2550
J. Phys. Chem. 1985,89, 2550-2555
Alumina: Sites and Mechanism for Benzaldehyde and Ammonia Reaction Miki Niwa,* Shinji Inagaki, and Yuichi Murakami Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464 Japan (Received: July 25, 1984)
Catalytically active alumina has been studied by a test reaction of benzaldehyde with ammonia. Benzaldehyde was easily adsorbed on alumina as benzoate ion with simultaneous formation of toluene and/or hydrogen. Since all the benzoate species were converted into benzonitrile by subsequent reaction with ammonia, the number of sites was measured by the usual pulse reaction at a selected condition. The density of sites for this reaction increased with the extent of dehydroxylation. The value obtained on alumina pretreated at 673 K, 2.2 f 0.2 nm-2, was independent of surface area and crystal phase, unless the surface was contaminated by anion residues such as SO-: and BO3’, or precalcined at such an extremely high temperature as 1673 K. This value is greater than those reported previously for adsorption of various molecules, and it is close to total AI-0 sites with an anion vacancy. It was thus confirmed that benzaldehyde was adsorbed on almost all the A 1 4 sites which consist of basic unsaturated oxygen and an A1 cation. Correlation of toluene formation with the surface acidity implied that toluene was formed on acid sites. An adsorption scheme, including toluene and H2 formation, is proposed in relation with the surface acid-base property.
Introduction Since alumina is important as a common support of an active component for various catalytic reactions, its surface structure has been a subject of interest to chemists. It provides us, however, rather complex problems especially about the site specification. A former study by Peril has demonstrated that the site quality is dependent on the evacuation temperature, and at least five kinds of hydroxide species were identified at the surface. Knozinger et aL2 proposed a more detailed model for active sites from the viewpoint of its structure. Because alumina has the structure of a defect spinel, the aluminum ions take positions of octahedral and tetrahedral vacant sites. Different net charges were anticipated due to different coordination numbers, which made it possible to classify the sites on alumina surface. In previous ~tudies,~” we found that ammoxidation of toluene proceeds on V205/A1203through a bifunctional mechanism. Toluene is oxidized on vanadium oxide, which is stabilized in a reduced state, V204,to produce benzaldehyde? After movement through the gas phase to the alumina ~ u r f a c e benzaldehyde ,~ is adsorbed as a benzoate ion? Finally, the benzoate ion reacts with ammonia to form benzonitrile. As understood from the mechanism, alumina plays an important role in this reaction, rather than being regarded merely as a support for the metal oxide. Attention should be focused not only on the high stability of the benzoate ion but also on its high activity with ammonia. Fink7 has stated that carboxylate ions are strongly adsorbed on alumina. This suggests that the benzoate ions on alumina might be a worthy system for investigation. We would like to know what kinds of sites are available for adsorption of benzaldehyde and how these sites catalyze the reactions. In the present study, alumina, pure or contaminated, calcined at different temperatures, was used to probe the site and mechanism for the reaction of benzaldehyde with ammonia on alumina. Involvement of surface acid-base properties is tested in this work by studying the benzonitrile formation from benzaldehyde and ammonia. Experimental Section Table I lists various kinds of A1203used in the present study. JRC-ALO 1-5 are reference catalysts obtained from the Catalysis (1) (a) Peri, J. B. J . Phys. Chem. 1965, 69, 211. (b) Ibid. 1965,69, 220. ( 2 ) (a) Knozinger, H.; Ratnasamy, P. Caral. Reo.-Sci. Eng. 1978, 17, 31.
(b) Boehm, H.-P.; Knozinger, H. “Catalysis Science and Technology”; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: West Berlin, 1983; p 39. (3) Murakami. Y.;Niwa, M.; Hattori, T.; Osawa, S.; Igushi, I.; Ando, H. J . Catal. 1977, 49, 8 3 . (4) Niwa, M.; Ando, H.; Murakami, Y. J . Carol. 1977, 49, 92. (5) Murakami, Y.; Ando, H.; Niwa, M. J . Caral. 1980, 67, 472. ( 6 ) Niwa, M.; Murakami, Y. J. Catal. 1982, 76, 9. ( 7 ) Fink, P. Rev. Roum. Chim.1969, 14, 811.
