Langmuir 1988,4, 147-152
sols containing particles with diameters well below 0.01 pm. Very small particle sizes are accessible because these materials have high densities (10.5 and 19.3g/cm3 for silver and gold), and retention in SFFF depends on particle mass. The high extinction efficiencies of these small particles makes detection by optical means practical, even at sample concentrations as low as the 0.005% of the gold sol described here. The silver and gold sols which we have examined are easily dispersed in dilute aqueous FL-70 surfactant. There seems to be little change in the state of dispersion in the course of the SFFF analysis, as determined spectrophotometrically, which makes it possible to detect particle growth as well as flocculation by this method. This represents an advantage over electron microscopy, where the
147
appearance of aggregation is often the result of the sample preparation method. The presence of gelatin adsorbed to the metal particles has little effect on the SFFF results because the gelatin contributes relatively little to either the mass of the particles or their optical extinction, even at 254 nm. Removal of the stabilizing gelatin, as by enzymatic action, may have little short-term effect on the sols, or may result in flocculation. In either case, SFFF is a convenient method for examining the stability of these materials.
Acknowledgment. We thank Raymond C.Schuck of these laboratories for preparing some of the Carey Lea silver sols. Registry No. Au, 7440-57-5; Ag, 7440-22-4.
Interaction of Benzene, Cyclohexene, and Cyclohexane with the Surface of Titanium Dioxide (Rutile) Yasuharu Suda Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan Received May 20, 1987. I n Final Form: August 18, 1987 The adsorption isotherms of organic cyclic molecules, benzene, cyclohexene, and cyclohexane, were measured at 25 O C on rutile samples covered with a controlled number of surface hydroxyl groups. The amount of irreversible adsorption increased with the number of a-electrons of adsorbate molecule and varied inversely with the surface hydroxyl content of the sample. These results suggest the participation of 7r-electrons of adsorbate molecules in the adsorption and give evidence for stronger interaction of a-electrons with surface Ti4+ions than with surface OH groups. Infrared spectra of adsorbed species were measured for surfaces of both dehydroxylated and hydroxylated rutiles. When benzene was adsorbed on the dehydroxylated surface, the absorption bands due to the C-C stretching vibration shifted to lower wavenumbers compared with those of the gaseous state, which supports the concept of charge transfer from benzene molecules to the dehydroxylatedsites, preferentially surface Ti4+ions. Reversible adsorption occurring on the hydroxylated rutile surface was ascribed to the interaction between the surface OH groups and ?r-electronsfor benzene and cyclohexene and to the contribution of dispersion force for cyclohexane having no a-electrons.
Introduction It is widely known that the surface hydroxyl groups produced by dissociative adsorption of water molecules have a significant effect on the surface properties of the solid. The characterization of surface hydroxyl groups on the solid, especially on metal oxides, has become a matter of great interest.'S2 A study of the interaction of solid surfaces with molecules having a range of natures such as molecular size, polarity, and functional groups provides a useful tool for characterization of their surface properties. In our previous work the effect of surface hydroxyls on the interaction of oxide surfaces with aliphatic organic molecules has been investigated for such typical metal oxides as zinc oxide3s4and titanium dioxide (rutile).6ie The adsorption of cyclic aromatic compounds on metal oxides is particularly interesting in view of the possible participation of a-electrons in the interaction. Benzene and its derivatives can form a variety of charge-transfer complexes depending on their electron-donating ability or (1) Parfitt, G. D. Prog. Surf. Membr. Sci. 1976, 11, 181. (2) Naaao. M. J. Phys. Chem. 1971, 75, 3822. (3) NGao, M.; Morcmoto, T. J. Phys. Chem. 1980,84, 2054. (4) Nagao, M.; Matauoka, K.; Hirai, H.; Morimoto, T. J. Phys. Chem. 1982,86,4188. (5) Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99. (6) Suda, Y.;Nagao, M. J . Chem. SOC.,Faraday Trans. 1 1987,83, 1739.
