Langmuir 1995,11, 4811-4817
4811
Infkared Spectroscopy Study of the Adsorption of Carbonyl Compounds on Severely Outgassed Silica: Spectroscopic and Thermodynamic Results Martine Allian, Enzo Borello, Piero Ugliengo, Guido Spanb, and Edoardo Garrone" Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, via P. Giuria 7, 10125 Torino, Italy Received June 26, 1995. I n Final Form: September 11, 1995@ Fourier transform infrared spectroscopy data are reported concerning the adsorption of formaldehyde, acetaldehyde, acetone, cyclopentanone, cyclohexanone, and 2-cyclohexen-1-oneon Aerosil outgassed at 800 "C. With the four heavier species, plain physisorption competes with the formation of H-bonded complexeswith the isolated surfacehydroxyl,which are best studied at vanishing coverage. The stretching mode of H-bonded hydroxyls shows in all cases an unusually complex shape (changingwhen physisorption takes place),because of the coupling between the OH stretch and the strongly anharmonic intermolecular modes. From the intensity of the VOH mode of the free hydroxyls as a function of pressure, the equilibrium constant for the H-bond formation is calculated. Several correlations are drawn for AVOH,Avco, and AGO. An estimate for A.W is made from AVOH,so that a fairly complete thermodynamic characterization of the H-bonding process is achieved.
Introduction The IR study of the interaction of molecules with the surface hydroxyls of amorphous silica is a classical subject. Knozinger has reviewed in 1976 the interaction of 95 adsorptives,l and others have been studied since. The general features are close to those of H-bonding in solution;2for instance, the shift in the OH stretching mode AVOHis the key observable, and all others may be related to it. H-bonding a t the silica surface is a favorite research theme in our laboratory: already in the 1960s the interaction of methanol was ~ t u d i e das , ~well as that of benzene and alkene^.^ Later, the interaction of CO and NO has been investigated at low t e m p e r a t ~ r e . ~More ,~ recently, the study of the interaction between isolated silica hydroxyls (here after referred to as SiOH) and small molecules has been carried out ab initio, mimicking the surface species by means of the silanol molecule HSSiOH. Such work has just been subjected to an extensive review;' one paper was in particular devoted to the interaction between H3SiOH and formaldehyde.8 As far as the adsorption of carbonyl compounds on amorphous silica is concerned, several spectroscopic papers have been p ~ b l i s h e d , ~ -the ' ~ first in 1954.9 A
* To whom correspondence may be addressed: fax 39-11-67e-mail, GARRONE@SILVER. 07-855;telephone 39-11-67-07-538; CH.UNITO.IT. Abstract published in Advance A C S Abstracts, November 15, 1995. (l)KnOzinger, H. The H-bond: recent advances in theory and experiment; Schuster, Zundel, Sandorfy Eds.; North-Holland; Amsterdam, 1976;Vol. 3,p 1269. (2)Pimentel, G. C.; McLellan, A. L. The Hydrogen Bond; W. H. Freeman and Co.: San Francisco, CA, 1960. (3)Borello, E.; Zecchina, A,; Morterra, C. J. Phys. Chem. 1967,71, 2938.Ann. Chim. 1963,53,690. (4)Zecchina,A.; Versino, C.; Appiano A,, Occhiena G.J.Phys. Chem. 1968,72, 1471. (5) Ghiotti, G,Garrone, E.; Morterra, C. J. Phys Chem 1979,83, 2863. (6)Ghiotti, G.; Garrone, E.; Boccuzzi, F. J. Phys. Chem. 1987,91, 5640. (7)Sauer, J.;Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Reu. 1994,94,2095. (8)Ugliengo, P.; Saunders,V. R.; Garrone, E. Chem. Phys. Lett. 1990, 169,501. (9)Sidorov, A. N.Dolk. Akad. Nauk SSSR 1964,95,1235. (lO)Hertl, W.,Hair, M. L. J . Chem. Phys. 1968,72,4676. @
puzzling complexity of the VOH band has been reported, but the explanations offered as to its nature do diverge. Such a reason has prompted us to resume in the present paper this old subject. There are, however, other reasons of interest. On one hand, the recent advent of FTIR instrumentation allows much more information to be drawn from IR spectra. For example, the fraction 8 of SiOH species engaged in H-bonding at a given pressure is readily measured through the integrated intensity of the corresponding OH stretching mode. On the other hand, it is possible to take advantage of the very simple nature of the surface of amorphous silica and to extract thermodynamic information from the IR spectra, by knowledge of 8 and the equilibrium pressure of the adsorbate. Such