbed was relatively small. Even so, the results point out that surface
Ellison TABLE 1: Percent Adsorption of C60 from Solutiona adsorption of C60 from cyclohexane material
SiO2/Al2O3
percent adsorbed
Y901 Y780 Y100 NaY NaX KL NaA ZSM-5 Si-MCM-41 Si-500 (150°C) Si-500 (600°C) Si-22 (150°C) Si-22 (600°C)
80 80 5.1 2.5 1.4 6.0 1.0 60 SiO2 SiO2 SiO2 SiO2 SiO2
>99 >99 92 90 21 84 3 negl. 11 negl. negl. negl. negl.
adsorption to Y901 fullerene
solvent
percent adsorbed
C60 C60 C60 C70 C70
toluene 1,2-DCB 1-MNP toluene cyclohexane
>99 82 45 >99 >99
a In all cases, 0.10 g of zeolite powder was equilibrated with a 5.0mL aliquot of 45 µM fullerene.
curvature may be important in the adsorption of C60 to silica. Mesoporous materials with channels more narrow than those tested here (i.e., 3.2 nm) could be more conducive to adsorption. Although this topic was not pursued here, it does seem worthy of further exploration because the organization of C60 in channels might be possible. The surface of Si-MCM-41 should resemble that of a hydrophobic silica gel because the material was calcined at 550 °C prior to use. Adsorption of C60 to Y901 was further tested in 1,2-DCB and 1-MNP, two solvents in which C60 is 10 times more soluble than in toluene. The results are shown in the bottom section of Table 1. In solution, C60 forms a ground-state complex with 1-MNP. This may account in part for the drop in C60 adsorption in this solvent. Of further interest was whether liquids such as pyridine, N,N-dimethylaniline, and nitrobenzene would remove C60 from the Y901 surface by competitive adsorption. These solvents were added in small amounts (5%) to cyclohexane and the solvent mixtures were equilibrated with C60-coated, selfsupported Y901 disks. None of these solvents were effective at stripping C60 from the surface. Adsorption of fullerene C70 to Y901 was also assessed in toluene and cyclohexane. The results show that fullerene adsorption on Y901 is not unique to C60. Similar effects were observed for C70. Spectroscopy of Adsorbed C60. Zeolite powders onto which C60 was adsorbed were violet-colored, similar to solutions of C60, and ground-state absorption spectra for these powders were solution-like. C70-coated USY was amber-colored, consistent with solutions of C70. The upper graph in Figure 2 shows the absorption spectrum of C60 on a self-supported disk of Y901 bathed in toluene. In toluene, the disk was nearly transparent. Similar spectra were observed for zeolite disks bathed in cyclohexane and to which C60 was adsorbed. These include NaX, NaY, Y901, and KL. One interesting effect was noted for NaY and is illustrated in the lower graph of Figure 2. After adsorbing C60 from cyclohexane to a self-supported disk of NaY and removing the cyclohexane by evacuation at 125 °C, the sample disk was violet-colored and gave spectrum A collected in the diffuse transmittance mode. Below 350 nm, there
Fullerene Adsorption on Zeolites
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11409
Figure 3. Decay profile and spectrum of 3C60 on evacuated Y901. The inset shows the spectrum of 3C60 at 12 µs following pulsed laser excitation. A double-exponential fit of the decay profile is shown. λex ) 355 nm.
