INFRAREDSTUDY OF SILIC$-ALUMINA
Nov., 1962
in chloroform but formed a suspension of very
small pa,rticles. Finally, fraction IB could be a complex derived from another carotene isomer. As mentioned earlier, the pure complex changes slowly in. the course of time, resulting in a product similar to that found in sample I which was prepared by using solid iodine. Experimental results indicate that the compound (sample II} between p-carotene and iodine is a charge transfer complex rather than a product resulting; from iodine addition to the double bonds in the carotene molecule. Such iodinated compounds would not dissociate into their components upon diasolution in chloroform. The absorption maximum of iodinated &carotene should be found at shorter wave lengths because of shortening of the conjugated chain. This wa8 found indeed by Savinov and Tretyakova.6 Such a shift is of the order of a few hundred Angstroms only and the shifted ;absorptions cannot be correlated to the new bands a t 2920 and 3600 A.. These bands cannot be assigned t o iodine either, since the absorptions of iodine in this region are of very low intensity aad of completely different character. Also TABLE I SOLUTION DETAILS 0%SPECTRA TAKEN PROX A 1.5 X OF SAUPLE II IN CHLOROFORM USINGA l-cw, CELL Soh no.
1 2 3 4
Iodine Concentrations are in moles/l. Conon. of LGg lo/I 4600
added iGdine
5
3 x 1.0-6 6 X lom6 1.2 x 1.0-4 2 . 4 x 10-4
6
A.
0.92 .85
.81 I72 .60 .62
3.6
x
10-4
7
4 . 8 x io-4
*
8
6.0
x
10-4
.42
46
3600
A,
0,34 .43 .48
2920
0.36 .43 .57
63
I75
.78 .90 -95 .99
.93 1.14
I
1.19 1.25
SURFACE
2223
a comparison of the infrared spectra of B-carotene triiodide and hexyl iodide showed that there is no absorption in the spectrum of the complex that (can be assigned to a carbon-iodine bond,1° The fact that there is an isosbestic point at 4125 A. in the spectra (Fig. 2) clearly indicates that there is an equilibrium between two absorbing species with overlapping absorptions. Apparently, the absorptions of iodine, originating from dissociation of the complex, at this wave length and at these concentrations, are too weak to influence the isosbestic point. Although for simplicity’s sake only four of the spectra are given in Fig. 2, it should be mentioned that the four other curves (2, 4, 5, and ‘7) also go through that same point at 4125 8. Probably there is a second isosbestio point a t about 5550 b. arising from overlap between a n absorption band of the complex with hmsx8200 and that of p-carotene. However, the point is so little different from the “background” that it could not be determined very accurately. From the experimental results presented here, one can conclude that treatment of p-carotene with iodine initially leads to a charge transfer complex, In the course of time, isomeri~ation~,~ as well as iodination of the carotene will take place.e Further work on this system will give more informationabout the equilibrium and also about the species represented as C40HJ’3 or, better perhaps, as (GoHae13),.In a later paper we expect to prove or disprove whether the absorption bands at 2920 and 3600 A.are due t o the IsAcknowledgments.-The authors wish to thank Dr, 0. H, LeBlanc for many discussions on the subject and IVlr. and Mrs. 0. Sovers for their help with the translation of one of the references. (10) 0 . lovere, unpublinhed work.
AN INFRARED STUDY OF -4SILICA-ALUMINA SURFACE BY R/IIcaaEL R. BASILA Gulf Research & Development Company, Pitisburgh, Pennsylvania Recehed M a y 2f, 196.8
In highly dehydrated samples of silica-alumina a single band is observed in the OH stretching re ion. The resulta imply that a sin,gle surface hydroxyl group type attached to silicon atoms predominates in these sampfes. The interaction of water with the surface of highly dehydrated silica-alumina is investigated, and it is established that fixedly adsorbed water is held on acidic sites far enough removed from surface hydroxyl groups that hydrogen-bonding to these groups does not occur. The ban& in the silica-aluminrt spectrum from 4000-1000 are tentat’ivelyassigned, including one which is t,hought to be due to the A10 linkage in an acidic surface group.