a procedure has been used to determine isosteric heats,1°but it appears to have wider applicability. Severely outgassed silica is indeed one of the simplest oxides, its surface being mainly composed of SiOSi bridges and SiOH groups, whose OH stretching mode (hereafter referred to as Y O H ) falls a t 3750 cm-l. The very small half-height width (Wuz) of such band (some 6 cm-l l7 ) is indicative of a strict homogeneity among the surface hydroxyls, which moreover do not interact with each other, in agreement with their low surface concentration (%l OW100 A2 only18 ). Thus, in the process of H-binding small molecules, surface hydroxyls may be regarded as a n ideal system in the thermodynamic sense. It has to be recalled, however, that some hydroxyls (at the most some 10%)do occur on the surface as geminal specie^,^^,^^ i.e., as couples of hydroxyls sitting on the same Si atom, as documented by NMR 27Sistudies.19 Such species are (11)Davydov, V. Ya.; Kiselev, A. V.; Kuznetsov, B. V. Zh. Fiz. Khim. 1966,39,2058. (12)Elkington, P.A., Curthoys, G. J. Phys. Chem. 1968,72,3475. (13)Davydov,V. Ya.; Kiselev, A. V.; Kuznetsov, B. V. J . Phys. Chem. 1970,44,1. (14)Folman, M.; Yates, D. J. C. Proc. Roy. SOC.London 1968,A246,
23 --.
(15)Young, R. P.; Sheppard, N. J . Cutal. 1967,7,223. (16)Busca, G.; Lamotte, J; Lavalley, J. C.; Lorenzelli,V. J. Am. Chem. SOC.1987,109,5197. (17)Knozinger, H.2.Phys. Chem. (Frankfurt) 1970,59,49. (18)Morrow, B. A,; Cody, I. A. J. Phys. Chem. 1973,77, 1465. (19)Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J.Am. Chem. SOC.1988,110,2023. . (20)Ferrari, A.M.; Ugliengo, P.; Garrone, E. J. Phys. Chem. 1993, 97,2671.
0743-746319512411-4811$09.00/00 1995 American Chemical Society
Allian et al.
4812 Langmuir, Vol. 11, No. 12, 1995 Table 1. Molecular Properties of the Adsorptives" BP VP IE PA VC-0 AGOcd ASoCd c1 -21 8622 10.88 7.45 1745 22.45 156.8 144.8 c2 20.8 1224 10.20 8.11 1745 17.6 c3 56.2 283 9.69 8.53 1735 14.0 143.8 c5 130.6 9.26 8.62 1748 6 9.14 8.70 1718 4.43 140.4 C6 155.6 C6= 169 1689
substance
.6
I r,,,,
a BP, boilingpoint ("C);VP, vapor pressure (mbar);IE, ionization energy (eV);PA, proton affinity (eV); VC-0,frequency of stretching modes in the gas phase (cm-l); AGocd, standard energy for condensation (kJ mol-I); A s O c d , standard entropy for condensation (Jmol-' K-l). Adsorptives are referred to accordingto their number of carbon atoms, e.g., C5 is cyclopentanone; C6= indicates 2-cyclohexen-1-one.
Wavenumbers (cm-I)
indistinguishable from SiOH groups as far as the OH stretching mode is concerned. Cluster model calculations20 show that the acidity of geminal hydroxyls is only marginally different from that of isolated ones, so that the set of surface hydroxyls can be still considered as constituing a n ideal ensemble in the interaction, notwithstanding the presence of geminals. The carbonyl compounds chosen for the present study are formaldehyde, acetaldehyde, and acetone (the most classical compounds in this family), cyclopentanone and cyclohexanone (to evidence the effect of the size of the cyclic carbonyl species),2-cyclohexen-1-one(to determine the effect of the double bond on the adjacent carbonylic group). Some relevant molecular properties of the adsorptives are gathered in Table 1.
-I
k1728
,11 i Wavenumbers (cm-1)
Experimental Section The self-supporting disk of amorphous silica (Aerosil Degussa P 25, specific surface area ~ 3 3 m2/g) 0 was placed into a quartz IR cell and activated under vacuum (up to Torr) a t 1073 K for 30 min. Spectra were taken at room temperature by means of a FTIR Perkin-Elmer 1760x (resolution 2 cm-l) before and after admission of the adsorbate at different pressures (depending on the carbonyl compound used). Experiments were usually run by decreasing the pressure, because equilibrium conditions are more rapidly obtained. Equilibrium pressures have been measured by means of either a mercury manometer or a vacuometer Thermovac TM 20. Chemicals were from Merck (99.5% for analysis). All substances are liquids, except formaldehyde, the commercial form ofwhich is the solid trimeric species (thermodynamicproperties, reported in Table 1 refer however to the monomeric liquid). In the following and in particular in the figures, the adsorptives are referred to according to their number of carbon atoms, e.g., C5 is cyclopentanone and C6= indicates 2-cyclohexen1-one.
