H. IMAI, Y. ONO,AND T. KEII
46
Adsorption of 1,l-Diphenyl-2-picrylhydrazyl on Solid Acid Catalysts by Haruo Imai, Yoshio Ono, and Tominaga Keii Department of Chemical Engineering, Tokyo Institute of Technology, Meguro, Tokyo, Japan (Received March 14, 1967)
Adsorption of l,l-diphenyl-2-picrylhydrazyl (DPPH) on some silica-alumina catalysts has been studied mainly by the electron spin resonance (em) method. The peak height of the esr spectrum of DPPH decreases upon addition of silica-alumina catalyst to benzene solution of DPPH. This decrease of the peak height is attributed to the chemisorption, not physical adsorption, of DPPH on the catalyst. From the investigation of some reactions of DPPH with a few acids, it is concluded that DPPH is adsorbed on Lewis acid sites, losing its unpaired electron, and on Brgnsted acid sites, obtaining a proton from silica-alumina catalyst. With the use of silica-alumina poisoned with ethyl acetate, it was found that the number of Lewis acid sites was 5 X 1012/cm2and that of Brfinsted acid sites was 1 X 1013/cm2.
Introduction Many adsorption methods of measuring the acidity of solid acid catalysts have been proposed.'-6 Among them, the method utilizing n-butylamine' is well known as the most useful one. However, this method also has some disadvantages in that the experimental procedure requires the use of optical spectrometry. In the present work, adsorption of 1,l-diphenyl-2picryl-2-hydrazyl (DPPH) on silica-alumina catalysts has been studied in connection with the surface acidity of the catalysts.
Experimental Section Silica-alumina (17% alumina) was prepared by coprecipitation with hydrolysis of ethyl orthosilicate and aluminum isopropoxide. Alumina and silica gel were obtained with hydrolysis of the corresponding compounds mentioned above. These were dried at 120" for 2 hr and then activated in air at 550" for 2 hr. Their specific surface areas, measured by the nitrogen adsorption method, were 280, 244, and 183 m2/g, respectively. Sodium-exchanged silica-alumina and aluminum were prepared by immersing the activated catalysts overnight in a 1 N sodium hydroxide aqueous solution and washing with water. The catalysts evacuated after overnight contact with ethyl acetate or n-butylamine were found to be poisoned. Silica gel treated with hydrogen fluoride was obtained by immersing silica gel in a 10% hydrogen fluoride solution and washing with water. DPPH was recrystallized from ethyl ether a t room temperature. The analytical grade benzene and carbon tetrachloride were dried with silica gel and used as solvents. The em measurements were carried out at room The Journal of Physical Chemistry
temperature with spectrometer JES 3110-X with 100 kc/sec field modulation. The cell used for the measurements is a quartz tube (8 mm in diameter) as illustrated in Figure 1. DPPH was used as a benzene solution M ) which was saturated completely (1.7 X with nitrogen. A part of about 0.4 cc of the DPPH solution was put into the cell, and the cell was set in the cavity of the esr spectrometer as illustrated in Figure 1A. A small amount of the catalyst, previously heated in vacuo for 2 hr at 500", was sealed into a glass ampoule. The catalyst was added to the cell by breaking the ampoule under a nitrogen stream from a branch of the cell. After mixing the catalyst with the solution for a few minutes, the esr measurements were carried out. Changes in the amount of DPPH were monitored by measuring the peak height of a first-derivative representation of the esr signal. In the optical spectrometric studies, the catalyst used was pressed into a plate about 0.1 mm in thickness.
Results and Discussion When a small amount of silica-alumina catalyst was added to the DPPH solution, the peak height decreased very rapidly and levelled off at a new value. The peak height decreased with successive addition of the cata-
(1) H. A. Benesi, J . A m . Chem. SOC.,7 8 , 5490 (1956). (2) H.P. Leftin and W. K . Hall, Actes Congr. Intern. Catalyse, le, Paris, 1, 1353 (1961). (3) R. L. Richardson and S. W. Benson, J . Phys. Chem., 61, 405 (1957). (4) (a) D.M. Brouwer, J . Catalysis, 1, 372 (1962); (b) W.K.Hall, ibid., 1, 53 (1962). (5) Y.Trambouze, Advan. Catalysis, 9, 544 (1957).
