THENAYUREOF ACIDICSITESox
A
3197
SILICA-ALUMINA
-
-
The Nature of the Acidic Sites on a Silica-Alumina.
Characterization by
Infrared Spectroscopic Studies of Trimethylamine and Pyridine Chemisorption'
by Michael It. Basila, Theodore R. Kantner, and Kee H. Rhee Gulf Research and Deoelopment C o m p a n y , Pittsburgh, Pennsylvania
(Received J u n e 16, 1964)
The chemisorption of trimethylamine and pyridine on a commercial silica-alumina catalyst was studied by infrared techniques. Trimethylamine was found to be unsuitable as an agent for differentiating between Lewis and Brpinsted sites because it is dissociatively adsorbed with concomitant generation of protons which form protonated chemisorbed species. Pyridine, on the other hand, is not dissociatively adsorbed and is, therefore, useful for determining the relative numbers of Lewis and Brpinsted acid sites by making use of the differences in the spectra of the pyridinium ion and coordinately bonded pyridine. Approximately equal numbers of Lewis and Brpinsted sites were observed on the highly dehydrated silica-alumina. The perturbing effects of potassium poisoning and irreversible HzOadsorption on the distribution of acid sites were studied. It was found that potassium poisoning weakens the majority of acid sites and eliminates apparent Brgnsted acidity. Irreversibly adsorbed H2O converts Lewis sites to Brpinsted sites; however, the interaction is relatively weak since the Ha0 can be removed by evacuation. These observations provide considerable insight into the nature of the acid sites and have led to the formulation of the following model. It is suggested that all of the primary acid sites on a silicaalumina are of the Lewis type centered on active surface aluminum atoms and that apparent Brpinsted sites are produced by a second-order interaction between the molecule chemisorbed on a Lewis site and a nearby surface hydroxyl group. This model qualitatively fits the experimental observations to date.
Introduction The question of the nature and importance of catalyst acidity with respect to catalytic reactions such as cracking and isomerization has been under intensive study for many years by a large number of workers. The various techniques for measuring catalyst acidity have been reviewed by Ryland, Tamele, and Wilson. They can be roughly divided into i,wo general categories, indicator techniques (with or without titration) and volumetric base adsorption techniques. Although a great deal of work has been done, little progress has been made in establishing the true nature of the catalytically important sites. One of the oldest yet still unsolved aspects of the problem is whether the acidity is of the Brpinsted (protonic) or Lewis (electron acceptor) type. One can find evidence in the literature to support either of these possibilities. I n recent years a number of spectroscopic tools have been ap-
plied to this problem, namely u l t r a v i ~ l e t , ~infrared,+'" -~ and electron spin resonance.j)l1-l4 The application of these techniques has added greatly to our knowledge (1) Presented at the symposium on "New Tools in Heterogeneous Catalytic Research," a t the 145th National Meeting of the American Chemical Society, New York, N. Y., Sept. 12, 1963. ( 2 ) L. B. Ryland, M. W. Tamele, and J. N. Wilson, "Catalysis," Vol. 7, Reinhold Publishing Corp., New York, N. Y., 1960, p. 1. (3) (a) H. P. Leftin, J . P h y s . Chern., 64, 1714 (1960); (b) H. P. Leftin and W. K. Hall, ibid., 64, 382 (1960); (e) ibid., 66, 1457 (1962); (d) Actes Congr. Intern. Catalyse, be, P a r i s , 1, 1353 (1961). (4) A. N. Webb, i b i d . , 1, 1289 (1981). (5) A. Terenin, V. Barachevsky, E. Kotov, and V. Kalmogarov, Speetrochim.' A c t a , 19, 1797 (1963), and references therein. (8) J. E. Mapes and R. P. Eischens, J . P h y s . Chern., 5 8 , 1059 (1954). (7) R. P. Eischens and W. A. Pliskin, Adoan. Catalysis, 10, 1 (1958). (8) D. E. Nicholson, A'atwre, 186, 630 (1960). (9) I. D. Chapman and M. L. Hair, .I. Catalysis, 2 , 145 (1963). (10) J. B. Peri, Actes Congr. Intern. Catalyse, Be, P a r i s , 1, 1333 (1961). (11) D. M. Brouwer, Chern. I n d . (London), 177 (1981).
Volume 68, Number 11
November, lQS/,
3198
of the nature of catalytic surfaces; however, it has also revealed the great complexity of the problem. The question of the type of acidity still remains unresolved, although the data do seem to favor the predominance of Lewis type acidity. Recently, Parryl6 has reported the utility of pyridine in the infrared study of catalyst acidity. He utilized the fact that pyridine interacting as a Lewis base (LPY) has a distinctly different spectrum from that of pyridine interacting as a Rrgnsted base (BPY).I6 His results indicate that only Lewis sites are present on alumina while both Lewis and Brgnsted sites are observed on silica-aluniina.15 This agrees with the results of Eischens and co-workers6J who studied the adsorption of ammonia on silica-alumina and alumina. In this work, the cheniisorptioii of pyridine and triniethylainiiie have been studied on a typical silicaalumina cracking catalyst for the purpose of characterizing the acidic sites. I n accord with the results of Parry1* and Eischens and co-workers,6 we find both Lewis and Br@nstedacid sites on silica-alumina. K e have also studied the perturbations of the strengths and distributions of these sites by agents such as RzO and potassiuni ions to provide further enlightenment on the nature of the sites.
