Langmuir 1999, 15, 477-481
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Probing Surface Acidity by 31P Nuclear Magnetic Resonance Spectroscopy of Arylphosphines Bing Hu and Ian D. Gay* Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada Received June 24, 1998. In Final Form: November 3, 1998 The adsorption of triphenylphosphine, tri(p-tolyl)phosphine, tri(m-tolyl)phosphine, and tri(p-chlorophenyl)phosphine has been studied on SiO2, Al2O3, and a SiO2-Al2O3 cracking catalyst. 31P solid-state NMR shows that all of these molecules are rotationally mobile in the physisorbed state. The interaction of the phosphines with Lewis and Brønsted acid sites on the surface can be observed. For these phosphines the chemical shifts of physisorbed and Lewis-bound species are too close to permit quantitative determination of the concentration of the latter. The concentration of Brønsted sites on SiO2-Al2O3 can, however, be measured. Using triphenylphosphine, we detect 0.18 µmol/m2 Brønsted sites, somewhat less than previously detected with stronger bases, indicating a spectrum of acid strengths on this catalyst. Steric effects appear to reduce the acid detectable with the tolylphosphines, and it is not clear that they are useful in studying the strength of acid sites on this catalyst. We find evidence for a small concentration of basic sites on the SiO2-Al2O3 catalyst.
Introduction The 31P NMR spectroscopy of adsorbed phosphines has become an important technique for the study of surface acidity. This is because NMR is in principle capable of more easy and more accurate concentration measurements than other spectroscopies commonly used for powdered catalysts, and because the high sensitivity of 31P permits this advantage to be exploited down to levels of a few percent of a monolayer. Most workers to date have studied trimethylphosphine,1-4 which is convenient to use, gives distinct lines for physisorbed, Lewis-bound, and Brønstedbound species, and with care4 permits quantitative determination of surface acid site concentration. A drawback to the probing of surfaces with alkylphosphines is that they are all rather strong bases and are likely to detect acid sites that are so weak as to be of little interest catalytically. It is thus of interest to probe acidic surfaces with weaker phosphine bases, and the obvious molecules to choose are the triarylphosphines. In aqueous solution, triphenylphosphine (PPh3, pKa ) 2.73) is 106 times weaker than trimethylphosphine (PMe3, pKa ) 8.65),5,6 and ring substitution can induce a moderate increase or decrease in strength, depending on the substituent. Thus arylphosphines should make it possible to study only the stronger acidic sites on surfaces. For the moment, we take pKa as characterizing the base strength of the phosphine. It should be noted that the sequence of base strengths measured in the gas phase7 is different from that in aqueous solution, and it is not immediately obvious which is preferable. We discuss this point in more detail below. (1) Lunsford, J. H.; Rothwell, W. D.; Shen, W. J. Am. Chem. Soc. 1985, 107, 1540. (2) Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 40. (3) Lunsford, J. H.; Tutunjian, P. N.; Chu, P.; Yeh, E. B.; Zalewski, D. J. J. Phys. Chem. 1989, 93, 2590. (4) Sheng, T. C.; Gay, I. D., J. Catal. 1994, 145, 10. (5) Golovin, M. N.; Rahman, M. M.; Belmonte, J. E.; Giering, W. D. Organometallics 1985, 4, 1981. (6) Allman, T.; Goel, R. G. Can. J. Chem. 1982, 60, 716. (7) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data 1984, 13, 695.
Triarylphosphines have two potential disadvantages: they are solids at room temperature, making sample preparation more difficult, and their large size may lead to steric limitations on the surface sites that can be probed. We have investigated these problems in a study8 of tricyclohexylphosphine (PCy3) on various surfaces. This solid phosphine presents the same handling and steric problems as the triarylphosphines, but it is a strong base, and in the absence of steric problems would be expected to detect as many sites as trialkylphosphines. We found that on commercial silica-alumina cracking catalyst hightemperature processing of the samples is necessary to induce diffusion of PCy3 through the pore structure at a reasonable rate. After processing at 200 °C, PCy3 detects slightly more acid sites than does PMe3, presumably due to the fact that it is a stronger base. These results indicate that steric limitations are not too serious on this type of surface. Experimental Section We have studied adsorption on silica gel, Davison grade 923, 484 m2/g, γ-alumina, Harshaw grade Al-3945, 225 m2/g, and a mixed silica-alumina catalyst, Davison grade 980, surface area 400 m2/g, 25 wt % Al2O3. The phosphines were obtained from Strem Chemicals. These were used as received, as none of them showed any P-containing impurity by cross-polarization magicangle spinning (CPMAS) NMR on the neat solids. Samples for NMR study were prepared as described in ref 8. Weighed samples of adsorbent and phosphine were vacuumdegassed at 450 °C and at room temperature, respectively, for 2 h. The two solids were subsequently mixed in vacuo in a NMR tube, which was then sealed off from the vacuum system. The sealed tubes were processed at temperatures up to 200 °C to disperse the phosphine on the adsorbent. (The vapor pressure of PPh3 is 666 Pa at 200 °C.9) NMR measurements were made at 3.5 T on a previously described8 spectrometer, with a 31P resonance frequency of 60.46 MHz. Radiofrequency field intensities of 45-50 kHz were used on both observe and decoupling channels, and magic-angle spinning of the sealed samples was carried out at 1.6-2.6 kHz. (8) Hu, B.; Gay, I. D. Langmuir 1995, 11, 3845. (9) Forward, M. V.; Bowden, S. T.; Jones, W. J. J. Chem. Soc. 1949, 3, S121.