0743-1463/88/2404-Ol47$01.50/0
electron-accepting ability.' The formation of chargetransfer complexes has been reported in some systems of metal oxides and benzene derivative^.^'^ Kiselev et al.'O have reported the charge-transfer interactions between surface hydroxyl groups on silica and a-electrons of benzene. We have also estimated the interaction energy between a-electrons and a zinc oxide surface from the heat-of-immersion in liquid benzene derivatives.15 In the present study, the adsorption isotherms of organic cyclic molecules on rutile samples with differing concentrations of surface hydroxyls are reported. The adsorbates are benzene, cyclohexene, and cyclohexane, differing in the number of a-electrons but otherwise similar in molecular size and structure. The effect of 7r-electrons on the interaction between the rutile surface and these cyclic molecules is discussed in connection with the surface hydroxyl content of the rutile samples. (7) hulliken, R. S.; Person, W. B. Molecular Complexes;Wiley-Interscience: New York, 1969. (8) Basila, M. R. J. Chem. Phys. 1961, 35, 1151. (9) Whalen, J. W. J. Phys. Chem. 1962,66, 511. (10)Galkin, G. A.; Kiselev, A. V.; Ligin, V. I. Trans. Faraday SOC. 1964, 60, 431. (11) Cusumano, J. A.; Low, M. J. D. J. Phys. Chem. 1970, 74, 793. (12) Cusumano, J. A.; Low, M. J. D. J. Phys. Chem. 1970, 74, 1950. (13) Cusumano, J. A.; Low, M. J. D. J. Catal. 1971, 23, 214. (14) Cusumano, J. A.; Low, M. J. D. J. Colloid Interface Sci. 1972,38, 245. (15) Suda, Y.; Morimoto, T. Langmuir 1985, I , 544.
0 1988 American Chemical Society
Suda
148 Langmuir, Vol. 4, No. 1, 1988
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Figure 1. Adsorption isotherms of benzene on rutile samples evacuated at various temperatures after complete hydroxylation: ( 0 , O ) 25 O C ; ( 0 ,H) 200 "C;(A, A) 600 "C. Open and filled marks represent the first and second adsorption, respectively. Experimental Section The original TiOz (rutile) sample was the same as that used in the previous ~ o r k .The ~ , ~sample was degassed at 600 "C for 4 h under a vacuum of 1 X Pa in order to remove surface contaminants, followed by exposure to saturated water vapor at room temperature to promote complete surface hydroxylation. By evacuating this fully hydroxylated sample at various temperatures between 25 and 600 "C, we obtained samples having a different number of surface hydroxyl groups. Organic cyclic compounds used as adsorbates were benzene (spectroscopicreagent grade from Merck), cyclohexene (guaranteed reagent grade from Kanto Chemicals), and cyclohexane (spectroscopic reagent grade from Nakarai Chemicals). These were purified further by distillation in the usual way and stored over 4A molecular sieves dried at 500 "C in vacuo. The surface hydroxyl content of each sample was measured by the successive-ignition-loss method?4 and it was expressed by the number of surface hydroxyl group remaining on the surface per unit area after evacuation of the fully hydroxylated sample. The surface hydroxyl contents were 7.88, 5.39, 3.75, 2.07,0.739, and 0.132 hydroxyl groups/nm2for the samplestreated at 25,100, 150,200,300, and 600 O C , respectively. The specific surface area of the sample pretreated at 600 O C was determined by applying the BET method to N2adsorption data and found to be 9.18 m2 g-1. The first adsorption isotherm of each organic molecule was determined at 25 "C for each rutile sample. After the isotherm was measured, the sample was exposed to the saturated vapor of the adsorbate to promote thorough adsorption. The sample was evacuated at 25 "C for 4 h under a vacuum of 1 X Pa, and then the second adsorption isotherm was measured at the same temperature as the first adsorption. Infrared spectra of adsorbed species were measured for both the dehydroxylated(600"C evacuated) and the fully hydroxylated (25 O C evacuated) samples. The spectra were recorded at ambient temperature by a Nippon-BunkoModel A302 diffraction grating spectrophotometer. Results and Discussion Figure 1 shows the fiist and second adsorption isotherms of benzene vapor on rutile samples covered with different concentrations of surface hydroxyls. Here, selected adsorption isotherms are represented in order to simplify the figure. As can be seen from Figure 1, the isotherm of benzene is type I1 in Brunauer's classification, indicating multilayer adsorption. For the sample treated at 25 "C, the first adsorption isotherm coincideswith the second one, which implies that benzene is adsorbed reversibly on the fully hydroxylated rutile surface. On the other hand, for
