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ACIDITYOF SURFACE HYDROXYL GROUPS
Acidity of Surface Hydroxyl Groups by Michael L. Hair and William Hertl Research and Developnent Laboratories, Corning Glass Works, Corning, New York 14830
(Received June 11, 1969)
Reports of hydroxyl band frequency shifts of phenols in solution indicate that the amount of shift can be related to the basicity of the solvent and the acidity of the phenol. Using this concept, the frequency shifts observed during adsorption on the hydroxyl groups of several oxide surfaces have been measured and compared with the shifts observed when several alcohols are dissolved in the same solvents. Comparison of these shifts with the known acidity constants of the alcohols enables pK, values for the various surface hydroxyl groups to be obtained. The values found were: magnesia, 15.5; boria, 8.8; silica, 7.1 ; silica-alumina, 7.1 ; and phosphorus, -0.4.
Experimental Section Introduction The solutions used were contained in a variable The nature of the hydrogen bonding interaction path length solution cell. between alcohols or phenols and organic solvents has Spectra were taken of various concentrations of the been the subject of numerous studies. Results obalcohols in carbon tetrachloride. With increasing contained prior to 1959 have been well summarized by centrations of solutes, an increasing fraction of the Pimentel and McClellan.’ One of the most useful tools solute molecules are mutually hydrogen bonded to employed in these studies has been infrared spectrosform dimeric and polymeric species. This is shown copy. On placing a hydroxyl-containing compound by the presence of two perturbed OH bands, the in solution, the band due to the OH vibration is moved relative intensities of which change with increasing to lower frequencies due to an H-bonding interaction concentration of solute. The concentrations of solutes between solute and solvent. used were chosen so that essentially no mutual hydroBellamy and Williams2 have shown that with ungen bonding was detectable. These concentrations hindered phenols and methanol, the perturbation of were methanol, 0.06 mol/l.; phenol, 0.10 mol/l.; and the OH band increases systematically with increasing trichlorophenol, 0.05 mol/l. To these solutions various basicity of the solvent. Also, the frequency shifts donor molecules were added and the position of the of unsubstituted phenol in a series of solvents can perturbed hydroxyl band(s) observed. be plotted against the corresponding values of methanol The oxide samples were mounted in a compensated and alkyl-substituted phenols in the same solvents cell so as to cancel out any gas-phase absorption bands. to give straight lines. The slopes of these plots were The experimental details for obtaining the spectra “different from that of phenol and nearer to that of of the samples in the presence of various gases have methanol, which is consistent with their weaker acidbeen described previously.4 ity.” Thus, in solution at least, OH band frequency The following oxide surfaces were prepared. (1) shift (Av) is a direct function of the acidity of the OH Silica (Cab-O-Sil, Cabot Co.) was pressed into a selfgroup and the basicity of the solvent. supporting disk and heated to 800” in air prior to This concept has also been discussed in the light using. (2) Magnesium hydroxide (J. T. Baker Chemof OH frequency shift data obtained when benzene is adsorbed on silica, magnesia, and molecular ~ i e v e s , ~ ical Co., N.F. grade) was pressed into a self-supporting an increasing OH frequency shift being interpreted disk and heated a t 500” for 15 min prior to using. as being due to a more acidic OH group. (3) Silica-alumina (American Cyanamid Co., Aerocat Triple A Cracking Catalyst Grade 75/85) was pressed The data obtained from the work described above into a self-supporting disk and heated a t either 400 or give only a qualitative measure of relative acidity or basicity, i.e., one can only determine if a given 520” for 0.5 hr in air, and then a t 200” in vacuo OH group is more or less acidic than another OH group, prior to using. (4)The phosphorus surface was preor if a given solvent is more or less basic than another pared by treating a silica disk with PCl, a t 120” until solvent. I n this paper we describe the correlation between the frequency shifts observed with two phenols (1) G. C. Pimentel and A. L. McClellan, “The Hydrogen Bond,” Freeman and Co., San Francisco, Calif., 1959. and methanol, and the known acidity values (pK.) (2) L. J. Bellamy and R. L. Williams, Proc. Roy. Soc., A254, 119, of these OH groups. With this correlation, we have (1960). used the observed OH frequency shifts on Mg-OH, (3) M. L.Hair, “Infrared Spectroscopy in Surface Chemistry,” DekB-OH, P-OH, and Si-OH surfaces to calculate the ker, New York, N. Y., 1967,p 136. acidity of these OH groups. (4) W. Hertl and M. L. Hair, J . Phys. Chem., 72,4676,136 (1968). Volume 74, Number 1
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MICHAELL. HAIRAND WILLIAMHERTL
Table I : Observed Hydroxyl Band Frequency Shifts (in cm-l) in Solution and on Surface Groups' Donor
Methanol
Hexane CCla Benzene Xylene Acetaldehyde Trimethylmethoxy silane Acetone Diethyl ether Pyridine Diethylamine Triethylamine Ammonia
... ...