10.1021/la980750a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/31/1998
478 Langmuir, Vol. 15, No. 2, 1999
Figure 1. 31P NMR spectra of PPh3: (A) Pure solid, (B) adsorbed on SiO2, 0.92 µmol/m2, and (C, D) adsorbed on Al2O3, 0.38 µmol/m2. For all spectra, MAS rotation rate is 1.8 kHz. For spectra B and D, sample was treated 1 h at 200 °C after preparation. For spectrum C, treatment was 1 h at 100 °C. All NMR measurements were made at ambient probe temperature of 27 ( 3 °C. In some cases cross-polarization was used for diagnostic tests; quantitative measurements were made with excitation by 90° pulses with dipolar decoupling of 1H, and in all such measurements it was verified that no significant change in peak ratios occurred for a 2-fold change in relaxation delay. In most cases the amount of each species was determined from its fractional peak area in the 31P NMR spectrum, together with the known amount of phosphine used in preparation of the sample. Overlapping lines were deconvolved when necessary by fitting Gaussian or Lorentzian peaks as appropriate, using a Levenberg-Marquardt algorithm (see e.g., Figure 2). We believe that intensity ratios determined in this way are correct within 5-10%, depending on noise level and degree of peak overlap. In addition, absolute intensity measurements were made in some cases by comparing total integrated intensity with that of a standard prepared from a weighed amount of reagent-grade (NH4)2HPO4 dispersed in powdered NaCl. In all cases the phosphorus found was within 10% of that expected from the sample preparation. Chemical shifts were measured relative to concentrated H3PO4, and were found to have a day-to-day reproducibility of (0.3 ppm.
Results and Discussion Triphenylphosphine. Figure 1 shows the spectra arising from the adsorption of PPh3 on SiO2 and Al2O3. In Figure 1A is shown the spectrum of pure solid triphenylphosphine. The main peak is at -9.3 ppm, in reasonable agreement with literature values of -7.210 and -10.311 ppm. The minor peaks are spinning side bands, arising from the chemical shift anisotropy of P. Their magnitude is as would be expected12 from literature11 values of the anisotropy. After adsorption (Figure 1B) on SiO2, the line broadens and shifts to -5.8 ppm, and the spinning side bands disappear. The spectrum shown in Figure 1B is at a coverage of 0.92 µmol/m2; the line remains at the same position within experimental error for coverages in the range of 0.25-1.7 µmol/m2 and narrows slightly at the highest coverages. No spinning side bands are observed at any coverage in this range. This resonance corresponds to physically adsorbed PPh3 on silica, where only weak interactions with the surface hydroxyls would be expected. The disappearance of spinning side bands means that the apparent chemical shift anisotropy has decreased radically; indeed, if magic-angle spinning is not used, the resonance remains symmetrical and is less than 4 ppm in width. This can only mean that the anisotropy (10) Clark, L.; Bemi, H. C.; Davis, J. A.; Fyfe, C. A.; Wasylishen, R. E. J. Am. Chem. Soc. 1982, 104, 438. (11) Penner, G. H.; Wasylishen, R. E. Can. J. Chem. 1989, 67, 1909. (12) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021.