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Figure 2. Adsorption isotherms of cyclohexene on rutile samples
evacuated at various temperatures after complete hydroxylation: symbols are the same as those in Figure 1. the dehydroxylated surface, there is a difference in the amount adsorbed between the first and second adsorptions, indicating an occurrence of irreversible adsorption. It is interesting to note that the amount of adsorption on the 600 "C treated sample is less than that on the 25 " C treated sample in the region of monolayer formation. For the samples treated a t moderate temperatures, 100, 150, and 200 "C,the first adsorption isotherm was almost the same as that on the 25 "C treated sample, whereas the isotherm for the 300 "C treated sample was situated between those for 25 "C and for 600 "C treated samples. The second adsorption isotherm shows a decreasing amount of adsorption with increasing temperature of the treatment of sample from 25 to 600 "C. The adsorption isotherms of cyclohexene vapor on rutile samples treated at 25,200, and 600 "C are shown in Figure 2. The shape of these isotherms resembles that of benzene. The agreement between the first and second adsorption isotherms for the 25 "C treated sample is evidence for reversible adsorption on this surface. An irreversible adsorption occurs on the samples treated a t 200 and 600 "C, where the amount of the first adsorption is larger than that of the second one. The amounts adsorbed in the monolayer region are in the order 200 > 25 > 600 ("C)treated samples for both fvst and second adsorptions. The amount of adsorption for the samples treated at other temperatures gradually increased with increasing temperature of the treatment to 200 "C,but it decreased by heat treatment a t higher temperatures. Figure 3 shows the adsorption isotherms of cyclohexane on rutile samples, in which the representative data are used as before. The variation of the amount adsorbed with the treatment temperature of the sample is fairly small. The amount of the first adsorption increases, while that of the second one decreases, with increasing temperature of the treatment. The adsorption isotherms of three kinds of adsorbates on the dehydroxylated (600 "Ctreated) and hydroxylated (25 "C treated) surface are redrawn in Figures 4 and 5, respectively,for easy comparisons among these adsorbates. On the dehydroxylated surface (Figure 41, the first adsorption isotherms almost agree with each other for every < 0.2), and beadsorbate in the monolayer region (PIP,, yond this the amount of adsorption is in the order benzene > cyclohexene > cyclohexane. The second adsorption decreases in the order cyclohexane > cyclohexene > benzene in the monolayer region. The amount of irreversible adsorption is found to decrease in the order benzene > cyclohexene > cyclohexane, in agreement with
Langmuir, Vol. 4, No. 1, 1988 149
Interaction of Cyclic Organics with Rutile
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Figure 3. Adsorption isotherms of cyclohexane on rutile samples evacuated at various temperatures after complete hydroxylation: symbols are the same as those in Figure 1.
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Figure 4. Adsorption isotherms of organic cyclic molecules on the dehydroxylated surface of rutile: (0, 0 ) benzene; (0,m) cyclohexene; (A,A) cyclohexane. open and fded marks represent the first and second adsorption, respectively.