Experimental Materials. The silica-alumina (SA) used in this work was the same sample of American Cyanamid Aerocat Triple A which was previously studied.'* It contains 25% alumina by v-eight on a dry basis and the surface area as determined by Kzadsorption was 430 in2/g. The base exchanged sample (KSA) was also the same as was studied in our previous work.18 The KSA surface area was also 430 m.2/g. It was prepared by soaking the uncalcined SA in a 1.5 111 aqueous potassiuin acetate solution for 48 hr. Chemical analysis indicated the addition of 0.21 g.-atoni of K/ g.-atom of A1 with the Al-Si ratio remaining the same as in the untreated sample. Deuterated silica-alumina (DSA) was prepared by repeatedly exposing the sample to DzO vapor, evacuating between exposures, until all of the OH stretching absorption in the sample disappeared. The sample was then dehydrated by the standard technique. The pyridine (PY) and trimrthylainine (T3IA) were hlallinckrodt A.R. and Eastman Kodak White Label grades, respectively. They were distilled in uucuo over P a 0 5through a glass woo1-PL06drying train, the heart cut being retained for use in each case. Techniques. The sample preparation and spectroscopic techniques were identical with those previously described.'Y The samples were used in wafer forni and T h e Journal of Physical Ch.emi8try
ill. R.BASILA, T. R. KAXTKER, 44KD K. H. RHEE
all operations were carried out on a conventional vacuum system capable of maintaining pressures in the mm. range. After a slow heating, with continuous evacuation, from ambient to 500' over a 3-hr. period, the samples were calcined in pure oxygen for a t least 4 hr. a t 500'. Following the calcination, the samples were evacuated a t 500' for at least 3 hr., but generally overnight. The addition of the nitrogen base was done, in most cases, by exposing the evacuated catalyst at 150' to the base at several mm. pressure for 1 hr. and pumping for 1 hr. at the same temperature. This procedure was followed to reduce the amount of physical adsorption. A number of additions mere niade at rooiii temperature, and the results of subsequent experiments were identical with those performed on samples which had been treated at 150'. Most of the spectra were recorded on a PerkinElmer Model 421 grating spectrophotometer. In a few instances a Perkin-Elmer Model 221 prism-grating instrument was used. In most samples the reference beam was attenuated during the initial part of the spectral scan, between 4000 and 2500 em.-', after which the attenuator was removed. The spectral slit width of the Model 421 was approximately 2 cm.-I aiid that of the Model 221 was of the order of 4 em. -1. The spectrometers were frequency calibrated using indene and atmospheric lvater vapor as standards. The frequencies quoted are thought to be accurate to h 1 cm. -l. The gravimetric data were determined on a conventional quartz spiral balance having a sensitivity of 1.5 mm./mg. The catalyst samples were wafers prepared in the same manner as those used in the spectroscopic determinations.
Results Trimethylamine Cheinisorption on Silica-Alumina. The utilization of T l I A as a means of distinguishing between Lewis and Brgnsted sites was suggested by the XHa and PY techniques of other u~orliers.6-s~'5The interaction of T3lA with a Brgnsted acid site was expected to lead to the formation of an NH+ group which could be detected by the characteristic Tu" (12) J. 3. Rooney and R. C. Pink, Proc. Chem. Soc., 70, 142 (1961): Trans. Faraday Soc., 58, 1632 (1962). (13) W. K. Hall, J . Catalysis, 1, 63 (1962). (14) J. K. Fogo, J . P h y s . Chem., 65, 1919 (1961). (15) E. P. Parry, J . Catalysis, 2, 371 (1963). (16) N. S . Gill, R. €1. Nuttall, D. E. Scaife, and D. W. A. Sharp, J . Inorg. LV:ucZ. Chem., 18, 79 (1961). (17) D. Cook, Can. J . Chem., 39, 2009 (1961). (18) AM.R. Basila, J . Phys. Chem., 66, 2223 (1962).
THENATUREOF ACIDICSITESON
A
stretching and deformation vibrations. The spectia of SA before and after the addition of TJIA a t 150' are shown in Fig. 1. The bands at 3745, 1633, and 1394 c m - ' in the SA spectrum have been previously assigned1* to the O H stretching vibration in surface silanol groups, an S i 0 overtone and a surface A 1 0 vibration, respectively. The latter assignment is quite tentative; however, it has been shown that it is definitely a vibration of a surface group.'* The bands of interest in the spectrum of TALA chemisorbed on SA are those at 3340, 3280, 3150, and 1588 cm.-' which are characteristic of the K H group
I
1 4000
/
I
3800
1
3600
WOO
-
2
3200
I
3199
SILICA-ALUMINA.
I
'
I
I
3000 2800 FRE9UENCY (crn-ll
1800
1600
I
1
1400
Figure 1. Trimethylamine chemisorbed on silica-alumina : ( a ) SA calcined and evacuated a t 500"; ( b ) SA exposed to trimethylamine a t 25 mm: and 25" for 0.5 hr. followed by 1.5-hr. evacuation a t 100" and 16-hr. evacuation a t 25".
in amines, uix., the S H stretching and deformation vibrations. The multiplicity of bands in the KH stretching region is typical of hydrogen-bonded n" gr0ups17~~~ and does n o t necessarily imply the presence of primary or of several types of secondary amines. It is not possible to determine on the basis of the frequencies or multiplicity of the NH stretching bands whether the KH group resides in a primary, secondary, or quaternary amine. The deformation frequency is considerably higher than is comnionly observed for trimethylamine salts (-1500 crn.-').l7 It is in the center of the range quoted by Bellamy20 for secondary amines and at the lower edge of the range for primary amines; however, any assignment to a particular type would be highly speculative. I n any case, the source of the hydrogen atoms which form the T\" groups is of particular interest. There are two possibilities: surface protons located in Br@nsted acid sites or protons produced by dissociative adsorption of the TAIA, In order to decide between these possibilities a sample of DSA was exposed to TIIA at room temperature. As can be seen in Fig. 2a, the deuteration is essentially
4000
36600
3200
2800
2400
MOO
1600
FREQUENCY /crn.ll
Figure 2. Trimethylamine adsorbed on deuterated silica-alumina: ( a ) DSA calcined and evacuated a t 500"; ( b ) DSA exposed to trimethylamine a t 25 mm. and 25" for 0.5 hr. folloTed by evacuation for 0.5 hr. at 25".
