Langmuir 1997, 13, 5107-5113
5107
IR Study of the Adsorption of Unsaturated Hydrocarbons on Highly Outgassed Silica: Spectroscopic and Thermodynamic Results Barbara Onida,†,‡ Martin Allian,† Enzo Borello,† Piero Ugliengo,† and Edoardo Garrone*,† Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy, and Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita` di Torino, via P. Giuria 7, 10125 Torino, Italy Received April 15, 1997. In Final Form: June 10, 1997X Fourier transform IR data are reported concerning the adsorption of ethyne, propyne, 1-butyne, 3-hexyne, benzene, and cyclohexene on Aerosil outgassed at 1073 K. With the three lightest acetylenic molecules, an ideal Langmuir-type interaction occurs. With the three six-carbon atom species, physisorption competes with the formation of H-bonded complexes, which are best studied at low coverage. From the intensity of the ν(OH) mode of the free hydroxyls as a function of pressure, the equilibrium constant for H-bond formation is calculated in different ways, according to the role of physisorption. Several correlations are drawn for ∆ν(OH) and ∆G°, based on the different basicities of the C-C triple bond as modulated by the alkyl substituents, and on the different C-C bond order in the other molecules. An estimate for ∆H° is made from ∆ν(OH) so that a fairly complete thermodynamic characterization of the H-bonding process is achieved. An explanation is proposed for the complex shape of the ν(OH) band in the case of 3-hexyne. The interplay between sheer H-bonding formation and dispersive contributions to the interaction is also discussed.
Introduction The interaction of molecules with the isolated hydroxyl of amorphous silica is a classical subject, showing features close to H-bonding in solution.1 The last review in the field goes back to 1976 and gathers data concerning 95 molecules.2 Since then, others have been studied, e.g., CO,3 NO,4 and N2O.5 Curiously, the interaction of acetylenic molecules has never been investigated. Some IR data on the interaction of acetylene and methylacetylene with the SiOH species present on the outer surfaces in zeolite crystals (extremely similar to those of amorphous silica) have been obtained as side-products in a study concerning the polymerization reactions of such molecules in H-ZSM5 systems6 promoted by the acidic Brønsted sites Si(OH)Al. A systematic study is however lacking: this is the subject of the present work. There is a renewed interest in H-bonded systems, the main reason being that H-bonding plays a central role in molecular recognition.7 In the last decade, much knowledge has been gained on gas-phase adducts by means of sophisticated rotational spectroscopies.8 Gas-solid in* To whom correspondence should be addressed: fax, 39-11-6707-855; Tel, 39-11-67-07-538; e-mail, GARRONE@ SILVER.CH.UNITO.IT. ‡ Politecnico di Torino. ‡ Universita ` di Torino. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Pimentel, G. C.; McLellan, A. L. The Hydrogen Bond; W. H. Freeman and Co.: San Francisco, CA, 1960. (2) Kno¨zinger, H. The H-bond: recent advances in theory and experiment; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North Holland: Amsterdam, 1976; Vol. 3, p 1269. (3) Ghiotti, G.; Garrone, E.; Morterra, C.; Boccuzzi, F. J. Phys. Chem. 1979, 83, 2863. (4) Ghiotti, G.; Garrone, E.; Boccuzzi, F. J. Phys. Chem. 1987, 91, 5640. (5) Garrone, E.; Ugliengo, P.; Ghiotti, G.; Borello, E. Spectrochim. Acta 1993, 49A, 1221. (6) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Borello, E. J. Chem. Soc., Faraday Trans., 1994, 94, 2095. (7) Buckingham, A. D.; Legon, A. C.; Roberts, S. M. Principles of molecular recognition; Blackie: Glasgow, 1993. (8) Legon, A. C.; Millen, D. J. Acc. Chem. Res. 1987, 20, 39. Legon, A. C. Chem. Soc. Rev. 1990, 19, 197.
