April, 1963
ACIDITYOF SOLIDCATALYSTS
tion to carbon capable of reducing thoria a t high temperatures. It appears, therefore, to be a necessary conclusion that the filaments employed by Schneider contained a carbon impurity sufficient to reduce some of the thoria according to reaction 6 followed by the subsequent evaporation of gaseous monoxide via reaction 8. Only these two of the proposed reactions are capable of explaining quantitatively the extent of reduction and subsequent loss in weight observed by
7 69
Schneider16if the carbon content of the filaments was of the order 0.05-0.1% by weight. This value is in reasonable agreement with the findings of Schlier. * 5 Since it has been shown that the extent of reduction of thoria by tungsten is indeed small, the previous discussion suggests that a carbon impurity in tungsten filaments is implicated in the production of thorium metal which is necessary for the enhancement of the electron emission of thoriated filaments.
DETERR/IINATION OF ACIDITY OF SOLID CATALYSTS BY AMMONIA CHEMISORPTION BY
YUTllKA
KVBOKAWA~
Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka, J a p a n Received A u g u s t 37, 1962 The determination of the strength of acid sites on silira-alumina and alumina catalysts has been carIied out by measuring the rate of desorption of ammonia chemisorbed on the catalysts. The activation energy of desorption is increased with decreasing amount adsorbed from 10 to 50 kcal./mole on the silica-alumina, indicating the heterogeneous nature of the distribution of acid sites. The acid strength of the alumina is found to be comparable to that of the silica-alumina. The energy distribution of acid sites on silica-alumina deteimined by the desorption method is markedly different from that obtained by the indicator method proposed by Benesi. The reason for such discrepancy is discussed.
Introduction In view of the importaiice of acidic catalysts in the petroleum industry, a considerable amount of research on measurements of the acidity of solid catalysts has been carried out by various workers. The most general method seems to be the titration of solid catalysts in a noli-aqueous medium with amines as proposed by Tamele.2a Benesi2tl expanded this method to make it possible to determine the acid strength distribution using a complete set of available Hammett indicators. As pointed out by Benesi, such a method is based upon some assumptions and it also has a number of limitations in its actual application. For measuring surface acidity, the investigation of the chemisorption of a basic gas such as ammonia at elevated temperatures seems to be very promising. Most of these studies are a t present restricted to measurements of the amount adsorbed a t a particular temperature.3 If, however, the energy values of ammonia chemisorption were determined over a wide range of coverage, the information concerning the acid strength distribution would be much improved. At present, the only work along these lines seems to be that of Zettlemoyer, et al.,4 who have determined the acid strength distribution from heat of immersion measurements. In previous work5 it has been shown that measurement of the desorption rate can give the energy relation for chemisorption 011 oxide catalysts over a wide range of coverage. It therefore has been undertaken to in( 1 ) Department of Chemistry, The Johns Hopkins University, Baltimore 18, Md. (2)(a) 31. W.Tamele, Discussions Faraday Sue.. 8, 270 (1950); (b) H. A. Benesi, J . Am. Chem. Soc., 78, 5490 (1956): J . P h y s . Chem.. 61, 970 (1957). (3) G. A . Mills, E. R . Boedecker, and A. G. Oblad, J . Am. Chem. Sue., 74, 1564 (1950). (4)(a) J . J. Chessick and A. C. Zettlemoyer, J . Phys. Chem., 64, 1217 '. C. F. Holm, and D. M. Blackburn (1058); 64, 1131 (1960); (b) A . Clark, 1 have recently determined tlie energy values of ammonia chemisorption on silica-alumina catalysts from adsorption equilibrium measurements ( J . Catalvsis, 1, 244 (1962)). (5) Y. Kubokau-a, Bull. Chem. S U CJ. a p a n , 33, 546, 550, 5 5 3 , 739, 747, 036 (1960).
vestigate the rate of desorption of ammonia chemisorbed 011 solid acid catalysts such as silica-alumina and alumina, Experimental Materials.-The silica-alumina catalyst containing 13% alumina was obtained from the Shokubaikasei Co. The alumina used in this work was the material for chromatographic use manufactured by the Wako Junyaku Kogyo Co. The impregnation of the catalysts with sodium hydroxide was carried out as follows: a solution containing the desired amount of sodium hydroxide was added to the catalysts, dried at looo, and sintered at 400". The final catalysts contained 3 mmoles of NaOH/g. By a similar impregnation procedure, sulfuric acid was mounted on pure silica gel for chromatographic use obtained from Mallinclirodt Chemical Works. The acid concentration was 1 meq./g. Ammonia was obtained from the thermal decomposition of ammonium chloride and purified by fractional distillation. Prior to the chemisorption experiments, the silica-alum ina and alumin a catalysts were evacuated at 500" for 12 hr., and the sulfuric acid mounted on silica gel at 250" for 8 hr. The surface areas determined by BET method using nitrogen adsorption were 448 m.2/g. for the silica-alumina and 190 m.2/g. for the alumina catalysts. Apparatus and Procedure.-The amount adsorbed was determined by using a conventional constant volume apparatus. The method for measuring the rate of desorption has been described in the previous paper,5 and will be repeated here only in outline. The chemisorbed gas was desorbed by malting use of a mercury diffusion pump and the desorbed gas was collected in a McLeod gage whose pressure was follofi-ed at definite intervals. It was confirmed that the oflserved rate of desorption is unaffected by the reverse process, i.e., readsorption. The activation energy of desorption was determined as follows. The temperature was lowered abruptly during the desorDtion experiment and the rates before the temperature drop were extrapolated to those for the smaller amounts adsorbed when the measurements were carried out after the temperature drop. Thus, the rates at the two temperatures corresponded to the same amount adsorbed and, the activation energy of desorption could be obtained.