Introduction In recent years, spectroscopic techniques have been applied to the direct examination of functional groups 011the surface of solids. The most studied functional groups have been surface hydroxyl groups’ Eischens and ‘liskinl have given an excellent review of the work done prior to 1967. Recently, surface hydroxyl groups have been studied on a number of solids; uix., silica and porous (1) R. P. Eischens and W. A. Pllskln, “Advances in Catalysis,” vol. 10,Aaademic Press, Inc., New Yosk, Na Y., 1968, ppd 1-56,
glassj2-* y - a l ~ m i n a ,and ~ titanium dioxide.10 A (2) R. S,McDonald, J . Phys. C h e n . , 62, 1168 (1958). (3) G. J. Young, J . cozzozd sei., is, 67 (1958). (4) A. Terenin and V. Fllimonov, “Hydrogen Bonding,” Pergamon
pr;;;!
~~.~Sr~pr,” (1960). a~a~~~~
8 , 806 (6) (a) M. Folman and D. J. C. Yates, Proc. Roy. SOC.(London), A M , 32 (b) J . Phys. them., 68, 183 (1959). (7) M. R. B a d a , J . Chem. ~ h w . S6, , 1151 (1961). ~ l ~ ~A.~Hockey l J . and B. 4. Pethica, Trans. Faraday Soc., 57, 2247
(195s);
(9) J. B. Peri and R. B. Hannan, J . Phya. Chsm.. 64, 1526 (10) D. J. C. Yatepl, %bid.,66, 746 (1961).
(1960).
2224
MIOHAEL R.BASILA
preliminary observation of the surface hydroxyl groups on silica-alumina was reported by Roev, at ~ 1 . l ' Most of the work has been concerned with the surface hydroxyl groups on silica and it is
Val. 66
obtained from Godfrey L. Cabot, Inc. The surface are5 was approximately 215 m.z/g. The p-xylene and mesitylene were Ewtinw White Label material# aad were uwd a@ received except for drying with Paor;and degwing on the
vacuum system. Sample Preparation.-In order t o facilitate handling and reduce the loss of transmission by scattering, the SA samwere pressed into thin wafers 25 mm. in diameter and ever, the hydrogen-bonded shifts of the OH ples 0.1 to 0.2 mm. thick, Pressures of the order of 8000 1b./inaz stretching vibration are considerably larger for were required, The concentration of SA in these samples the surface hydroxyl groupa than for molecular was ap roxirnately 7 mg./crn.? The cell design wa8 simito tiat of Peri and Mannan8 and the details need not be hydroxyl groups with the same donor,2J Peri and lar here except to say that it wag fitted with sodium Hannang have shown that there are three inde- given chloride windows. The cell was not permanently mounted pendent surface hydroxyl groups on yalumina in on the spectrometer, but wm transferred back and forth contrast to the one type observed on silica. Roev, between the spectrometer and the vacuum system for ~ipcctroscopicmeasurements and sample treatmmt, respecet d l . , have indicated that a single band in the OH tively. I n these o erations, the sample repositioning w&8 stretching region occurs a t 3750 cm.-t in the ailica- adequately reprofucible. For treatment a t elevated alumina spectrum11 I n the light of the r-alumina temperature, the dample wm moved to a sectlor\ of the cell results, it was of interest in this Laboratory to away from the windows around which a furnace was placed. study the surface hydroxyl groups on silica- The temperatures were controlled within & l o v , The vacuum system w5s conventional, and was capable of alumina in greater detail, evucuating the cell to pressures in the 10-6 znm. range. The rates of cracking, l 2 i s o m e r i ~ a t i 6 n and ~~~ All aamples were calcined in pure oxygen at atmon he& deuterium exchange18 €or hydrocarbons over a pressure for approximately four hours a t 500'. In t f e inisilica-alumina cracking catalyst are markedly tial dehydration the temperature was raised slowly while the was continuously pumped. The rate was such that affected by the presence of residual bound water in sample the temperature was raised from ambient t o 500' over a the catalyst, For example, the rate of exchange period of 3 hr. Following calcination it was evacuated for a t least 3 hr. at this temperature t b a final pressure less between isobutane and deuterated silica-dumina than mm. A sample BO treated is called a dehydrated is increased by a factor of 13 to 20 by the prior sample in the text, irreversible readsorption of a small amount of Dz0.18 Deuterated silica-alumina (DSA) wa8 prepared by a However, if more than the optimum amount is cyclic procesa in vacuo, The sample WBB repeatedly ex= readsorbed, an