0022-3654/85/2089-2550$01.50/0
TABLE I: List of Al,O, Used in the Present Study Al,O,
surface areajm’ g-’
crystal phase
impurity
A LOa
1 2
3 4 5
KHD 673’ 87 3 1073 1273 1473 1673
169 275 121 166 238 169 136 89 32
r)+
Y
SO,’- 2.00, SiO, 0.22% Na,O 0.3%
r)
Y Y
r)+
Y
Fe,O, 0.68, SO,’- 0.57%
X X X
C i t X
5 .O
ci
1.5
01
Na,O 0.25%‘
Reference catalyst A1,0, from Catalysis Society of Japan. ’levelNumber indicating the calcination temperature in K. ‘Impurity of uncalcined KHD. Each one included errors due to the a
loss of water upon calcination.
Society of Japan, and some data on them have been reported by the Society.* Other than these catalysts, alumina KHD, commercially supplied by Sumitomo Chemical Co., Ltd., was used after calcination in air at 673-1673 K. The crystal phase was X, X , a,or a mixture of them. Alumina ALO 1 was modified by the addition of Na or B, which were impregnated from solutions of CH3COONa or H3B03, respectively. These modified aluminas were calcined at 673 K for 2 h. The atomic ratio N a or B to A1 is shown by the number preceding the catalyst, e.g., 0.01 Na-ALO 1. A standard pulse reactor employed a Silicon DC 550 column for the separation of liquid products. Prior to the reactor, a liquid nitrogen trap was installed to remove water and oxygen impurities from the helium carrier gas. Benzaldehyde was purified by vacuum distillation, while ammonia (99.9 ~ 0 1 % was ) used without further purification. Benzaldehyde contained benzoic acid as main impurity below 0.5 mol %. It was verified however that its concentration up to 4 mol % did not affect the benzonitrile formation. In a separate experiment, hydrogen production was checked. Following the reactor were in series a liquid nitrogen trap, a CuO bed at 673 K, and a TCD cell. Product hydrogen only could pass through the liquid nitrogen trap; it was converted into water by the CuO and was detected by the TCD cell. Infrared spectra of A1203 were recorded on a Jasco IR-G spectrophotometer in the region of 4OOC-1000 cm-’. The IR cell (8) Murakami, Y.‘Preparation of Catalysts 111”; Ponceler, G., Grange, P., Jacobs, P. A,, Eds.; Elsevier: Amsterdam, 1983; p 775. @ 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2551
Study on Alumina Surface I
4000
(
3500
I
(
1
1
3000
'
I
~
"
I
"1800
I
1
1600
I
I
I
1
(
cm-1
1200
1400
)
Figure 1. IR spectra on ALO 1: (a) background; (b) by admission of benzaldehyde, followed by evacuation; (c) after reaction with ammonia for 30 min.
4000
3500
3000
1800
1600
1400
1200
Figure 2. IR spectra on ALO 1 (calcined at 1273 K): (a) background; (b) by admission of benzaldehyde, followed by evacuation.
was connected to a vacuum system, and spectra upon adsorption or evacuation were obtained in situ. Acidity of the alumina was measured by titrating the sample suspended in toluene with a toluene solution of 0.1N n-butylamine with various indicators. The alumina was precalcined at 673 K for 2 h.