complete. The spectrum of TMA on DSA in Fig. 2b exhibits bands at 3745, 3340, 3280, 3150, 2762, 2490, 2433, and 2300 cni.-' indicating the presence of OH, OD, NH, and ND groups. The formation of both NH and S D groups, coupled with the exchange of OD groups to form OH groups, indicates that a significant fraction of the TlIA has dissociated. This conclusion is based on the expectation that only S D and OD groups would have been observed if there were no dissociation. The possibility exists that the surface OH groups were produced by direct exchange between surface OD groups and the TAIA methyl groups. However, under our experimental conditions no bands due to CD stretching vibrations were observed which tends to eliminate this possibility. The above conclusion is supported by the fact that the spectrum of TMA chemisorbed on alumina is identical with Fig. 1 even though the results of ParryI5 and unpublished results from this laboratory indicate that alumina has no Brqhsted acidity. The possibility remains that the protons are not exchanged by the deuteration techniques used in the preparation of DSA; however, it will be shown later that this is not the case. Hence, it appears that T M A is of little value in detecting Brfinsted acidity on catalytic surfaces. This result suggests the possibility that a similar dissociation occurs in the chemisorption of PY and KH3 which gives rise to the protonated chemisorbed species which have been observed by other workers.6-8 Pyridine Chemisorption on Silica-Alunzina. The spectrum of PY adsorbed on SA at 150' by contact with 17 mm. of PY vapor for 1 hr. and pumping for 1 hr. a t 150" is shown in Fig. 3b. It is evident from the (19) L. J. Bellamy, "The Infrared Spectra of Complex Molecules," John Wiley and Sons, Inc., New T o r k , N. T., 1958, p. 259.
(20) L. J. Bellamy, ibid., p. 249.
Volume 68, Xumber 11 S o v e m h e r , 1964
3200
M. R. BASILA,T. R. KAXTKER, AR'D K. H. RHEE
.... Table I : The Assignments of Pyridine Physically
a
and Chemically Adsorbed on Silica-Alumina PY ,a
LPY,
BPY,
HPY.
cm.-l
em.-'
cm.?
cm.-l
*NH
...
...
VCH
3083 3054 3054 1580 1972 1482 1439
(3147) (3114) (3087) 1620 1577 1490 1450
3260 3188 (3147) (3114) (3087) 1638 1620 1490 1545
3065 3043 1614 1593 1490 1438
---Type---
7a 20b 16 7b 8a 8b 19a 1%
WH VCH
VCC(N)
*cc(N) WC(N) VCC(N)
...
"See ref. 21. I
4000
I
3600
I
I
3200
I
I
2800
,..,.
I
1800
I
I
1400
FREQUENCY (crn-l)
Figure 3. Pyridine adsorbed on silica-alumina: ( a ) SA calcined and evacuated a t 500"; ( b ) SA exposed to 17 mm. of pyridine a t 150" for 1 hr. followed by evacuation for 1 hr. a t 150"; (e) evacuated an additional 16 hr. a t 150".
extent of hydrogen bonding of the surface OH groups which is indicated by the broad band centered a t 3000 cm.-l and especially from the differences in spectra b and c that even at lt5O0 an impressive amount of PY is physically adsorbed. I n Fig. 3c virtually all of the physically adsorbed I'Y has been renioved by extended (16 hr.) pumping a t 150' as is evidenced by the lack of bands characteristic of the hydrogen-bonded species (discussed below) so that all of the bands are due to chemisorbed species. In the region 1700-1400 cm. -' which was studied by Parry's our spectra are essentially identical with his. Recently, the spectra of LPY and BPY in a number of molecular complexes have been studied and vibrational assignments have been made.I6,l7 These studies are in accord with the assignments of the chemisorbed LPY and BPY made by Parry15 in the 1700-1400-cni. --I region. In the present work, more complete assignnients of the chemisorbed species have been possible because of the wider frequency range studied. These assignments, which are based on the assignments of the molecular coniplexes,16b1*are given in Table I along with the assignment of molecular 1'Y itself. I n addition to the vibrational assignments of LPY and BPY, assignments are also given for hydrogen-bonded pyridine (HPY). These latter assignments are based on studies of the pyridine-water interaction by Sidorov.21 The presence of physically adsorbed PY which is apparently held on the surface by a hydrogen-bonding interaction with surface OH groups is detected by the observation of the characteristic 1593 and 1614 tin.-' bands as T h e Journal of Phgsical Chemistru
well as by the broad hydrogen-bonded OH stretching vibration which is centered around 3000 cm.-I. The chemically adsorbed species may also be hydrogen bonded to the surface hydroxyl groups as will be shown later, but they do not give rise to the bands characteristic of the physically adsorbed species. Cheniisorbed BPY is characterized by the bands at 3260 and 3188 cm.-' which are due to the K H + stretching vibrationI7 and by the bands a t 1638 and 1545 mi.- l which are due to the combined C-C stretching and in-plane C H and n" bending r n ~ d e s . ' ~ Chemisorbed '~~ LPY is characterized by the bands at 1452 and 1577 cm.-' which are due to the combined C-C stretching and inplane CH bending niodes.'6z22 As previously mentioned, the assignments of the 1638, 1577, 1545, 1490, and 1450 cm.-' bands are in accord with those of Parry.15 The additional assignments in Table I serve to corroborate his evidence that both Lewis and Brflnsted acidity are present on the surface of a silicaalumina. This conclusion agrees with the results of Eischens and c o - ~ o r k e r s ~and ~ 7Yicholson8 on the chemisorption of XH3 on silica-alumina. It should be mentioned that the C H stretching vibrations of LPY and BPY are indistinguishable; thus, the assignments of these vibrations shown in Table I are the same for each species and are quite tentative as indicated by the parentheses. One further point of interest in Table I is the fact that vlga has the same frequency in both LPY and BPY and is shifted only slightly (8 cm.-I) from its normal position in unperturbed PY. It was indicated earlier that the chemisorbed species are engaged in hydrogen-bonding interactions with (21) A. N. Sidorov, Opt. i Spectroskopiia, 8 , 51 (1960). (22) G. Zerbi, B. Crawford and J. Overend, J . Chem. Phus., 38, 127 (1963).