S0743-7463(97)00386-7 CCC: $14.00
teraction affords a way to study H-bonded systems with conventional vibrational spectroscopies under very simple circumstances. On the other hand, the interaction between an acetylenic molecule and the isolated hydroxyl of amorphous silica is simple enough to be tackled by abinitio calculations of model systems where the SiOH species is mimicked by the silanol molecule H3SiOH.9 Another reason for studying the interaction of base molecules with the SiOH groups is that it is relatively easy to extract thermodynamic information from the IR spectra, taking advantage of the very simple nature of the surface of amorphous silica. By knowledge of the adsorbate pressure and the corresponding fraction of SiOH species engaged in H-bonding (readily calculated through the integrated intensity of the corresponding OH stretching mode) the adsorption isotherm is constructed, and hence thermodynamic properties are calculated. This has been done by us in a preceeding paper, devoted to the interaction of silica SiOH groups with a family of carbonyl compounds.10 Highly outgassed silica is indeed one of the simplest oxides. Its surface is mainly composed of SiOSi bridges and surface hydroxyls groups, whose OH stretching mode (hereafter referred to as νOH) falls at 3747 cm-1. The very small half-height width (W1/2) of this band ( 0. Figure 5a reports the quantity [(1/θ0 - 1)/(p/p*)] as a function of pressure in the case of C6d and Bz. In such a representation, a linear behavior with positive slope is expected, the intercept yielding the value of K. This is indeed what was observed, though deviations occur at higher pressures. This indicates that (though naive) such a model describes the first stages of solvation and is therefore useful in determining K in the presence of physisorption. As already noted, the peak position varies with coverage in the pressure of physisorption. An accurate evaluation of the peak position at vanishing coverage may be carried out by plotting the frequency ν(ΟΗ) of the H-bonded hydroxyls as a function of (1 - θ0), as shown in Figure 5b. Two behaviors are noted, represented by C6 and Bz, respectively, in agreement with that observed for the adsorption isotherms. C6d shows the same behavior as Bz. The plot for C6 shows a plateau for low coverages, corresponding to the Henry region of the isotherm. For the other two molecules showing no Henry region, a continuous red shift with coverage is seen.
Adsorption of Unsaturated Hydrocarbons
(cm-1) and ∆G° (kJ mol-1) as a function of the
Figure 6. ∆ν(OH) number of carbon atoms in the acetylenic molecules.
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Figure 7. ∆ν(OH) (cm-1) and ∆G° (kJ mol-1) as a function of the bond order in the six-carbon atom molecules.
Discussion The surface hydroxyl of silica is a rather weak acid. Its pKa is estimated to be around 7,17 so that proton transfer is never observed, nor are the IR intricacies of really strong H-bonds, like the presence of Evans windows.18 As to the molecules studied, two types of considerations may be made. On the one hand, the basic strength of acetylenic molecules in the interaction may be estimated from ∆ν(OH) values, which are a function of the enthalpy of interaction ∆H°.2,19,20 On the other hand, in the series Bz, C6d, and C6, the C-C bond order varies from 1.5 to 2 to 3, and therefore a correlation between bond order and observed features of H bonding may be established. The values of ∆ν(OH) vary sensibly across the series of acetylenic compounds. To illustrate this, ∆ν(OH) values have been plotted in Figure 6 as a function of the number of carbon atoms in the molecule. The increase in ∆ν(OH) from C2 to C3 is due to the +I inductive effect of the methyl substituent, supplying electron density to the triple bond, and therefore rendering it more basic. The +I inductive effect of ethyl is only somewhat bigger. The increase in ∆ν(OH) from C2 to C6 is nearly exactly twice as much that from C2 to C4 showing that the +I effect of the ethyl substituents is additive. Also plotted in Figure 6 are the corresponding values of ∆G°, calculated from the equilibrium constant. It is quite evident that ∆G° values decrease with increasing number of carbon atoms, i.e., they are anticorrelated to ∆ν(OH) values. This is quite expected, because ∆ν(OH) and -∆G° ) RT ln K constitute two different measures of the strength of interaction, as discussed below. Figure 7 reports ∆ν(OH) as a function of the bond order in the series Bz, C6d, and C6. As far as the single C-C bond is concerned, the literature value of ∆ν(OH) for n-hexane has been considered (2). The red shift regularly increases with the C-C bond order. This seems to suggest, in the series considered, a steady increase of the electronic density at the midpoint of the C-C bond (i.e., of the electrostatic potential outside it, to which H-bonding is sensitive). It is however to be noted that acetylene and ethylene have very close molecular electrostatic potentials at the midpoint of the C-C bond and impart therefore similar shifts to the O-H stretching mode of the isolated hydroxyl of silica.9 Since the study of n-hexane is outside the scope of the present paper, the equilibrium constant (17) Hair, M. L.; Hertl, W. J. Phys. Chem. 1970, 74, 91. (18) Pelmenschikov, A. G.; van Santen, R. A. J. Phys. Chem. 1993, 97, 10678. (19) Hertl, W.; Hair, M. L. J. Chem. Phys. 1968, 72, 4676. (20) Curthoys, G.; Davydov, V. Ya.; Kiselev, A. V.; Kiselev, S. A.; Kuznetsov, B. V. J. Colloı¨d. Interface Sci. 1974, 48, 58.