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Figure 5. Adsorption isotherms of organic cyclic molecules on the hydroxylated surface of rutile: symbols are the same as those in Figure 4. a decreasing order of the number of a-electrons in the molecule. On the other hand, for every adsorbate only a reversible adsorption takes place on the fully hydroxylated surface (Figure 5). Two distinct features can be recognized in this figure. First, the adsorption isotherm of cyclohexene agrees with that of benzene in the lower pressure < 0.05),but with increasing pressure it comes range (PIP,,
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Figure 6. Dependence of the monolayer capacity of adsorbed organic cyclic molecules on the surface hydroxyl content of rutile sample: (0, 0 , 0 ) benzene; (0, W, 0 ) cyclohexene; (A,A, A) cyclohexane. Open, filled, and half-filled marks represent V,,, V,,, V,,, respectively.
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close to the isotherm of cyclohexane. Second, the adsorption of cyclohexane having no a-electrons is depressed in the lower pressure region. It is obvious from these facta that on the hydroxylated surface, especially in the initial stage of adsorption, a-electrons in the adsorbate molecules take part in the interaction with the solid surface. The amount of irreversible adsorption, Vi,,, is obtained by subtracting the monolayer capacity ( VmJ of the second adsorption from that of the first one ( VmJ and is plotted against the surface hydroxyl content of the sample in Figure 6. These monolayer capacities are calculated by applying the BET equation to the first and second adsorption isotherms. On the basis of the V,, values obtained for the dehydroxylated and fully hydroxylated surfaces, the average area occupied by an adsorbed molecule can be estimated to be 0.546 (0.452)for benzene, 0.539 (0.502)for cyclohexene, and 0.590 (0.600)nm2 molecule-' for cyclohexane (the figure in parentheses represents the value for the hydroxylated surface). Taking into account the cross sectional area of benzene molecule (0.426nm2),15the adsorbate molecules used in this study are assumed to be adsorbed in such a way that the cyclic molecule is lying flat with the plane of its ring parallel to the solid surface in somewhat loose packing. For the adsorbate molecule having a-electrons, the interaction of a-electrons with the surface Ti4+ions on the dehydroxylated ~urface'~,'~ or with the surface hydroxyls on the hydroxylated surface"15 is responsible for the adsorption phenomena in these systems. In the case of adsorption of cyclohexane that has neither a-electrons nor polarity, among various kinds of interaction energies the dispersion energy is predominant and the additional contribution from the interaction between the electrostatic field of the solid surface and the induced dipole of the molecule increased with increasing field strength by heat treatment of the sample at higher temperatures.15 This implies that the electrostatic field caused by multivalent ions, Ti4+ and 02-,on the dehydroxylated surface is stronger than that caused by OH- ions on the hydroxylated surface. On the dehydroxylated surface, Ti4+ ions act effectively as an adsorption site for cyclohexane in preference to 02-ions because of their greater charge and smaller size. The Vi, value is the largest on the dehydroxylated surface for every adsorbate, and it decreases with increasing surface hydroxyl content of the sample. It is reasonable to conclude from this fact that the interaction of a-electrons with surface Ti4+ions is stronger than
150 Langmuir, Vol. 4, No. 1, 1988 the interaction with surface hydroxyls. Furthermore, it may be easily accepted that the value of V , is in the order benzene > cyclohexene > cyclohexane in the whole range of surface hydroxyl content of the sample, corresponding to the decreasing number of a-electrons in the molecules. The value of V,, is almost the same for every adsorbate on the dehydroxylated surface, 1.7-1.8 molecules nm-2, regardless of the number of a-electrons in the molecule. Jones and Hockeyle have demonstrated that the exposed surfaces of rutile are composed of three planes, (110), (101), and (loo), in the ratio 3:1:1, on which the number of coordinatively unsaturated Ti4+ions is 5.1, 7.9, and 7.4 per nm2 for the respective planes. On the basis of these values and by assuming the adsorption model in which the surface Ti4+ions act as adsorption sites, the number of molecules adsorbed in the monolayer can be estimated to be 1.78 molecula for three kinds of adsorbates having almost the same molecular size. This value is in good agreement with the experimental values (1.69-1.83 molecules nm-2). It can, therefore, be considered that V,, values are determined by the number of surface titanium ions and their distribution as well as by the occupied area of adsorbed molecules. The variations of V,, with surface