SILICA-ALUMINA
3201
surface OH groups. This is evident from the results of a series of desorption experiments in which the chemisorbed P Y is removed in stages by evacuation for a number of hours (four or more) a t a series of temperatures. After each wacuation, the sample was cooled to room temperature and spectra were recorded. The results of these measurements are shown in Fig. 4. The relative amount of P Y remaining on the surface was determined by measuring the peak absorbance of the 1450-, 1490-, and i620-cm.-' bands and averaging the results to reduce the scatter. The LPY and BFY are apparently desorbed with equal probability. This is indicated by the identical functional dependence on temperature of the 1450-cm.-l band which is characteristic of LPY as compared to the 1490- and 1619cm. - l bands which contain contributions from both LPY and BPY. The number of free (nonhydrogen-
bonded) surface OH groups relative to the number which were present prior to the PY chemisorption was determined by measuring the peak absorbance of the 3745 cm. band. It was evident that roughly 10% of the surface OH groups remained hydrogen bonded after all of the HPY was removed. Figure 4a is a plot of the amount of chemisorbed PY remaining after evacuation a t a given temperature US. the temperature, and Fig. 4b is a plot of the amount of chemisorbed PY remaining us. the relative number of free surface OH groups after evacuation a t a given temperature. It is seen in Fig. 4b that the removal of a portion of the chemisorbed P Y is accompanied by an increase in the number of free (nonhydrogen-bonded) surface OH groups indicating that a hydrogen-bonding interaction exists between these two surface species. Furthermore, the relationship is linear, suggesting that the ratio of P Y to surface OH groups engaged in the interaction is constant. It has been possible to estimate this ratio by combining gravinietric adsorption measurements with the spectroscopic results. These experiments indicate that the chemisorbed PY-hydrogen-bonded surface OH ratio is approximately one; i.e., on the average, each chemisorbed P Y molecule is hydrogen bonded to a single surface OH group. It should be noted that the surface OH group concentration after the 500' evacuation is greater than that on the original surface. This increase is undoubtedly due to the decomposition of chemisorbed P Y a t the highest temperatures with subsequent generation of surface OH groups. It remains to be shown that the chemisorbed BPY is not produced by decomposition of chemisorbed PY a t low temperature with subsequent proton generation as was observed in the case of TRSA. PY was chemisorbed on DSA a t 150' and only OD and X D vibrations were observed ; hence, no detectable decomposition occurred. This experiment also shows that the surface hydrogen atoms which give rise to the BPY species are exchanged by the deuteration technique used, thus supporting the conclusion that TMA is dissociatively adsorbed. Pyridine Chemisorbed on Base-Exchanged SilicaAZumina. Alkali metals are known to have a detrimental effect on the cracking activity of silica-alumina catalyst^.^^,^^ In certain cases the addition of potassium has completely destroyed cracking activity.24 In Fig. 5 the spectrum of PY chemisorbed on KSA is given. It is evident from the absence of the char-
THENATUREOF ACIDICSITESON
5.4
5.8
I
A
6.2 6.6 7.0 7.4 (ARBITRARY UNITS )
l-
a
a
!z
m a
8
a
%4t,
Y
,=
2
a 4
0
,
,hi
100 200 300 400 TEMPERATURE ("C)
500
Figure 4. Pyridine desorption from silica-alumina: (a) the amount of pyridine remaining after evacuation a t elevated temperature us. temperelture; ( b ) the amount of pyridine remaining after evacuation a t elevated temperatures us. the number of nonhydrogen-bonded surface OH groups. The solid point represents the number of OH groups prior to pyridine chemisorption.
(23) D. Stright and J. D. Danforth, J . Phys. Chem., 57, 448 (1953). (24) G. A. Mills, E. R. Boedeker, and A. G. Oblad, J . Am. Chem. Sac., 72, 1554 (1950).
Volume 68, Number I 1
November. 1964
3202
11.R. BASILA,T. R. KAKTNER, AND E(.H. RHEE
of agreement between the sets of data is a measure of the internal consistency of the data. It is evident that the ratios are smaller for KSA than for SA, especially in going from 200 to 250'. Hence, it appears that K-poisoning severely weakens the acidic sites on SA. This agrees with the similar conclusions of HirschlerZ6and Hall, et d Z 7This explains our previous observation'8 that the amount of HzO irreversibly adsorbed a t 150' on KSA is considerably less than that adsorbed on SA.
Table I1 : Desorption of Pyridine from SA and KSA I
3400
I 3200
I
I 3000
I
I 2800
"
I
I 1700
I
I I 1500
1300
Ti,
Tz,
OC.
OC.