Results Infrared Study of Benzaldehyde and Ammonia Reaction. Infrared spectra were measured to identify the adsorbed species
on A1203and those modified by N a and B. Typical results on ALO 1 are shown in Figure 1. Upon adsorption of benzaldehyde, absorptions ascribable to benzoate ions appeared, as shown in Figure lb: the 1430-and 1540-cm-' bands are characteristics of symmetric and asymmetric stretching vibrations of carboxylate ions, respectively, and absorptions of C=C (1590, 1493, 1455 cm-I) and C-H (3050,3025 cm-I) were observed. These bands are in agreement with I R spectrum of aluminum benzoate, as reported by Chapman and Hair9 and Kuiper et al.1° The benzoate ion is bidentately bonded to an aluminum atom, and the structure can be described as Ph-Cf\A,
I
We have reported that the reaction of ammonia with the benzoate ion yields benzonitrile? In the present study, the reactivity of the adsorbed intermediate has been checked in detail. The benzoate ion can be completely removed by the contact with ammonia at 673 K for 30 min. The background IR spectrum was restored after the reaction, as shown in Figure IC. A similar experiment was done on ALO 1 which had been calcined at 1273 K, as shown in Figure 2. The intensity of the hydroxide stretching vibration obviously decreased and a sharp band was found at 3725 cm-'. The hydroxide observed on this sample was identified as being coordinated to an octahedral A1 site and termed a 11-a or 11-b group by Knozinger et a1.2 This kind of hydroxide is observed" on the surface of a-alumina (9) Chapman, I. D.; Hair, M. L. 'Proceedings of the 3rd International Congress on Catalysis"; Sachtler, W.M. H., Schuit, G. C. A,, Zwietering, P., Eds.; North-Holland: Amsterdam, 1965; Vol. 2, p 1091. (10) Kuiper, A. E. T.;Medema, J.; Van Bokhoven, J. J. G. M.; J . Catal. 1973, 29, 40. (11) Morterra, C.; Ghiotti, G.; Garrone, E.; Boccuzzi, F. J . Chem. SOC., Faraday Trans. 1 1976, 72, 2722.
Figure 3. IR spectra on ALO 1: (a) background; (b) after deuteration by D20vapor; (c) by admission of benzaldehyde, followed by evacuation.
calcined at a high temperature such as 1473 K, since all the A1 ions occupy octahedral sites in the a-A1203. Adsorption of benzaldehyde on such a well-characterized and simple surface yielded the bidentate benzoate ion as in the case of ALO 1 evacuated at 673 K. However, the asymmetric stretching vibration of the carboxylate shifted to 1570 cm-' from 1540 cm-' shown in Figure 1. Simultaneously, the hydroxide band became broad and shifted to a lower wavenumber. In order to follow the change of surface hydroxide in more detail, ALO-1alumina was deuterated by D 2 0 vapor in situ in the IR cell. Figure 3 shows the IR spectra obtained on the deuterated sample upon adsorption of benzaldehyde. The OD band observed at 2700 cm-' was decreased clearly by the formation of the benzoate species. The OD band at 2700 cm-' corresponds to hydroxide at about 3600 cm-I. It is therefore suggested that the hydroxide at ca. 3600 cm-I is reactive and easily removed or exchanged upon benzaldehyde adsorption. An infrared study was then performed on ALO-1 alumina doped with NaI2 and B," which have been reported in the lit(12) (a) Deo, A. V.; Chung, T.T.;Dalla Lana, I. G. J . Phys. Chem. 1971, 75,234. (b) Chuang, T.T.; Dalla Lana, I. G. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 773 (1972).
Niwa et al.
2552 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
- 16 9
.12 u
+d
V
gs
L
c
+d
s4 0
1
2 3 4 5 B . A . Dulse No.
6
1 2 3 NH3 pulse NO.
Figure 5. Example of pulse reaction for determining the amount of
benzoate species.