abrupt drop in the exchange rate posed t o De0 vapor8 and evacuated between expogures. The process was continued until d l of the absorption in the occws.la Similar effects are reported for the OH stretching region had disappeared. The sample then cracking and isomerization of a 2-methylpentme was dehydrated by the standard procedure. over silica-alumina.iZ Spectroscopic Techni ue spectra mere measured on Perkin-Elmer Mod8 &*!rating spectrophotometer, In this paper the results of infrared spectroscopic investigations of a commercial silica-alumina The instrument was continuously flushed with air from which the HsO and COS were removed. For most of the cracking catalyst are reported. These studies were Barnplea, it W&B neceaaary to attenuate the refermce beam primarily concerned with the nature of the mrface with screens t o ~ o m ~ e n s a tfor e the loss of transmianion hydroxyl groups and of the irreversibly readsorbed through the sample by ocattering, In all of the determinatione the reference beam attenuatioii ww necessary for the N20. The gradual dehydration from room tem- initial part of the scan only, in the region 4000-3000 cm.-'. perature to 500" also was Atudied. The break on the spectra of Fig. 1-6 indicates the point where the attenuating screen was removed from the reference Experimental beam. The spectra1 slit width of the instrument wa8 of the Materials,-The Bynthetic siliclcdumina, (SA) was a order of 2 cm -I. The scanning rate wa6 adjuated t o COV@ sample of American Cyanamid Aerocat Triple A which the spectral region 4000-1260 in 60 min. contained 28% alumina, by wtight on a dry baaia. The surface area of the #A determaned by Ns advorption W&B Results and Discvssion 430 m.e(g, A portion of the SA w ~ base s exchanged (KSA) Dehydrated SA.-The spectra from 4000-10OO by s o h g the uncdcined material in a 1.5 M aqueous solution of po$assium acetate for approximately 48 hr. Ohemi- om,-' of SA and DSA dehydrated a t 500" are given is indicated the addition of 0.21 mole of potassium in Fig, 1, For comparison purposes the spectrum E 2 a z g o f aluminum. The alumina content relative t6 of dehydrated S is given in Fig. 2. The positions silica remained the same. The fraction of the aluminum on the surface of SA waa estimated by contacting a sample of of prominent bands in these spectra are listed in uncalcined SA which had particle diameters less than 44 P Table I. The spectra of SA and S are seen to be with 5 1 N a ueom HNOs solution.for 1 hr. followed by very similar. This is to be expected since the SA thorough was%ing. The decrease in aluminum content sample is predominantly silica, The apparent amounted to 0.33 mole per mole of aluminum. The surface area of the uncalcined SA was 430 m.2/g. before and 560 background in the high frequency region of the SA spectra probably is due to scattering, since it m.2/g. after the treatment with acid. Assuming all of the dissolved aluminum to have been on the surface, one steadily decreases with decreasing frequency. The obtains a ratio of 0.63 mole of potassium per mole of surface primary differences between the SA and S spectra aluminum. The possibility exists that aluminum other than that on the surface was dissolved.14 Thus the above other than in relative intensities are the absence of ratio is a minimal value and the true value may be consider- the 3650 cm.-l band in SA and the absence of the ably higher. 1394 cm.-' band in S. Most of the bands of SA The amorphous silica (S) used was a sample of Cabosil in this frequency range can be assigned by com-
now well established that they behave very much like mdecular hydroxyl groups. In general, how-
M. Roev, V. N. Filimonov, and A. N. Terenin, Optika i Spektroskopiya, 4, 328 (1958). (12) S. G. Hindin, A. G. Oblad, and G. A. Mills, J. A m . Chem. Soc.. 77, 535 (1955). (13) R . G. Haldeman and P. H. Emmett, (bid., 78, 2922 (1956). (14) L. B. Rylend, M. W. Tamele, and J. N. Wilson, "Crttalysis," Yd. 7,Reinhold Publ. Corp., New York, N. Y., 1960, p. 48. (11) L.
parison with existing assignments for the S spectrum. The most recent assignments in the regions of interest have been made by Benesi and Jones15 and by McDonald.2 The bands a t 1638, 1868, and 1975 in the S spectrum have been assigned as (15) H.A. Benesi and A. C. Jones, J . P h y s . Chem.. 68, 179 (1959).
