1471
INFRARED STUDYOF BENZENE ADSORPTION ON AEROSIL
Infrared Study of Benzene Adsorption on Aerosil by Adriano Zecchina, Carlo Versino, Anna Appiano, and Giancarlo Occhiena Istituto d i Chimica Fisica dell’ Universitci d i Torino, Torino, Italy Accepted and Transmitted by The Faraday Society
( J u n e 25, 1967)
Adsorption of benzene on highly dehydroxylated Aerosil, having surface concentrations of hydroxyls included between 1 and 2 OH/100 Az, is studied by infrared spectroscopy. At low coverages, adsorption occurs mainly on hydroxyl sites, by a 1 : l interaction, involving the n-electron system of the ring. When the number of perturbed hydroxyls is a significant fraction of the whole, adsorption on completely dehydroxylated areas becomes important.
The adsorption of benzene on silica has been studied both from the calorimetric’ and s p e c t r o ~ c o p i cpoint ~~~ of view. The first kind of work pointed out that the isosteric adsorption heat is strongly affected by the degree of hydroxylation of the surface, and spectroscopic measurements showed that benzene interacts by its n-electron system with the surface hydroxyls. AS a consequence,2t3 a broad band appears at about 3630 cm-1, and a decrease in intensity of the stretching band of the unperturbed hydroxyls at 3750 cm-l occurs. The aim of this research is to study if a correlation exists between the number of adsorbed molecules and that of perturbed hydroxyls, in order to define exactly the type of interaction arising between the OH groups and benzene.
the whole spectrum. 1Modalities of correction are described in previous paper^,^,^ to which the reader is also addressed for the other experimental details. Thermal dehydration went on, in each experiment, at temperatures b700”, in such a way as to haveon the surface a large preponderance of isolated hydroxyl^.^,^ In these conditions, in fact, the infrared spectrum (Figure 1) shows either a nearly symmetric band at 3750 cm-l or a completely symmetric one, in the most dehydrated samples. The slight asymmetry indicates the presence of a very small fraction of nonisolated hydroxyls, i.e., reciprocally interacting hydroxyls.8 Their quantity cannot be exactly determined. In fact,
Experimental Section A Degussa Aerosil, having a specific surface of 310 m2/g, was employed. The Aerosil was compressed by hand into plates about 0.5 nim thick, 24 mm in diameter, and weighing about 50 mg. The slight pressure used gave samples of good infrared transparency, with a fairly high mechanical resistance. Benzene was dried on metallic sodium and purified by distillation. A relative pressure of 5 X represents the lowest limit in order to obtain a satisfactory spectrum of physisorbed benzene, without employing thicker samples and exceedingly wide slits. The infrared cell had a very small optical path (about 1 mm), which allowed us to study the spectrum of physisorbed gases, even under high equilibrium pressures, without appreciable trouble due to the vapor phase. Spectra were run at a constant temperature of 24 f l o ,and the heating effect of the beam was reduced by flowing dry and thermostated air around the cell walls in contact with the sampleU4 I n the measurement of the intensities, account was taken of the scattering effect produced by the adsorption, which causes a uniform loss of transparency along
1400
1500
1600 3000 3200 3400
3600 3800
4000
v , om-1.
Figure 1. Infrared spectrum of benzene adsorbed on highly dehydroxylated Aerosil; broken line is the background. (1) A . V. Kiselev, “Tenth Symposium of the Colston Research Society,” Butterworth and Co. Ltd., London, 1958, p 195. (2) M. 12. Basila, J . Chem. Phys., 35, 1151 (1961). (3) G. A. Galkin, A. V. Kiselev, and V. I. Lygin, Trans. Faraday SOC.,60, 431 (1964). (4) In this paper we always use optical densities on the maximum instead of integrated intensities. I n fact, owing to the difficulties inherent to the graphic separation of the bands a t 3750 and 3630 cm-1, such errors are made in the evaluation of the wings, that the obtained data do not represent a real improvement. (5) E. Borello, A. Zecchina, and C. Morterra, J. Phys. Chem., 71,
2938 (1967). (6) E. Borello, A. Zecchina, C. Morterra, and G. Ghiotti, {bid., 71, 2945 (1967). (7) J. J. Fripiat and J. Uytterhoeven, ibid., 66, 800 (1962). (8) J. A. Hockey and B. A. Pethica, Trans. Faraday Soc., 57, 2247 (1961).