150 200
200 250
---UT,/IT~)M~O--SA KSA
-- ( I T , / I T ~ ) ~ ~ z o - SA
KBA
FREQUENCY (ern-')
Figure 5. Effect of K-poisoning on the chemisorption of pyridine on silica-alumina: (a) SA exposed to pyridine a t 17 mm. and 150" for 1 hr. followed by evacuation for 16 hr. a t 150'; ( b ) KSA exposed to pyridine a t 17 mm. and 150" for 1 hr. followed by evacuation for 16 hr. at 150".
acteristic NH vibrations of BPY a t 3260, 3188, and 1545 cm.-' that K-poisoning has resulted in the elimination of the Brdnsted acid sites. It should also be noted that the concentration of surface OH groups is not markedly reduced on KSA. This agrees with the results of Haldeman and Emmett.26 Further insight into the effect of K-poisoning is provided by a study of the desorption of PY from KSA in stages as previously described. Gravimetric measurements indicate that the amount of PY initially adsorbed on KSA is somewhat less than that adsorbed on SA, i.e., after exposure to 17 mm. of P Y vapor for 1 hr. a t 150' and evacuation for 1 hr. a t the same temperature, SA retained 7.35 X lop2 mg./m.2 of PY, and KSA retained 5.37 X mg./m.2. Upon subsequent heating with evacuation a t a series of temperatures, it was evident that the PY was much less tenaciously held on KSA than on SA. This finding is illustrated in Table 11. The desorption was followed spectroscopically rather than gravimetrically utilizing the intensities, I , of the 1450 and 1620 cni.-' bands of chemisorbed PY. Since the intensities give only relative concentrations (upon comparison with the initial intensity), it was necessary to form ratios of the intensity of the band a t two temperatures in order that the two sets of data for SA and KSA be comparable. Thus, in Table 11, the intensity ratio of each band a t two temperatures, I T 2 / I T repreI, sents the ratio of the PY retained after evacuation a t T z to that previously retained after evacuation a t T I . One obtains the same information for each band so that the IT2/IT, ratios should be identical and the extent The Journal of Physical Chemistry
0.93 0.76
0.77 0.26
0.94 0.75
0.71 0.24
It should be pointed out that the degree of alkali metal poisoning may vary over a considerable range. For example, Parry15 observes only a partial poisoning of the Briiisted sites on silica-alumina. Similar differences in degree occur with respect to the activity of the poisoned catalyst.26 I n some earlier work1* we have observed the disappearance of the 1394 cm.-' band of Triple A silica-alumina as it result of Kpoisoning. We have subsequently found that, in cases where this band does not conipleteiy disappear, some Brinsted acid sites remain. Thus, one must be careful about interpreting the results of alkali nietal poisoning unless the extent of poisoning has been established. Distribution of Lewis and Br4nsted Acid Sites. One of the iiiost difficult problems in spectroscopic studies of chemisorbed molecules is in the determination of absorption coefficients of the bands of interest in their spectra. In cases where the adsorption experiment gives rise to two or more species, the situation is almost hopeless. The problem is not that, the absorption coefficient of a band-in a chemisorbed species would be drastically different from that in a comparable niolecular species, but that it generally happens that the comparable molecular species is not available for evaluation. In the present case, one is faced with the evaluation of absorption coefficients of characteristic bands of chemisorbed LPY and B P T in order to determine trhe (25) R. G. Haldeman and P. H. Emmett, J . Am. Chsm. Soc., 78, 2917 (1956). (26) A. E. Hirschler, J . Catalysis, 2, 428 (1963). (27) W. K. Hall, F. E. Lutinski. and H. R. Gerberich, ibid., in press.
THENATUREOF ACIDICSITESON
SILICA-ALUMINA
3203
relative numbers of Lewis and Brqinsted acid sites. If equal absorption coefficients are assumed for the 1450 and lc545 cni. -l bands, the ratio of the number of Lewis to the number of Brqinsted sites would simply be the ratio of the observed intensities, uiz., 10.5 :l. As will be seen later, this value is incorrect. While it is true that a number of molecular complexes of LPY and BPY can be prepared, it is generally the case that these complexes are crystalline and essentially insoluble in the available infrared transparent solvents. One hesitates to use solvents of very high polarity because of the problem of the solvent effect on the absorption coefficients. The direct determination of absorption coefficient is thus not possible. However, we have been able to evaluate the relative absorption coefficients of the 1543- and 1 4 3 0 - ~ m . -bands ~ by an indirect method based on the assumption that the absorption coefficients of the Vl$a mode of chemisorbed BPY and LPY are the same. It was noted earlier that the frequency of this mode is identical (1490 cni.-I) in LPY, BPY, and HPY and only slightly displaced from the normal frequency in unperturbed PY which suggests that the vibration is not sensitive to complex formation a t the X atom. Having made this assumption the absorption coeficients were calculated by niaking use of the selective elimination of BPY on KSA. The ratio of the intensities of the 1490-cm.-' band to that of the 1450-cni.-' band on KSA was taken to be identical with the comparable ratio of chemisorbed LFY. One can then combine this ratio with the ratios of the 1490-cm.-l band to the 1430- and 1545-ciii.-' bands from the spectrum of PY chemisorbed on SA to obtain the ratio of the absorption coefficients of the characteristic LPY and BPY bands. The ratio of the absorption coefficient of the 1450-~ni.-~ band to that of the 1345-cm.-' band is