Results and Preliminary Considerations Figure 1shows the IR spectra of acetaldehyde adsorbed on silica a t different pressures in the whole IR region. Upon increase of the equilibrium pressure, the 3750 cm-l band decreases, and a broad, structured absorption centered a t 3400 cm-l, due to H-bonded hydroxyls, is given rise. The band a t 1728 cm-l is due to the C=O stretching mode of the molecules engaged in H-bonding; some contribution from the gas phase is seen, responsible for the two bands a t frequencies higher than 1750 cm-l (P and Q branches, respectively). Other bands in Figure 1 are due to CH3 and the aldehydic C-H modes. Similar spectra are obtained for each carbonyl compound studied. In all cases, leaving aside standard features as the C-H stretching modes or similar, three spectral features turn out to be of interest: the C=O stretching mode, the stretching mode of H-bonded hydroxyls, and the intensity of the 3750 cm-l band.
Figure 1. FT-IR spectra of acetaldehyde adsorbed on silica outgassed at 800 "C at different pressures.
C=O Stretching Region. Figure 2 compares the absorptions observed in the CO stretching region with the six adsorptives. Sections a and b refer to the lighter and heavier species, respectively. In the latter case (section b), equilibrium pressures are of the order of few millibars only, so that no contribution of the gas phase to the IR spectra is seen. With, however, substantial relative pressures (plpo), because of the low values of PO (Table l), the process of H-bonding is accompanied by plain physisorption: the spectra in Figure 2b show therefore two C-0 bands due to adsorbed species, the higher frequency peak corresponding to physisorbed molecules in liquidlike state and the lower frequency peak to the molecules engaged in H-bonding. This latter is less pressure-dependent than the former peak, because the formation of hydrogen bonding is slightly more energetic. The reverse situation is met with the lighter species formaldehyde and acetaldehyde. Because of the weaker interactions, substantial equilibrium pressures have to be used, which cause a contribution to the IR spectrum of the gas phase. With, however, the related po values being rather high, the relative pressure plpo is small and no physisorption takes place. The case of acetaldehyde has already been illustrated in Figure 1. The adsorption of formaldehyde is weak and definitely pressure-dependent; for this reason, the study by Busca et al. has been conducted a t 170 K.16 We have repeated the experiment a t room temperature, for the sake of coherence with the other results (Figure 2a). Under these circumstances, the gaseous phase makes a large contribution to the spectrum, and the P and Q branches are clearly seen (the Q branch is the sharp one). The band due to H-bonded formaldehyde a t 1730 cm-l is clearly seen, superposed to the R branch.
Langmuir, Vol. 11, No. 12, 1995 4813
Carbonyl Compounds on Silica
I
a l
I
1750 1700 Wavenumbers (cm-1)
1
3400
3200
Wavenumbers(cm-1)
b
bl
21
180d
1
3600
I
,
,
3600
,
1750' ' ' 170d ' ' ' 16.50 Wavenumbers (cm-1)
'
'
'
Figure 2. FT-IR spectra in the YCO region of carbonyl compounds adsorbed on silica outgassed at 800 "C: C1, formaldehyde; C2, acetaldehyde; C3, acetone; C5, cyclopentanone; C6, cyclohexanone; C6=, 2-cyclohexen-1-one.