0 0 6 32 40
csz
Phenol
... ...
62
...
...
134
...
112 142 280 220, 400 251, 446
131, 236 280 460 ca. 650 ca. 700
...
...
Trichlorophenol
Silioa
...
SilioaMagnesia alumina
...
30 45 60 120 155 280 477
... ... ... 40 . . a
... 140, 290 345 600, 1010 ca. 830, 1030 910-1030
395 460 765 ca. 900 975 675
...
... ...
35 45 75
130 155
105
...
... ...
350 175
...
35
... ... ...
Phosphorus
...
... .., 430 ... ... ... ...
Boron
27 35 i 10
75
...
...
200 260 ca. 680
105 i 20 120 260 f 30
*..
. I .
850 f 150
... ... ...
295 i 30 380 f 30
... . I .
.*.
. . I
...
860
' The frequencies of the unperturbed surface hydroxyl groups are given in Table 11; for the alcohols, the unassociated hydroxyl frequencies in CCl, solution are methanol, 3630 om-'; phenol, 3598 cm-I; and trichlorophenol, 3525 om-'. all the silanol groups had disappeared, then hydrolyzing with water and heating the sample to 800" in air for 1 min. At the end of this procedure, P-OH groups are present as well as some silanol groups. After repeating this procedure four times, a fairly large number of P-OH groups and a small number of silanol groups remain on the surface. (5) The boron surface was prepared by treating a silica disk with BCla a t room temperature until all the silanol groups had disappeared. When water vapor was admitted to the cell a large fraction of the silanol groups reappeared as well as a band at 3706 cm-', which is due to the B-OH group. This procedure was repeated five times, a t which point the relative intensities of the B-OH to the Si-OH ceased to change. The final intensity of the B-OH was about 2.5 times as strong as the Si-OH. The sample was then heated to 800" in air for 1 min.
Results The results are summarized in Table I. I n Figure 1, the OH frequency shifts observed for the phenols and methanol are plotted, arbitrarily, against the shifts observed for the surface Si-OH groups. Straight line relationships are obtained. These do not pass through the origin, however, since the frequency shifts on silica were measured from the gas phase, whereas those of the alcohols were measured in a solution environment. I n solution the "unassociated" OH group must interact to a certain extent with the solvent, although this interaction is considerably less than the interaction with the added donors. In some cases (e.g., phenols acetone, methanol amines) a t least two perturbed hydroxyl bands appear, thus indicating a t least two kinds of interaction. I n Figure 1, the lines have been drawn through the points which we believe form a consistent series, i.e., through
+
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'/ 900 T 0
700 -
TRI-CHLORO PHENOL METHANOL
PHENOL
-
I
500-
.-0c f
3
-
2
100
200
400
600 ''Sig-OH
800
1000
(cm-')
Figure 1. Plots of observed alcohol hydroxyl band frequency shifts in the presence of various donors against hydroxyl band frequency shifts observed when the same donors are adsorbed on silanol groups.
those perturbed bands which arise from the same type of interaction. As only one perturbed band is observed with the surface group, it is clear that the interactions in solution are much more complex than those observed when a gaseous molecule adsorbs on a surface hydroxyl group. I n Figure 2, the hydroxyl band frequency shifts observed on the various oxides are plotted against the cortesponding shift observed on silica, and again straight lines are obtained. Using the relationship described above between the relative frequency shifts and the acidity of the OH groups, one can say qualitatively a t this point that with the alcohols, trichlorophenol is most acidic and methanol least acidic, and
ACIDITY OF SURFACE HYDROXYL GROUPS I
93
Bs -OH
A"Si,-OH
(ern-')
Figure 2. Plots of observed hydroxyl band frequency shifts when various donors are adsorbed on surface hydroxyl groups against the shifts observed when the same donors are adsorbed on silanol groups.