hydroxyl content of the sample are different among these three adsorbates. On the fully hydroxylated surface or partidy dehydroxylated surface carrying about 2 OH groups nm-2, the surface Ti4+ ions are screened by the protruding OH layers.16 Thus an adsorbate molecule with a six-memberedring approaching the surface can interact with the surface OH groups rather than with Ti4+ions. As can be expected from the cross sectional area of benzene described above, a stable adsorbed state of this molecule is assumed to be flat on the solid surface. Therefore, even if the dehydroxylationwould enhance the electrostatic field of the solid surface, this contribution to the interaction with a-electrons may not be as large as the OH-a interaction. In contrast to benzene, the cyclohexene molecule has a permanent dipole (0.332 D), and its a-electron is localized. With increasing strength of the surface electrostatic field by dehydroxylation, the interaction between the dipole of the molecule and the surface field becomes stronger, giving rise to an increase in the V , values. At surface hydroxyl content less than about 2 d H groups nm-2, both benzene and cyclohexene molecules can access the surface Ti4+ ions without interference from surface hydroxyls, so that the interaction between Ti4+ ions and a-electrons enhances adsorption. Thus the V,, value should be limited by the number of surface Ti4+ions, and finally it decreases to the value for dehydroxylated surface as stated above (1.7-1.8 molecules nm-2). In the case of cyclohexane having no a-electrons, the V,, value increases slightly with decreasing surface hydroxyl content of the sample and accordingly with increasing strength of the surface electrostatic field.16 Figure 7 shows the IR spectra of benzene adsorbed on the dehydroxylated and hydroxylated surfaces of rutile. When benzene is adsorbed on the dehydroxylated surface, three distinct absorption bands appear at 3095,3070, and 3040 cm-', the first of these being assigned to the stretching vibration of the C-H bond and the other two to composite modes (spectrum b, Figure 7)." A t this time, though not shown in the figure, the bands due to the out-of-plane deformational vibration of the C-H bond appeared at 1970 and 1815 cm-l, together with a sharp band at 1478 cm-'.
Suda ,_____-
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Figure 7. Infrared spectra of benzene adsorbed on the dehydroxylated (a-e) and hydroxylated (f-j) rutile surfaces: (a) background spectrum of rutile dehydroxylated at 600 "C; adsorption of benzene at equilibrium pressures of (b) 0.067, (c) 1.13, and (d) 6.00 kPa;(e) after evacuation of benzene vapor at ambient temperature; (0 background spectrum of the hydroxylated rutile; adsorption of benzene at (9) 0.067, (h) 1.36, and (i) 3.20 kPa; (j) after evacuatuion of benzene vapor at ambient temperature.
Gaseous benzene gives a characteristic absorption band at 1485 cm-', being assigned to the C-C stretching vibration of the ring." I t has been reported that the shift of this band is indicative of a formation of the bond of Tcomplex type between benzene molecule and metal atoms.18 Therefore, the present results showing a shift of the 1485-cm-l band toward a lower wavenumber support the adsorption mechanism through charge transfer between benzene molecule and the surface Ti4+ions on the dehydroxylated surface. The intensities of the bands in the 3000-3100-cm-' region increase with increasing pressure of benzene vapor (spectrums c , d, Figure 7). After evacuation of the sample disk at ambient beam temperature the absorption bands in the CH stretching region still remained slightly (spectrum e, Figure 7), indicating irreversible adsorption on this surface. On the hydroxylated rutile surface the bands in the OH stretching region appear at 3665,3520, and about 3400 cm-I (spectrum f, Figure 7), and are ascribed to the free OH groups, the OH groups perturbed by the formation of hydrogen bonding to molecular water,lg and mutually hydrogen-bonded OH groups20or the OH groups of molecular water present on the s u r f a ~ e , lrespectively. ~,~~ The presence of molecular water was confirmed by the appearance of the band due to the bending vibration of the water molecule at 1625 cm-'1.19121922When benzene is adsorbed on this hydroxylated surface, the absorption bands due to the aromatic C-H stretching vibration can be observed at the same wavenumber as in the case of adsorption on the dehydroxylated surface. The 3665-cm-l band of the free surface OH groups shifts to 3590 cm-', probably owing to the interaction with T-electrons in benzene molecules (spectrums h, i, Figure 7). The broad band centered at 3400 cm-' is not affected by the adsorption of benzene. Spectrum j in figure 7, recorded after evacuating (18)Shopov, D.;Palazov, A. Dokl. Bo&. Akad. Nauk 1969,22, 181. (19)Griffiths, D.M.;Rochester, C. H . J. Chem. SOC.,Faraday Trans. I 1977, 73, 150.