Figure 8. ∆νOH (cm-1) as a function of the proton affinity (P.A.) and ionization energy (I.E.) in eV for the acetylenic molecules.
concerning the interaction between SiOH and n-hexane was not determined. In Figure 6, therefore, only the data concerning ∆G° for Bz, C6d, and C6 are reported. As already observed in Figure 5, these values also turn out to be anticorrelated with ∆ν(OH) values. Similar considerations, concerning the acetylenic molecules and the family of six-carbon atom molecules with different C-C bond order, may be done for other features, e.g., by drawing correlations among ∆ν(OH) and the ionization energy (or proton affinity) and between spectroscopic and ∆G° values. Correlation between ∆ν(OH) and Molecular Properties. The plot in Figure 8 shows that a linear relationship exists between ∆ν(OH) and the ionization energies of the acetylenic molecules, and also between ∆ν(OH) and their proton affinities. Notice that such relationships only hold within the same family. The corresponding data, not reported, for C6d and Bz do not fall on the same lines. Such kinds of relationship have been proposed long ago in the literature. Cusumano and Low21 have tested the charge-transfer-no-bond theory of the H-bond proposed by Mulliken22 by showing that (∆ν(OH))1/2 is linearly related to the ionization energy within a series of molecules. Paukshtis et al.23 report a correlation (though somewhat coarse), between the shift in the OH stretching mode and the proton affinity of the basic molecule, for both the SiOH species and the Brønsted site in zeolites. 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 basic molecule.1 Such a procedure has been also (21) Cusumano, J. A.; Low, M. J. D. J. Catal. 1971, 23, 214. (22) Mulliken, R. J. Am. Chem. Soc. 1952, 74, 811. (23) Paukshtis, E. A.; Yurchenko, E. N. React. Kinet. Catal. Lett. 1981, 16, 131.
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applied to the silica surface silanol, both in the past24 and more recently.25 The correlations in Figure 8 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. There is ample evidence that the formation of weak and medium H-bonds like those under consideration is dominated by electrostatics,7,26 so that electron or proton transfer are but minor features. 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 H3SiOH/H-CtC-H,9 such electron transfer has been evaluated to be of the order of 0.0099 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 acetylene, such displacement is evaluated to be some 0.004 Å. In conclusion, the plots in Figure 8 probably have the meaning that, though very limited, both the electron transfer and the proton displacement follow the strength of interaction, as measured by ∆νOH. Correlation between ∆G° and Molecular Properties. In the series of acetylenic molecules, the absolute ∆G° values (Figure 6) are seen to increase approximately linearly with the number of carbon atoms. Two main contributions to the free energy of interaction may be envisaged, one due to H-bonding between the triple bond and the acidic hydroxyl, the other due to the nonspecific van der Waals-like interaction between the whole molecule and the surface. The former process is described by ∆νOH, whereas K (or ∆G°) describes the ensemble of both. It is not straightforward, however, to separate the two contributions (sheer H-bonding and dispersion interaction) in the energetics of adduct formation with SiOH groups. An increase in the number of carbon atoms in the molecule has the effect both of strengthening the SiOH/CtC bond and of increasing the mass of the adsorptive, with concomitant increase of van der Waals interactions. One naive way to assess the importance of dispersive interactions in the adsorption process is to compare ∆G° values with those for ∆G°cd, the standard free energy change for condensation (Figure 9a), a process only due to van der Waals interactions. A linear relationship is observed, with unit slope, ∆G° ≈ ∆G°cd - 5.0 kJ; i.e., the standard free energy change for interaction differs by a constant term from that for condensation. This result indicates probably a major role of dispersion forces in the energetics of adsorption process on the isolated hydroxyl of silica, even in those cases, like three of the present ones, where actual physisorption does not take place. In contrast, similar dispersive contributions to the energetics of interaction are expected with the three sixcarbon atom molecules, having very close boiling points and vapor pressures (Table 1). The regular trend in ∆G° observed in Figure 7 should thus be related only to the change in C-C bond order. Indeed, the comparison between ∆G°cd and ∆G° shows that these are of the same order of magnitude for Bz and C6d (-5.81, -5.05 and -5.45, -4.44 kJ, respectively), whereas ∆G° is much larger (in absolute value) than ∆G°cd for C6 (-12.14 and -5.99 kJ). This explains why physisorption accompanies (and competes with) H-bonding in the whole range of coverage (24) Davydov, V. Ya.; Kiselev, Kuznetsov, A. V.; B. V. J. Phys. Chem. 1970, 44, 1. (25) Sauer, J.; Ugliengo, P.; Garrone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095. (26) Buckingham, A. D.; Fowler, P. W.; Hutson, J. M. Chem. Rev., 1988, 88, 963.