INFRARED STUDYOF SILICA-ALUMINA SURFACE
Nov,, 1962:
2225
an Si0 overtone and two Xi0 combination bands, respectively.16 The similarity in frequency and the absence of a deuterimn isotope shift of the corresponding banda in the SA spectrum are consistent with thls assignment, and it is hereby adopted. TABLB I COMPARISON OB
VSA
(om-')
374 ti I
,
1975 1866 1633 1394
BAND'POSITIONS I N T H E SPECTRA O R SIbICAALUMINAAND SILICA VDEA (cm.-')
rs (om,-')
1866
3747 3650 1975 1868
1633 1394
1638 ..
2762 I ,
1975
McDonrild has assigned the 3747 cm.-l band in the spectrum of dehydrated S to the OH stretching vibration of surface hydroxyl groups.2 A similar band occurs in the SA spectrum a t 3745 cm.-l and is undoubtedly an OH stretching vibration. This band also has been observed by Roev, et al.ll Upon deuteratioin this band shifts to 2762 crn.-I, being the only kiand to exhibit an isotope shift. Since it has been shownle that the total hydrogen content of SA or S can be exchanged for deuterium by the technique used in this work, n0n.e of the rema ning bands in the SA spectrum can involve a vibration of a hydrogen containing group. The frequency ratio, Y B / P D , of 1.35 is very close t o the value of 1.37 calculated for a diatomic molecule, indicating that the normal coordinate of the vibration is strongly localized in the OH group. The occiscrence of a single band in the OH stretching region is rather surprising;. Two bands were expected corresponding to the two surface groups AlOH and SiOII. If hydroxyl groups were attached to the same fractions of surface A1 and Si atoms, the ratio of surface SiOH t o AlOH calculated on the assumption that the distribution of AI is uniform throughout the sam.ple would be approximately 2.5. Thus, if both groups were present, each should give rise to a band of detectable intensity provided that the magnitudes of the absorption coefficients were roughly equal. Peri and Hannan have shown lhat the r-alumina spectrum has three bands in the OH stretching region a t 3698, 3737, and 3795 c m - l corresponding to three independent hydroxyl group types.9 The factors giving rise to the three bands have not yet been determined so that there is no basis for predicting the frequency of the OH stretching vibration in the AlOH groups of SA. However, the middle frequency is close to the 3745 cm.-1 observed in the SA spectrum suggesting the possibility thak the OH stretching frequencies in the AlOH and SiOH groups lie close together so that they overlap strongly and give the appearance of a single band. The estimated half band widths of 12 f 1 and 10 f 1 for the corresponding bands in the SA and S spectra, respectively, would require accidental degeneracy and thus tend t o rule out this possibility, but not conclusively. These data suggest that one (16) R. G . Haldeman and P. H. E m m e t t , J . Am. Chem. Soc., 78, 2917 (1966).
Frequency Fig. l.--Wilica-alunina dehydrated a t 500 '. The dashed lines indicate the spectrum of deuterated silica-alumina.
,
-
I, %WO
-
,w
I
-*mo .
i boo
IMO
Frequency (cm.-l). Pig. Z.--Silica dehydrated at 500'.
r----"--7---
Frequency (cnio-l). Fig. 3.-The
interaction of p-xylene with dehydrated silica-alumina.
