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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979 nil,i31
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Communications to the Editor
not possible (around 2.1-2.4 mequiv of n-butylaminelg of mordenite); phenylazonaphythylamine gives only weak acidic coloration, which disappears with the first drops of butylamine; tolylazo-o-toluidin and phenylazodiphenylamine give acidic coloration, end points being a t 2.5-2.6 mequiv of n-butylamine/g of mordenite and 2.4-2.5 mequiv of n-butylaminelg of mordenite, respectively; 1,9-diphenyl-1,3,6,8-nonatetraen-5-one (dicinnamalacetone) gives acidic coloration, however, the end point cannot be determined since no color change happens during titration; anthraquinone does not give acidic coloration. The results indicate that certainly the indicator molecules must enter the zeolite channels to have access to the inner acidic centers which are considered to be the strongest. In the case of faujasite the indicator molecules obviously enter and leave the channels easily; with mordenite their mobility seems to be severely restricted, the bigger ones such as diphenylnonateraen being trapped inside. It should be mentioned further that H mordenite containing some percent of adsorbed water only gave a very weak coloration with the Hammett indicators. Water, in conclusion, seems to block effectively the pore openings of the mordenite. The use of Hammett indicators is, therefore, restricted to the pore diameter of the catalyst being investigated.
u
References and Notes 0 4
t
I
(1) (2) (3) (4) (5) (6)
Walling, Ch. J . Am. Chem. SOC. 1950, 72, 1164. Benesi, H. A. J . Am. Chem. SOC. 1956, 78, 5490. Hirschler, A. E. J . Catall968, 2 , 248; 1968, 1 1 , 274. Kladnig, W. J . Phys. Chem. 1976, 80,262. Barthomeuf, D. ACS Symp. Ser. 1977, No. 4 0 , 453. Mitchell, H. D.; Cross, L. C. "Tables of Interatomic Distances and Configurations in Molecules and Ions" Chemical Society, London, 1958. (7) Hammett, L. P.; Deyrup, A. J., J . Am. Chem. SOC.1932, 54, 2721. ( 8 ) This work was performed at the Instituto Venezolano de Investigaciones Cientificas, Centro de Petroleo y Quimica Caracas, Apartado 1827, Venezuela.
Worcester Polytechnic Institute' Department of Chemical Engineering Worcester, Massachusetts 0 1609
W. F. Kladnig
Received July 24, 1978; Revised Manuscript Received January 8, 1979
H'
'H
Zeolite Acidity Titration with Colored Indicators 11.98
19 2 %
Figure 2. Projections of molecular formulas of Hammett indicators as determined by use of ref 6. Criiical diameters of molecules are designed with: neutral red (3-amino-7-dimethyiamino-2-methylphenazin),a = 6.5 X 12.9 A; 4-p-ethoxyphenylazo-m-phenyienediamine,u = 6.5 X 16.2 A; 4-phenyiazo-l-naphthylamine, a = 7.8 X 11.9 A; 4-0-tolylazo-o-toluidin, a = 6.5 X 11.9 A; 1,9-diphenyl-l,3,6&nonatetraen5-one, rn = 5.4 X 19.2 A. (8-Nitrodiphenylamine, not shown in Figure 2, has been measured with u = 5.9 X 9.2 A,)
the indicators (Figure 1) cannot enter the 6.7-A pores. The indicators used had the following behavior: neutral red and p-ethoxyphenylazo-m-phenylenediamine give acidic coloration, but exact determination of the end point was 0022-3654/79/2083-0766$01 .OO/O
Publication costs assisted by the Centre National de la Recherche Scientifique
Sir: Measuring the acidity of solids with n-butylamine and colored indicators is of great interest.l Two major points have to be considered. First it has been shown that only some indicators give meaningful results.2 Secondly the principle of the titration is quite similar to what happens in solution. The indicator concentration has to be maintained low enough in order to have only a small fraction of the acid sites reacting with the indicators molecules, the major part of the acid centers being neutralized by the base molecule. Besides these fundamental features some complications occur in zeolites due to the size of the pores which in some cases is close to those of the base and of the indicators. In a very recent paper Kladnig' showed that in HY zeolites, and to a lesser extent in H mordenite, strong acid centers are titrated. He concludes that the corresponding indicators must enter the cavities and disagrees with a suggestion3 that the indicator molecules interactions with acidic centers should only take place a t a fraction of these sites. We would like to explain in more detail what we 0 1979 American Chemical
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The Journal of Physical Chemistry, Vol. 83, No. 6, 7979
Communications to the Editor
TABLE I acidity: mequiv/g
pore aperture,
zeolite
exptl
theory
a
offretite L
0.4 0.22
3 1.75
6.3 7.5
-
a Acidity measured with dimethylaminoazobenzene as indicator (total acidity).