Figure 4. IR spectra on 0.044 Na-ALO 1: (a) background; (b) by admission of benzaldehyde, followed by evacuation;(c) after reaction of ammonia for 30 min, and (d) for further 40 min.
erature. In the case of 0.044 Na-ALO 1 (Figure 4), new absorption bands at 1650 and 1380 cm-' were detected in addition to the benzoate ion (I). By reaction with ammonia for 15 min, these new absorptions almost disappeared, with a small amount of the benzoate ion (I) remaining unreacted, as shown in Figure 4c. However, the benzoate ion (I) remaining at the surface was removed by subsequent reaction NH3 for 40 min. Therefore, one can indicate that the species observed a t 1650 and 1380 cm-I possessed a higher reactivity than the benzoate ion (I). New absorptions were observed also on 0.015 Na-ALO 1 and KHD alumina calcined at 673 and 1273 K. In contrast, the IR spectra of B-ALO 1 possessed a strong absorption ascribable to the B033- ion at 1370 cm-I. It revealed, however, the benzoate (I) species likewise in the unmodified ALO 1. Since reaction of adsorbed molecules with ammonia yielded no compounds other than benzonitrile, these species must be intermediates to form benzonitrile. On the other hand, carboxylates gave rise to absorptions depending on the structure, unidentate or bidentate ligand to the central atom.I4 Compared with the previous data,15 new absorptiorc at 1650 and 1380 cm-' are ascribable to unidentate benzoate ion (11). Ph- - 0 - A I
i
0
I1
The structure of the benzoate ion could be related with the presence of Na impurity. It can be said that the unidentate benzoate ion (11) is found on the alumina with Na, since KHD alumina also possesses N a as an impurity. The IR spectrum of sodium benzoate which was prepared from sodium carbonate and benzoic acid, however, revealed the bidentate benzoate ion. The spectrum of sodium benzoate ion therefore suggests that the unidentate benzoate ion is bonded to an aluminum cation as the terminal atom of the species but not with sodium. Acidic sites on alumina were checked by ammonia adsorption on the alumina with the benzoate ion. One could detect ammonia adsorbed on L-type acid site at 1610 and 1240 cm-' as well as at 3300 cm-1, though the former bands were obscured by the (13) (a) Sato, M.; Aonuma, T.; Shiba, T. "Proceedings of the 3rd International Congress on Catalysis"; Sachtler, W. M. H., Schuit, G . C. A., Zwietering, P., North-Holland: Amsterdam, 1965; Vol. I, p 366. (b) Izumi, Y.; Shiba, T. Bull. Chem. SOC.Jpn. 1964, 37, 1797. (14) Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed.; Wiley: New York, 1977, p 232. (15) Robinson, S. D.; Uttley, M. F. J . Chem. SOC.,Dalton Trans. 1973, 1912.
ii 8 1.0
0,o
423
473
573 t o 1623 uene
523
673
B,A.adsorptlon temp. / K
Figure 6. Dependence of the density of benzoate on the adsorption temperature of benzaldehyde; that of toluene simultaneously formed is also plotted against the temperature.
I
O/ 0 ' 0 ' '
0.b8
' B
0.04 ' / AI
' +
'
I
'
0
-No
0.04 ' / AI
'
Figure 7. Variation of the site density of benzoate with Na and B ad-
ditions. benzoate ion. It was thus confirmed that the acid site was conserved unpoisoned by the adsorbed benzoate ion. Measurement of Site Density on A1203. The density of sites on A1203 yielding benzoate ions was measured by the pulse technique. First, the surface of alumina was saturated with the benzoate ion by repeated injections of benzaldehyde. Ammonia pulses were then injected, until benzonitrile was not detected. The typical result on ALO 1 is shown in Figure 5. Usually, six to seven pulses of aldehyde were needed to saturate the surface. Accompanied with the adsorption of benzaldehyde, toluene was formed, but the amount decreased with pulse number. Subsequently, three pulses of ammonia were required to convert all the adsorbed species into benzonitrile. Finally, its density was calculated from the sum of benzonitrile formed and the surface area. All the benzoate ions should be removed by these methods, since the IR study claimed perfect removal of the benzoate ion. Moreover, it was confirmed by IR spectroscopy that the alumina sample used in the pulse reaction did not contain any absorption of the benzoate ion. Figure 6 shows the dependence of the site density on the adsorption temperature of benzaldehyde with both pretreatment and ammonia reaction temperatures unchanged at 673 K. As for benzaldehyde adsorption, the density was almost constant at temperatures higher than 523 K, while the toluene formation
Study on Alumina Surface
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2553
TABLE II: Density of Site and Formation of Hzon AIz03 catalyst
density of sites/nm-2
1 2 3 4 5 1 (1273 K') 0.044b Na-ALO 1 0.022 Na-ALO 1 0.01 Na-ALO 1 0.05 B-ALO 1 0.1 B-ALO 1
2.1 1.5 2.3 2.4 1.9 2.3 2.1 2.0 2.1 0.7 0.1
H2 produced'/%
ALO 0.0 0.0
54.0 17.7
0.0 0.0 0.0
KHD 673c 873 1073 1273 1473 1683
2.1 2.4 2.4 1.6 2.2 0.3
21.0 18.9 15.5 28.9 48.9
B5 toluene
0,o
523
573
623
Na or B to AI. 'Number indicating the calcination temperature in K.