or the other of these group types is more easily dehydroxylated during the dehydration and that a single group type predominates in high degree of dehydration. On the basis of the frequency and half width of the band in the SA spectrum as compared to that in the S spectrum, the residual hydroxyl groups are tentatively identified as SiOH groups. Additional supporting evidence is provided by the ) the hydrogen-bonded frequency shifts (A v Q ~ of 3745 cm.-l band which are observed when pxylene and mesitylene are adsorbed on SA and S. In Fig. 3, the spectra of dehydrated SA before and after exposure to p-xylene vapor are given. The adsorption of p-xylene results in a very large increase in the background intensity. This is accompanied by a large decrease in the intensity of the 3745 cm.-l band and the appearance of a number of new bands. Most of these new bands are due to the physically adsorbed p-xylene and their frequencies are in quite good agreement with the corresponding frequencies in the spectrum of the pure liquid. The hydrogen-bonded OH stretching vibration is observed a t 3591 cm.-l. This band has very broad wings compared with the corresponding band in the S spectrum? Similar results were obtained with mesitylene as the adsorbate, the hydrogen-bonded O H stretching vibration occurring a t 3577 cm.-I. The AVOH values of 154 and 168 cm.-l for bonding with pxylene and mesitylene, respectively, are in good agreement with values of 154 and 166 cm.-l previously obtained for S.7 It should be mentioned that the AVOH obtained for SA, are single point
cy - 25%
b - IS0"C G - 200'C
3WO
4600
2504
3 m
d - 300OC e-400'C f - SOO'C
LOO0
I500
loo0
FREOLIENCY ( c M * ' \
Fig. 4.-The
Fig. 5.-The
dehydration of rehydrated silica-alumina The hPrizonta1 linas indicate the variation of the inkmity sf the 3745 cm. -1 band v i t h dehydratipn temperature,
%rQqUQDGy irreversible readsorption of water at 150' on dehydrated silica-alumina.
L , , tllfl , , , , loli
KO3
,
,
(
/
/
,
+
nhm
,
i
FreqUehCy (cm
j
¶Cm
im
m
-1).
Pi& 6,-The irreversible readsorpt,ion of water at 150* on dshydra,tedpotassium base exchanged silica-alumina,
inQWSi!i-&ktits, ivhereas it has been shown that the magnitude of AVOH depends on the fraction (f)
the boron exchange of an SA sample which previously had been treated with diborane. It is based on the assumption that the reactions of diborane with the AlOH groups and SiOH groups of 9A are the same as in the case of pure alumina ahd pbre silica, respectively. These results ~ o ~ l d ! imply that the OR 6tretchbg vibrations in AlOH a,nd 9iOR are accidently, degenerate giving rise t o the single band at 3745 Thue, although most of the data appear to support the assignment of the 3745 cm,-l band to a single group type, the possibility of a d d e n t a l degeneracy cannot be conclusively ruled out, The broad wings on the OA stretching vibration hydrogen-bonded Lo pxylene or mesitylene may be relevant to these considerations. The band a t 3650 cm.-l in the spectrum of S: h%s been assigned to hydrogen-bonded internal hydroxyl groups,' Earlier, it wag pointed out that this band vas not observed in the spectrum of SA, thus indicating the absence of these groups, Thia observation is in accord with the recent reshlts of Hall.
While this work was under may, a paper was published by Markova in which he presenteq a of surface hydroxyl groups interacting.7 The spectrum of a silica-alumina ia the OR stretching values given for $ were obtained by extrapolation region.zo Bands were observed at 8765, 3850, of o. p h t bf AUOH M. f t o f = 1.7 However, in and 3550 cm,-l in his spectrum, with the band a t Figh 3 the intensity of the 3745 cm,? band is seen 3850 cm.-l the most intense.zO In the light of the to be very small, so that f = 1 and the results for above discussion, the hand a t 3768 ern.-&undoubt&A are thought to be comparable with those €or 8, edly corresponds ts the band a t 3745 in Fig. 1, Unfortmately, no APOH data are available for while that a t 3650 cm. ia due to internal hydroxyl the hydroxyl groups on y-alumina for cornpariaon. groups, and finally the band at 35.50 cm,? is due t o Thus the AvoH data are consistent with the assign- hydrogen-bonded surface hydroxyl and adsorbed ment of the 3745 cm.-l band to the OH stretching HzO. Unfortunately, the details of his pretreatvibration in the SiOH group, but do not provide ment were not given, so that it is difficult to comconclusive evidence. pare his results with those obtained here. This assignment is supported by the results of All of the bands on the SA spectrum in the region O'Reilley, Leftin, and Hal1.I' These workers 4000-1250 cm.-l have been assigned except the studied the proton magnetic resonance spectrum 1394 cm.-l band. This assignment will be conof SA and S. Their data indicate that the majority sidered later. of protons occur in surface SiOH gr0ups.l' On the Dehydration.-The spectrum of dehydrated SA other hand, recent studies by Weiss, Knight, and which has been rehydrated by exposure to 18 Shapiro indicate that the ratio of AlOH to SiOH mm. of HzO vapor for 5 hr. and evacuated for 1 hr. groups on SA increases with increasing dehydra(18) ( a ) H. G. Weiss, J. A. Knight, and I. Shapira, J , Am, C h e m tion.I8 This result was obtained by measuring Soc., 81,1823 (1959); (b) 83, 1262 (19GO). (17) D.E.O'Reilley, H. P. Leftin, and W. K, Wall, J. Chern. Phys., 29, 970 (lg68).