Results and Discussion Acid Strength Distribution by Desorption Method.After ammonia was allowed to adsorb at about 250°, the temperature of the specimen was raised from - 50" to 450" in stages, at each of which the activation enerry of desorption was determined in the manlier given
Vol. 67
g
t 01
I
I
I
1
10 20 30 Amount desorbed (cc. (STP)/g.).
I 50
40
Fig. 1.-Activation energy of desorption of ammonia adsorbed on silica-alumina; -e--, NaOH impregnated; the amount adsorbed at room temperature and about 10 mm. before desorption decreased from 50.51 to 37.16 cc. after NaOH treatment. Figures indicatl ;he temperature of desorption. I
50
40
-
2 30
-8
> 2 c 3 2o 10
0
I
I
I
5 10 15 Amount desorbed (cc. (STP)/g.).
I
20
Fig. 2.-Activation energy of desorption of ammonia adsorbed on alumina; -0-, XaOH impregnated; the amount adsorbed at room temperature and about 10 mm. before desorption decreased from 22.26 to 12.57 cc. after XaOH treatment.
above, Le., by observing the rate change caused by an abrupt temperature drop during the desorption experiment. The results are shown in Fig. 1, which also contains the results of similar experiments with the catalyst impregnated with sodium hydroxide. It is seen that the curve of Ed (activation energy of desorption) against the desorbed amount is composed of two parts, one corresponding to the desorption below room temperature with E d lower than 10 kcal./niole and the other to the desorption with higher values of E d . For the former, the impregnation of the catalyst with sodium hydroxide has little or no effect on the amount adsorbed or on the value of E d , whereas for the latter, the impregnation reduces markedly the amount adsorbed to about 20% of the original value. This suggests that the former is mainly associated with a physical adsorption and the latter with a chemisorption on the acid sites. It may therefore be concluded that a marked chaiige in E d with
u
I
I
I
I
I
20 40 60 Amount desorbed (cc. (STP)/g.).
80
I 100
Fig. 3.-Activation energy o f desorption of ammonia adsorbed on sulfuric acid mounted on silica gel; the amount adsorbed a t room temperature and about 10 mm. Hg before desorption was 96.5 cc.
decreasing amount adsorbed as represented in Fig. 1 indicates the heterogeneous nature of the strengths of the acid sites and also that the energy distribution of acid sites can be obtained approximately from such plots of Ed against the amount ads~rbed.~.’Similar experiments were carried out with the alumina catalyst. The results are shown in Fig. 2. It is seen that the acid strength of the alumina is comparable to that of the silica-alumina catalyst. For comparison, the surface acidity of sulfuric acid mounted on pure silica gel was measured in a similar manner with the result shown in Fig. 3. It may be assumed that the desorption with E d higher than 10 kca1.l mole is associated with the acid sites produced by sulfuric acid, since all the ammonia adsorbed on nonmounted silica gel can be desorbed with E d lower than that value. From a coniparison of Fig. 1-3, it is concluded that the acid strength of alumina as well as of silica-alumina is higher than that of sulfuric acid mounted on silica gel. Such high acid strengths also have been reported by Benesi2 using the indicator method. Comparison of Desorption and Indicator Method.The acidity measurements by the butylamine titration method have been carried out with the same catalysts in a inanner similar to that described by Beiiesi. The results are represented in Table I. Although a complete set of Hammett indicators has not been used in the presTABLE I ACIDITYOF SOLIDACIDCATALYSTS BY AMINETITRATION Indicator
Dimethyl yellow Dicinnamalacetone Benzalacetophenone
(MEQ./G.) pKB.
3.3 -3.0 -5.6
Silica-alumina
0.48 .43 .43
Alumina
0 28 .25 .23
(6) Strictly speaking., the values of E d can he used as a measure of the strengths of the acid sites only in the oases where the Ed values are equal to the heats of adsorption. Even if they are different, however, such a treatment still may be correct in an approximate sense, since there seems to be little or no doubt that the heat of adsorption varies in parallel with the E d value. (7) According to the current physical model for the acid sites on siliraalumina, the total amount of acid sites is only a fraction of the total sites. I t may therefore be allowed to assume that such a marked variatlon in B d is attributed t o a heterogeneity of the surface rather than interaction between chemisorbed molecules or induced heterogeneity.