I
823
o 0
.e
0
0
L 5
2
3
4
Experimental run
I
100
content
773
Pretreatment t e w , / K Figure 9. Dependence of the site density of benzoate on the temperature of treatment.
0.0
-:OS
723
BN
'100 X H2 molecule formed/benzaldehyde reacted. bAtomic ratio
0.0 1 0
673
200 /
uml g - 1
Figure 10. Change of catalyst activity by repeated uses without intermittent treatment: (0)benzonitrile; (A)toluene formed.
Figure 8. Dependence of the site density of benzoate on sulfur content.
increased with increasing the temperature from 473 K. The sites accommodating the benzoate ions were almost filled at a temperature as high as 523 K. On the other hand, ammonia reaction at temperatures lower than 573 K yielded a smaller density than those obtained above 623 K. This indicates that the ammonia reaction temperature must be higher than 623 K in order to convert the adsorbed species into benzonitrile completely. Therefore, a catalyst bed temperature of 673 K was used for both molecules. Table I1 shows the site densities on various kinds of alumina pretreated at 673 K. Figure 7 shows the effects of the addition of N a and B on the site density of ALO-1 alumina. It was not affected by the addition of N a a t all, but was strongly suppressed by the addition of B. On the other hand, it seemed that the densities on ALO 1 to 5 were correlated with impurities contents of S042-, as shown in Figure 8. ALO 1, 3, and 4 had nearly the same density of sites, while ALO 2 and 5 which contained S042-possessed smaller values. Moreover, the site density on KHD alumina remained almost constant, except for the sample calcined at 1673 K, although the surface area was changed markedly by the calcination. One can conclude that the density of sites on A1203is 2.2 f 0.2 nm-2 and is almost independent of surface area or crystal phase. However, it is smaller on AI2O3containing boron or sulfur impurities, or calcined at such an extremely high temperature as 1673 K. In order to know the property of these sites, the influence of pretreatment temperature on site density was measured on ALO-1 alumina. In this experiment, benzaldehyde was injected at 673 K on the alumina pretreated at temperatures higher than 673 K, while on those pretreated below 673 K benzaldehyde was injected at the same temperature as used for the pretreatment. Since the adsorption temperature of benzaldehyde did not affect the amount of benzoate in the temperature region above 523 K, this method can be used to study the effect of changes only in the pretreatment temperature. The site density increased (Figure 9) with increasing
pretreatment temperature up to 823 K. ALO 1 had been prepared by the calcination at 973 K, but it was hydrated to some degree upon exposure to air at room temperature. elevation of pretreatment temperature should decrease the concentration of surface hydroxide and change the surface condition. One can easily conclude therefore that the site density depends on the extent of dehydration of the alumina surface. The influence of hydration of the alumina surface was checked further by addition of water vapor in the helium carrier gas. The site densities obtained in this experiment on ALO 1, 2, and 3 were 1.7, 0.9, and 1.2, respectively. In other words, the site density was suppressed by the flowing of wet helium gas. Irrespective of kinds of alumina, it was decreased by 20-37%. Reaction Profile and Acidity Strength. Hydrogen molecule formation on adsorption of the benzoate species was measured only under conditions of high benzaldehyde conversion, because the sensitivity of the detection was low. Hydrogen was detected upon injection of benzaldehyde on KHD alumina, Na-ALO 1 whose Na/Al ratio exceeded 0.022, and ALO 1 calcined at 1273 K, as shown in Table 11. However, it was not detected upon injection of ammonia at all. The change of catalyst activity was measured over the ALO 1 without any treatment between experimental runs. The activity of alumina did not change significantly in not only the formation of nitrile but also of toluene (Figure 10). The ratio of toluene to benzonitrile