(19) W. K. Hall, private wmmunicatiatl. ( 2 0 ) 2. A. Illarkova, Kzn&k.o i Katolda. 2, 435 (1Q61),
Xov., 1962
INFRARED STUDYOF SILICA-ALUMINA SURFACE
at room temperature is given in Fig. 4a.
The spectrum of hydrated SA ha,s bands a t 3740, 3500 (very broad), 1975, 1866, and 1633 (broad) cm. -1. Thia sample was subsequently dehydrated in stages a t 150, 200, 300, 400, and 500". At each of these temperatures spectra mere obtained after 2 hr. evacuation. These spectra are given in Fig. 4b through f . The interpretation of these spectra is difficult because of the breadth of the 3500 band and also because of the increase in background absorption over the entire spectral region. A similar large increase in background absorption is observed upon the adsorption of p-xylene or mesitylene as can be seen in Pig. 3. This latter phenomenon has been discussed by Roev, who attributes the background increase to extreme broadening of bands of the adsorbate or adsorbenLZ1 This broadening is thought to be due to photodesorption which occurs when hv > &, where hv is the energy of the absorbed photon and Q is the heat of I n general, upon dehydration the band a t 3746 cm.-l increases in intensity while those a t 3500 and 1633 cm.--l decrease in intensity and finally disappear. The largest intensity decreases in the 1633 cm.-l band occiir a t 150 and 200". Similar large decreases occur in the 3500 cm.-l band. This band is due to hydrogen-bonded OH stretching vibrations. The 1633 cm.-l band undoubtedly is due t o the hydrogen-bonded HOH bending mode in HzO, indicating that most of the changes in the 150 and 200" spectra are due to the desorption of strongly held physically adsorbed HBO. The accompanying increase in the 3745 cm.-l band indicates at least part of the desorbed H2O had been hydrogen-bonded to surface OH groups. The 200 and 300@spectra are essentially identical, except for small decreases in the 3500 and 1633 cm.-l regions which probably indicate removal of the final Itraces of physically adsorbed HzO. Dehydration at 400@produces a relatively large deband. This is accomcrease in the 3500 panied by an increase in the intensity of the 3745 cm.-l band and a small general decrease in background in the rwt of the spectrum. Dehydration at 500" produces a further small decrease in the 3500 cm.-l band and over the rest of the spectrum, but no detectable change in the 3745 cm.-l band. The changes in the spectra after dehydration at 400 and 500" may be due to the desorption of additional Hd3. This point will be discussed more fully in the next section. The final point of interest in these spectra is the absence of the '1394 cm.-l band in hydrated SA. This band can be seen to redevelop in the 400 and 500" spectra. Fixedly Adsorbed Water.-A, sample of dehydrated SA was exposed to 12 mm. of HzOvapor for 1 hr. and then evacuated for 1 hr., both a t 150". The HzO retained by the sample has been called " fixedly adsorbed" by Haldeman and Emmett.16 The influence of fixedly adsorbed H20on catalytic activity has been d e m o n ~ t r a t e d . ~ The ~ ~ ~ 3spectrum of SA before and after the addition of fixedly adsorbed !AZO is given in Fig. 5. It strongly re(21)
L. M. Roev, Doklady Aka& Nauk SBXR,
XSS, 561 (1900).