(20)Jaycock, M.J.; Waldsax, J . C. R. J. Chem. SOC.,Faraday Trans.
(16)Jones, P.;Hockey, J. A. Trans. Faraday SOC.1971, 67, 2679. (17)Herzberg, G. Molecular Spectra and Molecular Structure IZ. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966.
1 1974, 70,1501.
(21)Munuera, G.; Stone, F. S . Discuss. Faraday SOC.1971,52, 205. (22)Jackson, P.;Parfitt, G. D.J. Chem. SOC.,Faraday Trans. I 1972, 68,1443.
Interaction of Cyclic Organics with Rutile
Langmuir, Vol. 4, No. 1, 1988 151
Wavenumber/ lo2 cm-'
Figure 8. Infrared spectra of cyclohexane adsorbed on the dehydroxylated (a-e) and hydroxylated (f-j) rutile surfaces: a and fare the aame 88 those in Figure 7;adsorption of cyclohexane at (b) 0.067,(c) 1.20,(d) 2.53,(g) 0.13,(h) 1.33,and (i) 3.07 kPa; e and j represent the spectra after evacuation of cyclohexane vapor at ambient temperature.
Figure 9. Infrared spectra of cyclohexene adsorbed on the dehydroxylated (a-e) and hydroxylated (f-j) rutile surfaces: a and fare the same as those in Figure 7;adsorption of cyclohexene at (b) 0.067,(c) 1.27,(d) 2.73,(9) 0.067,(h) 1.47,and (i) 3.00 kPa; e and j represent the spectra after evacuation of cyclohexene vapor at ambient temperature.
the benzene vapor at ambient temperature, is the same as the background spectrum f taken before benzene adsorption. This result substantiates a reversible adsorption of benzene on the hydroxylated rutile surface, being consistent with the isotherm data shown in Figure 1. The IR spectra of cyclohexane adsorbed on the dehydroxylated and hydroxylated rutile surfaces are represented in Figure 8. Cyclohexane on the dehydroxylated surface gives absorption bands due to the asymmetric and symmetric C-H stretching vibrations at 2935 and 2860 cm-', respectively (spectrum b, Figure 8). The CH2 bending vibration band also appeared at 1447 cm-', though not shown in the figure. When the pressure of cyclohexane vapor is increased, the band at 2935 cm-' shifts to 2910 cm-' and a new band appears at 2965 cm-l (spectrums c, d, Figure 8). All these bands disappear, and the same spectrum as the background spectrum can be obtained after evacuating the vapor a t ambient temperature (spectrum e, Figure 8). The fairly intense band at 2965 an-' cannot be assigned to any type of vibration at present. The similar intense band at 2960 cm-' for cyclohexane vapor has been reported in the l i t e r a t ~ r ein , ~which ~ the assignment of this band has not yet been made. The cyclohexane molecule has two forms, the chair and boat form, the former being confirmed as the most stable at room temperature . It is apparent that the IR spectra of cyclohexane on the hydroxylated rutile surface resemble those on the dehydroxylated surface except in the OH stretching region. The free OH band at 3665 cm-l shifts to lower wavenumbers, finally to 3635 cm-', as the coverage of cyclohexane increases (spectrum i, Figure 8). The shift of this band is 30 cm-l, in comparison with 75 cm-' for the adsorption of benzene, which suggests a weak interaction between the surface OH groups and cyclohexane molecules. Both bands a t 3520 and 3400 cm-l are not affected by cyclohexane adsorption, as in the case of benzene adsorption. Spectrum j, Figure 8, taken after evacuation of the vapor, is analogous to the background spectrum f. From the cyclohexene adsorption on the dehydroxylated surface, the C-H stretching vibration band for the unsaturated carbon atoms in the ring can be assigned to the band at 3025 cm-l (Figure 9). In addition, four distinct absorption bands can be observed at 2935,2885,2865, and