Volume 72, Number 6 M a y 1988
1472
ZECCHINA,VERSINO,APPIANO,A N D OCCHIENA
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
ncsH8/100 Aa.
Figure 2. Decrease in concentration of the free surface hydroxyls 0s. the concentration of adsorbed benzene ( A n o ~ / 1 0 0A2 Initial numbers of free hydroxyls for every 100 A* are: 0 , 1.39; 0, 1.73; 1.92; 1.96; 0, 2.04.
us. ncs~,/lOOWz).
as previously shown,S for samples dehydrated a t temperatures higher than 700", there is a direct proportionality between weight loss and lowering of the optical density on the 3750-cm-' maximum. The proportionality coefficient, taking into account the experimental errors, is the same for all of the outgassing temperatures between 700 and 900". Thus, even if present, the hydroxyls giving rise to the above-mentioned asymmetry are so few in percenta,ge that their influence is negligible compared with the experimental errors. The intensity of the 3750-cm-' band can be varied using activation temperatures between 750 and 850" and different outgassing times. So it is possible to obtain samples with different concentrations of hydroxyls, generally included between 1 and 2 OH/100 Az. The concentration of the surface hydroxyls is measured from the intensity of the 3750-cm-l band, using as extinction coefficient 011 the maximum the value of 35 st 1 1. cm-' m01e-l.~ Sintering, and hence a decrease (from 310 to 250 m2/g) of the surface area, occurs only at temperatures higher than 950". This has been checked by determining the BET area of samples thermally treated in the same way as for spectroscopic measurements.
Discussion The low concentrations of surface hydroxyls used in this work are necessary, when a quantitative investigation on the dependence of the concentration of free hydroxyls from the coverage is undertaken. I n fact, for higher values of the concentration of OH The Journal of Physical Chemistry
0,
I,
groups, the number of mutually interacting hydroxyls is no more negligible. Such hydroxyls do not represent a well-defined surface species, because they give rise to an absorption that is strongly affected by thermal treatments. As previously pointed this can be ascribed to the fact that, on increasing the surface concentration of hydroxyls, the interactions arising among the various groups are of different intensity; in fact, on the heterogeneous surface of the silica, different distances among the OH groups are possible. For all of these reasons, we limited our investigation to samples largely dehydroxylated, and this is the main difference between our work and that of Galltin, et aZ.,3 who studied the adsorption on Aerosil thermally treated a t lower temperatures. The stretching band of the hydroxyls perturbed by benzene adsorption a t 3630 cm-l appears rather symmetric with a constant half-intensity band width of 95-100 em-' in the range of relative pressures investigated ( 5 X 10-3--0.6) and does not shift with coverage. Some other features of the spectrum of physisorbed benzene (Figure 1) are worthy of interest. The CH stretching bands a t about 3000 cm-' have practically the same frequency and half band width as in the liquid phase and in CC14 solution; their intensity increases with coverage. The nuclear stretching band at 14S3 em-' shows also the same coristancy in frequency and half band width compared with the liquid phase and CC1, solution and appears shifted of only 2 cm-' with respect t o the liquid phase. This remark agrees with what has been spectroscopically observed by Gallrin,
1473
INFRARED STUDYOF BENZENE ADSORPTION ON AEROSIL 0.6
0.5
0.4 ~
0
3
e,
0.3
23 I
n 0
0.2
0.1
0
0.1
0.2
0.5
0.4
0.3 O D of CsHu.
Figure 3. Optical density of perturbed hydroxyls at 3630 cm-1 as a function of the optical density of are: adsorbed benzene a t 1483 cm-1. Initial numbers of free hydoxyls for every 100 0 , 1.39; m, 1.73; 0, 1.92; B, 1.96; 0,2.04.