formed was 0.3-0.4 throughout these experimental runs. Small deactivation may be caused by the deposit of coke material, because the color of the catalyst turned into gray to light black after the experiment. Therefore, the reduction of alumina was disregarded. Furthermore, it was confirmed that the present reaction proceeded catalytically on the alumina surface. The surface acidity was measured on KHD and ALO- 1 aluminas. The inherent acidities of these aluminas were modified by calcining at higher temperatures or sodium doping. As compared with the acidity profile of the reaction product, correlations were observed between the formed toluene to benzonitrile ratio
2554
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Niwa et al.
PKO
Figure 11. Correlation between formed toluene to benzonitrile ratio and the strongest acidity: ALO 1 ( l ) , ALO 1 modified by 0.1 B (2). 0.01 Na (3), 0.022 Na (4), 0.044 Na ( 5 ) addition and calcining at 1273 K (6), and KHD calcined at 673 K (7), 873 K (8), 1023 K (9),1273 K (11). and 1473 K (12). and the strongest acidity on ALO-1 and KHD alumina (Figure 11). The formation of toluene increased with increasing the acidity strength. On the other hand, H2 formation was observed on the surface having an acidity strength which was characterized to be less than 1.5 of the pKa value. In other words, hydrogen molecule was formed in place of toluene in this region.
Discussion Site. Active sites of alumina are produced upon dehydroxylation of alumina surface. Because BO$- or Sod2-wtich enhanced the surface a ~ i d i t y ’ decreased ~J~ the site density, and anion species was stabilized, the site should be regarded as an electron-donating one. Therefore, one can conclude that the adsorption site for benzoate species is A1-0. However, there could be various possibilities of A 1 4 site, since alumina has a complex defect spinel structure. It is therefore interesting to know the structure of the site on the surface of aluminum oxide in detail. Alumina develops some amphoteric properties during dehydroxylation, and an anion vacancy (Lewis acid site) and a coordination unsaturated oxygen (cus oxygen, Lewis base site) are formed at the surface. Knozinger et al. calculated these densities? and the density of cus oxygen for the 11 1 face on the sample dehydroxylated at 673 K was reported to be 4.5 nm-2. For other faces, the calculated surface densities are 1.9 and 3.5 nm-2 for 110 and 100 faces. Cus oxygen thus produced could be regarded as sites available for the benzoate species. However, A1 cations in the vicinity of cus oxygen are also required for the adsorption of the bidentate benzoate species. In other words, two available sites, Le., cus oxygen and a neighboring aluminum cation, must act collectively as an adsorption site. Therefore, all the cus oxygen are not available for the adsorption of bidentate benzoate species. For example, the cus oxygen anion produced by regular dehydroxylation on 111 face A-layer is not usable, because the A1 cation is immersed under surrounding hydroxides (Figure 12a). On the other hand, cus oxygen anion on 111 face B-layer seems available for the adsorption site, because the aluminum cation is also exposed on the surface (Figure 12b). The A1 cation has to possess an anion vacancy as well as an oxygen anion to become available for the adsorption site of the benzoate species, irrespective of the coordination number of AI. Existence of such an unavailable cus oxygen is possible under the condition of low degree of dehydroxylation. Therefore, available sites for the present adsorption should be lower than that of the cus oxygen which is expected upon dehydroxylation. The average density on alumina surface, 2.2 nm-2, is therefore very close to that of AI-0 site with anion vacancy which is obtained upon dehydroxylation. In other words, almost all the exposed AI-0 site could adsorb the benzoate ion. (16) There are many papers claiming the high strength of acidity caused by the sulfur impurity. For example: Hino, M.; Arata, K.; J . Chem. Soc.. Chem. Commun. 1980, 851.