Onida et al.
Figure 9. Correlations for the standard changes in thermodynamic quantities of H-bonding. Section a: standard free enthalpy change of interaction ∆G° (kJ mol-1) as a function of the corresponding standard change for condensation ∆G°cd (kJ mol-1), the broken line is the bisectrix. Section b: the standard entropy change for H-bonding (∆S°) (kJ mol-1 K-1) and the standard entropy change for condensation (∆S°cd) (kJ mol-1 K-1) as functions of the number of carbon atoms in the molecule.
in the former two cases, whereas in the third the two phenomena are distinct and a physisorption-free range of coverage (Henry region) is observed. Correlation between ∆G° and the Other Standard Thermodynamic Quantities. No direct measurement of ∆H°, the enthalpy of interaction, has been carried out in the present work. The knowledge of ∆ν(OH), however, allows an evaluation of ∆H°, because experimental correlations between the two entities are available.19,20 Two approaches have been used in the literature to determine ∆H°. Hair and Hertl19 have measured isosteric heats qst of interaction with SiOH groups, by the use of the classical relationships
(d ln p/dT)θ ) -qst/RT2
(6)
∆H° ) qst + RT
(7)
The constancy of coverage θ was assured by monitoring in the IR the intensity of the 3747 cm-1 peak. Curthoys et al.20 have used instead direct calorimetry. In order to discriminate the actual process of interaction with SiOH groups from plain physisorption, ∆H° was defined as the difference between the heats of adsorption on a sample fully covered with hydroxyls and the same sample after dehydration. For all substances investigated, Curthoys et al.20 found the relationship. ∆H° ) -1.6 + 0.41(∆ν(OH))1/2 to hold. Hair and Hertl19 found different behavior with different families of compounds, according to the sp2 or sp3 nature of the atom engaged in H bonding: for instance, with adsorptives like carbonyls, in which the oxygen atom engaged in H-bonding is in sp2 hybridization, the proposed
Adsorption of Unsaturated Hydrocarbons
correlation is ∆H° ) -2.3 + 0.455(∆ν(OH))1/2. No data are reported for atoms in sp hybridization because no measurement on acetylenic systems has been carried out. Fortunately, the two formulas yield, in the range of ∆ν(OH) observed, results within few percent, so that one can confidently assume any of the two, even for the case not considered of sp-hybridized atoms. Starting from ∆ν(OH) values observed in the present work, a set of ∆H° values has been calculated (Table 2) and, by means of the standard relationship ∆G° ) ∆H° - T∆S°, also the standard change in entropy ∆S°. ∆H° values are probably comprehensive of both the contribution from sheer H-bonding and from dispersive interactions, although again it is impossible to discriminate between the two contributions. Inspection of Tables 1 and 2 shows that ∆H°cd and ∆H° are not clearly correlated. Note, however, that ∆H° values are systematically lower (in absolute values) than ∆H°cd, i.e., the presence of H-bonding does not compensate for the
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substitution of the whole-space liquid phase with the halfspace-filling silica. ∆S° values do correlate with the corresponding values for ∆S°cd, the standard entropy change for the condensation from the gas phase at 1 bar to the liquid phase, listed in Table 1. Figure 9b reports both ∆S° and ∆S°cd as a function of the number of carbon atoms in the molecule. When comparison is possible, ∆S° scales fairly well with ∆S°cd, the former being some 1/2 of the latter. Such an agreement corresponds to the qualitative concept that adsorbed molecules are in a state close to liquid-like. The smaller standard entropy change for the adsorption process in comparison with condensation, indicative of a large residual disorder, is also in agreement with the naive concept that the solid is only filling half of the available space and therefore allows more freedom to the adsorbed molecule. LA9703860