2840 cm-', the first two assigned to the asymmetric and the last two to the symmetric C-H stretching vibrations in the CH2ring system (spectrum b, Figure 9). In the lower wavenumber region not shown here, the band due to the C-C stretching vibration appeared at 1636 cm-', and the CH2deformationalmodes gave the bands at 1450 and 1436 cm-'. On subsequent cyclohexene adsorption, the most intense band at 2935 cm-l decreases in intensity and shifts to lower wavenumbers, together with an appearance of a band at 2958 cm-' (spectrum d, Figure 9). This absorption band cannot be ascribed to any type of vibration as in the case of cyclohexane adsorption at high coverage. Cyclohexene has also two types of conformation, the semichair and semiboat form, the former being stable at room temperature. The 2958-cm-l band might be caused by these steric effects. After evacuation of the cyclohexene vapor, almost all absorption bands, except the 2935-cm-' band, are removed (spectrum e, Figure 9). This result substantiates the idea of irreversible adsorption of cyclohexane on the dehydroxylated rutile surface. On the other hand, when cyclohexene is adsorbed on the hydroxylated surface, the free OH band at 3665 cm-' becomes broader and shifts to 3635 cm-'. At the same time, the 3530-cm-' band increases in intensity (spectrums g, h, i, Figure 9). From the consideration stated above that the free OH band at 3665 cm-' shifted to 3635 cm-' on adsorption of cyclohexane on the hydroxylated surface (spectrum i, Figure 8),it is reasonable to assume that the 3635-cm-' band in Figure 9 should be assigned to the OH groups perturbed by the CH2 fraction of cyclohexene molecules. The band at 3530 cm-l may be assigned to the OH groups interacting with ?r-electrons in the adsorbate molecules. The shift of this band from the position of the free OH band is 135 cm-', being greater than in the case of benzene adsorption where it is 75 cm-'. ?r-Electron density in the double bond between two carbon atoms in the cyclohexene molecule is larger than that in the benzene molecule where ?r-electrons are not localized in the ring. Therefore, the interaction of surface OH groups on the solid with ?r-electrons in cyclohexene is stronger than with those in benzene, giving rise to a greater shift of the free OH band by cyclohexene adsorption compared with benzene adsorption. According to the report by B0ehm,2~
17123
(23) Rasmussen, R.S.J . Chem. Phys. 1943, 11, 249.
(24) Boehm, H.P.Kolloid-Z. Z. Polym. 1968, 227, 17.
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Langmuir 1988, 4 , 152-158
the surface hydroxyl groups with a basic nature are added onto reactive C=C double bonds, e.g., cyclohexene, and a surface cyclohexanolate is formed, giving rise to an infrared absorption band at 1270 cm-' due to the C-0 vibration. However, no indications of such adsorbed species were found in the present study.
Conclusions In the present work it has been shown that the amount of irreversible adsorption of six-membered-ringmolecules on rutile samples varies in the order cyclohexane < cyclohexene < benzene and increases with decreasingsurface hydroxyl content of the sample. With the additional results of infrared spectra, it is evident that benzene and cyclohexene carrying u-electrons are adsorbed in a flat orientation on the dehydroxylated site, preferentially
surface Ti4+ions, through the formation of charge-transfer complexes, and that cyclohexane having no ?r-electronsis adsorbed by dispersion force. Reversible adsorption of benzene and cyclohexene on the hydroxylated surface is due to the OH-.?r interaction, the strength of which is greater for the former molecule than for the latter moletule, as exhibited in the shift of OH band in IR spectra.