47
ADSORPTION OF :L, 1-DIPHENYL-2-PICRYLHYDRAZYLON SOLID ACIDCATALYSTS
0
‘silica-alumina or
(A)
silica-gel
&------silica-gel
2
4
6
s x
10-4 M .
C, Concn of DPPH, X 10-4 M .
Figure 3. Adsorption isotherms of DPPH by silica-alumina (A) and silica gel (B) at 25’.
(B?
Figure 1. Apparatus.
JT
x IO m f e c u b
-4
2
- 3 4 Q
- 2 % +I
3 c
- 1
5
Figure 2. Change in the peak height of DPPH with the successive addition of silica-alumina (A) and silica gel (B).
lyst, as shown in Figure 2A, where the abscissa represents the total amount of the catalyst added to the solution. The peak height in the absence of the catalyst corresponds to 4.1 X 1017molecules/0.4 cc. The extrapolation of the straight line of Figure 2A to the abscissa indicates 10 mg of the catalyst, which shows that the slope of the line is 4.1 X 1017molecules/lO mg, i.e., 1.5 X lOI3 molecules/cm2. When carbon t e t w chloride was used as solvent instead of benzene, the same results were obtained. Accordingly, the value 1.5 X 10’3 molecules/cm2 may be considered as the saturation amount of chemisorbed DPPH on the surface of the silica-alumina a t the experimental temperature, 25”. The adsorption isotherm at 25” was measured by changing the initial concentration of the to 2 X mole/l. The obsolution from X X served isotherm (Figure 3) is a Langmuir type with a saturation value of 1.6 X 1013molecules/cm2. A similar experiment for silica gel showed no decrease in the peak height, as illustrated in Figure 2B. However, using a large amount of the solution (4 cc) and the cell set as in Figure lB, the adsorption isotherm as shown by B in Figure 3 was obtained. From these adsorption experiments, the adsorption of D P P H on the surface of the silica-alumina sample can be considered as “chemical” and that on the silica gel as “physical.” Furthermore, it should be noted that
the saturation amount of chemisorption, 1 X 1013 moleculesjcm2, corresponds to the total number of surface acid sites of the silica-alumina sample. It is very interesting to clarify the nature of this “chemisorption.” As is well known, the acid sites of the surface of a silica-alumina catalyst are divided into Lewis acid and Brgnsted acid. In order to know the interactions of DPPH with these acid sites, reactions of DPPH with the following acids have been carried out. Antimony pentachloride and aqueous acetic acid were used as examples of a Lewis acid and a Brdnsted acid, respectively. When DPPH was added to antimony pentachloride in carbon terachloride, the color of the solution changed from violet to green. No esr signal was obtained from this solution. However, the green solution changed its color again to violet and gave the esr signal of DPPH upon the addition of water, which decomposes antimony pentachloride. I n order to discover the stoichiometry of the reaction of DPPH with antimony pentachloride, esr measurements have been made for the solution using a variety of mole ratios, 0 < [SbCls]/[DPPH] < 1. These various solutions were made from DPPH in carbon tetrachloride in a concentration of 10-3 mole/l., and antimony pentachloride in carbon tetrachloride in a concentration of 10-3 mole/l. The observed relations between the esr peak heights of the solution mixtures and the mole ratios is shown in Figure 4; one mole of DPPH reacts with one mole of antimony pentachloride, ie., one DPPH loses its unpaired electron through the attack of one Lewis acid. It must be noted that the behavior of DPPH is very similar to that of perylene. (1) It is well known that perylene forms its cation radical with charge transfer to a Lewis acid such as AlCL. (2) The radical is also formed when perylene adsorbs on Lewis acid sites of silica-alumina or a l ~ m i n a ,and ~ , ~(3) perylene develops a violet color peculiar to the radical in concentrated sulfuric acid, while DPPH turns green in concentrated sulfuric acid as well as in a antimony pentachloridecarbon tetrachloride solution. These similarities strongly suggest that D P P H forms a charge-transfer (6) B. D. Flockhart and R. C. Pink, J. Catalysis, 4, 90 (1965). Volume 78, Number 1 January 1968
48
H. IMAI,Y. ONO,AND T. KEII
l4 12
h 0
0.2
0.4 0.6 SbCls/DPPH.