The case of acetone is intermediate. The spectra taken a t low pressures only show a component at 1716 cm-', due to the molecules H-bonded to SiOH groups. Those taken at higher pressures (like the one reported in Figure 2a) show contribution from the both the gas phase and the liquidlike physisorbed phase; this latter phase is responsible for a shoulder at 1735 cm-l. OH Stretching Mode of H-Bonded Hydroxyls. With all carbonyl compounds, this band is complex and appears a t least double. Moreover in some cases its shape is pressure-dependent. To emphasize such phenomenon, the spectra in the OH stretching region are reported in Figure 3 as normalized to the same intensity. In the case of acetaldehyde, all spectra yield the same band profile (Figure 3a). In contrast, with heavier molecules (i.e. acetone and cyclic molecules), when the pressure is increased, the band shifts to lower frequencies and its shape changes; the case of cyclopentanone is illustrated in Figure 3b. In these latter cases, physisorption is present (as documented by the presence of the liquidlike species in the C=O stretching region), and the changes in the shape of the OH stretching band are due to a "solvent" effect exerted on the SiOH.-.O=C complex by the other molecules of the carbonylic compound. The spectral features of the true SiOH--O=Ccomplexes are evidently best defined at vanishing coverage. Figure 4 gathers the spectra for the six molecules under study obtained a t the smallest coverage studied. From these spectra, AYOH values have been calculated. In the literature, two interpretations have been so far offered for the complexity of the OH stretching band under study. According to Busca et a1.,16two different types of OH with different acidity give rise to two types ofH-bonded complexes, absorbing a t two different frequencies. This
3400
3200
3c 0
Wavenumbers (cm-1)
1 00
Figure 3. Normalized FT-IR spectra in the YOH region of H-bonded hydroxyls: upper section,adsorptionof acetaldehyde; lower section, cyclopentanone.Curves 1t o 6 are at increasing coverage.
r
I
I
v
3600
3400
3200
Wavenumbers (cm- 1)
Figure 4. FT-IR spectra (YOH region) for carbonyl compounds at the smallest coverage attained. Symbols as in Figure 2.
explanation seems not adequate because in weaker interactions, e.g., with CO, SiOH groups appear of the same a ~ i d i t y .Moreover, ~ the same type of complexity is observed for the gas-phase adducts of a ~ e t o n e - H F . ~ l - ~ ~ Unless the presence of two types of gas-phase adducts is invoked,21the complexity of the stretching mode seems to be inherent to the bond between the acidic proton and the explanation has been carbonyl m ~ i e t y . A ~ more ~ , ~ subtle ~ advanced by Hertl and Hair,lo according to whom in the same region fall both the OH stretching mode of H-bonded hydroxyls and the first overtone of the C=O stretching (21) Couzi, M.; Le Calv6, J.;Huong, P. V.; Lacombe, J. J.Mol. Struct. 1970,5, 363.
(22)Amold, J., Millen, D. J. J. Chem. SOC.1966, 510. (23) Bouteiller, Y., Latajka, Z. J. Phys. Chem. 1992,97 (11, 145.
Allian et al.
4814 Langmuir, Vol. 11, No. 12, 1995
.1
8
P
P .o:
.El
n
-4
C 3500
3000 Wavenumbers (cm-1)
2500
I
2100
Figure 6. FT-IRspectra of acetone adsorbed on deuterated
silica. mode, the intensity of which is enhanced by the Fermi resonance. Such a n explanation does not hold, however, for the gas-phase adducts of acetone and HF,21,22 and may also be disproved by the following experiment. A silica sample, outgassed a t 800 "C, has been fully deuterated at room temperature by treatment with ND3, then adsorption of normal acetone has been carried out. The stretching mode of OD being a t 2762 cm-l, the first C-0 overtone of acetone falls well apart, and no Fermi resonance may occur. The results are reported in Figure 5. The right-hand side of the Figure shows a sharp band due to the SiOD species a t 2762 cm-l and a broad one arising from SiOD...O=C(CH3)2 species centered a t 2500 cm-l. This latter band is complex and structured no less than that due to species SiOH...O=C(CH3)2 . Such a band is visible centered at 3375 cm-l, as well as the stretching mode of SiOH a t 3750 cm-l, in the left-hand side of the figure, because acetone undergoes scrambling of its proton with silica hydroxyls a t room temperature. The comparison between the shape of the SiOH and SiOD species engaged in H-bonding shows two features. A small bump a t 3425 cm-l in the complex band envelope of H-bonded SiOH groups may be safely ascribed to the first C=O overtone of acetone in the complex, as a corresponding component is not present in the band envelope of D-bonded SiOD. From the frequencies