Hu and Gay
Figure 2. Gaussian fit of spectrum D from Figure 1. Upper, data points and fitting curve; lower, Gaussian components used in the fit.
is motionally averaged, and adsorbed molecules must be reorienting rapidly on the surface, on a submillisecond time scale, since the span of the chemical shift anisotropy is 51 ppm11 or 3.1 kHz in our field. The motion must be more random than just jumps about a 3-fold axis perpendicular to the surface, since that would lead to an axially symmetric anisotropy pattern, which is not observed. A similar result was observed for PCy3 on silica;8 it appears that a high degree of rotational motion is typical for these large phosphines on SiO2 at room temperature. Figure 1C,D shows spectra of PPh3 on Al2O3. In spectrum C the sample has been treated at 100 °C after preparation, and in spectrum D, at 200 °C. In the former case a symmetrical line is seen at -6.4 ppm. This is probably again physically adsorbed phosphine, although the line is broader than on silica. Treatment at 200 °C produces an unsymmetrical line, with substantial intensity to lower frequencies. This is clearly a composite line, and the spectrum of Figure 1D can in fact be well fitted by the superposition of two Gaussian lines, one at -9 ppm and a broader one at -18 ppm. This fit is shown in Figure 2. These results suggest that higher temperature treatment permits the diffusion of the phosphine to sites where different bonding modes are possible. A similar result was found for PCy3,8 although in that case 150 °C was sufficient to cause nearly total transformation of the physically adsorbed species, at a similar coverage. Alumina is expected to contain a high concentration of Lewis acid sites, which are readily detectable with PMe34 and PCy38. Unfortunately, PPh3 is not a good probe for Lewis sites, since the 31P shift is rather close to the value found above for physically adsorbed species. We find a shift of -8.4 ppm for the solid adduct with AlCl3, while a value of -7.3 ppm has been reported13 for the AlMe3 adduct in solution. Thus it is quite reasonable to assign our line at -9 ppm to PPh3 bound to the Lewis sites of alumina. The origin of the intensity near -18 ppm is less clear. Barron13 shows that shift differences between free and complexed phosphines correlate well with the Tolman cone angle of the phosphine. Accordingly, one possibility is a distortion of the phosphine induced by adsorption on alumina. However, the correlations in ref 13 suggest that a change in cone angle of about 20° would be required, which seems implausibly large. It is clear that the line at -18 ppm cannot arise from reaction with surface H+. No Brønsted acid is found on this alumina with any previously studied phosphine probes, and the resonance of PPh3H+ is in fact shifted in (13) Barron, A. R. J. Chem. Soc., Dalton Trans. 1988, 3047.
Probing Surface Acidity by
31P
NMR of Arylphosphines
Langmuir, Vol. 15, No. 2, 1999 479 Table 1. Acid Measurement with Triphenylphosphine
Figure 3. 31P NMR spectra of PPh3 on SiO2-Al2O3 cracking catalyst at coverage of 0.33 µmol/m2. MAS rate 1.8 kHz. (A) Treatment for 1 h at 100 °C; (B) treatment for 1 h at 150 °C; (C) treatment for 1 h at 200 °C.
Figure 4. 31P NMR spectra of PPh3 on SiO2-Al2O3 cracking catalyst at various coverages, after treatment for 1 h at 200 °C. MAS rate 1.84 ( 0.02 kHz. (A) 0.18 µmol/m2, (B) 0.33 µmol/m2, (C) 0.48 µmol/m2, (D) 0.77 µmol/m2, (E) 0.94 µmol/m2. Plots are normalized to the same absolute intensity, taking into account differing numbers of scans averaged. Varying noise levels reflect different numbers of scans.
the opposite direction. We find a shift of +4.2 ppm for this ion in aqueous solution. Figure 3 shows the spectra obtained for PPh3 on the SiO2-Al2O3 cracking catalyst. These consist of a line at -6.2 ppm, which transforms on thermal treatment into a line at +7.2 ppm. The latter line is completely eliminated by a decoupler delay of 70 µs, which indicates the presence of a P-H bond. This fact, together with the chemical shift, indicates that this line arises from PPh3H+, formed by the interaction of the phosphine with Brønsted acid sites on the surface. The increase in this species with heat treatment is similar to that found for PCy3 on the same catalyst8 and arises from activated diffusion of the phosphine to acid sites. No further spectral changes occurred for more prolonged thermal treatment. The total number of Brønsted sites on the surface can be determined by adsorbing progressively larger amounts of phosphine until all acid sites have reacted. Figure 4 shows spectra obtained on the SiO2-Al2O3 catalyst with differing amounts of adsorbed phosphine. At the lowest coverage the line of PPh3H+ is observed, together with broad features arising from Lewis sites. As coverage is increased, the latter are obscured by the line of physically adsorbed phosphine. Thus the concentration of Brønsted acid can be measured, but it is not practical to determine the amount of Lewis acid with PPh3. Table 1 gives the amount of Brønsted acid on the catalyst, calculated by multiplying the total amount of adsorbed phosphine by the fractional area of the PPh3H+ peak.