Wavenumber, X 104 om-1. 2.5 2.0
0.8
1.0
0
L 360
Figure 4. Stoichiometry of the reaction of DPPH with antimony pentachloride.
complex with the Lewis acid or Lewis sites of silicaalumina. The solution of DPPH in glacial acetic acid showed a violet color and the same spectrum as did DPPH in benzene. Upon adding a small amount of water, this violet color changed to red and the esr signal disappeared. The infrared spectrum of this red solution was completely identical with that of 1,l-diphenyl-2picrylhydrazine. The optical spectra of DPPH obtained in benzene, concentrated sulfuric acid, diluted aqueous acetic acid, and on the silica-alumina catalyst are shown in Figure 5. When the spectrum from the adsorbed DPPH (b) is compared with those from DPPH in diluted acetic acid (c) and concentrated sulfuric acid (d), the band at 700 mp can be assigned to DPPH adsorbed on Lewis acid sites, and the band from 350 to 450 mfi to the superimposed one from DPPH adsorbed on Lewis acid sites and on Brprnsted acid sites. From these experimental results, it can be concluded that DPPH is adsorbed on both Lewis sites and Brprnsted sites of the surface of the catalyst. In order to confirm this conclusion concerning the nature of the surface acidity, the chemisorption on some poisoned catalysts was examined, using the same procedure as was used to get Figure 2. The saturation amounts of chemisorption obtained with various samples are summarized in Table I. From the fact that no chemisorption occurred on silica-alumina or alumina poisoned with n-butylamine which is chemisorbed on both kinds of acid sites,' it is clear that DPPH is chemisorbed on acid sites. The fact that no chemisorDtion occurred on alumina poisoned with ethyl shows that Lewis sites on which is a Lewis the surface of alumina are responsible for chemisorption
The Journal of Physical Chemistry
1.6 I
11
\ L-.
,
1
400 500 Wavelength, mp.
600
'a ,
800
Figure 5. Visible absorption spectra of DPPH in benzene (a), on silica-alumina (b), in dil CH3COOH (c), and in concd HzS04 (d).
Table I : Chemisorption Amounts of DPPH (molecules/cme)
Nontreatment Poisoned with n-butylamine Poisoned with ethyl acetate Base exchange with aq NaOH Treated with aq HF
Silicaalumina 1 . 6 X 10" 0 1.0
x
10"
Alumina 2 . 8 X 1012 0 0
2 . 7 X 1012
2 . 3 X 1012
...
...
Silica gel 0
... ... . I .
5.6 X
1012
of DPPH. On the other hand, the fact that a considerable amount of chemisorption occurred on silicaalumina even after the same poisoning shows that the latter catalyst has Brprnsted sites also. The decrease of the adsorption amount by a sodium-exchange treatment indicates that some Lewis sites as well as Brpinsted sites were poisoned by the treatment, in accordance with the result obtained by Pink.6 Chemisorption occurs on silica gel pretreated with hydrogen fluoride, which is a Brprnsted acid. From these results, it is clear that DPPH is adsorbed chemically on both Lewis acid sites and Brpinsted acid sites. Supposing that Lewis sites on the surface of the silica-alumina were completely poisoned with ethyl acetate, the total number of Brprnsted sites should be 1.0 X 1013/cm2; the number of Lewis sites should be 5 X 1012/cm2. These values agree with those obtained by the use of other adsorbates: Lewis sites are 5 X 1012/cm2(triphenylmethane)2 and ; acid sites are 3 X lo1*/ 2 X 1012/cm2( ~ e r y l e n e ) ~total cm2 (n-butylamine) (7) T.Shiba, M, Sato, H.Hattori, and poro), 6 , so (1964).
K,Yoshida, Nhokuba{ (Sap-