of both the first overtone (3425 cm-') and the fundamental mode a t 1716 cm-l, the anharmonicity parameter for the acetone H-bonded to SiOH groups is calculated to be 7 cm-l, i.e., upon H-bond formation, negligible changes occur in the anharmonicity of the C=O stretch. Such observation, together with the limited intensity of the 3425 cm-l component, rules out the occurrence of any Fermi resonance between the C=O stretching overtone and the OH stretching fundamental in the SiOH**.O=C(CH3)2complexes. The explanation probably is that although the two modes actually do overlap in frequencies, the coupling between them is very small. The second observation is that a mere change in the mass of the proton definitely alters the shape of the band; in particular, new weak components become visible in the low-frequency tailing of , ~ 2 2 9 0cm-l, the band envelope a t ~ 2 1 6 0 ~, 2 2 3 0 and nearly regularly spaced by some 60 cm-'. This is in agreement with the explanation given below, involving the coupling between the 0 - H stretching mode and the highly anharmonic intermolecular stretching and bending modes. It has long been proposed2 that such a coupling is the cause of the broadness of the OH stretching mode of hydroxyls engaged in H-bonding. From a qualitative point
of view, such a n explanation is rather straightforward. A quantitative treatment is, instead, difficult. A stochastic treatment due to Bratos is a ~ a i l a b l e , ~showing * , ~ ~ that in the case of weak and medium-strength bonds, a broad, asymmetrically distorted Gaussian profile is expected,24 possibly showing Evans-type windows in the case of strong bondsUz5Only recently, ab initio results have become available concerning model systems close to those studied in the present paper. On one hand, some of US,^ when dealing with the system H3SiOH.-.OCH2, have studied the dependence of the intermolecular potential on the angles OH**.Oand H...O=C a t fixed distances. The corresponding maps show that the bending modes of the system are strongly anharmonic, and rather floppy, so that thermal motion is rather wide. On the other hand, Bouteiller and LatajkaZ3have studied theoretically some degrees of freedom of the gas-phase complex HF-acetone, in particular the H-F stretching mode and the H...O=C intermolecular one, by calculating the potential energy on a grid of points and by solving the corresponding nuclear Schrodinger equation numerically. The calculated IR spectrum (inclusive of intensities) reproduces well the main features of the spectra discussed so far. Two sets of quantum numbers are obviously involved, the one referring to the HF degree of freedom and the other to the intermolecular stretch. The spacing between the energy levels related to the intermolecular degree of freedom is rather small, and quantum levels other than the Y = 0 are populated. The strong anharmonicity of the intermolecular mode is such that the transition (00) (10) definitely differs from the (01) (11) and from the (02) (12). In conclusion, both previous computation work from this laboratory (concerning angular dependenceY and the calculations by Bouteiller and Latajka (dealing with interatomic distances)23indicate a complex nature for the band related to the perturbed hydroxyl species or similar. A complete ab initio treatment, inclusive of all degrees of freedom for the H-bonded complex, is out of reach with the present technology. It is not straightforward, therefore, to extract information from the fine structure of the spectra like those in Figures 3 and 4, though the nearly regular spacing of z6O cm-l in the low-frequency tailing of the band centered a t 2500 cm-l probably indicates the occurrence of a libration mode around such value. The Y O H mode of SiOH groups engaged in H-bonding is systematically observed to shift to lower frequencies when physisorption takes places (Figure 4). From a qualitative point of view, such a n effect is probably relatable to the hindrance of the intermolecular motions brought about by the surrounding molecules, comparable to the effect of lowering the temperature, which also shifts the Y O H band to lower frequencies.2 In some cases (Figure 4) also the displaced band shows a definite structure. Intensity of the 3760 cm-l Band. Let us write down the H-bonding interaction as a chemical process
-
-
SiOH
-
+ M=SiOH.-.M
characterized by the equilibrium constant K . Under the assumption of the ideality of the ensemble of SiOH groups, as discussed in the Introduction, this leads to the Langmuir equation
(24) Bratos, S. J. Chem. Phys. 1975,63, 3499. (25) Bratos, S.; Ratajczak, H J . Chem. Phys. 1982,76, 77.