which was obtained by assuming equal absorption coefficients. Hence, there appear to be approximately equal numbers of Lewis and Brqinsted acid sites on the SA surface which has been prepared by calcination and evacuation at 500". The validity of this result is, of course, subject to the assumption that the absorption coefficient of vlga is the same in LPY and BPY. Since v l g a does not involve a vibration of the n" group in BPY and the interaction is taking place at the S atom in both cases (which would presumably lead to about the same inductive effect on the intensity) it is felt that this assumption is reasonable. It is to be noted, however, that a deviation from the assumed equal absorption coefficients will produce an inversely proportional change in the ratio of Lewis to Brqinsted sites. Effect of Irreversibly Adsorbed HzO on the Acidity of Silica-Aluwzina. It has been shown that sniall amounts of HzO irreversibly adsorbed on silicaalumina enhance the catalyst activity for cracking,282 9 isomerization,20 and exchange reactions. 2 9 , 3 0 Mapes and Eischens6 have observed the conversion of chemisorbed NH3to NH4+upon exposure of a silica-alumina sample which contained" chemisorbed KH3 to HzO vapor. Parry15 has observed' a similar conversion of chemisorbed LPY to BPY upon contact with HZO vapor. Thus, the net effect of the added water appears to be the conversion of Lewis to Brqinsted acid sites. In Fig. 6 the results of our studies of the dual adsorption of PY and HzO on SA are shown. It was found that the order of adsorption does not affect the results so that only one set of experiments is shown. In this set, PY was chemisorbed a t 150" giving the spectrum a in Fig. 6. Upon exposure to 15 mm. of HzO a t 150" for 1 hr. followed by evacuation for 1 hr. at the same temperature, spectrum b results. Further contact with 15 nim. of HzO a t room temperature for 1 hr. with subsequent pumping for 1 hr. yields spectrum c. Evacuation a t 150" for 16 hr. removes most of the H,O and yields spectrum d which is almost identical with the original spectrum a. I t is evident from the increase in the intensity of the bands at 1490, 1345, and 1638 cm.-l with accompanying decrease in the 1450-cm.-' band that the concentration of BPY is increasing a t the expense of LPY. These observations are in agreement with those of Mapes and Eischens6 and Parry15 suggesting that the HZO is converting the Lewis acid sites to Brqinsted sites. This implies that the H,O
E1450
__ =
A
8.8
€1545
The magnitude of this ratio was checked by a study of the molecular complexes of PY with BF3 and HCI at low temperature. The assumption that the absorption coefficient of the 1490-cni.-' band is the same in LPY, BPIT,and PY was necessary in this determination, also. The value obtained for the ratio was within 15yo of that given above. The ratio obtained from 1,he cheinisorbed species, themselves, is obviously preferred for use in subsequent measurements. This ratio is combined with the relative intensity data to yield a value of 1.25 for the ratio of the number of Lewis sites to the number of Brqinsted sites on the SA surface which is considerably different than the value of 10.5
(28) R. C. Hansford, I n d . Eng. Chem., 39, 849 (1947). (29) S. G. Hindin, A. G. Oblad, and G. A. Mills, J. Am. Chem. SOC., 77, 535 (1955). (30) R. G. Haldeman and P. H. Emmett, i h i d . , 78, 2922 (1956).
Volume 68, Number I 1
November, 1964
3204
M. R. BASILA,T. R. KAKTNER, AND K. H. RHEE
I
I
I
I
(Fig. 6d). As indicated above, the order of adsorption has no effect on the results so that, in truth, PY displaces HzO. It is also to be noted that the addition of HzO is accompanied by an increase in the number of hydrogen-bonded surface OH groups as indicated by the decrease on the 3745-cm.-' band and concomitant increase in the broad band centered a t 3500 cm.-'. This is a significant departure from the normal behavior of irreversibly adsorbed H 2 0 on SA (which is adsorbed under the same conditions used in obtaining spectrum b) which engages in little (if any) hydrogen bonding to surface OH groups.l8 Thus, it would appear that< the HzO interacts siinultaneously with chemisorbed PY and a surface OH group, but does not displace the PY from the acid site; however, it is difficult to establish whether the primary interaction is with the PY or the surface OH. Similar effects of HzO on the electronic spectra of cheniisorbed carbonium ions have been observed by Leftin and Hal13c3dand others4j5wherein the characteristic absorption band of the chemisorbed carbonium ion disappears upon the addition of HzO and reappears after brief evacuation a t room temperature which desorbs the HzO. On the other hand, NH3 addition permanently eliminated the carbonium ion absorption by displacing it from the site so that it is desorbed. It had been suggested by Leftin and Hallad that the HzO is physically adsorbed and that it acts as a cocatalyst to effect the desorption of the carbonium ion; however, recent work on an ether-triphenyl'carboniunz ion complex in solution by Smith and Rao3I has shown that the ether-carbonium ion adduct does not exhibit the characteristic spectrum of the carbonium ion. Hall32has suggested that a similar complex is formed between HzO and cheniisorbed triphenylmethyl carbonium ion which causes the disappearance of the absorption band. Thus, it is probable that the carbonium ion is not desorbed upon contact with HZO vapor, which is consistent with our experiments on the PY-H20 system. Since it cannot be easily pumped off in 1 hr. a t 150°, but does eventually desorb on long pumping a t the same temperature, it is evident that the interaction of Hz0 with PY is stronger than that observed with the carbonium ion. Dual Adsorption of H 2 0 and PyTidine on Base-Exchanged Silica-Aluvzina. I t has been shown in our earlier work18 that K-poisoning leads to a severe reduction in the amount of irreversibly adsorbed HzO. accd,4
I
1800
I
I
I
1600 1400 FREQUENCY (crn-1)
Figure 6. Dual adsorption of pyridine and HzO on silicaalumina: (a) SA exposed to pyridine a t 17 mm. and 150" for 1 hr. followed by evacuation a t 150" for 16 hr.; ( b ) exposure to HzO a t 15 mm. and 150' for 1 hr. followed by evacuation at 150" for 1 hr.; ( c ) exposure to HtO a t 15 mm. and 25' for 1 hr. followed by evacuation for 1 hr. a t 25"; ( d ) evacuation for 16 hr. at 150'.