Figure 2. Ground-state absorption spectrum of C60 on self-supported zeolite disks. See the text for details.
was significant loss of analyzing light intensity due to scattering or the fact that evacuated samples are not transparent. Nevertheless, the spectrum is still resolvable. The usual absorption bands for C60 can be seen, including the weak absorption bands above 400 nm. When the sample was placed in MeOH, a yellow color promptly appeared that remained even after evacuation of the sample to remove the MeOH. The spectrum of the MeOHevacuated sample is shown by spectrum B. If this sample was placed in cyclohexane, then the spectrum reverted within 5 min to that of sample A, with loss of the yellow color and return of the violet color. These results are most likely caused by C60 aggregation or microcrystal formation on the NaY surface that yields a different absorption spectrum to the monomer. C60 is insoluble in polar solvents and remains on the NaY surface when the sample is bathed in MeOH. However, the adsorptive interaction with zeolite cations is apparently weakened by MeOH, causing C60 to pool on the surface. Cyclohexane promotes the dissolution of microcrystals and readsorption to cationic sites. Such effects were not observed for Y901, where the C60 absorption spectrum remained unaltered upon bathing the sample in MeOH. Spectrum B in Figure 2 might be a type of nanoscale effect in which C60 microcrystals behave differently than bulk C60. A similar coloration was observed by coating activated samples of NaX or NaY with C60 in toluene. This involved removing,
by use of a spatula, self-supported discs from a 1-mM C60 solution in toluene followed by evaporation of the solvent under vacuum at room temperature. The inset in the upper graph of Figure 2 shows the spectrum of C60 adsorbed to Si-MCM-41 at the vacuum-solid interface. C60 was placed on the surface by adsorption from cyclohexane. The spectrum is solution-like, and no yellow discoloration of the sample was observed. Previous studies have claimed electronic confinement effects that yield unusual absorption bands for C60 in the voids of Si-MCM-41.2,27 These effects were not observed in the present study. There is no reason to suspect that C60 is not located in the voids of MCM-41 because the pore diameter of the material used is more than 3 times the diameter of C60. In cyclohexane, C60 should be able to migrate by diffusion into the porous interior of Si-MCM-41. The results for MCM-41 are discussed in more detail below. Quenching of Triplet C60 on Y901. The photophysical properties of C60 have been well-characterized in solution.28,29 Basically, the triplet quantum yield of C60 is near unity because of rapid intersystem crossing. For this reason, C60 fluorescence is difficult to observe and, barring any ground-state complex formation, C60 photochemistry takes place mainly out of the triplet state. In the present study, quenching of the C60 triplet state (3C60) was assessed at the gas-solid and liquid-solid interface of Y901, and was also compared to excited-state quenching of other arenes. The absorption spectrum and decay profile of 3C60 at the vacuum-solid interface of Y901 is shown in Figure 3. The decay profile was best fit to a double exponential yielding decay times of 61 and 2.9 µs. The spectrum is similar to that found in solution. However, because of experimental limitations, the spectrum was collected at 12 µs following the laser pulse and therefore does not include the 2.9-µs component. The rate constant for quenching of 3C60 by O2 was assessed from a Stern-Volmer plot of the decay rate of the longer-lived component versus O2 pressure. Values for quenching of triplet phenanthrene (3Ph) located in the intraparticle void volume were also measured for comparison. Decay profiles for 3Ph were found to be single exponential. Quenching rate constants (kq) for 3C60 and 3Ph at the gas-solid interface were 1.2 × 105 s-1 Torr-1 and 1.4 × 105 s-1 Torr-1, respectively. Therefore, the
11410 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Ellison
TABLE 2: Quenching Rate Constants for Y901-Toluene and Bulk Toluene kq/108 s-1 M-1 probe/quencher 1
Py/O2 Py/O2 3 Ph/O2 3 C60/O2 3 C60/rubrene 3 C60/ferrocene 3
Y901-toluene
bulk toluene
18 n/aa n/ab 4.5 2.0 9.9
200 18 20 12 36 92
a The yield of 3Py was too low for measurements. b Phenanthrene is unretained in Y901-toluene.
approach of gaseous O2 to3C60 located on the particle exterior is quite similar to that for 3Ph located in the supercage. Singlet quenching was also assessed at the gas-solid interface. For such measurements, room-temperature fluorescence of C60 could not be detected, consistent with the low fluorescence quantum yield of C60 measured in solution (