Langmuir, Vol. 11, No. 12, 1995 4815
Carbonyl Compounds on Silica Table 2. Features of the €€-BondedMolecule@ substance AVOH WVZ Av(c-0) K AGO &lP ASa
c1 c2 c3 c5 C6 C6-
254 300 345 375 423 399
222 289 334 390 376 400
15 17 19 19 20 20
0.015 10.41 0.09 5.97 2.90 0.31 17.2 -7.06 -6.42 13.3 18.9 -7.28
20.7 104.4 23.3 98.3 25.7 96 27.2 67.4 29.5 77.5 28.4 70.8
AVOH,change in the frequency of OH stretchingmode (cm-l); WVZ,half-height width (cm-l); AV(C-01, change in the frequency of C=O stretchingmode (cm-'); K , equilibriumconstant (mbar);AGO, adsorption energy (kJ mol-'); AlT,estimatedadsorption enthalpy (W mol-'); AS", estimated adsorption entropy (J mol-' K-l)
where p* is the reference unit pressure (one millibar in the present case). The mole fraction 6' of hydroxyls engaged in H-bonding is readily estimated through the relationship 6' = 1 NA,,, where A is the integrated intensity of the free hydroxyl peak at a given coverage and A,, is the integrated intensity of the same peak for the bare sample. The above scheme holds in the absence of physisorption phenomena. Indeed, with acetaldehyde, the Langmuir equation is followed in the whole range of pressures and yieldsK= 0.09 (mbar). With formaldehyde we also expect the Langmuir equation to be followed. A full experimental check has, however, not been carried out: a single-point determination at 5 mbar has yielded K = 0.015(mbar). With the other carbonyl compounds, physisorption accompanies the H-bonding process, and the above scheme is no longer valid, as the SiOH...O=C species are then further solvated. As a consequence, the Langmuir equation only holds a t low pressures, and the equilibrium constants have been calculated in this region (Table 2). From the equilibrium constants, A G values have been calculated through the standard relationship AC? = RT In K. Table 1 reports, for comparison, the values of AGOcd, the change in the molar Gibbs free energy of the substance for the condensation process from the vapor phase at the standard pressure of 1 mbar to the liquid. AGocd is given by RT In (palp*),where p 0 is the vapor pressure.
Discussion The surface hydroxyl of silica is a rather weak acid, the pKa of which is estimated to be around 7,26so that proton transfer is never observed, nor are the IR intricacies of really strong H-bonds, like the presence of Evans wind o w ~ . ~ On ' the other hand, because the carbonylic compounds are relatively strong bases, the H-bonds under study are comparatively strong; if reference is made to the compilation by Knozinger,l only ammonia and pyridine impart to the V O H mode of the SiOH species definitely larger red shifts, whereas ethers cause shifts just somewhat larger than those of carbonylic compounds. The relatively conspicuous strength of the interaction may be the reason carbonylic compounds cause, when H-bonded to SiOH groups, a complex shape in the OH stretching mode, not observed in weaker interactions. It is worth noting, however, that in the case of NH3 and pyridine no fine structure of the OH stretching mode of hydroxyls engaged in H-bonding is observed, but instead a broad featureless band, and that ethers show structured bands, though less resolved than carbonylic compounds.2s The presence of components in the OH stretching mode of SiOH groups seems thus to be a function of the strength of ~~
~~
(26) Hair, M.L.;Hertl, W. J. Phys. Chem. 1970, 74, 91. (27) Pelmenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (28) Allian, M.; Garrone, E. Unpublished.
450 1
I
AVOR
350
250
1
2
3
NC
5
6
AVOH
450
11
eV 9t
8
250
350
Figure 6. AVOH(cm-1) as a function of the molecular properties: (a) number of carbon atoms (NC)in the molecules; (b) proton affinity (PA) and ionization energy (IE)in eV.
interaction, having a maximum for intermediate strength, corresponding to carbonylic compounds. It is commonly a c ~ e p t e d ' Jthat ~ ~ ~AVOH ~ is a function of the enthalpy of interaction W . In agreement with a n old quantum-mechanical description of H-bonding3O (Mulliken's charge-transfer-no-bond theory), it is usually assumed that (AvoH)* is a proper measure of the strength ofthe interaction AH". Recent ab initio calculations from our laboratory support such a n a s s ~ m p t i o ndespite , ~ ~ the fact that Mulliken's picture of H-bonding is now out of fashion. The values of AVOHvary sensibly across the series of carbonylic compound. To illustrate such a fact, AVOH values have been plotted in Figure 6a as a function of the number of carbon atoms in the molecule. The increase in the series C1,C2,C3 is due to the additive effect of the substitution of CH3 for H, as the +I inductive effect of the methyl substituent renders the C=O groups more negative. The +I inductive effect of ethyl and larger substituents is somewhat bigger, though fading away; this accounts for the further increase of avo^ when passing to cyclohexanone,which exhibits the largest AVOHvalue. Such a value fits the trend observed with C1,C2,C3 (broken curve in the Figure), whereas C5 does not. This is probably evidence that the strain in the five-membered ring causes a decrease in the negative charge of the oxygen atom in C5. When considering C6- a definite decrease in AVOH is observed with respect to C6. The availability of a n orbital delocalized over four atoms renders the oxygen atom less negative. A similar phenomenon occurs with acetonitrile and acrylonitrile, which impart the silica isolated hydroxyl AVOHvalues of 320 and 280 cm-l, respectively.12p2s The availability of a set of results for six adsorptives allows a number of different correlations to be drawn, (29) Curthoys, G.; Davydov, V. Ya.; Kiselev, A. V.; Kiselev, S. A., Kuznetsov, B. V. J. Collofd Interface Sei. 1974, 48, 58. (30) Mulliken, R.J. Am. Chem. Soc. 1952, 74, 811. (31)Ugliengo, P.;Ferrari, A. M.; Zecchina, A.; Garrone, E. In preparation.