2327
sernbles the spectrum in Fig. 4 of hydrated SA dehydrated at l50@, One important difference is observed, that being the absknce of a significant change in the intensity of the 3745 ern.-' band, This fact implies that the H20 is essentially immobile and that it is located on sites far enough removed from the SiOH groups to prevent hydrogen-bonding. The same experiment was performed with a KSA sample. These spectra are shown in Fig. 6, The new bands which are observed in the spectrum of KSA upon the addition of fixedly adsorbed E20 are the same as those observed in the SA spectrum, but their intensities are greatly diminished. The appearance of bands at 3500 (very broad) and 1633 cm.-l (hydrogen-bonded OH stretching and HOH bending vibrations, respectively) upon the addition of fixedly adsorbed HzO indicates that the adsorbed species has not undergone drastic chemical alteration, The large decrease in intensity of the bands in the KSA as compared to the S spectra suggests that the HzOis held on catalytically active sites which can be deactivated by the addition of K. This result differs somewhat from the findings of Haldeman and Emmett, who found no change in the amount of He0 fixedly adsorbed at 110' on SA or KSA.16 On the other hand, the observed increase in the deuterium exchange rate upon the addition of fixedly adsorbed HzO to KSA at 150' amounted t o only a small fraction of that observed on regular SA,13 which is consistent with the above observation. It is noteworthy that the 1394 cm.-l band disappears upon the addition of fixedly adsorbed HzO to SA and that it is absent in the spectrum of dehydrated KSA. The absence of an intensity decrease in the 3745 cm.-l band upon the addition of fixedly adsorbed HzO suggests an alternate interpretation of the spectral changes in the final stages of dehydration a t 400 and 500' in Big. 4 which are accompanied by an intensity decrease in this band. These data imply that surface hydroxyl groups which had been hydrogen-bonded to surface SiOH groups are beinq removed, If this were the case, then the rehydration of SA must result in some regeneration of OH groups. The identity of these groups is unknown. McDonald has observed similar changes in the dehydration and rehydration of 5.2 At this point it is of interest to consider the assignment of the 1394 cm.-' band. The following observations summarize the behavior of the 1394 cm.-l band: it is absent in the spectra of dehydrated S and KSA; it occurs in the spectrum oi dehydrated SA; it is absent in the spectrum of hydrated SA, but appears upon dehydration between 300 and 500"; it disappears upon addition of fixedly adsorbed HzO; it does not exhibit an isotope shift upon deuteration of SA. The behavior of this band upon the addition of mesitylene or p-xylene cannot be determined because of the overlapping bands of these adsorbates; however, it is known that the band disappears upon the chemisorption of pyridine.22 The sensitivity of this band to presence of an adsorbate indicates (22) M.
R. Basila, unpublished results.
2228
STANLEY BRUCEENSTEIX AYD L. M. MUNHERJEE
that it is associated with a surface group. The absence of a deuterium isotope ahift indicates that it is not a hydrogen containing group. The frequency of the band is in a range which could be assigned to an A10 vibration, possibly an overtone or combination band, The disappearance upon the addition of an adsorbate suggests a shift t o lower frequency where it cannot be observed due to overlapping with the strong Si0 fundamental, The absence of the band in dehydrated KSA implies that it is associated with acidic sites on SA which can be poisoned by K. On the basis of these observations, this band is tentatively assigned t o a vibration of a surface A10 group, probably an overtone or combination band. If this interpretation were correct, it would be consistent with the suggestion that, the acidic surface AI atom changes co6rdination number from four to five upon interaction with an H20 adsorbate molecule, since
Vol. 66
the A10 force constant would be smaller for the higher coordination number.23 The assignments of the bands in the region 40001250 cm.-l in the spectrum of dehydrated SA are reviewed in Table 11.