TABLE I1
A B C
base
indicator
X
Y
size
ZA
ZB ZC
presented shortly in ref 3. At first it has to be said that speaking of accessibility we shall consider only the acid sites in the largest channels and cavities. Looking, for example, a t HY zeolite which, among the usual zeolites, has the largest pore appertures, it is easy to see that approximately one indicator molecule may enter a supercage (the smallest indicator described by Kladnig, neutral red, is 6.5 X 12.9 A compared to the 12-13-A diameter of the cavity). Hence eight molecules would fill a unit cell and then 0.66 mmol for 1 g of zeolite. Even if there were two indicator molecules per cavity (1.32 mmol per g) it is quite less than the theoretical 4.66 mequiv of acidity per g. Therefore there are less interacting indicator molecules than there are acid sites. Moreover if so much indicator molecules were in the cavities the base molecules would have no more space to reach the acid sites. Besides these steric effects it is obvious that as for solutions only a small fraction of the acid sites must react with the indicator which is only a probe of the acidity. The number of indicator molecules in cavities has then to be very small, and may be, for instance, one every ten cavities, i.e., 0.06 mmol per g of zeolite or even less. For faujasite type zeolites with rather large cavities, easily accessible through their four windows, the indicator molecules may diffuse in the crystallites and the conclusion of Kladnig on this point is quite reasonable. The problem may be quite different for zeolites with other structures. There may be two types of disturbing effects. First the pore size is large enough to admit indicator molecules but the channels have no connection to each other. This is the case for L zeolite for which the rneasured acidity (Table I)4 is a factor of 8 lower than the theoretical value. The adsorbed indicator and base molecules probably block the channels and stop the titration. The second case is when there is a sieving of the indicator and base from entering the channel. This occurs probably for offretite (Table I)5 where the channel or gmelinite cage apertures are close to 6.3 and 5 A, respectively.6 Only the n-butylamine (4-5-A diameter) may enter easily. Since anyway acidity is titrated it is suggested that the acidic centers close to the crystallite surface and accessible to the indicators molecules may be able to play the role of acidity probe if one assumes that the base neutralizes equally the sites wherever they are located and that the acidity strength distributions on the crystallite interior and exterior are similar in form. This last point has been evidenced for offretite5 by comparing various results. Either with L or offretite zeolites the results of Table I do not reflect the particle surface acidity. This peculiar acidity ?wastitrated with tributylamine which does not enter the pores. It is 0.01 m e q ~ i v / g . ~ Table I1 sums up the various situations. The n-butylamine (size x ) is smaller than the indicator (size y). In 0022-3654/79/2083-0767$01 .OO/O
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case A the base does not enter the zeolite with pore size zA. No acidity except particle surface acidity is measured (titration with tributylamine). In case B the zeolite (pore size zB)admits only the base (offretite case). The acidity is titrable but probably steric hindrance a t the pore apertures seem to disturb the results. In case C the zeolite (pore size zc) admits both the base and the indicator. A disturbing effect may arise if the pores do not allow easy diffusion of the molecules (type L) and the acidity is decreased as in case B. Our own results with H mordenite belong to case B and with faujasite to case C.3 In conclusion when the pore size of the zeolite is small, acidity is still titrable, a t least partly, as long as the base enters the pores. Since only a small fraction of the sites has to react with indicators molecules, the size of the base is more critical than that of the indicator.
References and Notes (1) (2) (3) (4)
Kladnig, W. F., J . Phys. Chem. preceeding article in this issue. Drushel, I-I. V.; Sommers, A. L. Anal. Chem. 1966, 38, 1723. Barthomeuf, D. ACS Symp. Ser. 1977, 40, 453. Franco Parra, C.; Ballivet, D.; Barthomeuf, D.J . Catal., 1975, 40, 52. ( 5 ) Mirodatos, C.; Barthomeuf, D. J . Catal , in press. (6) Gard, J. A,; Tait, J. M. Adv. Chem. Ser., 1971, No. 101, 230. (7) Mirodatos, C.; Barthomeuf, D. to be published.
Denise Barthomeuf
Laboratoire de Catalyse Organique L. A. CNRS N. 237 6962 1 Villeurbanne, France Received August 24, 1978
Orbital Interaction in the Cycloaddition Reactions of Tetrasulfur Tetranitride, Tetraarsenic Tetrasulfide, and Tetraarsenic Tetraselenide with Olefins
Sir: Tetrasulfur tetranitride, S4N4,has been of gradually increasing importance as a starting material of sulfurnitrogen chemistry.' Among other reasons, it has recently drawn much attention as the precursor of polymeric sulfur nitride, (SN),,2 which is a low-dimensional metallic conductor, even becoming a superconductor a t 0.3 Me3 Meanwhile, the reaction of S4N4with ordinary organic molecules is also a matter of continued i n t e r e ~ t .Several ~ examples of cycloaddition reactions of S4N4with olefins have been originally reported by Becke-Goehring and S ~ h l a f e r They . ~ have naively supposed the products 1 from 1
RHC/S--" I,
CHR
/ N\
2
1
3 an analogy with the usual Diels-Alder reaction, the S4N4 acting as a diene. On the other hand, Gleiter has proposed using the MO concept, structure 2 which would be ob1979 American Chemical Society