4816 Langmuir, Vol. 11, No. 12, 1995
which comprise those (i)among AVOHand some molecular parameters (e.g. the ionization energy or proton affinity), (ii)among purely spectroscopic features, and (iii)between AVOHand AGO values. Correlation between AVOH and Molecular Properties. The plot in Figure 6b shows that a relationship exists between the AVOHvalues and, on one hand, the ionization energy of the molecules and, on the other hand, their proton affinities. Such relationships have been already proposed in the literature: Cusumano and have tested the charge-transfer-no-bond theory of the H-bond proposed by M ~ l l i k e npopular , ~ ~ in those days, by showing that AVO^)^^ is linearly related to the ionization energy within a family of alike molecules; Paukshtis et al.33report a correlation (though somewhat coarse), for both the SiOH species and the B r ~ n s t e dsite in zeolites, between the shift in the OH stretching mode and the proton affinity of the base molecule. In solution chemistry, it is more customary to relate the shift to the pKa of the conjugated acid, rather than the proton affinity of the base molecule.2 Such a procedure has been also applied to the silica surface silanol, both in the past34 and more r e ~ e n t l y . ~ The correlations in Figure 6b do not imply, however, that the main feature of the H-bonds formed is either the donation of electrons from the base molecule to the acidic partner or the donation of the acidic proton in the reverse sense. The two features, on one hand, cannot be the main one simultaneously: moreover, there is nowadays ample evidence that the formation ofweak and medium H-bonds like those under consideration is dominated by electrostatic interaction^.^"^^ The transfer of electrons from the base molecule to the acidic hydroxyl species takes place to a tiny extent; in the model calculation concerning the interaction H&3iOH*.*O=CH2,8such electron transfer has been evaluated to be of the order of 1.9 x charge units. Similarly, no real H-transfer takes place; a displacement of the proton toward the basic molecule does occur, but it is extremely limited. Again in the case of the model calculation between the silanol molecule and formaldehyde, such displacement is evaluated to be some 6x A. In conclusion, the plots in Figure 6b probably have the meaning that, though very limited, both the electron transfer and the proton displacement follow the strength of interaction, as measured by AVOH. Correlation between W m and AVOH.The data in Table 2 show that WVZ(the half-height width) and AVOH are close to each other, the former being systematically slightly less than the latter. The plot in Figure 7a suggest that the two sets ofdata are related by a straight line with a shope close to unity, though not passing through the origin. A full correlation curve, considering weaker interactions, should obviously include the origin and have accordingly a smaller slope in the region close to the origin. Indeed, for weaker interactions, the relationship between W112 and AVOHis known to be W112 x 3 1 4 A v ~ ~ . 2 Correlation between AVOHand AVCO.It is evident that the larger the strength of interaction AH", the larger both AVOHand AVCO,so that a correlation between the two latter entities is expected. Whereas, as stated above and (32) Cusumano, J. A.,Low, M. J. D. J. Catal. 1971,23,214. (33) Paukshtis, E.A.;Yurchenko, E. N. React. Kinet. Catul. Lett. 1981,16,131. (34) Sempels, R. E;Rouxhet, P. G. Bull. Soc. Chim.Belg. 1976,84, 261
(35) Lias, S. G.;Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.;Levin, R. D.; Mallard, W. G. J.Phys. Chem. Ref:Data 1988,17(1); Gas-Phase Ion and Neutral Thermochemistry. (36) (a) Legon, A. C.; Millen, D. J. Acc. Chem. Res. 1987,20,39. (b) Legon, A. C. Chem. Soc. Rev. 1990,19,197. (37) Buckingham, A. D.;Fowler, P. W.; Hutson, J. M. Chem. Reu. 1988,88,963.