April, 1963
FoILs
OF
POLYSTYRENESULFONIC ACIDAND ITS SALTS
ent work, the results obtained are in qualitative agreement with those of Benesi, apart from the high acid strength observed with the alumina. By means of the indicator method, Ballou, et aL,* also have found that alumina has high acid strength only when the adsorbed water vapor is completely eliminated. Such a high acid strength of alumina would be expected from the results of the desorption method described above, but there seems to be some disagreement between the conclusions obtained from the (desorption and indicator methods. From the result in Table I, it is concluded that almost all of the acid sites on silica-alumina are very strong, since essentially the same acidity value has been obtained at different pKa values. On the other hand, the desorption method indicates the presence of weak as (8) E. V. Ballou, R. T. Elarth, and R. 4.Flinn, J . Phys. Chem., 66, 1639 (1961).
771
well as strong acid sites on silica-alumina as seen in Fig. 1. The reason for such discrepancy is not apparent. It is a well known fact that the Benesi method is based upon the following assumption: the acid strength on solid catalyst can be discussed in terms of the H o function or pK, value of indicators, both referring to homogeneous media, in spite of the fact that the acidity is determined from the color change of the indicator adsorbed on solid catalysts. Further work is necessary to check whether such an assumption is indeed true. Acknowledgments.-It is a pleasure for the author to acknowledge Professor P. H. Emmett of Johns Hopkins University and Professor 0. Toyama of University of Osaka Prefecture, to whom he is indebted for valuable discussion and encouragement in this work. The author also wishes to thank Mr. K. Togano and Mr. S. Kobayashi for their assistance.
FOIILS OF POLYSTYRENESULFONIC ACID AND ITS SALTS. VIII. LOIT-TEMPERATURE INVESTIGATION OF THE INFRARED CONTINUOUS ABSORPTION SPECTRUM OF AQUEOUS ACID SOLUTIONS BY G. ZUNDELAND G.-M. SCHWAB Institute for Physical Chemistry, the LTniversity, M u n i c h , Germany Received August 37, 1962 I n studying aqueous acid solutions, a continuous absorption spectrum is observed in the infrared. The present work discuse,esthe question of whether this continuous absorption is connected with thermal vibrations in the hydrate complexes, or whether the high exchange probability of the excess proton in these hydrate complexes is a necessary condition for the existence of the absorption continuum. To decide this, we investigated polystyrenesulfonic acid a t 9O’K. by infrared spectroscopy. It was found that the continuous absorption is a t least as strong a t 90°K. as at 298°K. From this it follows that the continuous absorption has no connection with the thermal vibrations in the complexes. Finally, the high exchange probability of the excess proton is discussed as a cause for the existence of the continuum.
Continuous absorption has been observed in the infrared spectra of aqueous acid solutions, for the first time in the higher frequency regions as early as 1933 by Suhrmann and Breyerl and later by Meerlender2; in the lower frequency regions it was observed by Falk and Gigu&re3and by Ackermann4and later discussed by Wicke, Eigen and A ~ k e r m a n n Eigen ,~ and de Maeyer6 and A ~ k e r m a n n . ~ We have investigated this continuum more closely in polystyrenesulfortic acid foils7 and have shown the following: The polystyrenesulfonic acid foils contain according to the degree of hydration -SO,-H30+, respectively more strongly hydrated complexes as, for instance, -S03-H70:,+. In these complexes, the proton is largely free movable. Therefore, it is best described in proton boundary structures as done in Fig. 1 for the --S03-H703+ complex. The present study clarifies the temperature dependence of the absorption continuum. I n order to make understandable the question answered by the present (1) R. Suhrmenn and F. Breyer, 2. physik. Chem., B23, 193 (1933). (2) G.Meerlender, Dissertation (R.Suhrmann), Braunschweig, 1959. (3) M. Falk a n d P. A. GiiruBre, Can. J . Chem., 36, 1195 (1957). (4) Th. Aokermann, 2. physik. Chem. (Frankfurt), 27, 253 (1961). ( 5 ) E. Wicke, M. Eigen, and Th. Ackermann, 2. physik. C h e n . (Frankfurt), 1,340 (1954). (6) M. Eigen and L. de Maeyer, Proc. Roy. Soc. (London), 8247, 505 (1958). (7) G. Zundel, H. Woller, and G.-M. Schwab, 2. Elektrochem., 66, 129 (1962).
experiments we shall discuss first two alternative hypot heses. The Origin of the Absorption Continuum.-It is possible to imagine two ways in which the continuum could be produced. First Possibility.-Between the oxygen atom of “HzO” and that of “H3O+” in the hydrate complex is a potential with a small barrier. This potential barrier has to be small for the reason that t.he proton frequently changes from one position to the other. We may now suppose that the height and breadth of such a small potential barrier are particularly strongly affected by the thermal vibrational and rotary motions of the molecules in these complexes. The changes of the barrier effected by thermal motions could then be large in comparison with the height and breadth of the barrier a t low temperatures. A model which makes this conceivable as a cause for the continuum was discussed by us in ref. 7. Second Possibility.-The absorption continuum is caused by the high exchange probability of the excess proton within the complexes (see below). The low-temperature test permits of a decision between these two possibilities. The Experiments and their Results We investigatedaa two polystyrenesulfonic acid foils,8b one 5% cross-linkedEc (Fig. 2) and one 10%