displaces the PY from a priniary interaction with the surface acid sites such that the PY engages in a secondary interaction with the site through thc chemisorbed HzO by the formation of BPY. 'It is surprising, therefore, that the H2O is removed upon pumping 'at 150", whereas the amount of PY removed is negligible The Journal of Physical Chemistry
(31) W. B. Smith and P. S. Rao, J . Org. Chem., 26, 254 (1961). (32) W. K. Hall, private communication. The authors are grateful to Dr. Hall for calling attention to ref. 31 and for suggesting its significance with respect to his earlier work. 3c,d
THENATUREOF ACIDICSITESON
1800
A
3205
SILICA-ALUMINA
1600 1400 FREQUENCY ( cm")
Figure 7. Dual adsorpiion of pyridine and HzO on K-poisoned silica-alumina: (a) KSA exposed to pyridine a t 17 mm. and 150" for 1 hr. followed by evacuation for 16 hr. a t 150"; ( b ) exposed to HzOa t 15 mm. and 150" for 1 hr. followed by evacuation for 1 hr. a t 150".
Since the Brqinsted acidity is eliminated by K-poisoning, it was of interest to examine the effect'of irreversibly adsorbed HzO on PY chemisorbed on KSA. I n Fig. '7 the spectra of PY chemisorbed on KSA before and after contact with 15 mm. of HzO vapor at 150' (1-hr. exposure and 1-hr. evacuation) are shown. Again the addition of HzO leads to the conversion of LPY to BPY.
ments indicate approximately equal numbers of each type. (3) On the average, each chemisorbed P Y is hydrogen bonded to one surface OH group. (4) The desorptions of LPY and BPY exhibit the same functional dependence on temperature. (5) K-poisoning weakens the majority of the acid sites and eliminates apparent Brqinsted acidity. (6) Irreversibly adsorbed HzO converts LPY to BPY on both SA and KSA. This interaction is reversible and relatively weak since the HzO can be pumped away. (7) I n the dual adsorption of HzO and P Y on SA, the P Y is the stronger base and remains on the acid site. The HzO interacts with surface OH as well as with the chemisorbed PY. I n order to facilitate the interpretation of the above observations, the distribution of groups on the surface of SA has been estimated. For example, the approximate number of surface A1 a t o m has been determined by an acid leaching technique'&; hence, the ratio of O/Al a t the surface can be estimated by a knowledge of the surface area and the assumption that the surface 0 atoms are close-packed. The value obtained, 6.62, is identical with that obtained by assuming a homogeneous distribution of the A1 throughout the bulk of the solid. I n Table 111 a compilation of data which approximates the average distribution of surface groups is given. These data indicate that there are roughly two surface A1 atoms and five or six surface Si atoms per surface OH group. It is also evident that only a small fraction of the surface 0 atoms are situated in surface OH groups; similarly, only a small proportion of the surface A1 atoms are located in acidic sites strong enough to chemisorb PY a t 150". The most striking of the experimental observations
Table I11 : Average Distribution of Surface Groups on a Silica-Alumina"
Discussion Nature of the Acid Sites on Silica-Alumina. h number of conclusions concerning the nature of the acid sites on SA can be drawn from the results described in the foregoing sections and from our previous study of SA. 18 Before proceeding further it is worthwhile to review the pertinent experimental observations to be taken into considerat ion : (1) There are vel-y few, if any surface OH groups attached to A1 atom^.^^,^^ (2) The adsorption of NH3 and P Y on silica-alumina indicates that both Lewis and Brqinsted acidity are present on the ~ u r f a c e . ~ - ~Our ? l ~ measure-
(OH)s/Als = 0.55* Oa/Als = 6.62 Sis/Als = 2.50 (OH)s/Os = 0.084 K/Als 2 0.6c CsHjX/Als = 0,087' a Subscript S indicates a surface species. Based on a value of 1.24 X l0l4 surface OH/cm.2 for Triple A silica-alumina.34 For KSA only.18 Chemisorbed at 150'.
'
(33) W. K. Hall, H. P. Leftin, F. J. Cheselske, and D. E. O'Reilly, J . Catalysis, 2 , 506 (1963). (34) D. S. MacIver. private communication
Volume 68, Number 11
.Vozlember, 1965
3206
is the conversion of LPY to BPY through a relatively weak interaction with HzO. This reminds one of the weak interaction between XI33 and HzO in solution to form aniiiioniuin hydroxide. However, Sidorov21 has shown that the interaction between PY and H,O in solution is ~t typical hydrogen-bonding interaction. It is significant that no evidence for the formation of BPY was found in his study.21 The formation of BPY on the SA surface is, thus, a manifestation of the activation of the PY by the acid site. This observation is the basis of the following descriptive model which appears to qualitatively fit the experimental observations quite satisfactorily. It is proposed that all of the primary acid sites on a silica-alumina are of the Lewis type centered on active surface A1 atoms. Brginsted acidity occurs by a (‘second-order” interaction between the molecules chemisorbed on the Lewis type site and a nearby surface OH group. This interaction between the surface OH group and the chemisorbed molecule is essentially a hydrogen-bonding interaction. The strength of this interaction depends upon the interaction distance, t h e nature of the chemisorbed molecule and the degwe of activation of the molecule by the acid site. A critical dependence of the fraction of apparent Brginsted acid sites upon the strength of the primary Lewis sites and the critical interaction distance, CID (maximum interaction distance to produce the protonated chemisorbed species), is predicted from the model. In the preceding sections it has been concluded that an approximately equal distribution of LPY and BPY exists on SA, despite the fact that, on the average, each PY molecule is hydrogen bonded to a surface OH group. It was also found that this distribution is altered by the adsorption of HZO which significantly increases the number of BPY a t the expense of LPY. These observations imply (in terms of the above model) that the interaction distance of roughly half of the chemisorbed PY was greater than the C I D and that the addition of HzO increased the fraction of BPY by providing OH groups to interact with the chemisorbed PY a t less than the CID. On the other hand, one observes no apparent Brgnsted acidity on KSA, and it was further shown that the effect of K-poisoning was to reduce the strength of the acid sites. Thus, while the interaction distances are essentially the same as on SA, the decrease in strength of the site has resulted in a drastic decrcase in the amount of BPY. The subsequent addition of H20 produces an increase in BPY suggesting that the decrease in strength of the acid site has resulted in a smaller CID than can be achieved by the surface OH groups, but which can occur with adsorbed H20. The Journal of Physical Chemistry
R L R. BASILA,T. R. KANTNER, AND K. H. RHEE
The model thus qualitatively accounts for the effect of H20 on the distribution of Lewis and apparent Brginsted sites. The initial distribution of acid sites can also be rationalized in terms of the model. One must first have a rough idea of the geometrical arrangement of the surface groups. A possible representation of the relative distributions of the surface groups is shown in Fig. 8. This representation is not thought to be a valid model of the surface, but intended merely
0 . 4.0 A
Figure 8. Model of the basic structural unit of a silica-alumina surface.