Allian et al. 500 1
I C6=
300 200
";.*
4
1
A
C6
3
P2 ri
100 -. a
25
b C6
c 3 . 7 C6=
I I
10
:
15
(AVOHY
25
Figure 7. Correlation between spectroscopic features of adsorbed species: (a) half-height width (WVZ)(cm-') as a function of AYOH(cm-'1; (b) AYCO(cm-') as a function of ( A V O H ) ~ ~
(cm-l)Vz.
discussed to some detail in the following,it basically results that AHo = (AVOH)~'~, the dependence of AVCOon AHo is unknown. If one imagines to series-expand such a n unknown function, taking into account that AVCO is rather small (only of the order of some 20 cm-'1, it is feasible to stop the expansion to the linear term, i.e., to assume that AVCO is proportional to AH".In conclusion,one may expect that grossly AVCO (AvoH)'~. This is indeed what is observed in Figure 7b. The proportionality coefficient is close to 1, so that in practice AVCO= (AvoH)~". Correlation between AGO and Molecular Properties. The dependence of AGO upon the number of carbon atom in the adsorptive is shown in Figure 8a. AG'" decreases linearly down to C5, then levels off. In standard conditions (p* = 1mbar), adsorption ofthe lighter species is unfavored (AGO =- 0);the opposite holds with the heavier cyclic species. Figure 8b compares AGO values with those for AGocd;the former are seen to parallel closely the latter, being systematically lower by some 10 kJ. The meaning of Figure 8b probably is that van der Waals forces play a relevant role in the adsorption process on the isolated hydroxyl of silica, besides H-bonding. The similarity between the adsorption on SiOH groups and on nonspecific regions ofthe surface (thislatter only due to van der Waals forces) is documented by the simultaneous occurrence of the two processes. It is not straightfoward, however, to separate the two contributions (sheer H-bonding and van der Waals interaction) to the energetics of adsorption on SiOH groups. An increase in the number of carbon atoms in the molecule has the effect both of strengthening the H-.O=C bond and of increasing the mass ofthe adsorptive, with concomitant increase of van der Waals interactions. Correlationbetween AGO and the Other Standard ThermodynamicQuantities. No direct measurement of AHO, the enthalpy ofinteraction with SiOH groups, has been carried out in the present work. The knowledge of AVOH,however, allows the evaluation of AH", because experimental correlations between the two entities are have been used in the a ~ a i l a b l e .Two ~ ~ ~approaches ~~
Carbonyl Compounds on Silica
Langmuir, Vol. 11, No. 12, 1995 4817 200
.---- .
-3 150
t---"
c3
NC
c1
T 30
b
04 1
2
3
NC
5
6
Figure 9. Correlation between the standard entropy change for H-bonding (AS") (kJ mol-' K-l) and the standard entropy change for condensation ( M o d ) (kJ mol-' K-') a s a function of the number of carbon atoms in the molecule.
sp2 hybridization, the proposed correlation is AHo = -2.3 0.455(Avo~)". The two formulas fortunately yield, AGO for the range Of AYOHobserved, results within few percent. Starting from the AVOHvalues observed in the present Figure 8. Correlation for the standard free enthalpy change of H-bonding: (a) AGO (kJ mol-') a s a function of the number work, a set of AHO values has been calculated (Table 2); of carbon atoms; (b) AGO (kJ mol-l) as a function of A p e d (kJ these are seen to vary within the range 20-30 k J mol-'. mol-'). By means of the standard relationship AGO = AHo TAS", the standard change in entropy in the H-bonding literature to determine AHO. Hair and Hertl'O have process has then been calculated in each case (Table 2). measured isosteric heats qst of interaction with SiOH A steady decrease in ASo is observed with the number of groups, by the use of the classical relationships carbon atoms increasing up to four, the three heavier adsorptive yielding almost the same value. Such values (d InpldT), = -q,dRI" AHo = qst RT may be compared with the similar values for the standard The constancy of coverage f3was assured by monitoring entropy change for the condensation from the gas phase in the IR the intensity of the 3750 cm-l peak. Curthoys a t 1 mbar to the liquid phase, ASocd, which are listed in et al.29have used instead direct calorimetry. To disTable 1. Figure 9 reports both ASo and ASocdas a function criminate the actual process of interaction with SiOH of the number of carbon atoms in the molecule. It results groups from plain physisorption, AHo was defined as the that, when comparison is possible, ASo values scale very difference between the heats of adsorption on a sample well with ASocd values, the former being some 2/3 of the fully covered with hydroxyls and the same sample after latter. Such a n agreement corresponds to the qualitative dehydration. For all substances investigated, Curthoys concept that adsorbed molecules are in a liquidlike state et found the relationship AHo = -1.6 + 0 . 4 1 ( A v 0 ~ ) ~ ' ~ and support the overall validity of the thermodynamic to hold. Hair and HertllO found different behavior with data reported here. different families of compounds: with adsorptives like carbonyls, in which the atom engaged in H-bonding is in LA950510K
+
+