Acknowledgment. I thank Professor Tohru Takenaka of Kyoto University for his kind advice and helpful suggestions. I thank also Professors Mahiko Nagao and Tetsuo Morimoto of Okayama University for their discussions and constant encouragement throughout this work. Registry No. Benzene, 71-43-2; cyclohexene, 110-83-8; cyclohexane, 110-82-7;rutile, 1317-80-2;titanium dioxide, 13463-67-7.
Proton Transfer to Toluene in H-ZSM-5:TPD, IR, and NMR Studies W. E. Farneth and D. C. Roe E. I. du Pont de Nemours and Co., Central Research and Development Department, Wilmington, Delaware 19898
T. J . Gricus Kof'ke, C. J. Tabak, and R. J. Gorte* Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received July 10, 1987. In Final Form: August 19, 1987 The adsorption of toluene on H-ZSM-5 has been examined by using temperature-programmed desorption, thermogravimetric analysis, and transmission infrared spectroscopy. Toluene adsorbs preferentially with a coverage of one molecule per Al site in the zeolite and interads strongly with the hydroxyl group associated with the AI site. Isotope exchange between toluene-h8and D-ZSM-5 occurs exclusively at the A1 sites and at temperatures at least as low as 273 K. Proton NMR measurements for the toluene desorbing from D-ZSM-5 show that the isotope exchange occurs exclusively with the aromatic protons at the ortho and para positions, and the relative amounts of the ortho and para species are in good agreement with the product distributions obtained in homogeneous acids. These results give evidence for the formation of relatively stable, arenium-ion-typeintermediates in the zeolite structure.
Introduction There have been many papers published on the topic of zeolite acidity and acid-catalyzed reaction mechanisms in zeolites.' The most general conclusion from these studies is that carbenium-ion-like intermediates are involved in reaction pathways on zeolites just as they are in reactions carried out in acidic solutions. However, while evidence for the presence of carbenium ions during reaction is strong, direct observation of these intermediates has proven difficult, and the character of their interaction with the zeolite lattice has been deduced only circumstantially. Part of the problem is that chemistry has generally been examined under reactor-like conditions (Le., high temperatures and pressures) where competitive and consecutive reactions occur. In order to gain a clearer picture of the chemistry occurring in zeolites, we have been investigating the adsorption after evacuation of simple molecules that can undergo classical acid-catalyzed chemistry, such as alcohols (1) For a review, see: Jacobs, P. A. Carboniogenic Actiuity of Zeolites; Elsevier: Amsterdam, 1977.
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that can undergo dehydration24 or, in the work reported here, aromatic compounds that can participate in H/D exchange in H-ZSM-5. H-ZSM-5 was chosen for this work because this material has been shown to contain uniformly active Brransted acid sites in a ratio of one per framework Al atom after mild pretreatment.5p6 Our reason for using adsorption and desorption measurements rather than reaction rate measurements was to focus on the chemistry a t the acid site and to avoid the complex reaction and transport steps which occur in reactor environments. We have examined the chemisorbed species formed by the adsorption of toluene on D-ZSM-5 at low temperatures under vacuum using thermogravimetric analysis (TGA) and infrared spectroscopy. We have followed the decay of those species on heating and examined the isotope (2) Grady, M. C.; Gorte, R. J. J.Phys. Chem. 1986,89,1305. (3) Aronson, M. T.;Gorte, R. J.; Farneth, W. E. J.Catal. 1986,9& 434. (4) Aronson, M. T.; Gorte, R. J.; Farneth, W. E. J. Catal. 1987,105, 455. (5) Olson, D. H.;Haag,W. 0.; Lago, R.M. J . Catal. 1980, 61, 390.
(6)Derouane, E. G.; Baltusis, L.; Dessau, R. M.; Schmitt, K. D. Catalysis by Acids and Bases; Imelik, R., et al., Eds.;Elsevier: Amsterdam, 1986; p. 135.
0 1988 American Chemical Society