Figure 12. Structure model of alumina surface:2 (a) 11 1 face A-layer; (b) 11 1 face B-layer. Open circle, hydroxide; closed circle, aluminum cation; hatched circle, cus oxygen. TABLE I11 Site Density on Alumina for Various Adsorbates crystal treatment density/ mi2 study adsorbate phase temp/K 1073 0.05 Peril’ co2 Y 0.45 Fink17 773 Y Gregg and Ramsay’* K 0.2 1273 Take et aLi9 0.75 673 ?+Y acetate y 1.2 Fink7 773 formate y 0.6 Fink’ 773 Fink’ 0.62 773 co Y Della Gatta et aLZ0 0.2 1013 9 Della Gatta et aL20 0.06 1013 Y Parkyns21 1023 1.5 NO2 Y? 6 Parkyns2’ 2.3 1023 Site densities available for adsorption of various adsorbates were reported to be smaller than that of Lewis acid or base site, as summarized in Table 111. A large value was reported only in the case of NO2 adsorption by ParkymZ1Because of inconsistency between them, models of sites have been proposed to explain the adsorption of CO (x-site by Fink’) and C 0 2 (a-site by Peril’). Specific sites were required for the adsorption of these molecules. Such a specific site is not required to stabilize the benzoate ion on the alumina surface. Unexpectedly, a high surface density was obtained on the alumina which was calcined at such a high temperature as 1473 K, although the acidity of alumina was reportedZZto be suppressed by calcining above 1027 K. One possible explanation for the extraordinary activity is the conjugated roles of oxygen anion and aluminum cation for stabilizing the benzoate species. Not only the basicity of cus oxygen but also the acidity of cation may be (17) (18) (19) (20) 43, 90.
Peri, J. B. J . Phys. Chem. 1966, 70, 3168. Gregg, S.J.; Ramsay, J. D. F. J . Phys. Chem. 1969, 73, 1243. Take, J.; Nakanaga, K., private communication on ALO-1 alumina. Della Gatta, G.; Fubini, B.; Ghiotti, G.;Morterra, C. J . Catal. 1976,
(21) Parkyns, N . In “Proceedings of the 5th International Congress on Catalysis, 1972”; Hightower, J. W., Ed.; Elsevier: New York, 1973: Vol. I . p 12-255. (22) Tanabe, K. “Solid Acids and Bases”; Kodansha-Academic Press: Tokyo, 1970; p 45. (23) Tanabe, K.; Saito, K. J . Catal. 1974, 35, 247
The Journal of Physical Chemistry, Vol. 89, No. 12. 1985 2555
Study on Alumina Surface significant for the adsorption of the bidentate benzoate species. The small shift of the bidentate carboxylate observed on the ALO-1 alumina calcined at 1273 K may be explained by the weak Lewis acidity, because the shift to higher wavenumber on silica-alumina was explained by the increase of the electron charge in the A1 cation? Likewise, the presence of the unidentate benzoate species on Na-containing alumina should be correlated with the surface acidity. The Lewis acidity of A1 cation is probably weakened by N a additive so that the unidentate carboxylate is stabilized because of a weak interaction of benzoate with A1 cation. Reaction Mechanism. By-products toluene and hydrogen are indicative of the mechanism relevant to the benzaldehyde adsorption. Approximately, these have a complementary relation, because the hydrogen molecule is formed over the catalyst which yields a small amount of toluene or vice versa. As shown in Figure 11, toluene formation can be related with the surface acidity, and it is abundant on the alumina with the strong acidity. Such a correlation was observed on other kinds of oxide catalyst, since toluene was formed on amphoteric Ti02 and ZrOz but not on entirely basic MgO and Ce02.24 Therefore, it is considered that the formation of toluene occurs on the acidic site. The site for this reaction would be regarded as a bare aluminum site of L-type acid, because only the L-type acid site has been reported on alumina surface and its presence is confirmed also in this study. Tanabe and Saitoz3reported the conversion of benzaldehyde into benzyl benzoate on alkaline earth metal oxide catalysts at relatively lower temperature such as 400 K. They