adsorbed phosphine (µmol/m2)
Brønsted sites (µmol/m2)
shift of physisorbed phosphine (ppm)
0.11 0.18 0.24 0.33 0.48 0.77 0.94
0.07 0.11 0.14 0.15 0.18 0.09 0.08
-6.1 -6.2 -6.7 -6.8
As is evident from Figure 4 and Table 1, the apparent amount of PPh3H+ goes through a maximum with respect to phosphine coverage; this results from a decrease in the absolute intensity of the PPh3H+ peak at the highest coverages. The spectra of Figure 4D,E are among those for which an absolute intensity measurement was made (see Experimental Section) and the integrated total phosphorus signal is as expected from the sample preparation, within 5%. A similar phenomenon was observed with PMe3 on this catalyst4 but not with PCy3.8 For trimethylphosphine, this decrease was associated with a substantial systematic change in the position of the “physically adsorbed” peak with coverage, and the phenomenon could be satisfactorily interpreted as a fast exchange process between physically adsorbed species and a fraction of the PMe3H+ population. Moreover, the shift of the exchanging species could be satisfactorily calculated using as a physisorbed shift the value observed on pure SiO2. As Table 1 shows, there is a small change in the position of the physisorbed peak for PPh3, and we may ask whether the same phenomenon is acting here. To account via exchange for the loss of 0.10 µmol/m2 Brønsted intensity between the 0.48 and 0.94 µmol/m2 samples, we must assume that the “physisorbed” peak of the latter, corresponding to 0.86 µmol/m2, must contain 0.10 µmol/m2 Brønsted species, or 12%. In the case of fast exchange, the position of this peak should then be given by
δobs ) 0.12δB + 0.88δP where δB and δP are the chemical shifts of the PPh3H+ and physisorbed species, respectively. With the experimental values of -6.8 ppm for δobs and +7.2 ppm for δB, this equation shows that δP must have a value of -8.7 ppm. This is widely divergent from the value obtained above for PPh3 physisorbed on silica, -5.8 ppm. So the decrease in intensity of the PPh3H+ line cannot arise from exchange between this ion and a physisorbed species. If an exchange process is postulated, it must be between PPh3H+ and a species that is more like solid PPh3 (-9.3 ppm) than like a physisorbed species. Possibly some form of multilayer adsorption can occur at high phosphine coverages and give rise to such a species; however, it should be noted that there is no evidence for such a species on SiO2, where, as mentioned above, the chemical shift of the physisorbed species does not change up to a coverage nearly twice that used in Figure 4E. An alternative explanation for the disappearance of PPh3H+ can be seen in Figure 5. This spectrum shows a sample prepared in the usual manner but starting with solid PPh3HBr, rather than with PPh3. The amount of triphenylphosphonium bromide used corresponded to a coverage of 0.40 µmol/m2. As can be seen, the major peak is at +5.1 ppm, corresponding to PPh3H+, but a substantial peak, amounting to 13% of the total phosphorus appears at -5.4 ppm, due to adsorbed PPh3. This indicates that the surface must contain basic sites capable of removing
480 Langmuir, Vol. 15, No. 2, 1999
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Table 2. Acid Measurement with Tolyl Phosphines tri-(p-tolyl)phosphine
tri-(m-tolyl)phosphine
adsorbed phosphine (µmol/m2)
Brønsted sites (µmol/m2)
adsorbed phosphine (µmol/m2)
Brønsted sites (µmol/m2)
0.13 0.31 0.45 0.56 0.94 1.12
0.07 0.07 0.08 0.10 0.07 0.05
0.07 0.18 0.32 0.43 0.47 0.56
0.04 0.09 0.12 0.13 0.10 0.06
Figure 5. 31P NMR spectrum of sample prepared from PPh3HBr and silica-alumina at loading level of 0.40 µmol/m2.