Figure 3. Plot of the line slopes for the alcohols from Figure 1 against the pK, values for the alcohols. The arrows indicate the slopes of the lines given in Figure 2 for the various surface hydroxyl groups.
with the surface OH groups phosphorus is most acidic and magnesia least acidic (cf. slopes in Figure 1and 2).
hydroxyl groups for the various metal hydroxides, and literature pK, values for the corresponding free acids in solution ( i e . , silicic, phosphoric, and boric acids). The P-OH surface shows two bands due to unperturbed hydroxyl groups. These can possibly be assigned to single OH groups attached to the phosphorus and to OH groups in a geminal configuration attached to the phosphorus. When the various gases were added to the system, the perturbed bands were very broad. This could be due either to a distribution of OH bond energies or to the presence of overlapping perturbed bands due to the two types of hydroxyl groups. The frequency shifts were measured from the lower frequency band, which was more intense. When ammonia was added to the system, no bands assignable to NH4+ were observed, so that although the P-OH groups are much more acidic than Si-OH groups, apparently no ammonium ions are produced on the surface. The Mg-OH band a t 3752 cm-I shifts to lower frequencies (ca 3700 cm-l) when heated to a higher temperature than used here. This effect has been previously reported.* The precise frequency of the freely vibrating silanol group on the SiOhAlzOa was checked by mounting a silica disk in the reference beam of the spectrophotometer. The silanol frequencies were found to be identical. The frequency shifts observed when the various gases were added appeared to be the same as the shifts observed on pure silica. This is in agreement with, and extends, the observations made previously by Basila.6 However, when NH3 was added, a band attributable to NH4+ was observed. Thus, although
Discussion If the slopes of the lines in Figure 1are plotted against the known acidity values (pK.) of the alcohols (Figure 3) a straight line is obtained. Since all the slopes were obtained with respect to the silanol group, the pKa value of this group should be given by the value on the plot corresponding to a slope of 1.0. This is indicated by the labeled arrow in Figure 3 and gives a value of 7.1 0.5, which is in very good agreement with the value (about 7.0) obtained by t i t r a t i ~ n . ~This good agreement strongly suggests that this method of calculation is valid for obtaining the acidity values of the surface hydroxyl groups. The slopes of the plots for the surface hydroxyl band frequency shifts (Figure 2) can now be used in conjunction with the plot given in Figure 3 to obtain the pK, values of the various surface hydroxyl groups. The values of the slopes are indicated in Figure 3 by the labeled arrows. The pKa values to which these correspond (abscissa of Figure 3) are given in Table 11, together with the frequencies of the freely vibrating
*
Table I1 : Frequencies and Observed pKa Values of Surface Hydroxyl Groups
Oxide
Silica Magnesia Silica-alumina Phosphorous Boria
Frequency (in cm-1) of freely vibrsting OH group
3750 3752 3750 3670, 3700 3706
PKa
Literature pK. value for the acids (first dissociation step)
7.1 f 0 . 5 15.5 f 0.4 7.1
9.7
-0.4 8.8 f 0.6
2.0
... *..
9.1
(6) R. W. M a a t m a n , et al., J . Phys. Chem., 68,767 (1964). (6) M. R.Basila, J . Chem. Phys., 35, 1151 (1961).
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B-OH groups, indicating that the silanol groups are also stronger adsorption sites. It should be noted, however, that with the samples used here, the surface contains about 2.5 times as many B-OH groups as Si-OH groups so that the perturbed hydroxyl bands consist principally of perturbed B-OH groups. This observation is somewhat surprising, as titration data on porous glass-which contains a considerable quantity of boron on its surface7-indicates the presence of a more acidic group (pKa 5.7) on its surface in addition to the silanol (PKa = 7). The present data indicate that this increased acidity is probably associated with the solution interaction with Lewis acid sites rather than due to an acidic boranol group. It is of interest to compare the pKa values for the surface OH groups with the first dissociation step of the comparable free acid in solution (Table 11). It can be seen that the surface group has a lower pK, value than the “molecular” acid and also that there is no direct relationship between the two pKa values. If the pK, values for the surface hydroxyl groups are plotted against the Mulliken electronegativities I E) then a good inverse relationship is revealed (Figure 4).
-
+(I+EI
Figure 4. Plot of the pK. values of the surface hydroxyl groups against t8heMulliken electronegativity values.
the acidity of the OH group on this sample is the same as that observed on silica, it must be recognized that sites exist on the surface of these catalytic materials which are sufficiently acidic to protonate the ammonia molecule. The frequency shifts observed on B-OH are slightly less than on the Si-OH group, indicating that the B-OH groups are slightly less acidic than the Si-OH groups. For a given pressure of added gas a greater fraction of the silanol groups are perturbed than the
The Journal of Physical Chemistry
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Acknowledgment. The authors thank Miss E. R. Herritt for assistance in the experimental work. (7) I. Altug and M. L.Hair, J . Phys. Chen., 71,4260 (1967).