Az
et al.,3 who found that only the out-of-plane bending at 685 em-' is clearly perturbed by the adsorption. Such observations allow us to assume, with good reliability, that physisorption does not substantially alter, with respect to the CCl, solution, the extinction coefficient measured at 1483 cm-' on the maximum of the most intense absorption in the spectral range investigated. So surface concentrations of benzene have been calculated using an extinction coefficient of 62 f 1 1. em-' mole-'. In Figure 2 the decrease in concentration of the free OH groups is plotted us. the surface concentration of benzene for samples with starting surface hydroxyl concentrations: 2.04 f 0.06, 1.96 f 0.06, 1.92 f 0.06, 1.73 f 0.06, and 1.39 f 0.06 OH/100 Az. The straight line drawn in Figure 2 represents the theoretical one that the data ought to follow if the interaction were 1 : l for every coverage. A quite similar behavior is shown, obviously, by the relationship between the optical density at 3630 cm-' and the optical density at 1483 cm-l (Figure 3). As we can easily observe from Figure 2, in spite of the considerable scattering of the data, interaction is really of the 1: 1 type a t low coverages. The three samples with starting hydroxyl concentrations equal to 2.04, 1.96, and 1.92 give rise to a single curve, since the differences are probably of the same order of the scattering of the data. At higher coverages, a deviation from the proposed interaction scheme can clearly be observed, and the number of perturbed hydroxyls is less than the number
of benzene molecules adsorbed. More exactly, the greater the starting surface hydroxyl concentration is, the longer appears the rectilinear stretch of the 1:1 interaction. Moreover, when the intensity of the band at 3630 em-' is plotted us. the decrease of the band at 3750 cm-l, a series of points, individualizing rather well a straight line, are obtained (Figure 4),even for those coverages that in the preceding figures deviated from the straight line. The independence of these data from the coverage indicates that the band at 3630 cm-' does not shift, and its extinction coefficient on the maximum does not change, in agreement with the observed constancy of the band width. As a consequence, the interaction OH-CsHG involves a practically constant energy. The shift A v = 120 cm-l of the OH stretching band has, over all of the range of relative pressures investigated, the maximum value observed at the higher coverages by Galkin, et aLj3on more hydroxylated silicas, indicating also the maximum possible interaction energy. Thus, effects such as closer interaction or reorientation of the molecules with growing coverage are of little importance to our samples. The deviation from linearity in the plot of Figure 2 could then be explained considering a competitive adsorption on completely dehydrated areas. I n fact, the adsorption energies for the two types of sites are not very different, owing to the slight spectral shift of the perturbed hydroxyls ( A v = 120 em-'), from which we can roughly deduce an interaction energy of the order of a few kilocalories per mole. Since, following Galkin, Volume 72, Xzimber 6 May 1968
1474
ZECCHINA, VERRINO, AFPIANO,A N I ) OCCHIENA
0.3
d
0.2
0
a
2
.w
R
2 0.1
tion of the interaction between the benzene moleoules adsorbed on the hydroxyls and the surface as a whole. This quantity is not exactly valuable by means of spectroscopic measures, but following the data reported by Galkin, et a l l 3we can think that the gas-solid interaction energy of the molecules adsorbed on the OH groups is about 2 kcal/mole greater than that of the molecules adsorbed on a dehydrated zone. The slight difference justifies the fact that, when a part of the hydroxyls is bonded, the adsorption on dehydroxylated areas becomes important. The scheme of the 1: 1 interaction can be valid only for silica having concentrations of surface hydroxyls lower than 2 OH/100 In fact, for higher conceritrations the probability of finding hydroxyls at sufficiently short distance to interact at the same time with a single benzene molecule becomes higher. Therefore, for scarcely dehydroxylated silica, one must expect the stretching band of the perturbed hydroxyls to vary in frequency with the coverage, as a consequence of the formation of hydrogen bonds of variable intensity. Furthermore, we cannot exclude that, at relative the most hydroxylated pressures lower than 5 x samples can give rise to more complex interactions. Unfortunately, this fact cannot be revealed by infrared spectroscopy, owing to the low values of the extinction coefficient.
A2.
0
0.1
0.2 0.3 0.4 AOD of perturbed OH.
0.5
0.6
0.7
Figure 4. Decrease of the optical density of the free hydroxyls a t 3760 cm-1 as a function of the optical density of the perturbed hydroxyls a t 3630 cm-1. Initial numbers are: z0 , 1.39; of free hydroxyls for every 100 .i la, 1.73; 0, 1.92; . , 1.96; 0, 2.04.
et ~ l . Av , ~ gives a measure of the specific interaction
between aromatic ring and hydroxyl, the total amount of energy involved in the a,dsorption is greater than it is possible to deduce from the spectral shift, because we must take into account the nonspecific contribu-
The Journal of Physical Chemistw