H+ from PPh3H+. Since PPh3 is a rather weak base, its conjugate acid is correspondingly strong, so it should be able to protonate even weakly basic sites. Silica-alumina, though generally considered an acid catalyst, is known14 to possess basic sites that can be protonated by trichloroacetic acid. The above result shows that there must be not less than 0.05 µmol/m2 basic sites, equivalent to at least half the disappearing PPh3H+ in PPh3 adsorption experiments. Thus reaction of PPh3H+ with basic sites may account for its disappearance at high phosphine coverages. A surface can possess both Brønsted acid and base sites simultaneously only if they are separated in space and there is no mechanism permitting them to react with each other by proton transfer. It can be seen that an adsorbed layer of PPh3 might provide such a mechanism via successive proton transfers between PPh3H+ and PPh3. Assuming that acidic and basic sites are separated on the surface, we may expect low coverages of PPh3 to become protonated and the resulting phosphonium ion to be localized in the vicinity of the acid site. When PPh3 is added in excess of the surface acid concentration, a physisorbed layer appears and grows with subsequent addition of phosphine. At some point the physisorbed coverage will become high enough to form a path of PPh3 between acid and base sites, along which a proton could transfer by successive exchanges. There may be a thermodynamic barrier to the reaction of surface acid and base, arising from the resulting charge separation on the surface. Thermodynamically, the process of phosphine protonation at an acidic site, followed by reaction of PPh3H+ at a basic site, can be formulated as +
PPh3 + AOH f PPh3H + AO
-
PPh3H+ + B f PPh3 + BH+ so that the overall process corresponds to reaction of (14) Tanabe, K. In Solid Acids and Bases; Academic Press: New York, 1970; pp 43 and 62.
surface acid, AOH, with surface base, B. This leads to a charge separation on the surface, an endothermic contribution to the overall process. The presence of physisorbed phosphine will increase the dielectric constant of the surface layer, making the charge separation less endothermic. Thus it may be that the above overall process only becomes thermodynamically possible at high phosphine coverages. (The same argument can be made if the base is formulated as BOH, leading to B+ and H2O as reaction products.) Whatever the reason for decrease in the concentration of PPh3H+ at high phosphorus loadings, the phenomenon only occurs when there is a substantial concentration of physisorbed phosphine. In these circumstances, all of the surface Brønsted sites should have reacted with phosphine, and the maximum value observed for [PPh3H+] provides a good estimate of their concentration. Tolyl Phosphines. We have carried out a similar series of measurements with tri-(p-tolyl)phosphine and tri-(mtolyl)phosphine. These are slightly stronger bases than PPh3, in aqueous solution, having pKa values of 3.84 and 3.30, respectively. The qualitative behavior of these phosphines on SiO2, Al2O3, and SiO2-Al2O3 is in all respects analogous to that described above for triphenylphosphine. There are quantitative differences in the amount of Brønsted acid detected on SiO2-Al2O3 using tolyl phosphines, and our results for this are collected in Table 2. As can be seen in this table, both of the tolylphosphines detect less Brønsted acid than does PPh3. If they are stronger bases, this must indicate that some acid sites are not accessible to these larger molecules. The effect is slightly stronger for the p-tolyl than for the m-tolyl isomer. Tri(p-chlorophenyl)phosphine. We have studied one phosphine that is a weaker base in aqueous solution than PPh3, namely, P(p-ClC6H4)3, which has a pKa value of 1.03. Again the qualitative behavior is similar to the other arylphosphines. Two differences were noted: the shift of the physisorbed layer (-10.5 ppm) is to low frequency of the pure solid (-4.0 ppm), whereas the shift is in the opposite sense for the other phosphines; in addition, high coverages of P(p-ClC6H4)3 on SiO2-Al2O3 give rise to lines at 23-26 and 42 ppm upon heat treatment, which we interpret as arising from the phosphine oxide15,16 and its interaction products with surface acid sites.17 In Table 3 we give the amounts of Brønsted acid detected as P(pClC6H4)3H+ by use of this phosphine. Summary In this paper and in previous publications from our laboratory, we have determined apparent concentrations of Brønsted acid on the same SiO2-Al2O3 cracking catalyst, (15) Van Wazer, J. R.; Callis, C. F.; Shoolery, J. N.; Jones, R. C. J. Am. Chem. Soc. 1956, 78, 5715. (16) Jones, R. A. Y.; Katritzky, A. R. Angew. Chem. 1962, 74, 60. (17) Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. Soc. 1986, 108, 7119.