to provide a rough idea of the surface geometry. The surface is assumed to consist of several atomic layers, the outermost layer coiisist>ingof close-packcd 0 atoms under which is a layer containing Si and A1 atoms. The structural unit of interest consists of 13 0 atoms, two A1 atoms, five or six Si a t o m , and one H atom (located in a surface OH). It is further presunied that the acidic aluniinum is exposed by reinoval of an 0 atom from the close-packed structure. Only the A1 and 0 atoms are shown in Fig. 8 since thelocationof the Si atoms are not of particular interest. The hydrogen atom can be located on any of the 13 0 atoms in the structural unit; it is equally probable that it may be located adjacent to the acidic A1 atom, or to the nonrtcidic partner A1 atom. A considerable variation in interaction distance is thus possible. The relative size of a PY iiiolecule is also shown in Fig. 8 where it is depicted as a circular molecule centered on the acidic Al. It is most probable that the PY interacts through the r\’ atom,35so that its circle of influence is expected to be somewhat larger than shown in Fig. 8. It is also ~
~~
(35) L.
R.Snyder, J . Phys. Chem.. 67, 2344
(1963).
THEKATUREOF ACIDICSITESON
SILICA-ALUMINA
3207
probable that the BPY is produced by the interaction of the OH with the N atoms rather than with the Telectrons of PY. The interaction with the a-electrons would give simple hydrogen bonding. 36 From this crude representation it is evident that the sites most likely to give BPY are those which have the OH group adjacent to the acidic A1 atom. Since it is equally probable that the OH group would be adjacent to the acidic A1 or the nonacidic Al, one would predict an equal distribution of BPY and LPY, which is experiinentally observed. Thus,the model can be used qualitatively to rationalize the experimental results on the adsorption of nitrogen bases on silica-dumina. The catalytic iniportance of the apparent Brpinsted acidity is still open to question, however. From the gravinietric measurement of PY chemisorption on SA a t 150°, the number of acid sites per cmS2is estimated to be 5.6 X 10'3. It is becoming evident that the nurnber of Catalytically active sites is considerably smaller than this. Recent estimates by Hall and c o - w o r k e r ~ ~ places ~ ~ ~ ~the number in the 1 X 1OI2 to 5 X 1 O I 2 site/cni.2 range which is an order of magnitude smaller than the number estimated by P Y aldsorption. This discrepancy can probably be attributed to the difference in basicity between PY and hydrocarbons. PY, being a niuch stronger base, can adsorb on weakly acidic sites which presumably would be too weak to be catalytically active. The role (in terms of the above model) of apparent Brgnsted sites on catalytic reactions is also strongly dependent on this factor. Since the production of an apparent Brgnsted site depends on the degree of activation of the adsorbed molecule by the pri-
mary Lewis site, it is quite possible that the relative importance of these protonic sites would vary over a considerable range from one reaction to another. One might logically attempt to apply this model to the acidic sites on an alumina surface. However, the adsorption of. PY on alumina is significantly different, than on SA. Parry16 has shown that only LPY is observed and that the addition of H20 does not convert LPY to BPY on alumina. We repeated these experiments on a y-alumina and obtained the same results. We have not been able to rationalize these differences in behavior between SA and alumina, especially the lack of a conversion from LPY to BPY upon HzO adsorption since alumina is known to have atrong acidic sites. 38
A
Conclusion The infrared spectroscopic study of pyridine chemisorbed on a typical silica-alumina cracking catalyst has led to the conclusion that there are essenticlly equal numbers of Lewis and Brgnsted acid sites present on the surface. A model has been proposed in which all of the primary acid sites are of the Lewis type, and apparent Brpinsted sites are produced by the secondorder interaction between the molecule chemisorbed on the primary Lewis site and a nearby surface OH group. This model is consistent with the experimental observations to date and is felt worthy of further evaluation. (36) M. R. Basila, J . Chem. Phys., 3 5 , 1151 (1961). (37) W. K. Hall, J. G. Larson, and H. R. Gerberich, J . Am. Chem. SOC.,8 5 , 3711 (1963). (38) H. Pines and W. 0. Haag, zbtd., 82, 2471 (1960).
Volume 88,A'umber 11
h'ovember, 1964