considered that the reaction took place on base and acid sites which catalyzed the pseudo-Cannizzaro-type reaction. These pair sites adsorbed benzoate and benzylate species, respectively, and finally formed the ester compound. However, only the benzoate species was detected under the present condition. Neither benzyl alcohol nor benzyl benzoate was formed in this case. Consequently, the present reaction proceeds in a different mechanism from that of ester formation on MgO and CaO. As discussed above, the site accommodating the benzoate species is a basic electron donor, A1-0. The Cannizzaro-type reaction is catalyzed by the basic ion in the solution. On the basis of these similarities, one can describe the following scheme about the adsorption of benzoate ion 0
A I - 0 t Ph-CHO
- $)/ Ph-d'Al
t H
Simultaneously, benzaldehyde could interact with an acid site to which hydrogen would transfer from the former species, thus resulting in the formation of benzylate species, Le. H
AI t Ph-CHO
I H
temperature, as reported elsehwere.'O Consequently, the mechanism is confirmed experimentally at such a low temperature. However, the benzylate species is utterly unstable under the present condition, since this one is undetectable. It seems to become toluene by being further reduced. Neighboring hydroxide could reduce the species, because IR study on deuterated alumina indicated a decrease of the hydroxide intensity upon adsorption of benzaldehyde, and the hydroxide seemed to be restored on subsequent injection of ammonia. On the other hand, hydrogen liberated from the benzoate species has to become hydrogen molecule, if the acidity is so weak as not to sufficiently interact with benzaldehyde. The mechanism proposed here thus elucidates the relation'of product distribution with the surface acidity. Finally, the formation of benzonitrile from the benzoate species by the reaction with ammonia is probably described as
The acid site may be present close to the adsorbed species. The existence of a kind of acid-base pair site is not surprising, because both of them appeared simultaneously on the alumina surface due to the removal of water molecule in the calcination process. Ammonia adsorption is probably facilitated by the acid site where it is activated to react with the benzoate species. Dissociate adsorption of ammonia as N H 2 was not observed in the present study, although Periz5claimed a partial presence of the species. Ammonia would be activated via the dissociation as such species. One hydrogen remains in this scheme. Because hydrogen is not detected at all at ammonia pulse, this may be stabilized as hydroxide. As expected above, hydroxide used for the toluene formation may be thus restored to complete the catalytic reaction. The catalytic proceeding of this reaction on alumina is thus elucidated. One can finally summarize these previous equations into a stoichiometric formula in the case of no formation of hydrogen such as on A10-1 alumina. 3Ph-CHO
+ 2NH3
-
2Ph-CN
+ Ph-CH3 + 3 H 2 0
Accordingly, the amount of toluene formed should be half that of benzonitrile based on this stoichiometry. Toluene obtained experimentally on ALO 1 was 0.3-0.4 times as much as benzonitrile, which is almost consistent with the expected stoichiometry. However, the stoichiometry was not completely realized in many cases, even when the complementary hydrogen formation was taken into account. One possible reason is the experimental technique, because the unsteady pulse condition makes it difficult to carry out the stoichiometric reaction.
Acknowfedgment. This work was partially supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan (No. 59470097). Registry NO. AlzOj, 1344-28-1;PhCHO, 100-52-7;NH3, 7664-41-7.
The benzylate species was detected also on alumina at room ~
(24) Present authors, unreported results.
(25) Peri, J. B. J . Phys. Chem. 1965, 69, 231.