Probing Surface Acidity by
31P
NMR of Arylphosphines
Table 3. Acid Measurement with Tri(p-chlorophenyl)phosphine adsorbed phosphine (µmol/m2)
Brønsted sites (µmol/m2)
0.12 0.17 0.24 0.28
0.06 0.06 0.10 0.06
Table 4. Comparison of Acid Amounts Measured with Various Phosphines phosphine
pKa
Brønsted sites (µmol/m2)
reference
tricyclohexylphosphine trimethylphosphine triphenylphosphine tri(p-tolyl)phosphine tri(m-tolyl)phosphine tri(p-chlorophenyl)phosphine
9.70 8.65 2.73 3.84 3.30 1.03
0.26 0.21 0.18 0.10 0.13 0.10
8 4 this work this work this work this work
using a range of phosphines of different sizes and strengths. The results are summarized in Table 4. As noted in the Introduction, gas-phase basicities of phosphines are not in the same sequence as aqueous pK values,7 and in particular, PPh3 is a stronger gas-phase base than is PMe3, although gaseous PEt3 is stronger than either. One can readily imagine that hydrogen bonding and high dielectric constant make the acid-base interaction in H2O substantially different from that in a vacuum. Somewhat surprisingly, heats of protonation of phosphines in the less polar solvent dichloroethane18 correlate very well with pKa values, and hence less well with gas-phase basicities. Species adsorbed on a high-area porous solid are in an environment that is neither liquidlike nor vacuumlike; some stabilizing interactions (with the solid) are available, and the average dielectric constant will certainly be higher than unity. It is thus not clear which sequence of base strengths is appropriate. If a solid has a range of acid sites of varying strength, one can identify the stronger bases, for interaction with that solid, by observing which bases interact with more surface acid sites. This presupposes that steric effects are not important, making it desirable to compare bases of similar sizes. Thus a comparison of PCy3 and PPh3 (Table 4) is germane. PCy3 detects considerably more acid on our silica-alumina than does PPh3 and hence is a stronger base for interaction with this solid; one cannot argue that PCy3 accesses more sites due to smaller size. It would thus be interesting to know the gas-phase basicity of PCy3, but a direct experimental measurement does not seem to be available in the literature. An estimate can be made by using the two-parameter acid-base correlations of Drago and co-workers.19 These authors give (18) Bush, R. C.; Angelici, R. J. Inorg. Chem. 1988, 27, 681.
Langmuir, Vol. 15, No. 2, 1999 481
parameters for a variety of phosphines. By using their parameters and the gas-phases basicities for the five phosphines that are common to this paper and to ref 7, one can derive Drago parameters for gaseous H+ as a reagent. This correlation is very satisfactory, reproducing the gas-phase basicities with a standard deviation of 0.5 kcal/mol. One can then use the H+ parameters so derived to estimate the gas-phase basicity of PCy3 as 235.7 kcal/ mol, making it the strongest gas-phase base of those studied here (cf. PPh3 ) 222.5 kcal/mol). By the same method, one can estimate the gas-phase basicity of P(ptol)3 as 224.5 kcal/mol. We present the data of Table 4 in terms of aqueous pKa values. If the estimates in the previous paragraph are reasonable, these are in the same order as the gas-phase basicities for PCy3, PPh3, and P(p-tol)3; the situation in a porous solid is most likely intermediate between those in gaseous and aqueous environments. For the first three of these phosphines, the amount of Brønsted acid correlates inversely with the pKa of the phosphine. In accordance with previous work,20 most of the sites on SiO2-Al2O3 are strong acids, capable of protonating weak bases. A larger amount of acid is found with PCy3 than with PPh3, indicating that there are also weaker acid sites on this surface. The comparison of PCy3 and PPh3 is conclusive in this regard; because of the relative weakness of PMe3 in the gas phase, one might argue that there is a large number of strong sterically hindered sites accessible to this probe but not to the larger phosphines. We think it a more economical assumption that pKa is a better measure of basicity on the present surface. The tolylphosphines find less acid than PPh3, and since they are stronger bases, this must mean that steric effects are operative. Previous measurements of the pore size distribution for this catalyst8 by N2 adsorption show that all pores are greater than 30 Å in diameter, so penetration into the pore structure should not be a problem. Thus it seems that some local structural feature of some of the acid sites must prevent the approach of the tolyl phosphines. Since tri(p-chlorophenyl)phosphine is only slightly smaller than tri(p-tolyl)phosphine, it is not possible to argue whether the low amount of detected Brønsted sites arises from the weakness of this base or from steric effects. Acknowledgment. This work was supported by a research grant from the Natural Sciences and Engineering Research Council of Canada. LA980750A (19) Joerg, S.; Drago, R. S.; Sales, J. Organometallics 1998, 17, 589. (20) See Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases Elsevier: Amsterdam, 1989; pp 119-120 and references quoted therein.