Conclusions The foregoing discussion of the spectroscopic data has led to the following tentative conclusions:
1. In highly dehydrated samples, the surface hydroxyl groups are predominantly attached to silicon atoms, 2. The fixedly adsorbed water added at 150' is held on acidic surface sites which can be poisoned by K. 3, The fixedly adsorbed mater added at 150' retains its molecularity and is located on sites far enough removed from the surface hydroxyl groups that essentially no hydrogen-bonding to these groups occurs. TABLE I1 4. A weak band located at 1394 em.-' may be ASSIGNMENT OF WE BANDSIN THE DEHYDRATED SILICA- due to a vibration of the A10 linkage, possibly ALIXIXASPECTRUM an overtone or combination, in acidic surface Y groups. (om. -1) Assignment Acknowledgments.-The author is grateful t o 3745 OH stretch in surface SiOH groups Mr. T. R. Kantner for assistance with the experi1975 Si0 combination mental work, and to Dr. D. S. MacJver for several 1866 Si0 combination informative discussions. 1633 Si0 overtone 1394 Surface A10 overtone or combination (1) (23) Reference 14, p, 45,
EQUILIBRIA I N ET:TIHYLENEDIA&IINE. 11. HYDROGEN ELECTRODE STUDIES OF SOME ACIDS -4ND SODIUM SALTS BY STANLEY BRUCKENBTEIN AND L.nil, MUKHERJEE~ School of Chemistrv, Urbiverszly of Minne8ota, Minneapolie, Minn, RUEWE& May $1, 19UB
Hydrogen electrode studies of a series of pure acid solutions have yielded the dieeociation constants of four phenols relative t o hydrochloric acid. The behavior of these phenols indicates that the reaction HX X- = HXg-(KaxP-)occurs. Values of &m- found were 15 (phenol), 7 (thymol), 40 (0- henylphenol), and 46 (p-phenylphenol). Studies of the acid-Rodium salt mixtures permitted the determination of the reztive dissociation constants of these salts The pH of varioud 8OdlUm
+
salts has been found t o be independent of concentration and has Fielded another means of determining the relative dissociation constants for various acids. The results obtained for pKvx - pKacl are 5 40 (thymol), 4.35 (a-phenylphenol), 4,30 (phenol), 4.20 (p-phenylphenol), 2.05 (sodium thymolate), 2.10 (sodium o-phenylphenolate), 0.10 (phenylacetic acid), -0.35 (3-methyl-4phenylazophenol), -0.60 (hydrobromic acid), and - 1.4 (hvdriodic acid). The value of K x a x / K ~ xfor thymol is 2.1 X loaand 2.1 X 101for 0-phenylphenol. The difference between the negative logarithm of the nutoprotolysis constant of EDA and that of the sodium salt of EDA is 7,O.
-
Introduction A large number of compounds have been determined by titration as acids in ethylenediamine (EDA) as solvent using potentiometric methods to detect the equivalence point.3 However, only one quantitative potentiometric equilibrium study (1) This work was supported by the Office of Ordnance Research,
U. 5. Army. (2) From a thesis submitted by L. M. Mukherjee to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August, 1961. (3) (a) M. L. Moss, J. H. Elliot, and R. T. Hall, Anal. Chem.. 20, 784 (1948); (h) M. K a t s and R. A. Glenn, i h i d . , 24, 1167 (1952); (0) V. Z. Deal and G . E. A. Wyld, z h d , 27, 47 (1955); (d) A. J. Martin, ibid., 29, 79 (1957); ( e ) H.Brockman and E. ,Meyer, ~'alurzuzseenachaflen, 4 0 , 2 4 2 (1953);(f) H. Brookman and 5,Meyer, Chem. Ber., 87, 81 (1964).
of an acid and its conjugate base in EDA has been reported in the l i t e r a t ~ r e ,despite ~ the fact that such &dies are necessary to assess the limitations and advantages of EDA as a solvent for acidbase titrations. In this study, Schaap and coworkers titrated hydrogen bromide with sodium ethanolamine in the presence and absence of excess sodium bromide and were able to explain their results satisfactorily in terms of the ion-pair dissociation constants of hydrobromic acid, sodium bromide, and sodium ethanolamine using 5 X as the autoprotolysis constant ( K s )of EDA. Earlier potentiometric5 and conductometric5 stud(4) W. B. Sohaap, R. E. Bayer, J. R. Siefker, J. L. Kim P. W. Brewster, and F. C. Schmidt, Rec. Chem. P r o p . . 2 2 , 197 (1961).
